Advanced Bash-Scripting Guide
An in-depth exploration of the art of shell scripting
Mendel Cooper
thegrendel.abs@gmail.com
10
10 Mar 2014
Revision History
Revision 6.5 05 Apr 2012 Revised by: mc
'TUNGSTENBERRY' release
Revision 6.6 27 Nov 2012 Revised by: mc
'YTTERBIUMBERRY' release
Revision 10 10 Mar 2014 Revised by: mc
'PUBLICDOMAIN' release
This tutorial assumes no previous knowledge of scripting or
programming, yet progresses rapidly toward an intermediate/advanced
level of instruction . . . all the while sneaking in little nuggets
of UNIX® wisdom and lore. It serves as a textbook, a manual for
self-study, and as a reference and source of knowledge on shell
scripting techniques. The exercises and heavily-commented examples
invite active reader participation, under the premise that the only
way to really learn scripting is to write scripts.
This book is suitable for classroom use as a general introduction to
programming concepts.
This document is herewith granted to the Public Domain. No copyright!
________________________________________________________________
________________________________________________________________
Dedication
For Anita, the source of all the magic
Table of Contents
Part 1. Introduction
1. Shell Programming!
2. Starting Off With a Sha-Bang
Part 2. Basics
3. Special Characters
4. Introduction to Variables and Parameters
5. Quoting
6. Exit and Exit Status
7. Tests
8. Operations and Related Topics
Part 3. Beyond the Basics
9. Another Look at Variables
10. Manipulating Variables
11. Loops and Branches
12. Command Substitution
13. Arithmetic Expansion
14. Recess Time
Part 4. Commands
15. Internal Commands and Builtins
16. External Filters, Programs and Commands
17. System and Administrative Commands
Part 5. Advanced Topics
18. Regular Expressions
19. Here Documents
20. I/O Redirection
21. Subshells
22. Restricted Shells
23. Process Substitution
24. Functions
25. Aliases
26. List Constructs
27. Arrays
28. Indirect References
29. /dev and /proc
30. Network Programming
31. Of Zeros and Nulls
32. Debugging
33. Options
34. Gotchas
35. Scripting With Style
36. Miscellany
37. Bash, versions 2, 3, and 4
- Endnotes
38.1. Author's Note
38.2. About the Author
38.3. Where to Go For Help
38.4. Tools Used to Produce This Book
38.5. Credits
38.6. Disclaimer
Bibliography
A. Contributed Scripts
B. Reference Cards
C. A Sed and Awk Micro-Primer
C.1. Sed
C.2. Awk
D. Parsing and Managing Pathnames
E. Exit Codes With Special Meanings
F. A Detailed Introduction to I/O and I/O Redirection
G. Command-Line Options
G.1. Standard Command-Line Options
G.2. Bash Command-Line Options
H. Important Files
I. Important System Directories
J. An Introduction to Programmable Completion
K. Localization
L. History Commands
M. Sample .bashrc and .bash_profile Files
N. Converting DOS Batch Files to Shell Scripts
O. Exercises
O.1. Analyzing Scripts
O.2. Writing Scripts
P. Revision History
Q. Download and Mirror Sites
R. To Do List
S. Copyright
T. ASCII Table
Index
List of Tables
8-1. Operator Precedence
15-1. Job identifiers
33-1. Bash options
36-1. Numbers representing colors in Escape Sequences
B-1. Special Shell Variables
B-2. TEST Operators: Binary Comparison
B-3. TEST Operators: Files
B-4. Parameter Substitution and Expansion
B-5. String Operations
B-6. Miscellaneous Constructs
C-1. Basic sed operators
C-2. Examples of sed operators
E-1. Reserved Exit Codes
N-1. Batch file keywords / variables / operators, and their shell
equivalents
N-2. DOS commands and their UNIX equivalents
P-1. Revision History
List of Examples
2-1. cleanup: A script to clean up log files in /var/log
2-2. cleanup: An improved clean-up script
2-3. cleanup: An enhanced and generalized version of above scripts.
3-1. Code blocks and I/O redirection
3-2. Saving the output of a code block to a file
3-3. Running a loop in the background
3-4. Backup of all files changed in last day
4-1. Variable assignment and substitution
4-2. Plain Variable Assignment
4-3. Variable Assignment, plain and fancy
4-4. Integer or string?
4-5. Positional Parameters
4-6. wh, whois domain name lookup
4-7. Using shift
5-1. Echoing Weird Variables
5-2. Escaped Characters
5-3. Detecting key-presses
6-1. exit / exit status
6-2. Negating a condition using !
7-1. What is truth?
7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[
7-3. Arithmetic Tests using (( ))
7-4. Testing for broken links
7-5. Arithmetic and string comparisons
7-6. Testing whether a string is null
7-7. zmore
8-1. Greatest common divisor
8-2. Using Arithmetic Operations
8-3. Compound Condition Tests Using && and ||
8-4. Representation of numerical constants
8-5. C-style manipulation of variables
9-1. $IFS and whitespace
9-2. Timed Input
9-3. Once more, timed input
9-4. Timed read
9-5. Am I root?
9-6. arglist: Listing arguments with $* and $@
9-7. Inconsistent $* and $@ behavior
9-8. $* and $@ when $IFS is empty
9-9. Underscore variable
9-10. Using declare to type variables
9-11. Generating random numbers
9-12. Picking a random card from a deck
9-13. Brownian Motion Simulation
9-14. Random between values
9-15. Rolling a single die with RANDOM
9-16. Reseeding RANDOM
9-17. Pseudorandom numbers, using awk
10-1. Inserting a blank line between paragraphs in a text file
10-2. Generating an 8-character "random" string
10-3. Converting graphic file formats, with filename change
10-4. Converting streaming audio files to ogg
10-5. Emulating getopt
10-6. Alternate ways of extracting and locating substrings
10-7. Using parameter substitution and error messages
10-8. Parameter substitution and "usage" messages
10-9. Length of a variable
10-10. Pattern matching in parameter substitution
10-11. Renaming file extensions:
10-12. Using pattern matching to parse arbitrary strings
10-13. Matching patterns at prefix or suffix of string
11-1. Simple for loops
11-2. for loop with two parameters in each [list] element
11-3. Fileinfo: operating on a file list contained in a variable
11-4. Operating on a parameterized file list
11-5. Operating on files with a for loop
11-6. Missing in [list] in a for loop
11-7. Generating the [list] in a for loop with command substitution
11-8. A grep replacement for binary files
11-9. Listing all users on the system
11-10. Checking all the binaries in a directory for authorship
11-11. Listing the symbolic links in a directory
11-12. Symbolic links in a directory, saved to a file
11-13. A C-style for loop
11-14. Using efax in batch mode
11-15. Simple while loop
11-16. Another while loop
11-17. while loop with multiple conditions
11-18. C-style syntax in a while loop
11-19. until loop
11-20. Nested Loop
11-21. Effects of break and continue in a loop
11-22. Breaking out of multiple loop levels
11-23. Continuing at a higher loop level
11-24. Using continue N in an actual task
11-25. Using case
11-26. Creating menus using case
11-27. Using command substitution to generate the case variable
11-28. Simple string matching
11-29. Checking for alphabetic input
11-30. Creating menus using select
11-31. Creating menus using select in a function
12-1. Stupid script tricks
12-2. Generating a variable from a loop
12-3. Finding anagrams
15-1. A script that spawns multiple instances of itself
15-2. printf in action
15-3. Variable assignment, using read
15-4. What happens when read has no variable
15-5. Multi-line input to read
15-6. Detecting the arrow keys
15-7. Using read with file redirection
15-8. Problems reading from a pipe
15-9. Changing the current working directory
15-10. Letting let do arithmetic.
15-11. Showing the effect of eval
15-12. Using eval to select among variables
15-13. Echoing the command-line parameters
15-14. Forcing a log-off
15-15. A version of rot13
15-16. Using set with positional parameters
15-17. Reversing the positional parameters
15-18. Reassigning the positional parameters
15-19. "Unsetting" a variable
15-20. Using export to pass a variable to an embedded awk script
15-21. Using getopts to read the options/arguments passed to a script
15-22. "Including" a data file
15-23. A (useless) script that sources itself
15-24. Effects of exec
15-25. A script that exec's itself
15-26. Waiting for a process to finish before proceeding
15-27. A script that kills itself
16-1. Using ls to create a table of contents for burning a CDR disk
16-2. Hello or Good-bye
16-3. Badname, eliminate file names in current directory containing
bad characters and whitespace.
16-4. Deleting a file by its inode number
16-5. Logfile: Using xargs to monitor system log
16-6. Copying files in current directory to another
16-7. Killing processes by name
16-8. Word frequency analysis using xargs
16-9. Using expr
16-10. Using date
16-11. Date calculations
16-12. Word Frequency Analysis
16-13. Which files are scripts?
16-14. Generating 10-digit random numbers
16-15. Using tail to monitor the system log
16-16. Printing out the From lines in stored e-mail messages
16-17. Emulating grep in a script
16-18. Crossword puzzle solver
16-19. Looking up definitions in Webster's 1913 Dictionary
16-20. Checking words in a list for validity
16-21. toupper: Transforms a file to all uppercase.
16-22. lowercase: Changes all filenames in working directory to
lowercase.
16-23. du: DOS to UNIX text file conversion.
16-24. rot13: ultra-weak encryption.
16-25. Generating "Crypto-Quote" Puzzles
16-26. Formatted file listing.
16-27. Using column to format a directory listing
16-28. nl: A self-numbering script.
16-29. manview: Viewing formatted manpages
16-30. Using cpio to move a directory tree
16-31. Unpacking an rpm archive
16-32. Stripping comments from C program files
16-33. Exploring /usr/X11R6/bin
16-34. An "improved" strings command
16-35. Using cmp to compare two files within a script.
16-36. basename and dirname
16-37. A script that copies itself in sections
16-38. Checking file integrity
16-39. Uudecoding encoded files
16-40. Finding out where to report a spammer
16-41. Analyzing a spam domain
16-42. Getting a stock quote
16-43. Updating FC4
16-44. Using ssh
16-45. A script that mails itself
16-46. Generating prime numbers
16-47. Monthly Payment on a Mortgage
16-48. Base Conversion
16-49. Invoking bc using a here document
16-50. Calculating PI
16-51. Converting a decimal number to hexadecimal
16-52. Factoring
16-53. Calculating the hypotenuse of a triangle
16-54. Using seq to generate loop arguments
16-55. Letter Count"
16-56. Using getopt to parse command-line options
16-57. A script that copies itself
16-58. Exercising dd
16-59. Capturing Keystrokes
16-60. Preparing a bootable SD card for the Raspberry Pi
16-61. Securely deleting a file
16-62. Filename generator
16-63. Converting meters to miles
16-64. Using m4
17-1. Setting a new password
17-2. Setting an erase character
17-3. secret password: Turning off terminal echoing
17-4. Keypress detection
17-5. Checking a remote server for identd
17-6. pidof helps kill a process
17-7. Checking a CD image
17-8. Creating a filesystem in a file
17-9. Adding a new hard drive
17-10. Using umask to hide an output file from prying eyes
17-11. Backlight: changes the brightness of the (laptop) screen
backlight
17-12. killall, from /etc/rc.d/init.d
19-1. broadcast: Sends message to everyone logged in
19-2. dummyfile: Creates a 2-line dummy file
19-3. Multi-line message using cat
19-4. Multi-line message, with tabs suppressed
19-5. Here document with replaceable parameters
19-6. Upload a file pair to Sunsite incoming directory
19-7. Parameter substitution turned off
19-8. A script that generates another script
19-9. Here documents and functions
19-10. "Anonymous" Here Document
19-11. Commenting out a block of code
19-12. A self-documenting script
19-13. Prepending a line to a file
19-14. Parsing a mailbox
20-1. Redirecting stdin using exec
20-2. Redirecting stdout using exec
20-3. Redirecting both stdin and stdout in the same script with exec
20-4. Avoiding a subshell
20-5. Redirected while loop
20-6. Alternate form of redirected while loop
20-7. Redirected until loop
20-8. Redirected for loop
20-9. Redirected for loop (both stdin and stdout redirected)
20-10. Redirected if/then test
20-11. Data file names.data for above examples
20-12. Logging events
21-1. Variable scope in a subshell
21-2. List User Profiles
21-3. Running parallel processes in subshells
22-1. Running a script in restricted mode
23-1. Code block redirection without forking
23-2. Redirecting the output of process substitution into a loop.
24-1. Simple functions
24-2. Function Taking Parameters
24-3. Functions and command-line args passed to the script
24-4. Passing an indirect reference to a function
24-5. Dereferencing a parameter passed to a function
24-6. Again, dereferencing a parameter passed to a function
24-7. Maximum of two numbers
24-8. Converting numbers to Roman numerals
24-9. Testing large return values in a function
24-10. Comparing two large integers
24-11. Real name from username
24-12. Local variable visibility
24-13. Demonstration of a simple recursive function
24-14. Another simple demonstration
24-15. Recursion, using a local variable
24-16. The Fibonacci Sequence
24-17. The Towers of Hanoi
25-1. Aliases within a script
25-2. unalias: Setting and unsetting an alias
26-1. Using an and list to test for command-line arguments
26-2. Another command-line arg test using an and list
26-3. Using or lists in combination with an and list
27-1. Simple array usage
27-2. Formatting a poem
27-3. Various array operations
27-4. String operations on arrays
27-5. Loading the contents of a script into an array
27-6. Some special properties of arrays
27-7. Of empty arrays and empty elements
27-8. Initializing arrays
27-9. Copying and concatenating arrays
27-10. More on concatenating arrays
27-11. The Bubble Sort
27-12. Embedded arrays and indirect references
27-13. The Sieve of Eratosthenes
27-14. The Sieve of Eratosthenes, Optimized
27-15. Emulating a push-down stack
27-16. Complex array application: Exploring a weird mathematical
series
27-17. Simulating a two-dimensional array, then tilting it
28-1. Indirect Variable References
28-2. Passing an indirect reference to awk
29-1. Using /dev/tcp for troubleshooting
29-2. Playing music
29-3. Finding the process associated with a PID
29-4. On-line connect status
30-1. Print the server environment
30-2. IP addresses
31-1. Hiding the cookie jar
31-2. Setting up a swapfile using /dev/zero
31-3. Creating a ramdisk
32-1. A buggy script
32-2. Missing keyword
32-3. test24: another buggy script
32-4. Testing a condition with an assert
32-5. Trapping at exit
32-6. Cleaning up after Control-C
32-7. A Simple Implementation of a Progress Bar
32-8. Tracing a variable
32-9. Running multiple processes (on an SMP box)
34-1. Numerical and string comparison are not equivalent
34-2. Subshell Pitfalls
34-3. Piping the output of echo to a read
36-1. shell wrapper
36-2. A slightly more complex shell wrapper
36-3. A generic shell wrapper that writes to a logfile
36-4. A shell wrapper around an awk script
36-5. A shell wrapper around another awk script
36-6. Perl embedded in a Bash script
36-7. Bash and Perl scripts combined
36-8. Python embedded in a Bash script
36-9. A script that speaks
36-10. A (useless) script that recursively calls itself
36-11. A (useful) script that recursively calls itself
36-12. Another (useful) script that recursively calls itself
36-13. A "colorized" address database
36-14. Drawing a box
36-15. Echoing colored text
36-16. A "horserace" game
36-17. A Progress Bar
36-18. Return value trickery
36-19. Even more return value trickery
36-20. Passing and returning arrays
36-21. Fun with anagrams
36-22. Widgets invoked from a shell script
36-23. Test Suite
37-1. String expansion
37-2. Indirect variable references - the new way
37-3. Simple database application, using indirect variable
referencing
37-4. Using arrays and other miscellaneous trickery to deal four
random hands from a deck of cards
37-5. A simple address database
37-6. A somewhat more elaborate address database
37-7. Testing characters
37-8. Reading N characters
37-9. Using a here document to set a variable
37-10. Piping input to a read
37-11. Negative array indices
37-12. Negative parameter in string-extraction construct
A-1. mailformat: Formatting an e-mail message
A-2. rn: A simple-minded file renaming utility
A-3. blank-rename: Renames filenames containing blanks
A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted
password
A-5. copy-cd: Copying a data CD
A-6. Collatz series
A-7. days-between: Days between two dates
A-8. Making a dictionary
A-9. Soundex conversion
A-10. Game of Life
A-11. Data file for Game of Life
A-12. behead: Removing mail and news message headers
A-13. password: Generating random 8-character passwords
A-14. fifo: Making daily backups, using named pipes
A-15. Generating prime numbers using the modulo operator
A-16. tree: Displaying a directory tree
A-17. tree2: Alternate directory tree script
A-18. string functions: C-style string functions
A-19. Directory information
A-20. Library of hash functions
A-21. Colorizing text using hash functions
A-22. More on hash functions
A-23. Mounting USB keychain storage devices
A-24. Converting to HTML
A-25. Preserving weblogs
A-26. Protecting literal strings
A-27. Unprotecting literal strings
A-28. Spammer Identification
A-29. Spammer Hunt
A-30. Making wget easier to use
A-31. A podcasting script
A-32. Nightly backup to a firewire HD
A-33. An expanded cd command
A-34. A soundcard setup script
A-35. Locating split paragraphs in a text file
A-36. Insertion sort
A-37. Standard Deviation
A-38. A pad file generator for shareware authors
A-39. A man page editor
A-40. Petals Around the Rose
A-41. Quacky: a Perquackey-type word game
A-42. Nim
A-43. A command-line stopwatch
A-44. An all-purpose shell scripting homework assignment solution
A-45. The Knight's Tour
A-46. Magic Squares
A-47. Fifteen Puzzle
A-48. The Towers of Hanoi, graphic version
A-49. The Towers of Hanoi, alternate graphic version
A-50. An alternate version of the getopt-simple.sh script
A-51. The version of the UseGetOpt.sh example used in the Tab
Expansion appendix
A-52. Cycling through all the possible color backgrounds
A-53. Morse Code Practice
A-54. Base64 encoding/decoding
A-55. Inserting text in a file using sed
A-56. The Gronsfeld Cipher
A-57. Bingo Number Generator
A-58. Basics Reviewed
A-59. Testing execution times of various commands
A-60. Associative arrays vs. conventional arrays (execution times)
C-1. Counting Letter Occurrences
J-1. Completion script for UseGetOpt.sh
M-1. Sample .bashrc file
M-2. .bash_profile file
N-1. VIEWDATA.BAT: DOS Batch File
N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
T-1. A script that generates an ASCII table
T-2. Another ASCII table script
T-3. A third ASCII table script, using awk
Part 1. Introduction
Script: A writing; a written document. [Obs.]
--Webster's Dictionary, 1913 ed.
The shell is a command interpreter. More than just the insulating
layer between the operating system kernel and the user, it's also a
fairly powerful programming language. A shell program, called a
script, is an easy-to-use tool for building applications by "gluing
together" system calls, tools, utilities, and compiled binaries.
Virtually the entire repertoire of UNIX commands, utilities, and
tools is available for invocation by a shell script. If that were not
enough, internal shell commands, such as testing and loop constructs,
lend additional power and flexibility to scripts. Shell scripts are
especially well suited for administrative system tasks and other
routine repetitive tasks not requiring the bells and whistles of a
full-blown tightly structured programming language.
Table of Contents
- Shell Programming!
- Starting Off With a Sha-Bang
2.1. Invoking the script
2.2. Preliminary Exercises
________________________________________________________________
Chapter 1. Shell Programming!
No programming language is perfect. There is not even a single best
language; there are only languages well suited or perhaps poorly
suited for particular purposes.
--Herbert Mayer
A working knowledge of shell scripting is essential to anyone wishing
to become reasonably proficient at system administration, even if
they do not anticipate ever having to actually write a script.
Consider that as a Linux machine boots up, it executes the shell
scripts in /etc/rc.d to restore the system configuration and set up
services. A detailed understanding of these startup scripts is
important for analyzing the behavior of a system, and possibly
modifying it.
The craft of scripting is not hard to master, since scripts can be
built in bite-sized sections and there is only a fairly small set of
shell-specific operators and options [1] to learn. The syntax is
simple -- even austere -- similar to that of invoking and chaining
together utilities at the command line, and there are only a few
"rules" governing their use. Most short scripts work right the first
time, and debugging even the longer ones is straightforward.
In the early days of personal computing, the BASIC language en
abled
anyone reasonably computer proficient to write programs on an
early
generation of microcomputers. Decades later, the Bash scriptin
g
language enables anyone with a rudimentary knowledge of Linux
or
UNIX to do the same on modern machines.
We now have miniaturized single-board computers with amazing
capabilities, such as the [http://www.raspberrypi.org/]
Raspberry Pi.
Bash scripting provides a way to explore the capabilities of t
hese
fascinating devices.
A shell script is a quick-and-dirty method of prototyping a complex
application. Getting even a limited subset of the functionality to
work in a script is often a useful first stage in project
development. In this way, the structure of the application can be
tested and tinkered with, and the major pitfalls found before
proceeding to the final coding in C, C++, Java, Perl, or Python.
Shell scripting hearkens back to the classic UNIX philosophy of
breaking complex projects into simpler subtasks, of chaining together
components and utilities. Many consider this a better, or at least
more esthetically pleasing approach to problem solving than using one
of the new generation of high-powered all-in-one languages, such as
Perl, which attempt to be all things to all people, but at the cost
of forcing you to alter your thinking processes to fit the tool.
According to Herbert Mayer, "a useful language needs arrays,
pointers, and a generic mechanism for building data structures." By
these criteria, shell scripting falls somewhat short of being
"useful." Or, perhaps not. . . .
When not to use shell scripts
* Resource-intensive tasks, especially where speed is a factor
(sorting, hashing, recursion [2] ...)
* Procedures involving heavy-duty math operations, especially
floating point arithmetic, arbitrary precision calculations, or
complex numbers (use C++ or FORTRAN instead)
* Cross-platform portability required (use C or Java instead)
* Complex applications, where structured programming is a necessity
(type-checking of variables, function prototypes, etc.)
* Mission-critical applications upon which you are betting the
future of the company
* Situations where security is important, where you need to
guarantee the integrity of your system and protect against
intrusion, cracking, and vandalism
* Project consists of subcomponents with interlocking dependencies
* Extensive file operations required (Bash is limited to serial
file access, and that only in a particularly clumsy and
inefficient line-by-line fashion.)
* Need native support for multi-dimensional arrays
* Need data structures, such as linked lists or trees
* Need to generate / manipulate graphics or GUIs
* Need direct access to system hardware or external peripherals
* Need port or socket I/O
* Need to use libraries or interface with legacy code
* Proprietary, closed-source applications (Shell scripts put the
source code right out in the open for all the world to see.)
If any of the above applies, consider a more powerful scripting
language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a compiled
language such as C, C++, or Java. Even then, prototyping the
application as a shell script might still be a useful development
step.
We will be using Bash, an acronym [3] for "Bourne-Again shell" and a
pun on Stephen Bourne's now classic Bourne shell. Bash has become a
de facto standard for shell scripting on most flavors of UNIX. Most
of the principles this book covers apply equally well to scripting
with other shells, such as the Korn Shell, from which Bash derives
some of its features, [4] and the C Shell and its variants. (Note
that C Shell programming is not recommended due to certain inherent
problems, as pointed out in an October, 1993 Usenet post by Tom
Christiansen.)
What follows is a tutorial on shell scripting. It relies heavily on
examples to illustrate various features of the shell. The example
scripts work -- they've been tested, insofar as possible -- and some
of them are even useful in real life. The reader can play with the
actual working code of the examples in the source archive
(scriptname.sh or scriptname.bash), [5] give them execute permission
(chmod u+rx scriptname), then run them to see what happens. Should
the source archive not be available, then cut-and-paste from the
[http://www.tldp.org/LDP/abs/abs-guide.html.tar.gz] HTML or
[http://bash.deta.in/abs-guide.pdf] pdf rendered versions. Be aware
that some of the scripts presented here introduce features before
they are explained, and this may require the reader to temporarily
skip ahead for enlightenment.
Unless otherwise noted, [mailto:thegrendel.abs@gmail.com] the author
of this book wrote the example scripts that follow.
His countenance was bold and bashed not.
--Edmund Spenser
________________________________________________________________
Chapter 2. Starting Off With a Sha-Bang
Shell programming is a 1950s juke box . . .
--Larry Wall
In the simplest case, a script is nothing more than a list of system
commands stored in a file. At the very least, this saves the effort
of retyping that particular sequence of commands each time it is
invoked.
Example 2-1. cleanup: A script to clean up log files in /var/log
Cleanup
Run as root, of course.
cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Log files cleaned up."
There is nothing unusual here, only a set of commands that could just
as easily have been invoked one by one from the command-line on the
console or in a terminal window. The advantages of placing the
commands in a script go far beyond not having to retype them time and
again. The script becomes a program -- a tool -- and it can easily be
modified or customized for a particular application.
Example 2-2. cleanup: An improved clean-up script
!/bin/bash
Proper header for a Bash script.
Cleanup, version 2
Run as root, of course.
Insert code here to print error message and exit if not root.
LOG_DIR=/var/log
Variables are better than hard-coded values.
cd $LOG_DIR
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."
exit # The right and proper method of "exiting" from a script.
# A bare "exit" (no parameter) returns the exit status
#+ of the preceding command.
Now that's beginning to look like a real script. But we can go even
farther . . .
Example 2-3. cleanup: An enhanced and generalized version of above
scripts.
!/bin/bash
Cleanup, version 3
Warning:
This script uses quite a number of features that will be explained
By the time you've finished the first half of the book,
- there should be nothing mysterious about it.
LOG_DIR=/var/log
ROOT_UID=0 # Only users with $UID 0 have root privileges.
LINES=50 # Default number of lines saved.
E_XCD=86 # Can't change directory?
E_NOTROOT=87 # Non-root exit error.
Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
if [ -n "$1" ]
Test whether command-line argument is present (non-empty).
then
lines=$1
else
lines=$LINES # Default, if not specified on command-line.
fi
Stephane Chazelas suggests the following,
- as a better way of checking command-line arguments,
- but this is still a bit advanced for this stage of the tutorial.
E_WRONGARGS=85 # Non-numerical argument (bad argument format).
case "$1" in
"" ) lines=50;;
[!0-9]) echo "Usage: basename $0 lines-to-cleanup";
exit $E_WRONGARGS;;
esac
- Skip ahead to "Loops" chapter to decipher all this.
cd $LOG_DIR
if [ pwd != "$LOG_DIR" ] # or if [ "$PWD" != "$LOG_DIR" ]
# Not in /var/log?
then
echo "Can't change to $LOG_DIR."
exit $E_XCD
fi # Doublecheck if in right directory before messing with log file.
Far more efficient is:
cd /var/log || {
echo "Cannot change to necessary directory." >&2
exit $E_XCD;
}
tail -n $lines messages > mesg.temp # Save last section of message log file.
mv mesg.temp messages # Rename it as system log file.
cat /dev/null > messages
- No longer needed, as the above method is safer.
cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect.
echo "Log files cleaned up."
Note that there are other log files in /var/log not affected
exit 0
A zero return value from the script upon exit indicates success
Since you may not wish to wipe out the entire system log, this
version of the script keeps the last section of the message log
intact. You will constantly discover ways of fine-tuning previously
written scripts for increased effectiveness.
The sha-bang ( #!) [6] at the head of a script tells your system that
this file is a set of commands to be fed to the command interpreter
indicated. The #! is actually a two-byte [7] magic number, a special
marker that designates a file type, or in this case an executable
shell script (type man magic for more details on this fascinating
topic). Immediately following the sha-bang is a path name. This is
the path to the program that interprets the commands in the script,
whether it be a shell, a programming language, or a utility. This
command interpreter then executes the commands in the script,
starting at the top (the line following the sha-bang line), and
ignoring comments. [8]
!/bin/sh
!/bin/bash
!/usr/bin/perl
!/usr/bin/tcl
!/bin/sed -f
!/bin/awk -f
Each of the above script header lines calls a different command
interpreter, be it /bin/sh, the default shell (bash in a Linux
system) or otherwise. [9] Using #!/bin/sh, the default Bourne shell
in most commercial variants of UNIX, makes the script portable to
non-Linux machines, though you sacrifice Bash-specific features. The
script will, however, conform to the POSIX [10] sh standard.
Note that the path given at the "sha-bang" must be correct, otherwise
an error message -- usually "Command not found." -- will be the only
result of running the script. [11]
#! can be omitted if the script consists only of a set of generic
system commands, using no internal shell directives. The second
example, above, requires the initial #!, since the variable
assignment line, lines=50, uses a shell-specific construct. [12] Note
again that #!/bin/sh invokes the default shell interpreter, which
defaults to /bin/bash on a Linux machine.
Tip
This tutorial encourages a modular approach to constructing a script.
Make note of and collect "boilerplate" code snippets that might be
useful in future scripts. Eventually you will build quite an
extensive library of nifty routines. As an example, the following
script prolog tests whether the script has been invoked with the
correct number of parameters.
E_WRONG_ARGS=85
script_parameters="-a -h -m -z"
-a = all, -h = help, etc.
if [ $# -ne $Number_of_expected_args ]
then
echo "Usage: basename $0 $script_parameters"
basename $0 is the script's filename.
exit $E_WRONG_ARGS
fi
Many times, you will write a script that carries out one particular
task. The first script in this chapter is an example. Later, it might
occur to you to generalize the script to do other, similar tasks.
Replacing the literal ("hard-wired") constants by variables is a step
in that direction, as is replacing repetitive code blocks by
functions.
________________________________________________________________
2.1. Invoking the script
Having written the script, you can invoke it by sh scriptname, [13]
or alternatively bash scriptname. (Not recommended is using sh
<scriptname, since this effectively disables reading from stdin
within the script.) Much more convenient is to make the script itself
directly executable with a chmod.
Either:
chmod 555 scriptname (gives everyone read/execute permission)
[14]
or
chmod +rx scriptname (gives everyone read/execute permission)
chmod u+rx scriptname (gives only the script owner
read/execute permission)
Having made the script executable, you may now test it by
./scriptname. [15] If it begins with a "sha-bang" line, invoking the
script calls the correct command interpreter to run it.
As a final step, after testing and debugging, you would likely want
to move it to /usr/local/bin (as root, of course), to make the script
available to yourself and all other users as a systemwide executable.
The script could then be invoked by simply typing scriptname [ENTER]
from the command-line.
________________________________________________________________
2.2. Preliminary Exercises
1. System administrators often write scripts to automate common
tasks. Give several instances where such scripts would be useful.
2. Write a script that upon invocation shows the time and date,
lists all logged-in users, and gives the system uptime. The
script then saves this information to a logfile.
Part 2. Basics
Table of Contents
- Special Characters
- Introduction to Variables and Parameters
4.1. Variable Substitution
4.2. Variable Assignment
4.3. Bash Variables Are Untyped
4.4. Special Variable Types
- Quoting
5.1. Quoting Variables
5.2. Escaping
- Exit and Exit Status
- Tests
7.1. Test Constructs
7.2. File test operators
7.3. Other Comparison Operators
7.4. Nested if/then Condition Tests
7.5. Testing Your Knowledge of Tests
- Operations and Related Topics
8.1. Operators
8.2. Numerical Constants
8.3. The Double-Parentheses Construct
8.4. Operator Precedence
________________________________________________________________
Chapter 3. Special Characters
What makes a character special? If it has a meaning beyond its
literal meaning, a meta-meaning, then we refer to it as a special
character. Along with commands and keywords, special characters are
building blocks of Bash scripts.
Special Characters Found In Scripts and Elsewhere
Comments. Lines beginning with a # (with the exception of #!)
are comments and will not be executed.
This line is a comment.
Comments may also occur following the end of a command.
echo "A comment will follow." # Comment here.
^ Note whitespace before #
Comments may also follow whitespace at the beginning of a
line.
# A tab precedes this comment.
Comments may even be embedded within a pipe.
initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\
Delete lines containing '#' comment character.
sed -e 's/\./\. /g' -e 's/_/_ /g'` )
Excerpted from life.sh script
Caution
A command may not follow a comment on the same line. There is no
method of terminating the comment, in order for "live code" to begin
on the same line. Use a new line for the next command.
Note
Of course, a quoted or an escaped # in an echo statement does not
begin a comment. Likewise, a # appears in certain
parameter-substitution constructs and in numerical constant
expressions.
echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The # here does not begin a comment.
echo The # here begins a comment.
echo ${PATH#*:} # Parameter substitution, not a comment.
echo $(( 2#101011 )) # Base conversion, not a comment.
Thanks, S.C.
The standard quoting and escape characters (" ' ) escape the #.
Certain pattern matching operations also use the #.
;
Command separator [semicolon]. Permits putting two or more
commands on the same line.
echo hello; echo there
if [ -x "$filename" ]; then # Note the space after the semicolon.
echo "File $filename exists."; cp $filename $filename.bak
else # ^^
echo "File $filename not found."; touch $filename
fi; echo "File test complete."
Note that the ";" sometimes needs to be escaped.
;;
Terminator in a case option [double semicolon].
case "$variable" in
abc) echo "$variable = abc" ;;
xyz) echo "$variable = xyz" ;;
esac
;;&, ;&
Terminators in a case option (version 4+ of Bash).
.
"dot" command [period]. Equivalent to source (see Example
15-22). This is a bash builtin.
.
"dot", as a component of a filename. When working with
filenames, a leading dot is the prefix of a "hidden" file, a
file that an ls will not normally show.
bash$ touch .hidden-file
bash$ ls -l
total 10
-rw-r--r-- 1 bozo 4034 Jul 18 22:04 data1.addressbook
-rw-r--r-- 1 bozo 4602 May 25 13:58 data1.addressbook.bak
-rw-r--r-- 1 bozo 877 Dec 17 2000 employment.addressbook
bash$ ls -al
total 14
drwxrwxr-x 2 bozo bozo 1024 Aug 29 20:54 ./
drwx------ 52 bozo bozo 3072 Aug 29 20:51 ../
-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.addressbook
-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.addressbook.bak
-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.addressbook
-rw-rw-r-- 1 bozo bozo 0 Aug 29 20:54 .hidden-file
When considering directory names, a single dot represents the
current working directory, and two dots denote the parent
directory.
bash$ pwd
/home/bozo/projects
bash$ cd .
bash$ pwd
/home/bozo/projects
bash$ cd ..
bash$ pwd
/home/bozo/
The dot often appears as the destination (directory) of a file
movement command, in this context meaning current directory.
bash$ cp /home/bozo/current_work/junk/* .
Copy all the "junk" files to $PWD.
.
"dot" character match. When matching characters, as part of a
regular expression, a "dot" matches a single character.
"
partial quoting [double quote]. "STRING" preserves (from
interpretation) most of the special characters within STRING.
See Chapter 5.
'
full quoting [single quote]. 'STRING' preserves all special
characters within STRING. This is a stronger form of quoting
than "STRING". See Chapter 5.
,
comma operator. The comma operator [16] links together a
series of arithmetic operations. All are evaluated, but only
the last one is returned.
let "t2 = ((a = 9, 15 / 3))"
Set "a = 9" and "t2 = 15 / 3"
The comma operator can also concatenate strings.
for file in /{,usr/}bin/*calc
^ Find all executable files ending in "calc"
do
if [ -x "$file" ]
then
echo $file
fi
done
/bin/ipcalc
/usr/bin/kcalc
/usr/bin/oidcalc
/usr/bin/oocalc
Thank you, Rory Winston, for pointing this out.
,, ,
Lowercase conversion in parameter substitution (added in
version 4 of Bash).
\
escape [backslash]. A quoting mechanism for single characters.
\X escapes the character X. This has the effect of "quoting"
X, equivalent to 'X'. The \ may be used to quote " and ', so
they are expressed literally.
See Chapter 5 for an in-depth explanation of escaped
characters.
/
Filename path separator [forward slash]. Separates the
components of a filename (as in /home/bozo/projects/Makefile).
This is also the division arithmetic operator.
`
command substitution. The `command` construct makes available
the output of command for assignment to a variable. This is
also known as backquotes or backticks.
:
null command [colon]. This is the shell equivalent of a "NOP"
(no op, a do-nothing operation). It may be considered a
synonym for the shell builtin true. The ":" command is itself
a Bash builtin, and its exit status is true (0).
:
echo $? # 0
Endless loop:
while :
do
operation-1
operation-2
...
operation-n
done
Same as:
while true
do
...
done
Placeholder in if/then test:
if condition
then : # Do nothing and branch ahead
else # Or else ...
take-some-action
fi
Provide a placeholder where a binary operation is expected,
see Example 8-2 and default parameters.
: ${username=whoami}
${username=whoami} Gives an error without the leading :
unless "username" is a command or builtin...
: ${1?"Usage: $0 ARGUMENT"} # From "usage-message.sh example script.
Provide a placeholder where a command is expected in a here
document. See Example 19-10.
Evaluate string of variables using parameter substitution (as
in Example 10-7).
: ${HOSTNAME?} ${USER?} ${MAIL?}
Prints error message
- if one or more of essential environmental variables not set.
Variable expansion / substring replacement.
In combination with the > redirection operator, truncates a
file to zero length, without changing its permissions. If the
file did not previously exist, creates it.
: > data.xxx # File "data.xxx" now empty.
Same effect as cat /dev/null >data.xxx
However, this does not fork a new process, since ":" is a builtin.
See also Example 16-15.
In combination with the >> redirection operator, has no effect
on a pre-existing target file (: >> target_file). If the file
did not previously exist, creates it.
Note
This applies to regular files, not pipes, symlinks, and certain
special files.
May be used to begin a comment line, although this is not
recommended. Using # for a comment turns off error checking
for the remainder of that line, so almost anything may appear
in a comment. However, this is not the case with :.
: This is a comment that generates an error, ( if [ $x -eq 3] ).
The ":" serves as a field separator, in /etc/passwd, and in
the $PATH variable.
bash$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games
A colon is acceptable as a function name.
:()
{
echo "The name of this function is "$FUNCNAME" "
Why use a colon as a function name?
It's a way of obfuscating your code.
}
:
The name of this function is :
This is not portable behavior, and therefore not a recommended
practice. In fact, more recent releases of Bash do not permit
this usage. An underscore _ works, though.
A colon can serve as a placeholder in an otherwise empty
function.
not_empty ()
{
:
} # Contains a : (null command), and so is not empty.
!
reverse (or negate) the sense of a test or exit status [bang].
The ! operator inverts the exit status of the command to which
it is applied (see Example 6-2). It also inverts the meaning
of a test operator. This can, for example, change the sense of
equal ( = ) to not-equal ( != ). The ! operator is a Bash
keyword.
In a different context, the ! also appears in indirect
variable references.
In yet another context, from the command line, the ! invokes
the Bash history mechanism (see Appendix L). Note that within
a script, the history mechanism is disabled.
wild card [asterisk]. The * character serves as a "wild card"
for filename expansion in globbing. By itself, it matches
every filename in a given directory.
bash$ echo *
abs-book.sgml add-drive.sh agram.sh alias.sh
The * also represents any number (or zero) characters in a
regular expression.
arithmetic operator. In the context of arithmetic operations,
the * denotes multiplication.
** A double asterisk can represent the exponentiation operator
or extended file-match globbing.
?
test operator. Within certain expressions, the ? indicates a
test for a condition.
In a double-parentheses construct, the ? can serve as an
element of a C-style trinary operator. [17]
condition?result-if-true:result-if-false
(( var0 = var1<98?9:21 ))
^ ^
if [ "$var1" -lt 98 ]
then
var0=9
else
var0=21
fi
In a parameter substitution expression, the ? tests whether a
variable has been set.
?
wild card. The ? character serves as a single-character "wild
card" for filename expansion in globbing, as well as
representing one character in an extended regular expression.
$
Variable substitution (contents of a variable).
var1=5
var2=23skidoo
echo $var1 # 5
echo $var2 # 23skidoo
A $ prefixing a variable name indicates the value the variable
holds.
$
end-of-line. In a regular expression, a "$" addresses the end
of a line of text.
${}
Parameter substitution.
$' ... '
Quoted string expansion. This construct expands single or
multiple escaped octal or hex values into ASCII [18] or
Unicode characters.
$*, $@
positional parameters.
$?
exit status variable. The $? variable holds the exit status of
a command, a function, or of the script itself.
$$
process ID variable. The $$ variable holds the process ID [19]
of the script in which it appears.
()
command group.
(a=hello; echo $a)
Important
A listing of commands within parentheses starts a subshell.
Variables inside parentheses, within the subshell, are not visible to
the rest of the script. The parent process, the script, cannot read
variables created in the child process, the subshell.
a=123
( a=321; )
echo "a = $a" # a = 123
"a" within parentheses acts like a local variable.
array initialization.
Array=(element1 element2 element3)
{xxx,yyy,zzz,...}
Brace expansion.
echo "{These,words,are,quoted}" # " prefix and suffix
"These" "words" "are" "quoted"
cat {file1,file2,file3} > combined_file
Concatenates the files file1, file2, and file3 into combined_file.
cp file22.{txt,backup}
Copies "file22.txt" to "file22.backup"
A command may act upon a comma-separated list of file specs
within braces. [20] Filename expansion (globbing) applies to
the file specs between the braces.
Caution
No spaces allowed within the braces unless the spaces are quoted or
escaped.
echo {file1,file2}\ :{\ A," B",' C'}
file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C
{a..z}
Extended Brace expansion.
echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z
Echoes characters between a and z.
echo {0..3} # 0 1 2 3
Echoes characters between 0 and 3.
base64_charset=( {A..Z} {a..z} {0..9} + / = )
Initializing an array, using extended brace expansion.
From vladz's "base64.sh" example script.
The {a..z} extended brace expansion construction is a feature
introduced in version 3 of Bash.
{}
Block of code [curly brackets]. Also referred to as an inline
group, this construct, in effect, creates an anonymous
function (a function without a name). However, unlike in a
"standard" function, the variables inside a code block remain
visible to the remainder of the script.
bash$ { local a;
a=123; }
bash: local: can only be used in a
function
a=123
{ a=321; }
echo "a = $a" # a = 321 (value inside code block)
Thanks, S.C.
The code block enclosed in braces may have I/O redirected to
and from it.
Example 3-1. Code blocks and I/O redirection
!/bin/bash
Reading lines in /etc/fstab.
File=/etc/fstab
{
read line1
read line2
} < $File
echo "First line in $File is:"
echo "$line1"
echo
echo "Second line in $File is:"
echo "$line2"
exit 0
Now, how do you parse the separate fields of each line?
Hint: use awk, or . . .
. . . Hans-Joerg Diers suggests using the "set" Bash builtin.
Example 3-2. Saving the output of a code block to a file
!/bin/bash
rpm-check.sh
Queries an rpm file for description, listing,
- and whether it can be installed.
Saves output to a file.
This script illustrates using a code block.
SUCCESS=0
E_NOARGS=65
if [ -z "$1" ]
then
echo "Usage: basename $0 rpm-file"
exit $E_NOARGS
fi
{ # Begin code block.
echo
echo "Archive Description:"
rpm -qpi $1 # Query description.
echo
echo "Archive Listing:"
rpm -qpl $1 # Query listing.
echo
rpm -i --test $1 # Query whether rpm file can be installed.
if [ "$?" -eq $SUCCESS ]
then
echo "$1 can be installed."
else
echo "$1 cannot be installed."
fi
echo # End code block.
} > "$1.test" # Redirects output of everything in block to file.
echo "Results of rpm test in file $1.test"
See rpm man page for explanation of options.
exit 0
Note
Unlike a command group within (parentheses), as above, a code block
enclosed by {braces} will not normally launch a subshell. [21]
It is possible to iterate a code block using a non-standard for-loop.
{}
placeholder for text. Used after xargs -i (replace strings
option). The {} double curly brackets are a placeholder for
output text.
ls . | xargs -i -t cp ./{} $1
^^ ^^
From "ex42.sh" (copydir.sh) example.
{} ;
pathname. Mostly used in find constructs. This is not a shell
builtin.
Definition: A pathname is a filename that includes the complete path.
As an example, /home/bozo/Notes/Thursday/schedule.txt. This is
sometimes referred to as the absolute path.
Note
The ";" ends the -exec option of a find command sequence. It needs to
be escaped to protect it from interpretation by the shell.
[ ]
test.
Test expression between [ ]. Note that [ is part of the shell
builtin test (and a synonym for it), not a link to the
external command /usr/bin/test.
[[ ]]
test.
Test expression between [[ ]]. More flexible than the
single-bracket [ ] test, this is a shell keyword.
See the discussion on the [[ ... ]] construct.
[ ]
array element.
In the context of an array, brackets set off the numbering of
each element of that array.
Array[1]=slot_1
echo ${Array[1]}
[ ]
range of characters.
As part of a regular expression, brackets delineate a range of
characters to match.
$[ ... ]
integer expansion.
Evaluate integer expression between $[ ].
a=3
b=7
echo $[$a+$b] # 10
echo $[$a*$b] # 21
Note that this usage is deprecated, and has been replaced by
the (( ... )) construct.
(( ))
integer expansion.
Expand and evaluate integer expression between (( )).
See the discussion on the (( ... )) construct.
&> >& >> < <>
redirection.
scriptname >filename redirects the output of scriptname to
file filename. Overwrite filename if it already exists.
command &>filename redirects both the stdout and the stderr of
command to filename.
Note
This is useful for suppressing output when testing for a condition.
For example, let us test whether a certain command exists.
bash$ type bogus_command &>/dev/null
bash$ echo $?
1
Or in a script:
command_test () { type "$1" &>/dev/null; }
^
cmd=rmdir # Legitimate command.
command_test $cmd; echo $? # 0
cmd=bogus_command # Illegitimate command
command_test $cmd; echo $? # 1
command >&2 redirects stdout of command to stderr.
scriptname >>filename appends the output of scriptname to file
filename. If filename does not already exist, it is created.
[i]<>filename opens file filename for reading and writing, and
assigns file descriptor i to it. If filename does not exist,
it is created.
process substitution.
(command)>
<(command)
In a different context, the "<" and ">" characters act as
string comparison operators.
In yet another context, the "<" and ">" characters act as
integer comparison operators. See also Example 16-9.
<<
redirection used in a here document.
<<<
redirection used in a here string.
<, >
ASCII comparison.
veg1=carrots
veg2=tomatoes
if [[ "$veg1" < "$veg2" ]]
then
echo "Although $veg1 precede $veg2 in the dictionary,"
echo -n "this does not necessarily imply anything "
echo "about my culinary preferences."
else
echo "What kind of dictionary are you using, anyhow?"
fi
<, >
word boundary in a regular expression.
bash$ grep '\<the\>' textfile
|
pipe. Passes the output (stdout) of a previous command to the
input (stdin) of the next one, or to the shell. This is a
method of chaining commands together.
echo ls -l | sh
Passes the output of "echo ls -l" to the shell,
- with the same result as a simple "ls -l".
cat *.lst | sort | uniq
Merges and sorts all ".lst" files, then deletes duplicate lines.
A pipe, as a classic method of interprocess communication, sends the
stdout of one process to the stdin of another. In a typical case, a
command, such as cat or echo, pipes a stream of data to a filter, a
command that transforms its input for processing. [22]
cat $filename1 $filename2 | grep $search_word
For an interesting note on the complexity of using UNIX pipes, see
the UNIX FAQ, Part 3.
The output of a command or commands may be piped to a script.
!/bin/bash
uppercase.sh : Changes input to uppercase.
tr 'a-z' 'A-Z'
Letter ranges must be quoted
- to prevent filename generation from single-letter filenames.
exit 0
Now, let us pipe the output of ls -l to this script.
bash$ ls -l | ./uppercase.sh
-RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT
-RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT
-RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE
Note
The stdout of each process in a pipe must be read as the stdin of the
next. If this is not the case, the data stream will block, and the
pipe will not behave as expected.
cat file1 file2 | ls -l | sort
The output from "cat file1 file2" disappears.
A pipe runs as a child process, and therefore cannot alter script
variables.
variable="initial_value"
echo "new_value" | read variable
echo "variable = $variable" # variable = initial_value
If one of the commands in the pipe aborts, this prematurely
terminates execution of the pipe. Called a broken pipe, this
condition sends a SIGPIPE signal.
|
force redirection (even if the noclobber option is set). This
will forcibly overwrite an existing file.
||
OR logical operator. In a test construct, the || operator
causes a return of 0 (success) if either of the linked test
conditions is true.
&
Run job in background. A command followed by an & will run in
the background.
bash$ sleep 10 &
[1] 850
[1]+ Done sleep 10
Within a script, commands and even loops may run in the
background.
Example 3-3. Running a loop in the background
!/bin/bash
background-loop.sh
for i in 1 2 3 4 5 6 7 8 9 10 # First loop.
do
echo -n "$i "
done & # Run this loop in background.
# Will sometimes execute after second loop.
echo # This 'echo' sometimes will not display.
for i in 11 12 13 14 15 16 17 18 19 20 # Second loop.
do
echo -n "$i "
done
echo # This 'echo' sometimes will not display.
======================================================
The expected output from the script:
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
Sometimes, though, you get:
11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 bozo $
(The second 'echo' doesn't execute. Why?)
Occasionally also:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
(The first 'echo' doesn't execute. Why?)
Very rarely something like:
11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20
The foreground loop preempts the background one.
exit 0
Nasimuddin Ansari suggests adding sleep 1
- after the echo -n "$i" in lines 6 and 14,
Caution
A command run in the background within a script may cause the script
to hang, waiting for a keystroke. Fortunately, there is a remedy for
this.
&&
AND logical operator. In a test construct, the && operator
causes a return of 0 (success) only if both the linked test
conditions are true.
option, prefix. Option flag for a command or filter. Prefix
for an operator. Prefix for a default parameter in parameter
substitution.
COMMAND -[Option1][Option2][...]
ls -al
sort -dfu $filename
if [ $file1 -ot $file2 ]
then # ^
echo "File $file1 is older than $file2."
fi
if [ "$a" -eq "$b" ]
then # ^
echo "$a is equal to $b."
fi
if [ "$c" -eq 24 -a "$d" -eq 47 ]
then # ^ ^
echo "$c equals 24 and $d equals 47."
fi
param2=${param1:-$DEFAULTVAL}
^
--
The double-dash -- prefixes long (verbatim) options to
commands.
sort --ignore-leading-blanks
Used with a Bash builtin, it means the end of options to that
particular command.
Tip
This provides a handy means of removing files whose names begin with
a dash.
bash$ ls -l
-rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname
bash$ rm -- -badname
bash$ ls -l
total 0
The double-dash is also used in conjunction with set.
set -- $variable (as in Example 15-18)
redirection from/to stdin or stdout [dash].
bash$ cat -
abc
abc
...
Ctl-D
As expected, cat - echoes stdin, in this case keyboarded user
input, to stdout. But, does I/O redirection using - have
real-world applications?
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
Move entire file tree from one directory to another
[courtesy Alan Cox a.cox@swansea.ac.uk, with a minor change]
- cd /source/directory
Source directory, where the files to be moved are.
- &&
"And-list": if the 'cd' operation successful,
then execute the next command.
- tar cf - .
The 'c' option 'tar' archiving command creates a new archive,
the 'f' (file) option, followed by '-' designates the target file
as stdout, and do it in current directory tree ('.').
- |
Piped to ...
- ( ... )
a subshell
- cd /dest/directory
Change to the destination directory.
- &&
"And-list", as above
- tar xpvf -
Unarchive ('x'), preserve ownership and file permissions ('p'),
and send verbose messages to stdout ('v'),
reading data from stdin ('f' followed by '-').
Note that 'x' is a command, and 'p', 'v', 'f' are options.
Whew!
More elegant than, but equivalent to:
cd source/directory
tar cf - . | (cd ../dest/directory; tar xpvf -)
Also having same effect:
cp -a /source/directory/* /dest/directory
Or:
cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
If there are hidden files in /source/directory.
bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf -
--uncompress tar file-- | --then pass it to "tar"--
If "tar" has not been patched to handle "bunzip2",
- this needs to be done in two discrete steps, using a pipe.
The purpose of the exercise is to unarchive "bzipped" kernel source.
Note that in this context the "-" is not itself a Bash
operator, but rather an option recognized by certain UNIX
utilities that write to stdout, such as tar, cat, etc.
bash$ echo "whatever" | cat -
whatever
Where a filename is expected, - redirects output to stdout
(sometimes seen with tar cf), or accepts input from stdin,
rather than from a file. This is a method of using a
file-oriented utility as a filter in a pipe.
bash$ file
Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...
By itself on the command-line, file fails with an error
message.
Add a "-" for a more useful result. This causes the shell to
await user input.
bash$ file -
abc
standard input: ASCII text
bash$ file -
!/bin/bash
standard input: Bourne-Again shell script text executable
Now the command accepts input from stdin and analyzes it.
The "-" can be used to pipe stdout to other commands. This
permits such stunts as prepending lines to a file.
Using diff to compare a file with a section of another:
grep Linux file1 | diff file2 -
Finally, a real-world example using - with tar.
Example 3-4. Backup of all files changed in last day
!/bin/bash
Backs up all files in current directory modified within last 24 hours
- in a "tarball" (tarred and gzipped file).
BACKUPFILE=backup-$(date +%m-%d-%Y)
Embeds date in backup filename.
Thanks, Joshua Tschida, for the idea.
archive=${1:-$BACKUPFILE}
If no backup-archive filename specified on command-line,
- it will default to "backup-MM-DD-YYYY.tar.gz."
tar cvf - find . -mtime -1 -type f -print > $archive.tar
gzip $archive.tar
echo "Directory $PWD backed up in archive file "$archive.tar.gz"."
Stephane Chazelas points out that the above code will fail
- if there are too many files found
- or if any filenames contain blank characters.
He suggests the following alternatives:
find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"
using the GNU version of "find".
find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' ;
portable to other UNIX flavors, but much slower.
exit 0
Caution
Filenames beginning with "-" may cause problems when coupled with the
"-" redirection operator. A script should check for this and add an
appropriate prefix to such filenames, for example ./-FILENAME,
$PWD/-FILENAME, or $PATHNAME/-FILENAME.
If the value of a variable begins with a -, this may likewise create
problems.
var="-n"
echo $var
Has the effect of "echo -n", and outputs nothing.
previous working directory. A cd - command changes to the
previous working directory. This uses the $OLDPWD
environmental variable.
Caution
Do not confuse the "-" used in this sense with the "-" redirection
operator just discussed. The interpretation of the "-" depends on the
context in which it appears.
Minus. Minus sign in an arithmetic operation.
=
Equals. Assignment operator
a=28
echo $a # 28
In a different context, the "=" is a string comparison
operator.
Plus. Addition arithmetic operator.
In a different context, the + is a Regular Expression
operator.
Option. Option flag for a command or filter.
Certain commands and builtins use the + to enable certain
options and the - to disable them. In parameter substitution,
the + prefixes an alternate value that a variable expands to.
%
modulo. Modulo (remainder of a division) arithmetic operation.
let "z = 5 % 3"
echo $z # 2
In a different context, the % is a pattern matching operator.
~
home directory [tilde]. This corresponds to the $HOME internal
variable. ~bozo is bozo's home directory, and ls ~bozo lists
the contents of it. ~/ is the current user's home directory,
and ls ~/ lists the contents of it.
bash$ echo ~bozo
/home/bozo
bash$ echo ~
/home/bozo
bash$ echo ~/
/home/bozo/
bash$ echo ~:
/home/bozo:
bash$ echo ~nonexistent-user
~nonexistent-user
~+
current working directory. This corresponds to the $PWD
internal variable.
~-
previous working directory. This corresponds to the $OLDPWD
internal variable.
=~
regular expression match. This operator was introduced with
version 3 of Bash.
^
beginning-of-line. In a regular expression, a "^" addresses
the beginning of a line of text.
^, ^^
Uppercase conversion in parameter substitution (added in
version 4 of Bash).
Control Characters
change the behavior of the terminal or text display. A control
character is a CONTROL + key combination (pressed
simultaneously). A control character may also be written in
octal or hexadecimal notation, following an escape.
Control characters are not normally useful inside a script.
+ Ctl-A
Moves cursor to beginning of line of text (on the
command-line).
+ Ctl-B
Backspace (nondestructive).
+ Ctl-C
Break. Terminate a foreground job.
+ Ctl-D
Log out from a shell (similar to exit).
EOF (end-of-file). This also terminates input from stdin.
When typing text on the console or in an xterm window, Ctl-D
erases the character under the cursor. When there are no
characters present, Ctl-D logs out of the session, as
expected. In an xterm window, this has the effect of closing
the window.
+ Ctl-E
Moves cursor to end of line of text (on the command-line).
+ Ctl-F
Moves cursor forward one character position (on the
command-line).
+ Ctl-G
BEL. On some old-time teletype terminals, this would
actually ring a bell. In an xterm it might beep.
+ Ctl-H
Rubout (destructive backspace). Erases characters the cursor
backs over while backspacing.
!/bin/bash
Embedding Ctl-H in a string.
a="^H^H" # Two Ctl-H's -- backspaces
# ctl-V ctl-H, using vi/vim
echo "abcdef" # abcdef
echo
echo -n "abcdef$a " # abcd f
Space at end ^ ^ Backspaces twice.
echo
echo -n "abcdef$a" # abcdef
No space at end ^ Doesn't backspace (why?).
# Results may not be quite as expected.
echo; echo
Constantin Hagemeier suggests trying:
a=$'\010\010'
a=$'\b\b'
a=$'\x08\x08'
But, this does not change the results.
Now, try this.
rubout="^H^H^H^H^H" # 5 x Ctl-H.
echo -n "12345678"
sleep 2
echo -n "$rubout"
sleep 2
+ Ctl-I
Horizontal tab.
+ Ctl-J
Newline (line feed). In a script, may also be expressed in
octal notation -- '\012' or in hexadecimal -- '\x0a'.
+ Ctl-K
Vertical tab.
When typing text on the console or in an xterm window, Ctl-K
erases from the character under the cursor to end of line.
Within a script, Ctl-K may behave differently, as in Lee Lee
Maschmeyer's example, below.
+ Ctl-L
Formfeed (clear the terminal screen). In a terminal, this
has the same effect as the clear command. When sent to a
printer, a Ctl-L causes an advance to end of the paper
sheet.
+ Ctl-M
Carriage return.
!/bin/bash
Thank you, Lee Maschmeyer, for this example.
read -n 1 -s -p \
$'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'
# Of course, '0d' is the hex equivalent of Control-M.
echo >&2 # The '-s' makes anything typed silent,
#+ so it is necessary to go to new line explicitly.
read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'
# '0a' is the hex equivalent of Control-J, linefeed.
echo >&2
read -n 1 -s -p $'And Control-K\x0bgoes straight down.'
echo >&2 # Control-K is vertical tab.
A better example of the effect of a vertical tab is:
var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'
echo "$var"
This works the same way as the above example. However:
echo "$var" | col
This causes the right end of the line to be higher than the left end.
It also explains why we started and ended with a line feed --
- to avoid a garbled screen.
As Lee Maschmeyer explains:
In the [first vertical tab example] . . . the vertical tab
- makes the printing go straight down without a carriage return.
This is true only on devices, such as the Linux console,
- that can't go "backward."
The real purpose of VT is to go straight UP, not down.
It can be used to print superscripts on a printer.
The col utility can be used to emulate the proper behavior of VT.
exit 0
+ Ctl-N
Erases a line of text recalled from history buffer [23] (on
the command-line).
+ Ctl-O
Issues a newline (on the command-line).
+ Ctl-P
Recalls last command from history buffer (on the
command-line).
+ Ctl-Q
Resume (XON).
This resumes stdin in a terminal.
+ Ctl-R
Backwards search for text in history buffer (on the
command-line).
+ Ctl-S
Suspend (XOFF).
This freezes stdin in a terminal. (Use Ctl-Q to restore
input.)
+ Ctl-T
Reverses the position of the character the cursor is on with
the previous character (on the command-line).
+ Ctl-U
Erase a line of input, from the cursor backward to beginning
of line. In some settings, Ctl-U erases the entire line of
input, regardless of cursor position.
+ Ctl-V
When inputting text, Ctl-V permits inserting control
characters. For example, the following two are equivalent:
echo -e '\x0a'
echo
Ctl-V is primarily useful from within a text editor.
+ Ctl-W
When typing text on the console or in an xterm window, Ctl-W
erases from the character under the cursor backwards to the
first instance of whitespace. In some settings, Ctl-W erases
backwards to first non-alphanumeric character.
+ Ctl-X
In certain word processing programs, Cuts highlighted text
and copies to clipboard.
+ Ctl-Y
Pastes back text previously erased (with Ctl-U or Ctl-W).
+ Ctl-Z
Pauses a foreground job.
Substitute operation in certain word processing
applications.
EOF (end-of-file) character in the MSDOS filesystem.
Whitespace
functions as a separator between commands and/or variables.
Whitespace consists of either spaces, tabs, blank lines, or
any combination thereof. [24] In some contexts, such as
variable assignment, whitespace is not permitted, and results
in a syntax error.
Blank lines have no effect on the action of a script, and are
therefore useful for visually separating functional sections.
$IFS, the special variable separating fields of input to
certain commands. It defaults to whitespace.
Definition: A field is a discrete chunk of data expressed as a string
of consecutive characters. Separating each field from adjacent fields
is either whitespace or some other designated character (often
determined by the $IFS). In some contexts, a field may be called a
record.
To preserve whitespace within a string or in a variable, use
quoting.
UNIX filters can target and operate on whitespace using the
POSIX character class [:space:].
________________________________________________________________
Chapter 4. Introduction to Variables and Parameters
Variables are how programming and scripting languages represent data.
A variable is nothing more than a label, a name assigned to a
location or set of locations in computer memory holding an item of
data.
Variables appear in arithmetic operations and manipulation of
quantities, and in string parsing.
________________________________________________________________
4.1. Variable Substitution
The name of a variable is a placeholder for its value, the data it
holds. Referencing (retrieving) its value is called variable
substitution.
$
Let us carefully distinguish between the name of a variable
and its value. If variable1 is the name of a variable, then
$variable1 is a reference to its value, the data item it
contains. [25]
bash$ variable1=23
bash$ echo variable1
variable1
bash$ echo $variable1
23
The only times a variable appears "naked" -- without the $
prefix -- is when declared or assigned, when unset, when
exported, in an arithmetic expression within double
parentheses (( ... )), or in the special case of a variable
representing a signal (see Example 32-5). Assignment may be
with an = (as in var1=27), in a read statement, and at the
head of a loop (for var2 in 1 2 3).
Enclosing a referenced value in double quotes (" ... ") does
not interfere with variable substitution. This is called
partial quoting, sometimes referred to as "weak quoting."
Using single quotes (' ... ') causes the variable name to be
used literally, and no substitution will take place. This is
full quoting, sometimes referred to as 'strong quoting.' See
Chapter 5 for a detailed discussion.
Note that $variable is actually a simplified form of
${variable}. In contexts where the $variable syntax causes an
error, the longer form may work (see Section 10.2, below).
Example 4-1. Variable assignment and substitution
!/bin/bash
ex9.sh
Variables: assignment and substitution
a=375
hello=$a
^ ^
No space permitted on either side of = sign when initializing variables.
What happens if there is a space?
"VARIABLE =value"
^
% Script tries to run "VARIABLE" command with one argument, "=value".
"VARIABLE= value"
^
% Script tries to run "value" command with
- the environmental variable "VARIABLE" set to "".
echo hello # hello
Not a variable reference, just the string "hello" ...
echo $hello # 375
^ This is a variable reference.
echo ${hello} # 375
Likewise a variable reference, as above.
Quoting . . .
echo "$hello" # 375
echo "${hello}" # 375
echo
hello="A B C D"
echo $hello # A B C D
echo "$hello" # A B C D
As we see, echo $hello and echo "$hello" give different results.
=======================================
Quoting a variable preserves whitespace.
=======================================
echo
echo '$hello' # $hello
^ ^
Variable referencing disabled (escaped) by single quotes,
- which causes the "$" to be interpreted literally.
Notice the effect of different types of quoting.
hello= # Setting it to a null value.
echo "$hello (null value) = $hello" # $hello (null value) =
Note that setting a variable to a null value is not the same as
- unsetting it, although the end result is the same (see below).
It is permissible to set multiple variables on the same line,
- if separated by white space.
Caution, this may reduce legibility, and may not be portable.
var1=21 var2=22 var3=$V3
echo
echo "var1=$var1 var2=$var2 var3=$var3"
May cause problems with legacy versions of "sh" . . .
echo; echo
numbers="one two three"
^ ^
other_numbers="1 2 3"
^ ^
If there is whitespace embedded within a variable,
- then quotes are necessary.
other_numbers=1 2 3 # Gives an error message.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers" # other_numbers = 1 2 3
Escaping the whitespace also works.
mixed_bag=2\ ---\ Whatever
^ ^ Space after escape ().
echo "$mixed_bag" # 2 --- Whatever
echo; echo
echo "uninitialized_variable = $uninitialized_variable"
Uninitialized variable has null value (no value at all!).
uninitialized_variable= # Declaring, but not initializing it --
#+ same as setting it to a null value, as above.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
uninitialized_variable=23 # Set it.
unset uninitialized_variable # Unset it.
echo "uninitialized_variable = $uninitialized_variable"
# uninitialized_variable =
# It still has a null value.
echo
exit 0
Caution
An uninitialized variable has a "null" value -- no assigned value at
all (not zero!).
if [ -z "$unassigned" ]
then
echo "$unassigned is NULL."
fi # $unassigned is NULL.
Using a variable before assigning a value to it may cause problems.
It is nevertheless possible to perform arithmetic operations on an
uninitialized variable.
echo "$uninitialized" # (blank line)
let "uninitialized += 5" # Add 5 to it.
echo "$uninitialized" # 5
Conclusion:
An uninitialized variable has no value,
- however it evaluates as 0 in an arithmetic operation.
See also Example 15-23.
________________________________________________________________
4.2. Variable Assignment
=
the assignment operator (no space before and after)
Caution
Do not confuse this with = and -eq, which test, rather than assign!
Note that = can be either an assignment or a test operator, depending
on context.
Example 4-2. Plain Variable Assignment
!/bin/bash
Naked variables
echo
When is a variable "naked", i.e., lacking the '$' in front?
When it is being assigned, rather than referenced.
Assignment
a=879
echo "The value of "a" is $a."
Assignment using 'let'
let a=16+5
echo "The value of "a" is now $a."
echo
In a 'for' loop (really, a type of disguised assignment):
echo -n "Values of "a" in the loop are: "
for a in 7 8 9 11
do
echo -n "$a "
done
echo
echo
In a 'read' statement (also a type of assignment):
echo -n "Enter "a" "
read a
echo "The value of "a" is now $a."
echo
exit 0
Example 4-3. Variable Assignment, plain and fancy
!/bin/bash
a=23 # Simple case
echo $a
b=$a
echo $b
Now, getting a little bit fancier (command substitution).
a=echo Hello! # Assigns result of 'echo' command to 'a' ...
echo $a
Note that including an exclamation mark (!) within a
- command substitution construct will not work from the command-line,
- since this triggers the Bash "history mechanism."
Inside a script, however, the history functions are disabled by default.
a=ls -l # Assigns result of 'ls -l' command to 'a'
echo $a # Unquoted, however, it removes tabs and newlines.
echo
echo "$a" # The quoted variable preserves whitespace.
# (See the chapter on "Quoting.")
exit 0
Variable assignment using the $(...) mechanism (a newer method
than backquotes). This is likewise a form of command
substitution.
From /etc/rc.d/rc.local
R=$(cat /etc/redhat-release)
arch=$(uname -m)
________________________________________________________________
4.3. Bash Variables Are Untyped
Unlike many other programming languages, Bash does not segregate its
variables by "type." Essentially, Bash variables are character
strings, but, depending on context, Bash permits arithmetic
operations and comparisons on variables. The determining factor is
whether the value of a variable contains only digits.
Example 4-4. Integer or string?
!/bin/bash
int-or-string.sh
a=2334 # Integer.
let "a += 1"
echo "a = $a " # a = 2335
echo # Integer, still.
b=${a/23/BB} # Substitute "BB" for "23".
# This transforms $b into a string.
echo "b = $b" # b = BB35
declare -i b # Declaring it an integer doesn't help.
echo "b = $b" # b = BB35
let "b += 1" # BB35 + 1
echo "b = $b" # b = 1
echo # Bash sets the "integer value" of a string to 0.
c=BB34
echo "c = $c" # c = BB34
d=${c/BB/23} # Substitute "23" for "BB".
# This makes $d an integer.
echo "d = $d" # d = 2334
let "d += 1" # 2334 + 1
echo "d = $d" # d = 2335
echo
What about null variables?
e='' # ... Or e="" ... Or e=
echo "e = $e" # e =
let "e += 1" # Arithmetic operations allowed on a null variable?
echo "e = $e" # e = 1
echo # Null variable transformed into an integer.
What about undeclared variables?
echo "f = $f" # f =
let "f += 1" # Arithmetic operations allowed?
echo "f = $f" # f = 1
echo # Undeclared variable transformed into an integer.
However ...
let "f /= $undecl_var" # Divide by zero?
let: f /= : syntax error: operand expected (error token is " ")
Syntax error! Variable $undecl_var is not set to zero here!
But still ...
let "f /= 0"
let: f /= 0: division by 0 (error token is "0")
Expected behavior.
Bash (usually) sets the "integer value" of null to zero
- when performing an arithmetic operation.
But, don't try this at home, folks!
It's undocumented and probably non-portable behavior.
Conclusion: Variables in Bash are untyped,
- with all attendant consequences.
exit $?
Untyped variables are both a blessing and a curse. They permit more
flexibility in scripting and make it easier to grind out lines of
code (and give you enough rope to hang yourself!). However, they
likewise permit subtle errors to creep in and encourage sloppy
programming habits.
To lighten the burden of keeping track of variable types in a script,
Bash does permit declaring variables.
________________________________________________________________
4.4. Special Variable Types
Local variables
Variables visible only within a code block or function (see
also local variables in functions)
Environmental variables
Variables that affect the behavior of the shell and user
interface
Note
In a more general context, each process has an "environment", that
is, a group of variables that the process may reference. In this
sense, the shell behaves like any other process.
Every time a shell starts, it creates shell variables that correspond
to its own environmental variables. Updating or adding new
environmental variables causes the shell to update its environment,
and all the shell's child processes (the commands it executes)
inherit this environment.
Caution
The space allotted to the environment is limited. Creating too many
environmental variables or ones that use up excessive space may cause
problems.
bash$ eval "seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'"
bash$ du
bash: /usr/bin/du: Argument list too long
Note: this "error" has been fixed, as of kernel version 2.6.23.
(Thank you, Stéphane Chazelas for the clarification, and for
providing the above example.)
If a script sets environmental variables, they need to be
"exported," that is, reported to the environment local to the
script. This is the function of the export command.
Note
A script can export variables only to child processes, that is, only
to commands or processes which that particular script initiates. A
script invoked from the command-line cannot export variables back to
the command-line environment. Child processes cannot export variables
back to the parent processes that spawned them.
Definition: A child process is a subprocess launched by another
process, its parent.
Positional parameters
Arguments passed to the script from the command line [26] :
$0, $1, $2, $3 . . .
$0 is the name of the script itself, $1 is the first argument,
$2 the second, $3 the third, and so forth. [27] After $9, the
arguments must be enclosed in brackets, for example, ${10},
${11}, ${12}.
The special variables $* and $@ denote all the positional
parameters.
Example 4-5. Positional Parameters
!/bin/bash
Call this script with at least 10 parameters, for example
./scriptname 1 2 3 4 5 6 7 8 9 10
MINPARAMS=10
echo
echo "The name of this script is "$0"."
Adds ./ for current directory
echo "The name of this script is "basename $0"."
Strips out path name info (see 'basename')
echo
if [ -n "$1" ] # Tested variable is quoted.
then
echo "Parameter #1 is $1" # Need quotes to escape #
fi
if [ -n "$2" ]
then
echo "Parameter #2 is $2"
fi
if [ -n "$3" ]
then
echo "Parameter #3 is $3"
fi
...
if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}.
then
echo "Parameter #10 is ${10}"
fi
echo "-----------------------------------"
echo "All the command-line parameters are: "$*""
if [ $# -lt "$MINPARAMS" ]
then
echo
echo "This script needs at least $MINPARAMS command-line arguments!"
fi
echo
exit 0
Bracket notation for positional parameters leads to a fairly
simple way of referencing the last argument passed to a script
on the command-line. This also requires indirect referencing.
args=$# # Number of args passed.
lastarg=${!args}
Note: This is an indirect reference to $args ...
Or: lastarg=${!#} (Thanks, Chris Monson.)
This is an indirect reference to the $# variable.
Note that lastarg=${!$#} doesn't work.
Some scripts can perform different operations, depending on
which name they are invoked with. For this to work, the script
needs to check $0, the name it was invoked by. [28] There must
also exist symbolic links to all the alternate names of the
script. See Example 16-2.
Tip
If a script expects a command-line parameter but is invoked without
one, this may cause a null variable assignment, generally an
undesirable result. One way to prevent this is to append an extra
character to both sides of the assignment statement using the
expected positional parameter.
variable1_=$1_ # Rather than variable1=$1
This will prevent an error, even if positional parameter is absent.
critical_argument01=$variable1_
The extra character can be stripped off later, like so.
variable1=${variable1_/_/}
Side effects only if $variable1_ begins with an underscore.
This uses one of the parameter substitution templates discussed later.
(Leaving out the replacement pattern results in a deletion.)
A more straightforward way of dealing with this is
- to simply test whether expected positional parameters have been passed.
if [ -z $1 ]
then
exit $E_MISSING_POS_PARAM
fi
However, as Fabian Kreutz points out,
- the above method may have unexpected side-effects.
A better method is parameter substitution:
${1:-$DefaultVal}
See the "Parameter Substition" section
- in the "Variables Revisited" chapter.
---
Example 4-6. wh, whois domain name lookup
!/bin/bash
ex18.sh
Does a 'whois domain-name' lookup on any of 3 alternate servers:
ripe.net, cw.net, radb.net
Place this script -- renamed 'wh' -- in /usr/local/bin
Requires symbolic links:
ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic
ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows
E_NOARGS=75
if [ -z "$1" ]
then
echo "Usage: basename $0 [domain-name]"
exit $E_NOARGS
fi
Check script name and call proper server.
case basename $0 in # Or: case ${0##*/} in
"wh" ) whois $1@whois.tucows.com;;
"wh-ripe" ) whois $1@whois.ripe.net;;
"wh-apnic" ) whois $1@whois.apnic.net;;
"wh-cw" ) whois $1@whois.cw.net;;
* ) echo "Usage: `basename $0` [domain-name]";;
esac
exit $?
---
The shift command reassigns the positional parameters, in
effect shifting them to the left one notch.
$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.
The old $1 disappears, but $0 (the script name) does not
change. If you use a large number of positional parameters to
a script, shift lets you access those past 10, although
{bracket} notation also permits this.
Example 4-7. Using shift
!/bin/bash
shft.sh: Using 'shift' to step through all the positional parameters.
Name this script something like shft.sh,
- and invoke it with some parameters.
sh shft.sh a b c def 83 barndoor
until [ -z "$1" ] # Until all parameters used up . . .
do
echo -n "$1 "
shift
done
echo # Extra linefeed.
But, what happens to the "used-up" parameters?
echo "$2"
Nothing echoes!
When $2 shifts into $1 (and there is no $3 to shift into $2)
So, it is not a parameter copy, but a move.
exit
See also the echo-params.sh script for a "shiftless"
- alternative method of stepping through the positional params.
The shift command can take a numerical parameter indicating
how many positions to shift.
!/bin/bash
shift-past.sh
shift 3 # Shift 3 positions.
n=3; shift $n
Has the same effect.
echo "$1"
exit 0
======================== #
$ sh shift-past.sh 1 2 3 4 5
4
However, as Eleni Fragkiadaki, points out,
- attempting a 'shift' past the number of
- positional parameters ($#) returns an exit status of 1,
- and the positional parameters themselves do not change.
This means possibly getting stuck in an endless loop. . . .
For example:
until [ -z "$1" ]
do
echo -n "$1 "
shift 20 # If less than 20 pos params,
done #+ then loop never ends!
When in doubt, add a sanity check. . . .
shift 20 || break
^^^^^^^^
Note
The shift command works in a similar fashion on parameters passed to
a function. See Example 36-18.
________________________________________________________________
Chapter 5. Quoting
Quoting means just that, bracketing a string in quotes. This has the
effect of protecting special characters in the string from
reinterpretation or expansion by the shell or shell script. (A
character is "special" if it has an interpretation other than its
literal meaning. For example, the asterisk * represents a wild card
character in globbing and Regular Expressions).
bash$ ls -l [Vv]*
-rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT
-rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh
-rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh
bash$ ls -l '[Vv]*'
ls: [Vv]*: No such file or directory
In everyday speech or writing, when we "quote" a phrase, we set it
apart and give it special meaning. In a Bash script, when we quote a
string, we set it apart and protect its literal meaning.
Certain programs and utilities reinterpret or expand special
characters in a quoted string. An important use of quoting is
protecting a command-line parameter from the shell, but still letting
the calling program expand it.
bash$ grep '[Ff]irst' *.txt
file1.txt:This is the first line of file1.txt.
file2.txt:This is the First line of file2.txt.
Note that the unquoted grep [Ff]irst *.txt works under the Bash
shell. [29]
Quoting can also suppress echo's "appetite" for newlines.
bash$ echo $(ls -l)
total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 1
2:57 u.sh
bash$ echo "$(ls -l)"
total 8
-rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh
-rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh
________________________________________________________________
5.1. Quoting Variables
When referencing a variable, it is generally advisable to enclose its
name in double quotes. This prevents reinterpretation of all special
characters within the quoted string -- except $, ` (backquote), and \
(escape). [30] Keeping $ as a special character within double quotes
permits referencing a quoted variable ("$variable"), that is,
replacing the variable with its value (see Example 4-1, above).
Use double quotes to prevent word splitting. [31] An argument
enclosed in double quotes presents itself as a single word, even if
it contains whitespace separators.
List="one two three"
for a in $List # Splits the variable in parts at whitespace.
do
echo "$a"
done
one
two
three
echo "---"
for a in "$List" # Preserves whitespace in a single variable.
do # ^ ^
echo "$a"
done
one two three
A more elaborate example:
variable1="a variable containing five words"
COMMAND This is $variable1 # Executes COMMAND with 7 arguments:
"This" "is" "a" "variable" "containing" "five" "words"
COMMAND "This is $variable1" # Executes COMMAND with 1 argument:
"This is a variable containing five words"
variable2="" # Empty.
COMMAND $variable2 $variable2 $variable2
# Executes COMMAND with no arguments.
COMMAND "$variable2" "$variable2" "$variable2"
# Executes COMMAND with 3 empty arguments.
COMMAND "$variable2 $variable2 $variable2"
# Executes COMMAND with 1 argument (2 spaces).
Thanks, Stéphane Chazelas.
Tip
Enclosing the arguments to an echo statement in double quotes is
necessary only when word splitting or preservation of whitespace is
an issue.
Example 5-1. Echoing Weird Variables
!/bin/bash
weirdvars.sh: Echoing weird variables.
echo
var="'(]\{}$""
echo $var # '(]{}$"
echo "$var" # '(]{}$" Doesn't make a difference.
echo
IFS=''
echo $var # '(] {}$" \ converted to space. Why?
echo "$var" # '(]{}$"
Examples above supplied by Stephane Chazelas.
echo
var2="\\""
echo $var2 # "
echo "$var2" # \"
echo
But ... var2="\\"" is illegal. Why?
var3='\\'
echo "$var3" # \\
Strong quoting works, though.
************************************************************ #
As the first example above shows, nesting quotes is permitted.
echo "$(echo '"')" # "
^ ^
At times this comes in useful.
var1="Two bits"
echo "$var1 = "$var1"" # $var1 = Two bits
^ ^
Or, as Chris Hiestand points out ...
if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]
^ ^ ^ ^ ^ ^ ^ ^
then
...
fi
************************************************************ #
Single quotes (' ') operate similarly to double quotes, but do not
permit referencing variables, since the special meaning of $ is
turned off. Within single quotes, every special character except '
gets interpreted literally. Consider single quotes ("full quoting")
to be a stricter method of quoting than double quotes ("partial
quoting").
Note
Since even the escape character () gets a literal interpretation
within single quotes, trying to enclose a single quote within single
quotes will not yield the expected result.
echo "Why can't I write 's between single quotes"
echo
The roundabout method.
echo 'Why can'''t I write '"'"'s between single quotes'
|-------| |----------| |-----------------------|
Three single-quoted strings, with escaped and quoted single quotes between.
This example courtesy of Stéphane Chazelas.
________________________________________________________________
5.2. Escaping
Escaping is a method of quoting single characters. The escape ()
preceding a character tells the shell to interpret that character
literally.
Caution
With certain commands and utilities, such as echo and sed, escaping a
character may have the opposite effect - it can toggle on a special
meaning for that character.
Special meanings of certain escaped characters
used with echo and sed
\n
means newline
\r
means return
\t
means tab
\v
means vertical tab
\b
means backspace
\a
means alert (beep or flash)
\0xx
translates to the octal ASCII equivalent of 0nn, where nn is a
string of digits
Important
The $' ... ' quoted string-expansion construct is a mechanism that
uses escaped octal or hex values to assign ASCII characters to
variables, e.g., quote=$'\042'.
Example 5-2. Escaped Characters
!/bin/bash
escaped.sh: escaped characters
First, let's show some basic escaped-character usage. ###
Escaping a newline.
echo ""
echo "This will print
as two lines."
This will print
as two lines.
echo "This will print \
as one line."
This will print as one line.
echo; echo
echo "============="
echo "\v\v\v\v" # Prints \v\v\v\v literally.
Use the -e option with 'echo' to print escaped characters.
echo "============="
echo "VERTICAL TABS"
echo -e "\v\v\v\v" # Prints 4 vertical tabs.
echo "=============="
echo "QUOTATION MARK"
echo -e "\042" # Prints " (quote, octal ASCII character 42).
echo "=============="
The $'\X' construct makes the -e option unnecessary.
echo; echo "NEWLINE and (maybe) BEEP"
echo $'\n' # Newline.
echo $'\a' # Alert (beep).
# May only flash, not beep, depending on terminal.
We have seen $'\nnn" string expansion, and now . . .
=================================================================== #
Version 2 of Bash introduced the $'\nnn' string expansion construct.
=================================================================== #
echo "Introducing the $' ... ' string-expansion construct . . . "
echo ". . . featuring more quotation marks."
echo $'\t \042 \t' # Quote (") framed by tabs.
Note that '\nnn' is an octal value.
It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t' # Quote (") framed by tabs.
Thank you, Greg Keraunen, for pointing this out.
Earlier Bash versions allowed '\x022'.
echo
Assigning ASCII characters to a variable.
quote=$'\042' # " assigned to a variable.
echo "$quote Quoted string $quote and this lies outside the quotes."
echo
Concatenating ASCII chars in a variable.
triple_underline=$'\137\137\137' # 137 is octal ASCII code for '_'.
echo "$triple_underline UNDERLINE $triple_underline"
echo
ABC=$'\101\102\103\010' # 101, 102, 103 are octal A, B, C.
echo $ABC
echo
escape=$'\033' # 033 is octal for escape.
echo ""escape" echoes as $escape"
no visible output.
echo
exit 0
A more elaborate example:
Example 5-3. Detecting key-presses
!/bin/bash
Author: Sigurd Solaas, 20 Apr 2011
Used in ABS Guide with permission.
Requires version 4.2+ of Bash.
key="no value yet"
while true; do
clear
echo "Bash Extra Keys Demo. Keys to try:"
echo
echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
echo "* The four arrow keys"
echo "* Tab, enter, escape, and space key"
echo "* The letter and number keys, etc."
echo
echo " d = show date/time"
echo " q = quit"
echo "================================"
echo
Convert the separate home-key to home-key_num_7:
if [ "$key" = $'\x1b\x4f\x48' ]; then
key=$'\x1b\x5b\x31\x7e'
Quoted string-expansion construct.
fi
Convert the separate end-key to end-key_num_1.
if [ "$key" = $'\x1b\x4f\x46' ]; then
key=$'\x1b\x5b\x34\x7e'
fi
case "$key" in
$'\x1b\x5b\x32\x7e') # Insert
echo Insert Key
;;
$'\x1b\x5b\x33\x7e') # Delete
echo Delete Key
;;
$'\x1b\x5b\x31\x7e') # Home_key_num_7
echo Home Key
;;
$'\x1b\x5b\x34\x7e') # End_key_num_1
echo End Key
;;
$'\x1b\x5b\x35\x7e') # Page_Up
echo Page_Up
;;
$'\x1b\x5b\x36\x7e') # Page_Down
echo Page_Down
;;
$'\x1b\x5b\x41') # Up_arrow
echo Up arrow
;;
$'\x1b\x5b\x42') # Down_arrow
echo Down arrow
;;
$'\x1b\x5b\x43') # Right_arrow
echo Right arrow
;;
$'\x1b\x5b\x44') # Left_arrow
echo Left arrow
;;
$'\x09') # Tab
echo Tab Key
;;
$'\x0a') # Enter
echo Enter Key
;;
$'\x1b') # Escape
echo Escape Key
;;
$'\x20') # Space
echo Space Key
;;
d)
date
;;
q)
echo Time to quit...
echo
exit 0
;;
*)
echo You pressed: '"$key"'
;;
esac
echo
echo "================================"
unset K1 K2 K3
read -s -N1 -p "Press a key: "
K1="$REPLY"
read -s -N2 -t 0.001
K2="$REPLY"
read -s -N1 -t 0.001
K3="$REPLY"
key="$K1$K2$K3"
done
exit $?
See also Example 37-1.
"
gives the quote its literal meaning
echo "Hello" # Hello
echo ""Hello" ... he said." # "Hello" ... he said.
$
gives the dollar sign its literal meaning (variable name
following \$ will not be referenced)
echo "$variable01" # $variable01
echo "The book cost $7.98." # The book cost $7.98.
\
gives the backslash its literal meaning
echo "\" # Results in \
Whereas . . .
echo "" # Invokes secondary prompt from the command-line.
# In a script, gives an error message.
However . . .
echo '' # Results in \
Note
The behavior of \ depends on whether it is escaped, strong-quoted,
weak-quoted, or appearing within command substitution or a here
document.
# Simple escaping and quoting
echo \z # z
echo \z # \z
echo '\z' # \z
echo '\z' # \z
echo "\z" # \z
echo "\z" # \z
# Command substitution
echo echo \z # z
echo echo \\z # z
echo echo \\\z # \z
echo echo \\\\z # \z
echo echo \\\\\\z # \z
echo echo \\\\\\\z # \z
echo echo "\z" # \z
echo echo "\\z" # \z
# Here document
cat <<EOF
\z
EOF # \z
cat <<EOF
\z
EOF # \z
These examples supplied by Stéphane Chazelas.
Elements of a string assigned to a variable may be escaped, but the
escape character alone may not be assigned to a variable.
variable=\
echo "$variable"
Will not work - gives an error message:
test.sh: : command not found
A "naked" escape cannot safely be assigned to a variable.
What actually happens here is that the "" escapes the newline and
- the effect is variable=echo "$variable"
variable=\
23skidoo
echo "$variable" # 23skidoo
# This works, since the second line
#+ is a valid variable assignment.
variable=\
^ escape followed by space
echo "$variable" # space
variable=\
echo "$variable" # \
variable=\\
echo "$variable"
Will not work - gives an error message:
test.sh: : command not found
First escape escapes second one, but the third one is left "naked",
- with same result as first instance, above.
variable=\\
echo "$variable" # \
# Second and fourth escapes escaped.
# This is o.k.
Escaping a space can prevent word splitting in a command's argument
list.
file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"
List of files as argument(s) to a command.
Add two files to the list, and list all.
ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list
echo "------------------------------------------------------------------------
-"
What happens if we escape a couple of spaces?
ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
Error: the first three files concatenated into a single argument to 'ls -l'
because the two escaped spaces prevent argument (word) splitting.
The escape also provides a means of writing a multi-line command.
Normally, each separate line constitutes a different command, but an
escape at the end of a line escapes the newline character, and the
command sequence continues on to the next line.
(cd /source/directory && tar cf - . ) | \
(cd /dest/directory && tar xpvf -)
Repeating Alan Cox's directory tree copy command,
but split into two lines for increased legibility.
As an alternative:
tar cf - -C /source/directory . |
tar xpvf - -C /dest/directory
See note below.
(Thanks, Stéphane Chazelas.)
Note
If a script line ends with a |, a pipe character, then a , an
escape, is not strictly necessary. It is, however, good programming
practice to always escape the end of a line of code that continues to
the following line.
echo "foo
bar"
foo
bar
echo
echo 'foo
bar' # No difference yet.
foo
bar
echo
echo foo\
bar # Newline escaped.
foobar
echo
echo "foo\
bar" # Same here, as \ still interpreted as escape within weak quotes.
foobar
echo
echo 'foo\
bar' # Escape character \ taken literally because of strong quoting.
foo\
bar
Examples suggested by Stéphane Chazelas.
________________________________________________________________
Chapter 6. Exit and Exit Status
... there are dark corners in the Bourne shell, and people use all of
them.
--Chet Ramey
The exit command terminates a script, just as in a C program. It can
also return a value, which is available to the script's parent
process.
Every command returns an exit status (sometimes referred to as a
return status or exit code). A successful command returns a 0, while
an unsuccessful one returns a non-zero value that usually can be
interpreted as an error code. Well-behaved UNIX commands, programs,
and utilities return a 0 exit code upon successful completion, though
there are some exceptions.
Likewise, functions within a script and the script itself return an
exit status. The last command executed in the function or script
determines the exit status. Within a script, an exit nnn command may
be used to deliver an nnn exit status to the shell (nnn must be an
integer in the 0 - 255 range).
Note
When a script ends with an exit that has no parameter, the exit
status of the script is the exit status of the last command executed
in the script (previous to the exit).
!/bin/bash
COMMAND_1
. . .
COMMAND_LAST
Will exit with status of last command.
exit
The equivalent of a bare exit is exit $? or even just omitting the
exit.
!/bin/bash
COMMAND_1
. . .
COMMAND_LAST
Will exit with status of last command.
exit $?
!/bin/bash
COMMAND1
. . .
COMMAND_LAST
Will exit with status of last command.
$? reads the exit status of the last command executed. After a
function returns, $? gives the exit status of the last command
executed in the function. This is Bash's way of giving functions a
"return value." [32]
Following the execution of a pipe, a $? gives the exit status of the
last command executed.
After a script terminates, a $? from the command-line gives the exit
status of the script, that is, the last command executed in the
script, which is, by convention, 0 on success or an integer in the
range 1 - 255 on error.
Example 6-1. exit / exit status
!/bin/bash
echo hello
echo $? # Exit status 0 returned because command executed successfully.
lskdf # Unrecognized command.
echo $? # Non-zero exit status returned -- command failed to execute.
echo
exit 113 # Will return 113 to shell.
# To verify this, type "echo $?" after script terminates.
By convention, an 'exit 0' indicates success,
- while a non-zero exit value means an error or anomalous condition.
See the "Exit Codes With Special Meanings" appendix.
$? is especially useful for testing the result of a command in a
script (see Example 16-35 and Example 16-20).
Note
The !, the logical not qualifier, reverses the outcome of a test or
command, and this affects its exit status.
Example 6-2. Negating a condition using !
true # The "true" builtin.
echo "exit status of "true" = $?" # 0
! true
echo "exit status of "! true" = $?" # 1
Note that the "!" needs a space between it and the command.
!true leads to a "command not found" error
The '!' operator prefixing a command invokes the Bash history mechanism.
true
!true
No error this time, but no negation either.
It just repeats the previous command (true).
=========================================================== #
Preceding a pipe with ! inverts the exit status returned.
ls | bogus_command # bash: bogus_command: command not found
echo $? # 127
! ls | bogus_command # bash: bogus_command: command not found
echo $? # 0
Note that the ! does not change the execution of the pipe.
Only the exit status changes.
=========================================================== #
Thanks, Stéphane Chazelas and Kristopher Newsome.
Caution
Certain exit status codes have reserved meanings and should not be
user-specified in a script.
________________________________________________________________
Chapter 7. Tests
Every reasonably complete programming language can test for a
condition, then act according to the result of the test. Bash has the
test command, various bracket and parenthesis operators, and the
if/then construct.
________________________________________________________________
7.1. Test Constructs
* An if/then construct tests whether the exit status of a list of
commands is 0 (since 0 means "success" by UNIX convention), and
if so, executes one or more commands.
* There exists a dedicated command called [ (left bracket special
character). It is a synonym for test, and a builtin for
efficiency reasons. This command considers its arguments as
comparison expressions or file tests and returns an exit status
corresponding to the result of the comparison (0 for true, 1 for
false).
* With version 2.02, Bash introduced the [[ ... ]] extended test
command, which performs comparisons in a manner more familiar to
programmers from other languages. Note that [[ is a keyword, not
a command.
Bash sees [[ $a -lt $b ]] as a single element, which returns an
exit status.
* The (( ... )) and let ... constructs return an exit status,
according to whether the arithmetic expressions they evaluate
expand to a non-zero value. These arithmetic-expansion constructs
may therefore be used to perform arithmetic comparisons.
(( 0 && 1 )) # Logical AND
echo $? # 1 ***
And so ...
let "num = (( 0 && 1 ))"
echo $num # 0
But ...
let "num = (( 0 && 1 ))"
echo $? # 1 ***
(( 200 || 11 )) # Logical OR
echo $? # 0 ***
...
let "num = (( 200 || 11 ))"
echo $num # 1
let "num = (( 200 || 11 ))"
echo $? # 0 ***
(( 200 | 11 )) # Bitwise OR
echo $? # 0 ***
...
let "num = (( 200 | 11 ))"
echo $num # 203
let "num = (( 200 | 11 ))"
echo $? # 0 ***
The "let" construct returns the same exit status
- as the double-parentheses arithmetic expansion.
Caution
Again, note that the exit status of an arithmetic expression is not
an error value.
var=-2 && (( var+=2 ))
echo $? # 1
var=-2 && (( var+=2 )) && echo $var
# Will not echo $var!
* An if can test any command, not just conditions enclosed within
brackets.
if cmp a b &> /dev/null # Suppress output.
then echo "Files a and b are identical."
else echo "Files a and b differ."
fi
The very useful "if-grep" construct:
if grep -q Bash file
then echo "File contains at least one occurrence of Bash."
fi
word=Linux
letter_sequence=inu
if echo "$word" | grep -q "$letter_sequence"
The "-q" option to grep suppresses output.
then
echo "$letter_sequence found in $word"
else
echo "$letter_sequence not found in $word"
fi
if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
then echo "Command succeeded."
else echo "Command failed."
fi
* These last two examples courtesy of Stéphane Chazelas.
Example 7-1. What is truth?
!/bin/bash
Tip:
If you're unsure how a certain condition might evaluate,
echo
echo "Testing "0""
if [ 0 ] # zero
then
echo "0 is true."
else # Or else ...
echo "0 is false."
fi # 0 is true.
echo
echo "Testing "1""
if [ 1 ] # one
then
echo "1 is true."
else
echo "1 is false."
fi # 1 is true.
echo
echo "Testing "-1""
if [ -1 ] # minus one
then
echo "-1 is true."
else
echo "-1 is false."
fi # -1 is true.
echo
echo "Testing "NULL""
if [ ] # NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi # NULL is false.
echo
echo "Testing "xyz""
if [ xyz ] # string
then
echo "Random string is true."
else
echo "Random string is false."
fi # Random string is true.
echo
echo "Testing "$xyz""
if [ $xyz ] # Tests if $xyz is null, but...
# it's only an uninitialized variable.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
echo "Testing "-n $xyz""
if [ -n "$xyz" ] # More pedantically correct.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
xyz= # Initialized, but set to null value.
echo "Testing "-n $xyz""
if [ -n "$xyz" ]
then
echo "Null variable is true."
else
echo "Null variable is false."
fi # Null variable is false.
echo
When is "false" true?
echo "Testing "false""
if [ "false" ] # It seems that "false" is just a string ...
then
echo ""false" is true." #+ and it tests true.
else
echo ""false" is false."
fi # "false" is true.
echo
echo "Testing "$false"" # Again, uninitialized variable.
if [ "$false" ]
then
echo ""$false" is true."
else
echo ""$false" is false."
fi # "$false" is false.
# Now, we get the expected result.
What would happen if we tested the uninitialized variable "$true"?
echo
exit 0
Exercise. Explain the behavior of Example 7-1, above.
if [ condition-true ]
then
command 1
command 2
...
else # Or else ...
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi
Note
When if and then are on same line in a condition test, a semicolon
must terminate the if statement. Both if and then are keywords.
Keywords (or commands) begin statements, and before a new statement
on the same line begins, the old one must terminate.
if [ -x "$filename" ]; then
Else if and elif
elif
elif is a contraction for else if. The effect is to nest an
inner if/then construct within an outer one.
if [ condition1 ]
then
command1
command2
command3
elif [ condition2 ]
Same as else if
then
command4
command5
else
default-command
fi
The if test condition-true construct is the exact equivalent of if [
condition-true ]. As it happens, the left bracket, [ , is a token
[33] which invokes the test command. The closing right bracket, ] ,
in an if/test should not therefore be strictly necessary, however
newer versions of Bash require it.
Note
The test command is a Bash builtin which tests file types and
compares strings. Therefore, in a Bash script, test does not call the
external /usr/bin/test binary, which is part of the sh-utils package.
Likewise, [ does not call /usr/bin/[, which is linked to
/usr/bin/test.
bash$ type test
test is a shell builtin
bash$ type '['
[ is a shell builtin
bash$ type '[['
[[ is a shell keyword
bash$ type ']]'
]] is a shell keyword
bash$ type ']'
bash: type: ]: not found
If, for some reason, you wish to use /usr/bin/test in a Bash script,
then specify it by full pathname.
Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[
!/bin/bash
echo
if test -z "$1"
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if /usr/bin/test -z "$1" # Equivalent to "test" builtin.
^^^^^^^^^^^^^ # Specifying full pathname.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if [ -z "$1" ] # Functionally identical to above code blocks.
if [ -z "$1" should work, but...
- Bash responds to a missing close-bracket with an error message.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if /usr/bin/[ -z "$1" ] # Again, functionally identical to above.
if /usr/bin/[ -z "$1" # Works, but gives an error message.
Note:
This has been fixed in Bash, version 3.x.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
exit 0
The [[ ]] construct is the more versatile Bash version of [ ]. This
is the extended test command, adopted from ksh88.
No filename expansion or word splitting takes place between [[ and
]], but there is parameter expansion and command substitution.
file=/etc/passwd
if [[ -e $file ]]
then
echo "Password file exists."
fi
Using the [[ ... ]] test construct, rather than [ ... ] can prevent
many logic errors in scripts. For example, the &&, ||, <, and >
operators work within a [[ ]] test, despite giving an error within a
[ ] construct.
Arithmetic evaluation of octal / hexadecimal constants takes place
automatically within a [[ ... ]] construct.
[[ Octal and hexadecimal evaluation ]]
Thank you, Moritz Gronbach, for pointing this out.
decimal=15
octal=017 # = 15 (decimal)
hex=0x0f # = 15 (decimal)
if [ "$decimal" -eq "$octal" ]
then
echo "$decimal equals $octal"
else
echo "$decimal is not equal to $octal" # 15 is not equal to 017
fi # Doesn't evaluate within [ single brackets ]!
if [[ "$decimal" -eq "$octal" ]]
then
echo "$decimal equals $octal" # 15 equals 017
else
echo "$decimal is not equal to $octal"
fi # Evaluates within [[ double brackets ]]!
if [[ "$decimal" -eq "$hex" ]]
then
echo "$decimal equals $hex" # 15 equals 0x0f
else
echo "$decimal is not equal to $hex"
fi # [[ $hexadecimal ]] also evaluates!
Note
Following an if, neither the test command nor the test brackets ( [ ]
or [[ ]] ) are strictly necessary.
dir=/home/bozo
if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message.
echo "Now in $dir."
else
echo "Can't change to $dir."
fi
The "if COMMAND" construct returns the exit status of COMMAND.
Similarly, a condition within test brackets may stand alone without
an if, when used in combination with a list construct.
var1=20
var2=22
[ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"
home=/home/bozo
[ -d "$home" ] || echo "$home directory does not exist."
The (( )) construct expands and evaluates an arithmetic expression.
If the expression evaluates as zero, it returns an exit status of 1,
or "false". A non-zero expression returns an exit status of 0, or
"true". This is in marked contrast to using the test and [ ]
constructs previously discussed.
Example 7-3. Arithmetic Tests using (( ))
!/bin/bash
arith-tests.sh
Arithmetic tests.
The (( ... )) construct evaluates and tests numerical expressions.
Exit status opposite from [ ... ] construct!
(( 0 ))
echo "Exit status of "(( 0 ))" is $?." # 1
(( 1 ))
echo "Exit status of "(( 1 ))" is $?." # 0
(( 5 > 4 )) # true
echo "Exit status of "(( 5 > 4 ))" is $?." # 0
(( 5 > 9 )) # false
echo "Exit status of "(( 5 > 9 ))" is $?." # 1
(( 5 == 5 )) # true
echo "Exit status of "(( 5 == 5 ))" is $?." # 0
(( 5 = 5 )) gives an error message.
(( 5 - 5 )) # 0
echo "Exit status of "(( 5 - 5 ))" is $?." # 1
(( 5 / 4 )) # Division o.k.
echo "Exit status of "(( 5 / 4 ))" is $?." # 0
(( 1 / 2 )) # Division result < 1.
echo "Exit status of "(( 1 / 2 ))" is $?." # Rounded off to 0.
# 1
(( 1 / 0 )) 2>/dev/null # Illegal division by 0.
^^^^^^^^^^^
echo "Exit status of "(( 1 / 0 ))" is $?." # 1
What effect does the "2>/dev/null" have?
What would happen if it were removed?
Try removing it, then rerunning the script.
======================================= #
(( ... )) also useful in an if-then test.
var1=5
var2=4
if (( var1 > var2 ))
then #^ ^ Note: Not $var1, $var2. Why?
echo "$var1 is greater than $var2"
fi # 5 is greater than 4
exit 0
________________________________________________________________
7.2. File test operators
Returns true if...
-e
file exists
-a
file exists
This is identical in effect to -e. It has been "deprecated,"
[34] and its use is discouraged.
-f
file is a regular file (not a directory or device file)
-s
file is not zero size
-d
file is a directory
-b
file is a block device
-c
file is a character device
device0="/dev/sda2" # / (root directory)
if [ -b "$device0" ]
then
echo "$device0 is a block device."
fi
/dev/sda2 is a block device.
device1="/dev/ttyS1" # PCMCIA modem card.
if [ -c "$device1" ]
then
echo "$device1 is a character device."
fi
/dev/ttyS1 is a character device.
-p
file is a pipe
function show_input_type()
{
[ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}
show_input_type "Input" # STDIN
echo "Input" | show_input_type # PIPE
This example courtesy of Carl Anderson.
-h
file is a symbolic link
-L
file is a symbolic link
-S
file is a socket
-t
file (descriptor) is associated with a terminal device
This test option may be used to check whether the stdin [ -t 0
] or stdout [ -t 1 ] in a given script is a terminal.
-r
file has read permission (for the user running the test)
-w
file has write permission (for the user running the test)
-x
file has execute permission (for the user running the test)
-g
set-group-id (sgid) flag set on file or directory
If a directory has the sgid flag set, then a file created
within that directory belongs to the group that owns the
directory, not necessarily to the group of the user who
created the file. This may be useful for a directory shared by
a workgroup.
-u
set-user-id (suid) flag set on file
A binary owned by root with set-user-id flag set runs with
root privileges, even when an ordinary user invokes it. [35]
This is useful for executables (such as pppd and cdrecord)
that need to access system hardware. Lacking the suid flag,
these binaries could not be invoked by a non-root user.
-rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd
A file with the suid flag set shows an s in its permissions.
-k
sticky bit set
Commonly known as the sticky bit, the save-text-mode flag is a
special type of file permission. If a file has this flag set,
that file will be kept in cache memory, for quicker access.
[36] If set on a directory, it restricts write permission.
Setting the sticky bit adds a t to the permissions on the file
or directory listing. This restricts altering or deleting
specific files in that directory to the owner of those files.
drwxrwxrwt 7 root 1024 May 19 21:26 tmp/
If a user does not own a directory that has the sticky bit
set, but has write permission in that directory, she can only
delete those files that she owns in it. This keeps users from
inadvertently overwriting or deleting each other's files in a
publicly accessible directory, such as /tmp. (The owner of the
directory or root can, of course, delete or rename files
there.)
-O
you are owner of file
-G
group-id of file same as yours
-N
file modified since it was last read
f1 -nt f2
file f1 is newer than f2
f1 -ot f2
file f1 is older than f2
f1 -ef f2
files f1 and f2 are hard links to the same file
!
"not" -- reverses the sense of the tests above (returns true
if condition absent).
Example 7-4. Testing for broken links
!/bin/bash
broken-link.sh
Used in ABS Guide with permission.
A pure shell script to find dead symlinks and output them quoted
- so they can be fed to xargs and dealt with :)
- eg. sh broken-link.sh /somedir /someotherdir|xargs rm
This, however, is a better method:
find "somedir" -type l -print0|\
xargs -r0 file|\
grep "broken symbolic"|
sed -e 's/^|: broken symbolic.$/"/g'
- but that wouldn't be pure Bash, now would it.
Caution: beware the /proc file system and any circular links!
If no args are passed to the script set directories-to-search
- to current directory. Otherwise set the directories-to-search
[ $# -eq 0 ] && directorys=pwd || directorys=$@
Setup the function linkchk to check the directory it is passed
- for files that are links and don't exist, then print them quoted.
If one of the elements in the directory is a subdirectory then
- send that subdirectory to the linkcheck function.
linkchk () {
for element in $1/*; do
[ -h "$element" -a ! -e "$element" ] && echo \"$element\"
[ -d "$element" ] && linkchk $element
# Of course, '-h' tests for symbolic link, '-d' for directory.
done
}
Send each arg that was passed to the script to the linkchk() function
- if it is a valid directoy. If not, then print the error message
for directory in $directorys; do
if [ -d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi
done
exit $?
Example 31-1, Example 11-8, Example 11-3, Example 31-3, and Example
A-1 also illustrate uses of the file test operators.
________________________________________________________________
7.3. Other Comparison Operators
A binary comparison operator compares two variables or quantities.
Note that integer and string comparison use a different set of
operators.
integer comparison
-eq
is equal to
if [ "$a" -eq "$b" ]
-ne
is not equal to
if [ "$a" -ne "$b" ]
-gt
is greater than
if [ "$a" -gt "$b" ]
-ge
is greater than or equal to
if [ "$a" -ge "$b" ]
-lt
is less than
if [ "$a" -lt "$b" ]
-le
is less than or equal to
if [ "$a" -le "$b" ]
<
is less than (within double parentheses)
(("$a" < "$b"))
<=
is less than or equal to (within double parentheses)
(("$a" <= "$b"))
is greater than (within double parentheses)
(("$a" > "$b"))
=
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
string comparison
=
is equal to
if [ "$a" = "$b" ]
Caution
Note the whitespace framing the =.
if [ "$a"="$b" ] is not equivalent to the above.
==
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
Note
The == comparison operator behaves differently within a
double-brackets test than within single brackets.
[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
[ $a == z* ] # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
Thanks, Stéphane Chazelas
!=
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]]
construct.
<
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 27-11 for an application of this comparison
operator.
-z
string is null, that is, has zero length
String='' # Zero-length ("null") string variable.
if [ -z "$String" ]
then
echo "$String is null."
else
echo "$String is NOT null."
fi # $String is null.
-n
string is not null.
Caution
The -n test requires that the string be quoted within the test
brackets. Using an unquoted string with ! -z, or even just the
unquoted string alone within test brackets (see Example 7-6) normally
works, however, this is an unsafe practice. Always quote a tested
string. [37]
Example 7-5. Arithmetic and string comparisons
!/bin/bash
a=4
b=5
Here "a" and "b" can be treated either as integers or strings.
There is some blurring between the arithmetic and string comparisons,
- since Bash variables are not strongly typed.
Bash permits integer operations and comparisons on variables
- whose value consists of all-integer characters.
Caution advised, however.
echo
if [ "$a" -ne "$b" ]
then
echo "$a is not equal to $b"
echo "(arithmetic comparison)"
fi
echo
if [ "$a" != "$b" ]
then
echo "$a is not equal to $b."
echo "(string comparison)"
"4" != "5"
ASCII 52 != ASCII 53
fi
In this particular instance, both "-ne" and "!=" work.
echo
exit 0
Example 7-6. Testing whether a string is null
!/bin/bash
str-test.sh: Testing null strings and unquoted strings,
- but not strings and sealing wax, not to mention cabbages and kings . . .
Using if [ ... ]
If a string has not been initialized, it has no defined value.
This state is called "null" (not the same as zero!).
if [ -n $string1 ] # string1 has not been declared or initialized.
then
echo "String "string1" is not null."
else
echo "String "string1" is null."
fi # Wrong result.
Shows $string1 as not null, although it was not initialized.
echo
Let's try it again.
if [ -n "$string1" ] # This time, $string1 is quoted.
then
echo "String "string1" is not null."
else
echo "String "string1" is null."
fi # Quote strings within test brackets!
echo
if [ $string1 ] # This time, $string1 stands naked.
then
echo "String "string1" is not null."
else
echo "String "string1" is null."
fi # This works fine.
The [ ... ] test operator alone detects whether the string is null.
However it is good practice to quote it (if [ "$string1" ]).
As Stephane Chazelas points out,
if [ $string1 ] has one argument, "]"
if [ "$string1" ] has two arguments, the empty "$string1" and "]"
echo
string1=initialized
if [ $string1 ] # Again, $string1 stands unquoted.
then
echo "String "string1" is not null."
else
echo "String "string1" is null."
fi # Again, gives correct result.
Still, it is better to quote it ("$string1"), because . . .
string1="a = b"
if [ $string1 ] # Again, $string1 stands unquoted.
then
echo "String "string1" is not null."
else
echo "String "string1" is null."
fi # Not quoting "$string1" now gives wrong result!
exit 0 # Thank you, also, Florian Wisser, for the "heads-up".
Example 7-7. zmore
!/bin/bash
zmore
View gzipped files with 'more' filter.
E_NOARGS=85
E_NOTFOUND=86
E_NOTGZIP=87
if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]
$1 can exist, but be empty: zmore "" arg2 arg3
then
echo "Usage: basename $0 filename" >&2
Error message to stderr.
exit $E_NOARGS
Returns 85 as exit status of script (error code).
fi
filename=$1
if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.
then
echo "File $filename not found!" >&2 # Error message to stderr.
exit $E_NOTFOUND
fi
if [ ${filename##*.} != "gz" ]
Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit $E_NOTGZIP
fi
zcat $1 | more
Uses the 'more' filter.
May substitute 'less' if desired.
exit $? # Script returns exit status of pipe.
Actually "exit $?" is unnecessary, as the script will, in any case,
- return the exit status of the last command executed.
compound comparison
-a
logical and
exp1 -a exp2 returns true if both exp1 and exp2 are true.
-o
logical or
exp1 -o exp2 returns true if either exp1 or exp2 is true.
These are similar to the Bash comparison operators && and ||, used
within double brackets.
[[ condition1 && condition2 ]]
The -o and -a operators work with the test command or occur within
single test brackets.
if [ "$expr1" -a "$expr2" ]
then
echo "Both expr1 and expr2 are true."
else
echo "Either expr1 or expr2 is false."
fi
Caution
But, as rihad points out:
[ 1 -eq 1 ] && [ -n "echo true 1>&2" ] # true
[ 1 -eq 2 ] && [ -n "echo true 1>&2" ] # (no output)
^^^^^^^ False condition. So far, everything as expected.
However ...
[ 1 -eq 2 -a -n "echo true 1>&2" ] # true
^^^^^^^ False condition. So, why "true" output?
Is it because both condition clauses within brackets evaluate?
[[ 1 -eq 2 && -n "echo true 1>&2" ]] # (no output)
No, that's not it.
Apparently && and || "short-circuit" while -a and -o do not.
Refer to Example 8-3, Example 27-17, and Example A-29 to see compound
comparison operators in action.
________________________________________________________________
7.4. Nested if/then Condition Tests
Condition tests using the if/then construct may be nested. The net
result is equivalent to using the && compound comparison operator.
a=3
if [ "$a" -gt 0 ]
then
if [ "$a" -lt 5 ]
then
echo "The value of \"a\" lies somewhere between 0 and 5."
fi
fi
Same result as:
if [ "$a" -gt 0 ] && [ "$a" -lt 5 ]
then
echo "The value of "a" lies somewhere between 0 and 5."
fi
Example 37-4 and Example 17-11 demonstrate nested if/then condition
tests.
________________________________________________________________
7.5. Testing Your Knowledge of Tests
The systemwide xinitrc file can be used to launch the X server. This
file contains quite a number of if/then tests. The following is
excerpted from an "ancient" version of xinitrc (Red Hat 7.1, or
thereabouts).
if [ -f $HOME/.Xclients ]; then
exec $HOME/.Xclients
elif [ -f /etc/X11/xinit/Xclients ]; then
exec /etc/X11/xinit/Xclients
else
# failsafe settings. Although we should never get here
# (we provide fallbacks in Xclients as well) it can't hurt.
xclock -geometry 100x100-5+5 &
xterm -geometry 80x50-50+150 &
if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then
netscape /usr/share/doc/HTML/index.html &
fi
fi
Explain the test constructs in the above snippet, then examine an
updated version of the file, /etc/X11/xinit/xinitrc, and analyze the
if/then test constructs there. You may need to refer ahead to the
discussions of grep, sed, and regular expressions.
________________________________________________________________
Chapter 8. Operations and Related Topics
8.1. Operators
assignment
variable assignment
Initializing or changing the value of a variable
=
All-purpose assignment operator, which works for both
arithmetic and string assignments.
var=27
category=minerals # No spaces allowed after the "=".
Caution
Do not confuse the "=" assignment operator with the = test operator.
= as a test operator
if [ "$string1" = "$string2" ]
then
command
fi
if [ "X$string1" = "X$string2" ] is safer,
- to prevent an error message should one of the variables be empty.
(The prepended "X" characters cancel out.)
arithmetic operators
plus
minus
multiplication
/
division
**
exponentiation
Bash, version 2.02, introduced the "**" exponentiation operator.
let "z=5**3" # 5 * 5 * 5
echo "z = $z" # z = 125
%
modulo, or mod (returns the remainder of an integer division
operation)
bash$ expr 5 % 3
2
5/3 = 1, with remainder 2
This operator finds use in, among other things, generating
numbers within a specific range (see Example 9-11 and Example
9-15) and formatting program output (see Example 27-16 and
Example A-6). It can even be used to generate prime numbers,
(see Example A-15). Modulo turns up surprisingly often in
numerical recipes.
Example 8-1. Greatest common divisor
!/bin/bash
gcd.sh: greatest common divisor
Uses Euclid's algorithm
The "greatest common divisor" (gcd) of two integers
- is the largest integer that will divide both, leaving no remainder.
Euclid's algorithm uses successive division.
In each pass,
The gcd = dividend, on the final pass.
For an excellent discussion of Euclid's algorithm, see
- Jim Loy's site, http://www.jimloy.com/number/euclids.htm.
Argument check
ARGS=2
E_BADARGS=85
if [ $# -ne "$ARGS" ]
then
echo "Usage: basename $0 first-number second-number"
exit $E_BADARGS
fi
gcd ()
{
dividend=$1 # Arbitrary assignment.
divisor=$2 #! It doesn't matter which of the two is larger.
# Why not?
remainder=1 # If an uninitialized variable is used inside
#+ test brackets, an error message results.
until [ "$remainder" -eq 0 ]
do # ^^^^^^^^^^ Must be previously initialized!
let "remainder = $dividend % $divisor"
dividend=$divisor # Now repeat with 2 smallest numbers.
divisor=$remainder
done # Euclid's algorithm
} # Last $dividend is the gcd.
gcd $1 $2
echo; echo "GCD of $1 and $2 = $dividend"; echo
Exercises :
- Check command-line arguments to make sure they are integers,
- and exit the script with an appropriate error message if not.
- Rewrite the gcd () function to use local variables.
exit 0
+=
plus-equal (increment variable by a constant) [38]
let "var += 5" results in var being incremented by 5.
-=
minus-equal (decrement variable by a constant)
*=
times-equal (multiply variable by a constant)
let "var *= 4" results in var being multiplied by 4.
/=
slash-equal (divide variable by a constant)
%=
mod-equal (remainder of dividing variable by a constant)
Arithmetic operators often occur in an expr or let expression.
Example 8-2. Using Arithmetic Operations
!/bin/bash
Counting to 11 in 10 different ways.
n=1; echo -n "$n "
let "n = $n + 1" # let "n = n + 1" also works.
echo -n "$n "
: $((n = $n + 1))
":" necessary because otherwise Bash attempts
- to interpret "$((n = $n + 1))" as a command.
echo -n "$n "
(( n = n + 1 ))
A simpler alternative to the method above.
Thanks, David Lombard, for pointing this out.
echo -n "$n "
n=$(($n + 1))
echo -n "$n "
: $[ n = $n + 1 ]
":" necessary because otherwise Bash attempts
- to interpret "$[ n = $n + 1 ]" as a command.
Works even if "n" was initialized as a string.
echo -n "$n "
n=$[ $n + 1 ]
Works even if "n" was initialized as a string.
- Avoid this type of construct, since it is obsolete and nonportable.
Thanks, Stephane Chazelas.
echo -n "$n "
Now for C-style increment operators.
Thanks, Frank Wang, for pointing this out.
let "n++" # let "++n" also works.
echo -n "$n "
(( n++ )) # (( ++n )) also works.
echo -n "$n "
: $(( n++ )) # : $(( ++n )) also works.
echo -n "$n "
: $[ n++ ] # : $[ ++n ] also works
echo -n "$n "
echo
exit 0
Note
Integer variables in older versions of Bash were signed long (32-bit)
integers, in the range of -2147483648 to 2147483647. An operation
that took a variable outside these limits gave an erroneous result.
echo $BASH_VERSION # 1.14
a=2147483646
echo "a = $a" # a = 2147483646
let "a+=1" # Increment "a".
echo "a = $a" # a = 2147483647
let "a+=1" # increment "a" again, past the limit.
echo "a = $a" # a = -2147483648
# ERROR: out of range,
# + and the leftmost bit, the sign bit,
# + has been set, making the result negative.
As of version >= 2.05b, Bash supports 64-bit integers.
Caution
Bash does not understand floating point arithmetic. It treats numbers
containing a decimal point as strings.
a=1.5
let "b = $a + 1.3" # Error.
t2.sh: let: b = 1.5 + 1.3: syntax error in expression
(error token is ".5 + 1.3")
echo "b = $b" # b=1
Use bc in scripts that that need floating point calculations or math
library functions.
bitwise operators. The bitwise operators seldom make an appearance in
shell scripts. Their chief use seems to be manipulating and testing
values read from ports or sockets. "Bit flipping" is more relevant to
compiled languages, such as C and C++, which provide direct access to
system hardware. However, see vladz's ingenious use of bitwise
operators in his base64.sh (Example A-54) script.
bitwise operators
<<
bitwise left shift (multiplies by 2 for each shift position)
<<=
left-shift-equal
let "var <<= 2" results in var left-shifted 2 bits (multiplied
by 4)
bitwise right shift (divides by 2 for each shift position)
=
right-shift-equal (inverse of <<=)
&
bitwise AND
&=
bitwise AND-equal
|
bitwise OR
|=
bitwise OR-equal
~
bitwise NOT
^
bitwise XOR
^=
bitwise XOR-equal
logical (boolean) operators
!
NOT
if [ ! -f $FILENAME ]
then
...
&&
AND
if [ $condition1 ] && [ $condition2 ]
Same as: if [ $condition1 -a $condition2 ]
Returns true if both condition1 and condition2 hold true...
if [[ $condition1 && $condition2 ]] # Also works.
Note that && operator not permitted inside brackets
Note
&& may also be used, depending on context, in an and list to
concatenate commands.
||
OR
if [ $condition1 ] || [ $condition2 ]
Same as: if [ $condition1 -o $condition2 ]
Returns true if either condition1 or condition2 holds true...
if [[ $condition1 || $condition2 ]] # Also works.
Note that || operator not permitted inside brackets
Note
Bash tests the exit status of each statement linked with a logical
operator.
Example 8-3. Compound Condition Tests Using && and ||
!/bin/bash
a=24
b=47
if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]
then
echo "Test #1 succeeds."
else
echo "Test #1 fails."
fi
ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ]
Note: if [[ $a -eq 24 && $b -eq 24 ]] works.
The double-bracket if-test is more flexible
- than the single-bracket version.
(The "&&" has a different meaning in line 17 than in line 6.)
Thanks, Stephane Chazelas, for pointing this out.
if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
then
echo "Test #2 succeeds."
else
echo "Test #2 fails."
fi
The -a and -o options provide
- an alternative compound condition test.
Thanks to Patrick Callahan for pointing this out.
if [ "$a" -eq 24 -a "$b" -eq 47 ]
then
echo "Test #3 succeeds."
else
echo "Test #3 fails."
fi
if [ "$a" -eq 98 -o "$b" -eq 47 ]
then
echo "Test #4 succeeds."
else
echo "Test #4 fails."
fi
a=rhino
b=crocodile
if [ "$a" = rhino ] && [ "$b" = crocodile ]
then
echo "Test #5 succeeds."
else
echo "Test #5 fails."
fi
exit 0
The && and || operators also find use in an arithmetic
context.
bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
1 0 1 0
miscellaneous operators
,
Comma operator
The comma operator chains together two or more arithmetic
operations. All the operations are evaluated (with possible
side effects. [39]
let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1" ^^^^^^ # t1 = 11
Here t1 is set to the result of the last operation. Why?
let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2".
echo "t2 = $t2 a = $a" # t2 = 5 a = 9
The comma operator finds use mainly in for loops. See Example
11-13.
________________________________________________________________
8.2. Numerical Constants
A shell script interprets a number as decimal (base 10), unless that
number has a special prefix or notation. A number preceded by a 0 is
octal (base 8). A number preceded by 0x is hexadecimal (base 16). A
number with an embedded # evaluates as BASE#NUMBER (with range and
notational restrictions).
Example 8-4. Representation of numerical constants
!/bin/bash
numbers.sh: Representation of numbers in different bases.
Decimal: the default
let "dec = 32"
echo "decimal number = $dec" # 32
Nothing out of the ordinary here.
Octal: numbers preceded by '0' (zero)
let "oct = 032"
echo "octal number = $oct" # 26
Expresses result in decimal.
Hexadecimal: numbers preceded by '0x' or '0X'
let "hex = 0x32"
echo "hexadecimal number = $hex" # 50
echo $((0x9abc)) # 39612
^^ ^^ double-parentheses arithmetic expansion/evaluation
Expresses result in decimal.
Other bases: BASE#NUMBER
BASE between 2 and 64.
NUMBER must use symbols within the BASE range, see below.
let "bin = 2#111100111001101"
echo "binary number = $bin" # 31181
let "b32 = 32#77"
echo "base-32 number = $b32" # 231
let "b64 = 64#@_"
echo "base-64 number = $b64" # 4031
This notation only works for a limited range (2 - 64) of ASCII characters.
10 digits + 26 lowercase characters + 26 uppercase characters + @ + _
echo
echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))
# 1295 170 44822 3375
Important note:
Using a digit out of range of the specified base notation
let "bad_oct = 081"
(Partial) error message output:
bad_oct = 081: value too great for base (error token is "081")
Octal numbers use only digits in the range 0 - 7.
exit $? # Exit value = 1 (error)
Thanks, Rich Bartell and Stephane Chazelas, for clarification.
________________________________________________________________
8.3. The Double-Parentheses Construct
Similar to the let command, the (( ... )) construct permits
arithmetic expansion and evaluation. In its simplest form, a=$(( 5 +
3 )) would set a to 5 + 3, or 8. However, this double-parentheses
construct is also a mechanism for allowing C-style manipulation of
variables in Bash, for example, (( var++ )).
Example 8-5. C-style manipulation of variables
!/bin/bash
c-vars.sh
Manipulating a variable, C-style, using the (( ... )) construct.
echo
(( a = 23 )) # Setting a value, C-style,
#+ with spaces on both sides of the "=".
echo "a (initial value) = $a" # 23
(( a++ )) # Post-increment 'a', C-style.
echo "a (after a++) = $a" # 24
(( a-- )) # Post-decrement 'a', C-style.
echo "a (after a--) = $a" # 23
(( ++a )) # Pre-increment 'a', C-style.
echo "a (after ++a) = $a" # 24
(( --a )) # Pre-decrement 'a', C-style.
echo "a (after --a) = $a" # 23
echo
Note that, as in C, pre- and post-decrement operators
- have different side-effects.
n=1; let --n && echo "True" || echo "False" # False
n=1; let n-- && echo "True" || echo "False" # True
Thanks, Jeroen Domburg.
echo
(( t = a<45?7:11 )) # C-style trinary operator.
^ ^ ^
echo "If a < 45, then t = 7, else t = 11." # a = 23
echo "t = $t " # t = 7
echo
Easter Egg alert!
Chet Ramey seems to have snuck a bunch of undocumented C-style
- constructs into Bash (actually adapted from ksh, pretty much).
In the Bash docs, Ramey calls (( ... )) shell arithmetic,
- but it goes far beyond that.
Sorry, Chet, the secret is out.
See also "for" and "while" loops using the (( ... )) construct.
These work only with version 2.04 or later of Bash.
exit
See also Example 11-13 and Example 8-4.
________________________________________________________________
8.4. Operator Precedence
In a script, operations execute in order of precedence: the higher
precedence operations execute before the lower precedence ones. [40]
Table 8-1. Operator Precedence
Operator Meaning Comments
HIGHEST PRECEDENCE
var++ var-- post-increment, post-decrement C-style operators
++var --var pre-increment, pre-decrement
! ~ negation logical / bitwise, inverts sense of following operator
** exponentiation arithmetic operation
- / % multiplication, division, modulo arithmetic operation
-
- addition, subtraction arithmetic operation
<< >> left, right shift bitwise
-z -n unary comparison string is/is-not null
-e -f -t -x, etc. unary comparison file-test
< -lt > -gt <= -le >= -ge compound comparison string and integer
-nt -ot -ef compound comparison file-test
== -eq != -ne equality / inequality test operators, string and
integer
& AND bitwise
^ XOR exclusive OR, bitwise
| OR bitwise
&& -a AND logical, compound comparison
|| -o OR logical, compound comparison
?: trinary operator C-style
= assignment (do not confuse with equality test)
*= /= %= += -= <<= >>= &= combination assignment times-equal,
divide-equal, mod-equal, etc.
, comma links a sequence of operations
LOWEST PRECEDENCE
In practice, all you really need to remember is the following:
* The "My Dear Aunt Sally" mantra (multiply, divide, add, subtract)
for the familiar arithmetic operations.
* The compound logical operators, &&, ||, -a, and -o have low
precedence.
* The order of evaluation of equal-precedence operators is usually
left-to-right.
Now, let's utilize our knowledge of operator precedence to analyze a
couple of lines from the /etc/init.d/functions file, as found in the
Fedora Core Linux distro.
while [ -n "$remaining" -a "$retry" -gt 0 ]; do
This looks rather daunting at first glance.
Separate the conditions:
while [ -n "$remaining" -a "$retry" -gt 0 ]; do
--condition 1-- ^^ --condition 2-
If variable "$remaining" is not zero length
- variable "$retry" is greater-than zero
- the [ expresion-within-condition-brackets ] returns success (0)
- and the while-loop executes an iteration.
==============================================================
Evaluate "condition 1" and "condition 2" before
- ANDing them. Why? Because the AND (-a) has a lower precedence
- than the -n and -gt operators,
- and therefore gets evaluated last.
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
Again, separate the conditions:
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
--condition 1--------- ^^ --condition 2-----
If file "/etc/sysconfig/i18n" exists
- variable $NOLOCALE is zero length
- the [ test-expresion-within-condition-brackets ] returns success (0)
- and the commands following execute.
As before, the AND (-a) gets evaluated last
- because it has the lowest precedence of the operators within
==============================================================
Note:
${NOLOCALE:-} is a parameter expansion that seems redundant.
But, if $NOLOCALE has not been declared, it gets set to null,
This makes a difference in some contexts.
Tip
To avoid confusion or error in a complex sequence of test operators,
break up the sequence into bracketed sections.
if [ "$v1" -gt "$v2" -o "$v1" -lt "$v2" -a -e "$filename" ]
Unclear what's going on here...
if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] && [[ -e "$filename" ]]
Much better -- the condition tests are grouped in logical sections.
Part 3. Beyond the Basics
Table of Contents
- Another Look at Variables
9.1. Internal Variables
9.2. Typing variables: declare or typeset
9.3. $RANDOM: generate random integer
- Manipulating Variables
10.1. Manipulating Strings
10.2. Parameter Substitution
- Loops and Branches
11.1. Loops
11.2. Nested Loops
11.3. Loop Control
11.4. Testing and Branching
- Command Substitution
- Arithmetic Expansion
- Recess Time
________________________________________________________________
Chapter 9. Another Look at Variables
Used properly, variables can add power and flexibility to scripts.
This requires learning their subtleties and nuances.
________________________________________________________________
9.1. Internal Variables
Builtin variables:
variables affecting bash script behavior
$BASH
The path to the Bash binary itself
bash$ echo $BASH
/bin/bash
$BASH_ENV
An environmental variable pointing to a Bash startup file to
be read when a script is invoked
$BASH_SUBSHELL
A variable indicating the subshell level. This is a new
addition to Bash, version 3.
See Example 21-1 for usage.
$BASHPID
Process ID of the current instance of Bash. This is not the
same as the $$ variable, but it often gives the same result.
bash4$ echo $$
11015
bash4$ echo $BASHPID
11015
bash4$ ps ax | grep bash4
11015 pts/2 R 0:00 bash4
But ...
!/bin/bash4
echo "$$ outside of subshell = $$" # 9602
echo "$BASH_SUBSHELL outside of subshell = $BASH_SUBSHELL" # 0
echo "$BASHPID outside of subshell = $BASHPID" # 9602
echo
( echo "$$ inside of subshell = $$" # 9602
echo "$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL" # 1
echo "$BASHPID inside of subshell = $BASHPID" ) # 9603
Note that $$ returns PID of parent process.
$BASH_VERSINFO[n]
A 6-element array containing version information about the
installed release of Bash. This is similar to $BASH_VERSION,
below, but a bit more detailed.
Bash version info:
for n in 0 1 2 3 4 5
do
echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done
BASH_VERSINFO[0] = 3 # Major version no.
BASH_VERSINFO[1] = 00 # Minor version no.
BASH_VERSINFO[2] = 14 # Patch level.
BASH_VERSINFO[3] = 1 # Build version.
BASH_VERSINFO[4] = release # Release status.
BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture
# (same as $MACHTYPE).
$BASH_VERSION
The version of Bash installed on the system
bash$ echo $BASH_VERSION
3.2.25(1)-release
tcsh% echo $BASH_VERSION
BASH_VERSION: Undefined variable.
Checking $BASH_VERSION is a good method of determining which
shell is running. $SHELL does not necessarily give the correct
answer.
$CDPATH
A colon-separated list of search paths available to the cd
command, similar in function to the $PATH variable for
binaries. The $CDPATH variable may be set in the local
~/.bashrc file.
bash$ cd bash-doc
bash: cd: bash-doc: No such file or directory
bash$ CDPATH=/usr/share/doc
bash$ cd bash-doc
/usr/share/doc/bash-doc
bash$ echo $PWD
/usr/share/doc/bash-doc
$DIRSTACK
The top value in the directory stack [41] (affected by pushd
and popd)
This builtin variable corresponds to the dirs command, however
dirs shows the entire contents of the directory stack.
$EDITOR
The default editor invoked by a script, usually vi or emacs.
$EUID
"effective" user ID number
Identification number of whatever identity the current user
has assumed, perhaps by means of su.
Caution
The $EUID is not necessarily the same as the $UID.
$FUNCNAME
Name of the current function
xyz23 ()
{
echo "$FUNCNAME now executing." # xyz23 now executing.
}
xyz23
echo "FUNCNAME = $FUNCNAME" # FUNCNAME =
# Null value outside a function.
See also Example A-50.
$GLOBIGNORE
A list of filename patterns to be excluded from matching in
globbing.
$GROUPS
Groups current user belongs to
This is a listing (array) of the group id numbers for current
user, as recorded in /etc/passwd and /etc/group.
root# echo $GROUPS
0
root# echo ${GROUPS[1]}
1
root# echo ${GROUPS[5]}
6
$HOME
Home directory of the user, usually /home/username (see
Example 10-7)
$HOSTNAME
The hostname command assigns the system host name at bootup in
an init script. However, the gethostname() function sets the
Bash internal variable $HOSTNAME. See also Example 10-7.
$HOSTTYPE
host type
Like $MACHTYPE, identifies the system hardware.
bash$ echo $HOSTTYPE
i686
$IFS
internal field separator
This variable determines how Bash recognizes fields, or word
boundaries, when it interprets character strings.
$IFS defaults to whitespace (space, tab, and newline), but may
be changed, for example, to parse a comma-separated data file.
Note that $* uses the first character held in $IFS. See
Example 5-1.
bash$ echo "$IFS"
(With $IFS set to default, a blank line displays.)
bash$ echo "$IFS" | cat -vte
^I$
$
(Show whitespace: here a single space, ^I [horizontal tab],
and newline, and display "$" at end-of-line.)
bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
(Read commands from string and assign any arguments to pos params.)
Set $IFS to eliminate whitespace in pathnames.
IFS="$(printf '\n\t')" # Per David Wheeler.
Caution
$IFS does not handle whitespace the same as it does other characters.
Example 9-1. $IFS and whitespace
!/bin/bash
ifs.sh
var1="a+b+c"
var2="d-e-f"
var3="g,h,i"
IFS=+
The plus sign will be interpreted as a separator.
echo $var1 # a b c
echo $var2 # d-e-f
echo $var3 # g,h,i
echo
IFS="-"
The plus sign reverts to default interpretation.
The minus sign will be interpreted as a separator.
echo $var1 # a+b+c
echo $var2 # d e f
echo $var3 # g,h,i
echo
IFS=","
The comma will be interpreted as a separator.
The minus sign reverts to default interpretation.
echo $var1 # a+b+c
echo $var2 # d-e-f
echo $var3 # g h i
echo
IFS=" "
The space character will be interpreted as a separator.
The comma reverts to default interpretation.
echo $var1 # a+b+c
echo $var2 # d-e-f
echo $var3 # g,h,i
======================================================== #
However ...
$IFS treats whitespace differently than other characters.
output_args_one_per_line()
{
for arg
do
echo "[$arg]"
done # ^ ^ Embed within brackets, for your viewing pleasure.
}
echo; echo "IFS=" ""
echo "-------"
IFS=" "
var=" a b c "
^ ^^ ^^^
output_args_one_per_line $var # output_args_one_per_line echo " a b c "
[a]
[b]
[c]
echo; echo "IFS=:"
echo "-----"
IFS=:
var=":a::b:c:::" # Same pattern as above,
^ ^^ ^^^ #+ but substituting ":" for " " ...
output_args_one_per_line $var
[]
[a]
[]
[b]
[c]
[]
[]
Note "empty" brackets.
The same thing happens with the "FS" field separator in awk.
echo
exit
(Many thanks, Stéphane Chazelas, for clarification and above
examples.)
See also Example 16-41, Example 11-8, and Example 19-14 for
instructive examples of using $IFS.
$IGNOREEOF
Ignore EOF: how many end-of-files (control-D) the shell will
ignore before logging out.
$LC_COLLATE
Often set in the .bashrc or /etc/profile files, this variable
controls collation order in filename expansion and pattern
matching. If mishandled, LC_COLLATE can cause unexpected
results in filename globbing.
Note
As of version 2.05 of Bash, filename globbing no longer distinguishes
between lowercase and uppercase letters in a character range between
brackets. For example, ls [A-M]* would match both File1.txt and
file1.txt. To revert to the customary behavior of bracket matching,
set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or
~/.bashrc.
$LC_CTYPE
This internal variable controls character interpretation in
globbing and pattern matching.
$LINENO
This variable is the line number of the shell script in which
this variable appears. It has significance only within the
script in which it appears, and is chiefly useful for
debugging purposes.
*** BEGIN DEBUG BLOCK ***
last_cmd_arg=$_ # Save it.
echo "At line number $LINENO, variable "v1" = $v1"
echo "Last command argument processed = $last_cmd_arg"
*** END DEBUG BLOCK ***
$MACHTYPE
machine type
Identifies the system hardware.
bash$ echo $MACHTYPE
i686
$OLDPWD
Old working directory ("OLD-Print-Working-Directory", previous
directory you were in).
$OSTYPE
operating system type
bash$ echo $OSTYPE
linux
$PATH
Path to binaries, usually /usr/bin/, /usr/X11R6/bin/,
/usr/local/bin, etc.
When given a command, the shell automatically does a hash
table search on the directories listed in the path for the
executable. The path is stored in the environmental variable,
$PATH, a list of directories, separated by colons. Normally,
the system stores the $PATH definition in /etc/profile and/or
~/.bashrc (see Appendix H).
bash$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin
PATH=${PATH}:/opt/bin appends the /opt/bin directory to the
current path. In a script, it may be expedient to temporarily
add a directory to the path in this way. When the script
exits, this restores the original $PATH (a child process, such
as a script, may not change the environment of the parent
process, the shell).
Note
The current "working directory", ./, is usually omitted from the
$PATH as a security measure.
$PIPESTATUS
Array variable holding exit status(es) of last executed
foreground pipe.
bash$ echo $PIPESTATUS
0
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo ${PIPESTATUS[1]}
127
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127
The members of the $PIPESTATUS array hold the exit status of
each respective command executed in a pipe. $PIPESTATUS[0]
holds the exit status of the first command in the pipe,
$PIPESTATUS[1] the exit status of the second command, and so
on.
Caution
The $PIPESTATUS variable may contain an erroneous 0 value in a login
shell (in releases prior to 3.0 of Bash).
tcsh% bash
bash$ who | grep nobody | sort
bash$ echo ${PIPESTATUS[*]}
0
The above lines contained in a script would produce the expected 0 1
0 output.
Thank you, Wayne Pollock for pointing this out and supplying the
above example.
Note
The $PIPESTATUS variable gives unexpected results in some contexts.
bash$ echo $BASH_VERSION
3.00.14(1)-release
bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0 0 0
bash$ echo ${PIPESTATUS[@]}
141 127 0
Chet Ramey attributes the above output to the behavior of ls. If ls
writes to a pipe whose output is not read, then SIGPIPE kills it, and
its exit status is 141. Otherwise its exit status is 0, as expected.
This likewise is the case for tr.
Note
$PIPESTATUS is a "volatile" variable. It needs to be captured
immediately after the pipe in question, before any other command
intervenes.
bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0 0 0
bash$ echo ${PIPESTATUS[@]}
0 127 0
bash$ echo ${PIPESTATUS[@]}
0
Note
The pipefail option may be useful in cases where $PIPESTATUS does not
give the desired information.
$PPID
The $PPID of a process is the process ID (pid) of its parent
process. [42]
Compare this with the pidof command.
$PROMPT_COMMAND
A variable holding a command to be executed just before the
primary prompt, $PS1 is to be displayed.
$PS1
This is the main prompt, seen at the command-line.
$PS2
The secondary prompt, seen when additional input is expected.
It displays as ">".
$PS3
The tertiary prompt, displayed in a select loop (see Example
11-30).
$PS4
The quartenary prompt, shown at the beginning of each line of
output when invoking a script with the -x [verbose trace]
option. It displays as "+".
As a debugging aid, it may be useful to embed diagnostic
information in $PS4.
P4='$(read time junk < /proc/$$/schedstat; echo "@@@ $time @@@ " )'
Per suggestion by Erik Brandsberg.
set -x
Various commands follow ...
$PWD
Working directory (directory you are in at the time)
This is the analog to the pwd builtin command.
!/bin/bash
E_WRONG_DIRECTORY=85
clear # Clear the screen.
TargetDirectory=/home/bozo/projects/GreatAmericanNovel
cd $TargetDirectory
echo "Deleting stale files in $TargetDirectory."
if [ "$PWD" != "$TargetDirectory" ]
then # Keep from wiping out wrong directory by accident.
echo "Wrong directory!"
echo "In $PWD, rather than $TargetDirectory!"
echo "Bailing out!"
exit $E_WRONG_DIRECTORY
fi
rm -rf *
rm .[A-Za-z0-9]* # Delete dotfiles.
rm -f .[^.]* ..?* to remove filenames beginning with multiple dots.
(shopt -s dotglob; rm -f *) will also work.
Thanks, S.C. for pointing this out.
A filename (basename) may contain all characters in the 0 - 255 range,
Deleting files beginning with weird characters, such as -
- is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)
result=$? # Result of delete operations. If successful = 0.
echo
ls -al # Any files left?
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo
Various other operations here, as necessary.
exit $result
$REPLY
The default value when a variable is not supplied to read.
Also applicable to select menus, but only supplies the item
number of the variable chosen, not the value of the variable
itself.
!/bin/bash
reply.sh
REPLY is the default value for a 'read' command.
echo
echo -n "What is your favorite vegetable? "
read
echo "Your favorite vegetable is $REPLY."
REPLY holds the value of last "read" if and only if
echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of $REPLY is still $REPLY."
$REPLY is still set to its previous value because
- the variable $fruit absorbed the new "read" value.
echo
exit 0
$SECONDS
The number of seconds the script has been running.
!/bin/bash
TIME_LIMIT=10
INTERVAL=1
echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo
while [ "$SECONDS" -le "$TIME_LIMIT" ]
do # $SECONDS is an internal shell variable.
if [ "$SECONDS" -eq 1 ]
then
units=second
else
units=seconds
fi
echo "This script has been running $SECONDS $units."
On a slow or overburdened machine, the script may skip a count
#+ every once in a while.
sleep $INTERVAL
done
echo -e "\a" # Beep!
exit 0
$SHELLOPTS
The list of enabled shell options, a readonly variable.
bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs
$SHLVL
Shell level, how deeply Bash is nested. [43] If, at the
command-line, $SHLVL is 1, then in a script it will increment
to 2.
Note
This variable is not affected by subshells. Use $BASH_SUBSHELL when
you need an indication of subshell nesting.
$TMOUT
If the $TMOUT environmental variable is set to a non-zero
value time, then the shell prompt will time out after $time
seconds. This will cause a logout.
As of version 2.05b of Bash, it is now possible to use $TMOUT
in a script in combination with read.
Works in scripts for Bash, versions 2.05b and later.
TMOUT=3 # Prompt times out at three seconds.
echo "What is your favorite song?"
echo "Quickly now, you only have $TMOUT seconds to answer!"
read song
if [ -z "$song" ]
then
song="(no answer)"
Default response.
fi
echo "Your favorite song is $song."
There are other, more complex, ways of implementing timed
input in a script. One alternative is to set up a timing loop
to signal the script when it times out. This also requires a
signal handling routine to trap (see Example 32-5) the
interrupt generated by the timing loop (whew!).
Example 9-2. Timed Input
!/bin/bash
timed-input.sh
TMOUT=3 Also works, as of newer versions of Bash.
TIMER_INTERRUPT=14
TIMELIMIT=3 # Three seconds in this instance.
# May be set to different value.
PrintAnswer()
{
if [ "$answer" = TIMEOUT ]
then
echo $answer
else # Don't want to mix up the two instances.
echo "Your favorite veggie is $answer"
kill $! # Kills no-longer-needed TimerOn function
#+ running in background.
# $! is PID of last job running in background.
fi
}
TimerOn()
{
sleep $TIMELIMIT && kill -s 14 $$ &
Waits 3 seconds, then sends sigalarm to script.
}
Int14Vector()
{
answer="TIMEOUT"
PrintAnswer
exit $TIMER_INTERRUPT
}
trap Int14Vector $TIMER_INTERRUPT
Timer interrupt (14) subverted for our purposes.
echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer
Admittedly, this is a kludgy implementation of timed input.
However, the "-t" option to "read" simplifies this task.
See the "t-out.sh" script.
However, what about timing not just single user input,
If you need something really elegant ...
- consider writing the application in C or C++,
- using appropriate library functions, such as 'alarm' and 'setitimer.'
exit 0
An alternative is using stty.
Example 9-3. Once more, timed input
!/bin/bash
timeout.sh
Written by Stephane Chazelas,
- and modified by the document author.
INTERVAL=5 # timeout interval
timedout_read() {
timeout=$1
varname=$2
old_tty_settings=stty -g
stty -icanon min 0 time ${timeout}0
eval read $varname # or just read $varname
stty "$old_tty_settings"
See man page for "stty."
}
echo; echo -n "What's your name? Quick! "
timedout_read $INTERVAL your_name
This may not work on every terminal type.
The maximum timeout depends on the terminal.
- (it is often 25.5 seconds).
echo
if [ ! -z "$your_name" ] # If name input before timeout ...
then
echo "Your name is $your_name."
else
echo "Timed out."
fi
echo
The behavior of this script differs somewhat from "timed-input.sh."
At each keystroke, the counter resets.
exit 0
Perhaps the simplest method is using the -t option to read.
Example 9-4. Timed read
!/bin/bash
t-out.sh [time-out]
Inspired by a suggestion from "syngin seven" (thanks).
TIMELIMIT=4 # 4 seconds
read -t $TIMELIMIT variable <&1
^^^
In this instance, "<&1" is needed for Bash 1.x and 2.x,
but unnecessary for Bash 3+.
echo
if [ -z "$variable" ] # Is null?
then
echo "Timed out, variable still unset."
else
echo "variable = $variable"
fi
exit 0
$UID
User ID number
Current user's user identification number, as recorded in
/etc/passwd
This is the current user's real id, even if she has
temporarily assumed another identity through su. $UID is a
readonly variable, not subject to change from the command line
or within a script, and is the counterpart to the id builtin.
Example 9-5. Am I root?
!/bin/bash
am-i-root.sh: Am I root or not?
ROOT_UID=0 # Root has $UID 0.
if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up?
then
echo "You are root."
else
echo "You are just an ordinary user (but mom loves you just the same)."
fi
exit 0
============================================================= #
Code below will not execute, because the script already exited.
An alternate method of getting to the root of matters:
ROOTUSER_NAME=root
username=id -nu # Or... username=whoami
if [ "$username" = "$ROOTUSER_NAME" ]
then
echo "Rooty, toot, toot. You are root."
else
echo "You are just a regular fella."
fi
See also Example 2-3.
Note
The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are
not Bash builtins. These are, however, often set as environmental
variables in one of the Bash or login startup files. $SHELL, the name
of the user's login shell, may be set from /etc/passwd or in an
"init" script, and it is likewise not a Bash builtin.
tcsh% echo $LOGNAME
bozo
tcsh% echo $SHELL
/bin/tcsh
tcsh% echo $TERM
rxvt
bash$ echo $LOGNAME
bozo
bash$ echo $SHELL
/bin/tcsh
bash$ echo $TERM
rxvt
Positional Parameters
$0, $1, $2, etc.
Positional parameters, passed from command line to script,
passed to a function, or set to a variable (see Example 4-5
and Example 15-16)
$#
Number of command-line arguments [44] or positional parameters
(see Example 36-2)
$*
All of the positional parameters, seen as a single word
Note
"$*" must be quoted.
$@
Same as $*, but each parameter is a quoted string, that is,
the parameters are passed on intact, without interpretation or
expansion. This means, among other things, that each parameter
in the argument list is seen as a separate word.
Note
Of course, "$@" should be quoted.
Example 9-6. arglist: Listing arguments with $* and $@
!/bin/bash
arglist.sh
Invoke this script with several arguments, such as "one two three" ...
E_BADARGS=85
if [ ! -n "$1" ]
then
echo "Usage: basename $0 argument1 argument2 etc."
exit $E_BADARGS
fi
echo
index=1 # Initialize count.
echo "Listing args with "$*":"
for arg in "$" # Doesn't work properly if "$" isn't quoted.
do
echo "Arg #$index = $arg"
let "index+=1"
done # $* sees all arguments as single word.
echo "Entire arg list seen as single word."
echo
index=1 # Reset count.
# What happens if you forget to do this?
echo "Listing args with "$@":"
for arg in "$@"
do
echo "Arg #$index = $arg"
let "index+=1"
done # $@ sees arguments as separate words.
echo "Arg list seen as separate words."
echo
index=1 # Reset count.
echo "Listing args with $* (unquoted):"
for arg in $*
do
echo "Arg #$index = $arg"
let "index+=1"
done # Unquoted $* sees arguments as separate words.
echo "Arg list seen as separate words."
exit 0
Following a shift, the $@ holds the remaining command-line
parameters, lacking the previous $1, which was lost.
!/bin/bash
Invoke with ./scriptname 1 2 3 4 5
echo "$@" # 1 2 3 4 5
shift
echo "$@" # 2 3 4 5
shift
echo "$@" # 3 4 5
Each "shift" loses parameter $1.
"$@" then contains the remaining parameters.
The $@ special parameter finds use as a tool for filtering
input into shell scripts. The cat "$@" construction accepts
input to a script either from stdin or from files given as
parameters to the script. See Example 16-24 and Example 16-25.
Caution
The $* and $@ parameters sometimes display inconsistent and puzzling
behavior, depending on the setting of $IFS.
Example 9-7. Inconsistent $* and $@ behavior
!/bin/bash
Erratic behavior of the "$*" and "$@" internal Bash variables,
- depending on whether or not they are quoted.
Demonstrates inconsistent handling of word splitting and linefeeds.
set -- "First one" "second" "third:one" "" "Fifth: :one"
Setting the script arguments, $1, $2, $3, etc.
echo
echo 'IFS unchanged, using "$*"'
c=0
for i in "$*" # quoted
do echo "$((c+=1)): [$i]" # This line remains the same in every instance.
# Echo args.
done
echo ---
echo 'IFS unchanged, using $*'
c=0
for i in $* # unquoted
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS unchanged, using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS unchanged, using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---
IFS=:
echo 'IFS=":", using "$*"'
c=0
for i in "$*"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $*'
c=0
for i in $*
do echo "$((c+=1)): [$i]"
done
echo ---
var=$*
echo 'IFS=":", using "$var" (var=$*)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $var (var=$*)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
var="$*"
echo 'IFS=":", using $var (var="$*")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$var" (var="$*")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---
var=$@
echo 'IFS=":", using $var (var=$@)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$var" (var=$@)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
var="$@"
echo 'IFS=":", using "$var" (var="$@")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $var (var="$@")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo
Try this script with ksh or zsh -y.
exit 0
This example script written by Stephane Chazelas,
- and slightly modified by the document author.
Note
The $@ and $* parameters differ only when between double quotes.
Example 9-8. $* and $@ when $IFS is empty
!/bin/bash
If $IFS set, but empty,
- then "$*" and "$@" do not echo positional params as expected.
mecho () # Echo positional parameters.
{
echo "$1,$2,$3";
}
IFS="" # Set, but empty.
set a b c # Positional parameters.
mecho "$*" # abc,,
^^
mecho $* # a,b,c
mecho $@ # a,b,c
mecho "$@" # a,b,c
The behavior of $* and $@ when $IFS is empty depends
- on which Bash or sh version being run.
It is therefore inadvisable to depend on this "feature" in a script.
Thanks, Stephane Chazelas.
exit
Other Special Parameters
$-
Flags passed to script (using set). See Example 15-16.
Caution
This was originally a ksh construct adopted into Bash, and
unfortunately it does not seem to work reliably in Bash scripts. One
possible use for it is to have a script self-test whether it is
interactive.
$!
PID (process ID) of last job run in background
LOG=$0.log
COMMAND1="sleep 100"
echo "Logging PIDs background commands for script: $0" >> "$LOG"
So they can be monitored, and killed as necessary.
echo >> "$LOG"
Logging commands.
echo -n "PID of "$COMMAND1": " >> "$LOG"
${COMMAND1} &
echo $! >> "$LOG"
PID of "sleep 100": 1506
Thank you, Jacques Lederer, for suggesting this.
Using $! for job control:
possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
Forces completion of an ill-behaved program.
Useful, for example, in init scripts.
Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.
Or, alternately:
This example by Matthew Sage.
Used with permission.
TIMEOUT=30 # Timeout value in seconds
count=0
possibly_hanging_job & {
while ((count < TIMEOUT )); do
eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'
# /proc is where information about running processes is found.
# "-d" tests whether it exists (whether directory exists).
# So, we're waiting for the job in question to show up.
((count++))
sleep 1
done
eval '[ -d "/proc/$!" ] && kill -15 $!'
# If the hanging job is running, kill it.
}
-------------------------------------------------------------- #
However, this may not not work as specified if another process
- begins to run after the "hanging_job" . . .
In such a case, the wrong job may be killed.
Ariel Meragelman suggests the following fix.
TIMEOUT=30
count=0
Timeout value in seconds
possibly_hanging_job & {
while ((count < TIMEOUT )); do
eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))'
lastjob=$!
((count++))
sleep 1
done
eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob'
}
exit
$_
Special variable set to final argument of previous command
executed.
Example 9-9. Underscore variable
!/bin/bash
echo $_ # /bin/bash
# Just called /bin/bash to run the script.
# Note that this will vary according to
#+ how the script is invoked.
du >/dev/null # So no output from command.
echo $_ # du
ls -al >/dev/null # So no output from command.
echo $_ # -al (last argument)
:
echo $_ # :
$?
Exit status of a command, function, or the script itself (see
Example 24-7)
$$
Process ID (PID) of the script itself. [45] The $$ variable
often finds use in scripts to construct "unique" temp file
names (see Example 32-6, Example 16-31, and Example 15-27).
This is usually simpler than invoking mktemp.
________________________________________________________________
9.2. Typing variables: declare or typeset
The declare or typeset builtins, which are exact synonyms, permit
modifying the properties of variables. This is a very weak form of
the typing [46] available in certain programming languages. The
declare command is specific to version 2 or later of Bash. The
typeset command also works in ksh scripts.
declare/typeset options
-r readonly
(declare -r var1 works the same as readonly var1)
This is the rough equivalent of the C const type qualifier. An
attempt to change the value of a readonly variable fails with
an error message.
declare -r var1=1
echo "var1 = $var1" # var1 = 1
(( var1++ )) # x.sh: line 4: var1: readonly variable
-i integer
declare -i number
The script will treat subsequent occurrences of "number" as an integer.
number=3
echo "Number = $number" # Number = 3
number=three
echo "Number = $number" # Number = 0
Tries to evaluate the string "three" as an integer.
Certain arithmetic operations are permitted for declared
integer variables without the need for expr or let.
n=6/3
echo "n = $n" # n = 6/3
declare -i n
n=6/3
echo "n = $n" # n = 2
-a array
declare -a indices
The variable indices will be treated as an array.
-f function(s)
declare -f
A declare -f line with no arguments in a script causes a
listing of all the functions previously defined in that
script.
declare -f function_name
A declare -f function_name in a script lists just the function
named.
-x export
declare -x var3
This declares a variable as available for exporting outside
the environment of the script itself.
-x var=$value
declare -x var3=373
The declare command permits assigning a value to a variable in
the same statement as setting its properties.
Example 9-10. Using declare to type variables
!/bin/bash
func1 ()
{
echo This is a function.
}
declare -f # Lists the function above.
echo
declare -i var1 # var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1 # Integer declaration eliminates the need for 'let'.
echo "var1 incremented by 1 is $var1."
Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1 # Results in error message, with no change to variable.
echo "var1 is still $var1"
echo
declare -r var2=13.36 # 'declare' permits setting a variable property
#+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
var2=13.37 # Generates error message, and exit from script.
echo "var2 is still $var2" # This line will not execute.
exit 0 # Script will not exit here.
Caution
Using the declare builtin restricts the scope of a variable.
foo ()
{
FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar # Prints bar.
However . . .
foo (){
declare FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar # Prints nothing.
Thank you, Michael Iatrou, for pointing this out.
________________________________________________________________
9.2.1. Another use for declare
The declare command can be helpful in identifying variables,
environmental or otherwise. This can be especially useful with
arrays.
bash$ declare | grep HOME
HOME=/home/bozo
bash$ zzy=68
bash$ declare | grep zzy
zzy=68
bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
bash$ echo ${Colors[@]}
purple reddish-orange light green
bash$ declare | grep Colors
Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
________________________________________________________________
9.3. $RANDOM: generate random integer
Anyone who attempts to generate random numbers by deterministic means
is, of course, living in a state of sin.
--John von Neumann
$RANDOM is an internal Bash function (not a constant) that returns a
pseudorandom [47] integer in the range 0 - 32767. It should not be
used to generate an encryption key.
Example 9-11. Generating random numbers
!/bin/bash
$RANDOM returns a different random integer at each invocation.
Nominal range: 0 - 32767 (signed 16-bit integer).
MAXCOUNT=10
count=1
echo
echo "$MAXCOUNT random numbers:"
echo "-----------------"
while [ "$count" -le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integer
s.
do
number=$RANDOM
echo $number
let "count += 1" # Increment count.
done
echo "-----------------"
If you need a random int within a certain range, use the 'modulo' operator.
This returns the remainder of a division operation.
RANGE=500
echo
number=$RANDOM
let "number %= $RANGE"
^^
echo "Random number less than $RANGE --- $number"
echo
If you need a random integer greater than a lower bound,
- then set up a test to discard all numbers below that.
FLOOR=200
number=0 #initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
done
echo "Random number greater than $FLOOR --- $number"
echo
Let's examine a simple alternative to the above loop, namely
let "number = $RANDOM + $FLOOR"
That would eliminate the while-loop and run faster.
But, there might be a problem with that. What is it?
Combine above two techniques to retrieve random number between two limits.
number=0 #initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE" # Scales $number down within $RANGE.
done
echo "Random number between $FLOOR and $RANGE --- $number"
echo
Generate binary choice, that is, "true" or "false" value.
BINARY=2
T=1
number=$RANDOM
let "number %= $BINARY"
Note that let "number >>= 14" gives a better random distribution
- (right shifts out everything except last binary digit).
if [ "$number" -eq $T ]
then
echo "TRUE"
else
echo "FALSE"
fi
echo
Generate a toss of the dice.
SPOTS=6 # Modulo 6 gives range 0 - 5.
# Incrementing by 1 gives desired range of 1 - 6.
# Thanks, Paulo Marcel Coelho Aragao, for the simplification.
die1=0
die2=0
Would it be better to just set SPOTS=7 and not add 1? Why or why not?
Tosses each die separately, and so gives correct odds.
let "die1 = $RANDOM % $SPOTS +1" # Roll first one.
let "die2 = $RANDOM % $SPOTS +1" # Roll second one.
# Which arithmetic operation, above, has greater precedence --
#+ modulo (%) or addition (+)?
let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo
exit 0
Example 9-12. Picking a random card from a deck
!/bin/bash
pick-card.sh
This is an example of choosing random elements of an array.
Pick a card, any card.
Suites="Clubs
Diamonds
Hearts
Spades"
Denominations="2
3
4
5
6
7
8
9
10
Jack
Queen
King
Ace"
Note variables spread over multiple lines.
suite=($Suites) # Read into array variable.
denomination=($Denominations)
num_suites=${#suite[*]} # Count how many elements.
num_denominations=${#denomination[*]}
echo -n "${denomination[$((RANDOM%num_denominations))]} of "
echo ${suite[$((RANDOM%num_suites))]}
$bozo sh pick-cards.sh
Jack of Clubs
Thank you, "jipe," for pointing out this use of $RANDOM.
exit 0
Example 9-13. Brownian Motion Simulation
!/bin/bash
brownian.sh
Author: Mendel Cooper
Reldate: 10/26/07
License: GPL3
This script models Brownian motion:
- the random wanderings of tiny particles in a fluid,
- as they are buffeted by random currents and collisions.
- This is colloquially known as the "Drunkard's Walk."
It can also be considered as a stripped-down simulation of a
- Galton Board, a slanted board with a pattern of pegs,
- down which rolls a succession of marbles, one at a time.
- At the bottom is a row of slots or catch basins in which
- the marbles come to rest at the end of their journey.
Think of it as a kind of bare-bones Pachinko game.
As you see by running the script,
- most of the marbles cluster around the center slot.
- This is consistent with the expected binomial distribution.
As a Galton Board simulation, the script
- disregards such parameters as
- board tilt-angle, rolling friction of the marbles,
- angles of impact, and elasticity of the pegs.
To what extent does this affect the accuracy of the simulation?
PASSES=500 # Number of particle interactions / marbles.
ROWS=10 # Number of "collisions" (or horiz. peg rows).
RANGE=3 # 0 - 2 output range from $RANDOM.
POS=0 # Left/right position.
RANDOM=$$ # Seeds the random number generator from PID
#+ of script.
declare -a Slots # Array holding cumulative results of passes.
NUMSLOTS=21 # Number of slots at bottom of board.
Initialize_Slots () { # Zero out all elements of the array.
for i in $( seq $NUMSLOTS )
do
Slots[$i]=0
done
echo # Blank line at beginning of run.
}
Show_Slots () {
echo; echo
echo -n " "
for i in $( seq $NUMSLOTS ) # Pretty-print array elements.
do
printf "%3d" ${Slots[$i]} # Allot three spaces per result.
done
echo # Row of slots:
echo " |||||||||||||||||||||__|"
echo " ||"
echo # Note that if the count within any particular slot exceeds 99,
#+ it messes up the display.
# Running only(!) 500 passes usually avoids this.
}
Move () { # Move one unit right / left, or stay put.
Move=$RANDOM # How random is $RANDOM? Well, let's see ...
let "Move %= RANGE" # Normalize into range of 0 - 2.
case "$Move" in
0 ) ;; # Do nothing, i.e., stay in place.
1 ) ((POS--));; # Left.
2 ) ((POS++));; # Right.
* ) echo -n "Error ";; # Anomaly! (Should never occur.)
esac
}
Play () { # Single pass (inner loop).
i=0
while [ "$i" -lt "$ROWS" ] # One event per row.
do
Move
((i++));
done
SHIFT=11 # Why 11, and not 10?
let "POS += $SHIFT" # Shift "zero position" to center.
(( Slots[$POS]++ )) # DEBUG: echo $POS
echo -n "$POS "
}
Run () { # Outer loop.
p=0
while [ "$p" -lt "$PASSES" ]
do
Play
(( p++ ))
POS=0 # Reset to zero. Why?
done
}
main ()
Initialize_Slots
Run
Show_Slots
exit $?
Exercises:
- Show the results in a vertical bar graph, or as an alternative,
- Alter the script to use /dev/urandom instead of $RANDOM.
Will this make the results more random?
- Provide some sort of "animation" or graphic output
for each marble played.
Jipe points out a set of techniques for generating random numbers
within a range.
Generate random number between 6 and 30.
rnumber=$((RANDOM%25+6))
Generate random number in the same 6 - 30 range,
- but the number must be evenly divisible by 3.
rnumber=$(((RANDOM%30/3+1)*3))
Note that this will not work all the time.
It fails if $RANDOM%30 returns 0.
Frank Wang suggests the following alternative:
rnumber=$(( RANDOM%27/3*3+6 ))
Bill Gradwohl came up with an improved formula that works for
positive numbers.
rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))
Here Bill presents a versatile function that returns a random number
between two specified values.
Example 9-14. Random between values
!/bin/bash
random-between.sh
Random number between two specified values.
Script by Bill Gradwohl, with minor modifications by the document author.
Corrections in lines 187 and 189 by Anthony Le Clezio.
Used with permission.
randomBetween() {
Generates a positive or negative random number
#+ between $min and $max
#+ and divisible by $divisibleBy.
Gives a "reasonably random" distribution of return values.
Bill Gradwohl - Oct 1, 2003
syntax() {
Function embedded within function.
echo
echo "Syntax: randomBetween [min] [max] [multiple]"
echo
echo -n "Expects up to 3 passed parameters, "
echo "but all are completely optional."
echo "min is the minimum value"
echo "max is the maximum value"
echo -n "multiple specifies that the answer must be "
echo "a multiple of this value."
echo " i.e. answer must be evenly divisible by this number."
echo
echo "If any value is missing, defaults area supplied as: 0 32767 1"
echo -n "Successful completion returns 0, "
echo "unsuccessful completion returns"
echo "function syntax and 1."
echo -n "The answer is returned in the global variable "
echo "randomBetweenAnswer"
echo -n "Negative values for any passed parameter are "
echo "handled correctly."
}
local min=${1:-0}
local max=${2:-32767}
local divisibleBy=${3:-1}
Default values assigned, in case parameters not passed to function.
local x
local spread
Let's make sure the divisibleBy value is positive.
[ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy))
Sanity check.
if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o ${min} -eq ${max} ]; then
syntax
return 1
fi
See if the min and max are reversed.
if [ ${min} -gt ${max} ]; then
# Swap them.
x=${min}
min=${max}
max=${x}
fi
If min is itself not evenly divisible by $divisibleBy,
#+ then fix the min to be within range.
if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then
if [ ${min} -lt 0 ]; then
min=$((min/divisibleBy*divisibleBy))
else
min=$((((min/divisibleBy)+1)*divisibleBy))
fi
fi
If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then
if [ ${max} -lt 0 ]; then
max=$((((max/divisibleBy)-1)*divisibleBy))
else
max=$((max/divisibleBy*divisibleBy))
fi
fi
---------------------------------------------------------------------
Now, to do the real work.
Note that to get a proper distribution for the end points,
#+ the range of random values has to be allowed to go between
#+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1.
The slight increase will produce the proper distribution for the
#+ end points.
Changing the formula to use abs(max-min)+1 will still produce
#+ correct answers, but the randomness of those answers is faulty in
#+ that the number of times the end points ($min and $max) are returned
#+ is considerably lower than when the correct formula is used.
---------------------------------------------------------------------
spread=$((max-min))
Omair Eshkenazi points out that this test is unnecessary,
#+ since max and min have already been switched around.
[ ${spread} -lt 0 ] && spread=$((0-spread))
let spread+=divisibleBy
randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min))
return 0
However, Paulo Marcel Coelho Aragao points out that
#+ when $max and $min are not divisible by $divisibleBy,
#+ the formula fails.
He suggests instead the following formula:
rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy))
}
Let's test the function.
min=-14
max=20
divisibleBy=3
Generate an array of expected answers and check to make sure we get
- at least one of each answer if we loop long enough.
declare -a answer
minimum=${min}
maximum=${max}
if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then
if [ ${minimum} -lt 0 ]; then
minimum=$((minimum/divisibleBy*divisibleBy))
else
minimum=$((((minimum/divisibleBy)+1)*divisibleBy))
fi
fi
If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then
if [ ${maximum} -lt 0 ]; then
maximum=$((((maximum/divisibleBy)-1)*divisibleBy))
else
maximum=$((maximum/divisibleBy*divisibleBy))
fi
fi
We need to generate only positive array subscripts,
- so we need a displacement that that will guarantee
disp=$((0-minimum))
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
answer[i+disp]=0
done
Now loop a large number of times to see what we get.
loopIt=1000 # The script author suggests 100000,
#+ but that takes a good long while.
for ((i=0; i<${loopIt}; ++i)); do
Note that we are specifying min and max in reversed order here to
#+ make the function correct for this case.
randomBetween ${max} ${min} ${divisibleBy}
Report an error if an answer is unexpected.
[ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ]
\
&& echo MIN or MAX error - ${randomBetweenAnswer}!
[ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] \
&& echo DIVISIBLE BY error - ${randomBetweenAnswer}!
Store the answer away statistically.
answer[randomBetweenAnswer+disp]=$((answer[randomBetweenAnswer+disp]+1))
done
Let's check the results
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
[ ${answer[i+disp]} -eq 0 ] \
&& echo "We never got an answer of $i." \
|| echo "${i} occurred ${answer[i+disp]} times."
done
exit 0
Just how random is $RANDOM? The best way to test this is to write a
script that tracks the distribution of "random" numbers generated by
$RANDOM. Let's roll a $RANDOM die a few times . . .
Example 9-15. Rolling a single die with RANDOM
!/bin/bash
How random is RANDOM?
RANDOM=$$ # Reseed the random number generator using script process ID.
PIPS=6 # A die has 6 pips.
MAXTHROWS=600 # Increase this if you have nothing better to do with your tim
e.
throw=0 # Number of times the dice have been cast.
ones=0 # Must initialize counts to zero,
twos=0 #+ since an uninitialized variable is null, NOT zero.
threes=0
fours=0
fives=0
sixes=0
print_result ()
{
echo
echo "ones = $ones"
echo "twos = $twos"
echo "threes = $threes"
echo "fours = $fours"
echo "fives = $fives"
echo "sixes = $sixes"
echo
}
update_count()
{
case "$1" in
- ((ones++));; # Since a die has no "zero", this corresponds to 1.
- ((twos++));; # And this to 2.
- ((threes++));; # And so forth.
- ((fours++));;
- ((fives++));;
- ((sixes++));;
esac
}
echo
while [ "$throw" -lt "$MAXTHROWS" ]
do
let "die1 = RANDOM % $PIPS"
update_count $die1
let "throw += 1"
done
print_result
exit $?
The scores should distribute evenly, assuming RANDOM is random.
With $MAXTHROWS at 600, all should cluster around 100,
Keep in mind that RANDOM is a pseudorandom generator,
- and not a spectacularly good one at that.
Randomness is a deep and complex subject.
Sufficiently long "random" sequences may exhibit
- chaotic and other "non-random" behavior.
Exercise (easy):
Rewrite this script to flip a coin 1000 times.
Choices are "HEADS" and "TAILS."
As we have seen in the last example, it is best to reseed the RANDOM
generator each time it is invoked. Using the same seed for RANDOM
repeats the same series of numbers. [48] (This mirrors the behavior
of the random() function in C.)
Example 9-16. Reseeding RANDOM
!/bin/bash
seeding-random.sh: Seeding the RANDOM variable.
v 1.1, reldate 09 Feb 2013
MAXCOUNT=25 # How many numbers to generate.
SEED=
random_numbers ()
{
local count=0
local number
while [ "$count" -lt "$MAXCOUNT" ]
do
number=$RANDOM
echo -n "$number "
let "count++"
done
}
echo; echo
SEED=1
RANDOM=$SEED # Setting RANDOM seeds the random number generator.
echo "Random seed = $SEED"
random_numbers
RANDOM=$SEED # Same seed for RANDOM . . .
echo; echo "Again, with same random seed ..."
echo "Random seed = $SEED"
random_numbers # . . . reproduces the exact same number series.
#
# When is it useful to duplicate a "random" series?
echo; echo
SEED=2
RANDOM=$SEED # Trying again, but with a different seed . . .
echo "Random seed = $SEED"
random_numbers # . . . gives a different number series.
echo; echo
RANDOM=$$ seeds RANDOM from process id of script.
It is also possible to seed RANDOM from 'time' or 'date' commands.
Getting fancy...
SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }'| sed s/^0*//)
Pseudo-random output fetched
- from /dev/urandom (system pseudo-random device-file),
- then converted to line of printable (octal) numbers by "od",
- then "awk" retrieves just one number for SEED,
- finally "sed" removes any leading zeros.
RANDOM=$SEED
echo "Random seed = $SEED"
random_numbers
echo; echo
exit 0
Note
The /dev/urandom pseudo-device file provides a method of generating
much more "random" pseudorandom numbers than the $RANDOM variable. dd
if=/dev/urandom of=targetfile bs=1 count=XX creates a file of
well-scattered pseudorandom numbers. However, assigning these numbers
to a variable in a script requires a workaround, such as filtering
through od (as in above example, Example 16-14, and Example A-36), or
even piping to md5sum (see Example 36-16).
There are also other ways to generate pseudorandom numbers in a
script. Awk provides a convenient means of doing this.
Example 9-17. Pseudorandom numbers, using awk
!/bin/bash
random2.sh: Returns a pseudorandom number in the range 0 - 1,
- to 6 decimal places. For example: 0.822725
Uses the awk rand() function.
AWKSCRIPT=' { srand(); print rand() } '
Command(s)/parameters passed to awk
Note that srand() reseeds awk's random number generator.
echo -n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"
What happens if you leave out the 'echo'?
exit 0
Exercises:
- Using a loop construct, print out 10 different random numbers.
(Hint: you must reseed the srand() function with a different seed
- Using an integer multiplier as a scaling factor, generate random numbers
- in the range of 10 to 100.
- Same as exercise #2, above, but generate random integers this time.
The date command also lends itself to generating pseudorandom integer
sequences.
________________________________________________________________
Chapter 10. Manipulating Variables
10.1. Manipulating Strings
Bash supports a surprising number of string manipulation operations.
Unfortunately, these tools lack a unified focus. Some are a subset of
parameter substitution, and others fall under the functionality of
the UNIX expr command. This results in inconsistent command syntax
and overlap of functionality, not to mention confusion.
String Length
${#string}
expr length $string
These are the equivalent of strlen() in C.
expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ} # 15
echo expr length $stringZ # 15
echo expr "$stringZ" : '.*' # 15
Example 10-1. Inserting a blank line between paragraphs in a text
file
!/bin/bash
paragraph-space.sh
Ver. 2.1, Reldate 29Jul12 [fixup]
Inserts a blank line between paragraphs of a single-spaced text file.
Usage: $0 <FILENAME
MINLEN=60 # Change this value? It's a judgment call.
Assume lines shorter than $MINLEN characters ending in a period
- terminate a paragraph. See exercises below.
while read line # For as many lines as the input file has ...
do
echo "$line" # Output the line itself.
len=${#line}
if [[ "$len" -lt "$MINLEN" && "$line" =~ [*{.}]$ ]]
if [[ "$len" -lt "$MINLEN" && "$line" =~ [*.] ]]
An update to Bash broke the previous version of this script. Ouch!
Thank you, Halim Srama, for pointing this out and suggesting a fix.
then echo # Add a blank line immediately
fi #+ after a short line terminated by a period.
done
exit
Exercises:
- The script usually inserts a blank line at the end
- of the target file. Fix this.
- Line 17 only considers periods as sentence terminators.
Modify this to include other common end-of-sentence characters,
Length of Matching Substring at Beginning of String
expr match "$string" '$substring'
$substring is a regular expression.
expr "$string" : '$substring'
$substring is a regular expression.
stringZ=abcABC123ABCabc
|------|
12345678
echo expr match "$stringZ" 'abc[A-Z]*.2' # 8
echo expr "$stringZ" : 'abc[A-Z]*.2' # 8
Index
expr index $string $substring
Numerical position in $string of first character in $substring
that matches.
stringZ=abcABC123ABCabc
123456 ...
echo expr index "$stringZ" C12 # 6
# C position.
echo expr index "$stringZ" 1c # 3
'c' (in #3 position) matches before '1'.
This is the near equivalent of strchr() in C.
Substring Extraction
${string:position}
Extracts substring from $string at $position.
If the $string parameter is "*" or "@", then this extracts the
positional parameters, [49] starting at $position.
${string:position:length}
Extracts $length characters of substring from $string at
$position.
stringZ=abcABC123ABCabc
0123456789.....
0-based indexing.
echo ${stringZ:0} # abcABC123ABCabc
echo ${stringZ:1} # bcABC123ABCabc
echo ${stringZ:7} # 23ABCabc
echo ${stringZ:7:3} # 23A
# Three characters of substring.
Is it possible to index from the right end of the string?
echo ${stringZ:-4} # abcABC123ABCabc
Defaults to full string, as in ${parameter:-default}.
However . . .
echo ${stringZ:(-4)} # Cabc
echo ${stringZ: -4} # Cabc
Now, it works.
Parentheses or added space "escape" the position parameter.
Thank you, Dan Jacobson, for pointing this out.
The position and length arguments can be "parameterized," that
is, represented as a variable, rather than as a numerical
constant.
Example 10-2. Generating an 8-character "random" string
!/bin/bash
rand-string.sh
Generating an 8-character "random" string.
if [ -n "$1" ] # If command-line argument present,
then #+ then set start-string to it.
str0="$1"
else # Else use PID of script as start-string.
str0="$$"
fi
POS=2 # Starting from position 2 in the string.
LEN=8 # Extract eight characters.
str1=$( echo "$str0" | md5sum | md5sum )
Doubly scramble ^^^^^^ ^^^^^^
- by piping and repiping to md5sum.
randstring="${str1:$POS:$LEN}"
Can parameterize ^^^^ ^^^^
echo "$randstring"
exit $?
bozo$ ./rand-string.sh my-password
1bdd88c4
No, this is is not recommended
- as a method of generating hack-proof passwords.
If the $string parameter is "*" or "@", then this extracts a
maximum of $length positional parameters, starting at
$position.
echo ${*:2} # Echoes second and following positional parameters.
echo ${@:2} # Same as above.
echo ${*:2:3} # Echoes three positional parameters, starting at second.
expr substr $string $position $length
Extracts $length characters from $string starting at
$position.
stringZ=abcABC123ABCabc
123456789......
1-based indexing.
echo expr substr $stringZ 1 2 # ab
echo expr substr $stringZ 4 3 # ABC
expr match "$string" '($substring)'
Extracts $substring at beginning of $string, where $substring
is a regular expression.
expr "$string" : '($substring)'
Extracts $substring at beginning of $string, where $substring
is a regular expression.
stringZ=abcABC123ABCabc
=======
echo expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)' # abcABC1
echo expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)' # abcABC1
echo expr "$stringZ" : '\(.......\)' # abcABC1
All of the above forms give an identical result.
expr match "$string" '.*($substring)'
Extracts $substring at end of $string, where $substring is a
regular expression.
expr "$string" : '.*($substring)'
Extracts $substring at end of $string, where $substring is a
regular expression.
stringZ=abcABC123ABCabc
======
echo expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)' # ABCabc
echo expr "$stringZ" : '.*\(......\)' # ABCabc
Substring Removal
${string#substring}
Deletes shortest match of $substring from front of $string.
${string##substring}
Deletes longest match of $substring from front of $string.
stringZ=abcABC123ABCabc
|----| shortest
|----------| longest
echo ${stringZ#a*C} # 123ABCabc
Strip out shortest match between 'a' and 'C'.
echo ${stringZ##a*C} # abc
Strip out longest match between 'a' and 'C'.
You can parameterize the substrings.
X='a*C'
echo ${stringZ#$X} # 123ABCabc
echo ${stringZ##$X} # abc
# As above.
${string%substring}
Deletes shortest match of $substring from back of $string.
For example:
Rename all filenames in $PWD with "TXT" suffix to a "txt" suffix.
For example, "file1.TXT" becomes "file1.txt" . . .
SUFF=TXT
suff=txt
for i in $(ls *.$SUFF)
do
mv -f $i ${i%.$SUFF}.$suff
Leave unchanged everything except the shortest pattern match
#+ starting from the right-hand-side of the variable $i . . .
done ### This could be condensed into a "one-liner" if desired.
Thank you, Rory Winston.
${string%%substring}
Deletes longest match of $substring from back of $string.
stringZ=abcABC123ABCabc
|| shortest
|------------| longest
echo ${stringZ%b*c} # abcABC123ABCa
Strip out shortest match between 'b' and 'c', from back of $stringZ.
echo ${stringZ%%b*c} # a
Strip out longest match between 'b' and 'c', from back of $stringZ.
This operator is useful for generating filenames.
Example 10-3. Converting graphic file formats, with filename
change
!/bin/bash
cvt.sh:
Converts all the MacPaint image files in a directory to "pbm" format.
Uses the "macptopbm" binary from the "netpbm" package,
- which is maintained by Brian Henderson (bryanh@giraffe-data.com).
Netpbm is a standard part of most Linux distros.
OPERATION=macptopbm
SUFFIX=pbm # New filename suffix.
if [ -n "$1" ]
then
directory=$1 # If directory name given as a script argument...
else
directory=$PWD # Otherwise use current working directory.
fi
Assumes all files in the target directory are MacPaint image files,
- with a ".mac" filename suffix.
for file in $directory/* # Filename globbing.
do
filename=${file%.*c} # Strip ".mac" suffix off filename
#+ ('.*c' matches everything
#+ between '.' and 'c', inclusive).
$OPERATION $file > "$filename.$SUFFIX"
# Redirect conversion to new filename.
rm -f $file # Delete original files after converting.
echo "$filename.$SUFFIX" # Log what is happening to stdout.
done
exit 0
Exercise:
As it stands, this script converts all the files in the current
Modify it to work only on files with a ".mac" suffix.
*** And here's another way to do it. *** #
!/bin/bash
Batch convert into different graphic formats.
Assumes imagemagick installed (standard in most Linux distros).
INFMT=png # Can be tif, jpg, gif, etc.
OUTFMT=pdf # Can be tif, jpg, gif, pdf, etc.
for pic in *"$INFMT"
do
p2=$(ls "$pic" | sed -e s/.$INFMT//)
echo $p2
convert "$pic" $p2.$OUTFMT
done
exit $?
Example 10-4. Converting streaming audio files to ogg
!/bin/bash
ra2ogg.sh: Convert streaming audio files (*.ra) to ogg.
Uses the "mplayer" media player program:
http://www.mplayerhq.hu/homepage
Uses the "ogg" library and "oggenc":
http://www.xiph.org/
This script may need appropriate codecs installed, such as sipr.so ...
Possibly also the compat-libstdc++ package.
OFILEPREF=${1%%ra} # Strip off the "ra" suffix.
OFILESUFF=wav # Suffix for wav file.
OUTFILE="$OFILEPREF""$OFILESUFF"
E_NOARGS=85
if [ -z "$1" ] # Must specify a filename to convert.
then
echo "Usage: basename $0 [filename]"
exit $E_NOARGS
fi
mplayer "$1" -ao pcm:file=$OUTFILE
oggenc "$OUTFILE" # Correct file extension automatically added by oggenc.
rm "$OUTFILE" # Delete intermediate *.wav file.
# If you want to keep it, comment out above line.
exit $?
Note:
On a Website, simply clicking on a *.ram streaming audio file
- usually only downloads the URL of the actual *.ra audio file.
You can then use "wget" or something similar
- to download the *.ra file itself.
Exercises:
As is, this script converts only *.ra filenames.
Add flexibility by permitting use of *.ram and other filenames.
If you're really ambitious, expand the script
- to do automatic downloads and conversions of streaming audio files.
Given a URL, batch download streaming audio files (using "wget")
- and convert them on the fly.
A simple emulation of getopt using substring-extraction
constructs.
Example 10-5. Emulating getopt
!/bin/bash
getopt-simple.sh
Author: Chris Morgan
Used in the ABS Guide with permission.
getopt_simple()
{
echo "getopt_simple()"
echo "Parameters are '$*'"
until [ -z "$1" ]
do
echo "Processing parameter of: '$1'"
if [ ${1:0:1} = '/' ]
then
tmp=${1:1} # Strip off leading '/' . . .
parameter=${tmp%%=*} # Extract name.
value=${tmp##*=} # Extract value.
echo "Parameter: '$parameter', value: '$value'"
eval $parameter=$value
fi
shift
done
}
Pass all options to getopt_simple().
getopt_simple $*
echo "test is '$test'"
echo "test2 is '$test2'"
exit 0 # See also, UseGetOpt.sh, a modified version of this script.
sh getopt_example.sh /test=value1 /test2=value2
Parameters are '/test=value1 /test2=value2'
Processing parameter of: '/test=value1'
Parameter: 'test', value: 'value1'
Processing parameter of: '/test2=value2'
Parameter: 'test2', value: 'value2'
test is 'value1'
test2 is 'value2'
Substring Replacement
${string/substring/replacement}
Replace first match of $substring with $replacement. [50]
${string//substring/replacement}
Replace all matches of $substring with $replacement.
stringZ=abcABC123ABCabc
echo ${stringZ/abc/xyz} # xyzABC123ABCabc
# Replaces first match of 'abc' with 'xyz'.
echo ${stringZ//abc/xyz} # xyzABC123ABCxyz
# Replaces all matches of 'abc' with # 'xyz'.
echo ---------------
echo "$stringZ" # abcABC123ABCabc
echo ---------------
# The string itself is not altered!
Can the match and replacement strings be parameterized?
match=abc
repl=000
echo ${stringZ/$match/$repl} # 000ABC123ABCabc
^ ^ ^^^
echo ${stringZ//$match/$repl} # 000ABC123ABC000
Yes! ^ ^ ^^^ ^^^
echo
What happens if no $replacement string is supplied?
echo ${stringZ/abc} # ABC123ABCabc
echo ${stringZ//abc} # ABC123ABC
A simple deletion takes place.
${string/#substring/replacement}
If $substring matches front end of $string, substitute
$replacement for $substring.
${string/%substring/replacement}
If $substring matches back end of $string, substitute
$replacement for $substring.
stringZ=abcABC123ABCabc
echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc
# Replaces front-end match of 'abc' with 'XY
Z'.
echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ
# Replaces back-end match of 'abc' with 'XYZ
'.
________________________________________________________________
10.1.1. Manipulating strings using awk
A Bash script may invoke the string manipulation facilities of awk as
an alternative to using its built-in operations.
Example 10-6. Alternate ways of extracting and locating substrings
!/bin/bash
substring-extraction.sh
String=23skidoo1
012345678 Bash
123456789 awk
Note different string indexing system:
Bash numbers first character of string as 0.
Awk numbers first character of string as 1.
echo ${String:2:4} # position 3 (0-1-2), 4 characters long
# skid
The awk equivalent of ${string:pos:length} is substr(string,pos,length).
echo | awk '
{ print substr("'"${String}"'",3,4) # skid
}
'
Piping an empty "echo" to awk gives it dummy input,
- and thus makes it unnecessary to supply a filename.
echo "----"
And likewise:
echo | awk '
{ print index("'"${String}"'", "skid") # 3
} # (skid starts at position 3)
' # The awk equivalent of "expr index" ...
exit 0
________________________________________________________________
10.1.2. Further Reference
For more on string manipulation in scripts, refer to Section 10.2 and
the relevant section of the expr command listing.
Script examples:
1. Example 16-9
2. Example 10-9
3. Example 10-10
4. Example 10-11
5. Example 10-13
6. Example A-36
7. Example A-41
________________________________________________________________
10.2. Parameter Substitution
Manipulating and/or expanding variables
${parameter}
Same as $parameter, i.e., value of the variable parameter. In
certain contexts, only the less ambiguous ${parameter} form
works.
May be used for concatenating variables with strings.
your_id=${USER}-on-${HOSTNAME}
echo "$your_id"
echo "Old $PATH = $PATH"
PATH=${PATH}:/opt/bin # Add /opt/bin to $PATH for duration of script.
echo "New $PATH = $PATH"
${parameter-default}, ${parameter:-default}
If parameter not set, use default.
var1=1
var2=2
var3 is unset.
echo ${var1-$var2} # 1
echo ${var3-$var2} # 2
^ Note the $ prefix.
echo ${username-whoami}
Echoes the result of whoami, if variable $username is still unset.
Note
${parameter-default} and ${parameter:-default} are almost equivalent.
The extra : makes a difference only when parameter has been declared,
but is null.
!/bin/bash
param-sub.sh
Whether a variable has been declared
- affects triggering of the default option
- even if the variable is null.
username0=
echo "username0 has been declared, but is set to null."
echo "username0 = ${username0-whoami}"
Will not echo.
echo
echo username1 has not been declared.
echo "username1 = ${username1-whoami}"
Will echo.
username2=
echo "username2 has been declared, but is set to null."
echo "username2 = ${username2:-whoami}"
^
Will echo because of :- rather than just - in condition test.
Compare to first instance, above.
Once again:
variable=
variable has been declared, but is set to null.
echo "${variable-0}" # (no output)
echo "${variable:-1}" # 1
^
unset variable
echo "${variable-2}" # 2
echo "${variable:-3}" # 3
exit 0
The default parameter construct finds use in providing
"missing" command-line arguments in scripts.
DEFAULT_FILENAME=generic.data
filename=${1:-$DEFAULT_FILENAME}
If not otherwise specified, the following command block operates
- on the file "generic.data".
Begin-Command-Block
...
...
...
End-Command-Block
From "hanoi2.bash" example:
DISKS=${1:-E_NOPARAM} # Must specify how many disks.
Set $DISKS to $1 command-line-parameter,
- or to $E_NOPARAM if that is unset.
See also Example 3-4, Example 31-2, and Example A-6.
Compare this method with using an and list to supply a default
command-line argument.
${parameter=default}, ${parameter:=default}
If parameter not set, set it to default.
Both forms nearly equivalent. The : makes a difference only
when $parameter has been declared and is null, [51] as above.
echo ${var=abc} # abc
echo ${var=xyz} # abc
$var had already been set to abc, so it did not change.
${parameter+alt_value}, ${parameter:+alt_value}
If parameter set, use alt_value, else use null string.
Both forms nearly equivalent. The : makes a difference only
when parameter has been declared and is null, see below.
echo "###### ${parameter+alt_value} ########"
echo
a=${param1+xyz}
echo "a = $a" # a =
param2=
a=${param2+xyz}
echo "a = $a" # a = xyz
param3=123
a=${param3+xyz}
echo "a = $a" # a = xyz
echo
echo "###### ${parameter:+alt_value} ########"
echo
a=${param4:+xyz}
echo "a = $a" # a =
param5=
a=${param5:+xyz}
echo "a = $a" # a =
Different result from a=${param5+xyz}
param6=123
a=${param6:+xyz}
echo "a = $a" # a = xyz
${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg and abort the
script with an exit status of 1.
Both forms nearly equivalent. The : makes a difference only
when parameter has been declared and is null, as above.
Example 10-7. Using parameter substitution and error messages
!/bin/bash
Check some of the system's environmental variables.
This is good preventative maintenance.
If, for example, $USER, the name of the person at the console, is not set,
- the machine will not recognize you.
: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
The ${variablename?} construction can also check
- for variables set within the script.
ThisVariable=Value-of-ThisVariable
Note, by the way, that string variables may be set
- to characters disallowed in their names.
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable".
echo; echo
: ${ZZXy23AB?"ZZXy23AB has not been set."}
Since ZZXy23AB has not been set,
- then the script terminates with an error message.
You can specify the error message.
: ${variablename?"ERROR MESSAGE"}
Same result with: dummy_variable=${ZZXy23AB?}
dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
echo ${ZZXy23AB?} >/dev/null
Compare these methods of checking whether a variable has been set
echo "You will not see this message, because script already terminated."
HERE=0
exit $HERE # Will NOT exit here.
In fact, this script will return an exit status (echo $?) of 1.
Example 10-8. Parameter substitution and "usage" messages
!/bin/bash
usage-message.sh
: ${1?"Usage: $0 ARGUMENT"}
Script exits here if command-line parameter absent,
- with following error message.
usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
echo "These two lines echo only if command-line parameter given."
echo "command-line parameter = "$1""
exit 0 # Will exit here only if command-line parameter present.
Check the exit status, both with and without command-line parameter.
If command-line parameter present, then "$?" is 0.
If not, then "$?" is 1.
Parameter substitution and/or expansion. The following expressions
are the complement to the match in expr string operations (see
Example 16-9). These particular ones are used mostly in parsing file
path names.
Variable length / Substring removal
${#var}
String length (number of characters in $var). For an array,
${#array} is the length of the first element in the array.
Note
Exceptions:
+ ${#*} and ${#@} give the number of positional parameters.
+ For an array, ${#array[*]} and ${#array[@]} give the number
of elements in the array.
Example 10-9. Length of a variable
!/bin/bash
length.sh
E_NO_ARGS=65
if [ $# -eq 0 ] # Must have command-line args to demo script.
then
echo "Please invoke this script with one or more command-line arguments."
exit $E_NO_ARGS
fi
var01=abcdEFGH28ij
echo "var01 = ${var01}"
echo "Length of var01 = ${#var01}"
Now, let's try embedding a space.
var02="abcd EFGH28ij"
echo "var02 = ${var02}"
echo "Length of var02 = ${#var02}"
echo "Number of command-line arguments passed to script = ${#@}"
echo "Number of command-line arguments passed to script = ${#*}"
exit 0
${var#Pattern}, ${var##Pattern}
${var#Pattern} Remove from $var the shortest part of $Pattern
that matches the front end of $var.
${var##Pattern} Remove from $var the longest part of $Pattern
that matches the front end of $var.
A usage illustration from Example A-7:
Function from "days-between.sh" example.
Strips leading zero(s) from argument passed.
strip_leading_zero () # Strip possible leading zero(s)
{ #+ from argument passed.
return=${1#0} # The "1" refers to "$1" -- passed arg.
} # The "0" is what to remove from "$1" -- strips zeros.
Manfred Schwarb's more elaborate variation of the above:
strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
{ # Bash will interpret such numbers as octal values.
shopt -s extglob # Turn on extended globbing.
local val=${1##+(0)} # Use local variable, longest matching series of 0's.
shopt -u extglob # Turn off extended globbing.
_strip_leading_zero2=${val:-0}
# If input was 0, return 0 instead of "".
}
Another usage illustration:
echo basename $PWD # Basename of current working directory.
echo "${PWD##*/}" # Basename of current working directory.
echo
echo basename $0 # Name of script.
echo $0 # Name of script.
echo "${0##*/}" # Name of script.
echo
filename=test.data
echo "${filename##*.}" # data
# Extension of filename.
${var%Pattern}, ${var%%Pattern}
${var%Pattern} Remove from $var the shortest part of $Pattern
that matches the back end of $var.
${var%%Pattern} Remove from $var the longest part of $Pattern
that matches the back end of $var.
Version 2 of Bash added additional options.
Example 10-10. Pattern matching in parameter substitution
!/bin/bash
patt-matching.sh
Pattern matching using the # ## % %% parameter substitution operators.
var1=abcd12345abc6789
pattern1=a*c # * (wild card) matches everything between a - c.
echo
echo "var1 = $var1" # abcd12345abc6789
echo "var1 = ${var1}" # abcd12345abc6789
# (alternate form)
echo "Number of characters in ${var1} = ${#var1}"
echo
echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c')
echo "--------------"
echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789
Shortest possible match, strips out first 3 characters abcd12345abc6789
^^^^^ |-|
echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789
Longest possible match, strips out first 12 characters abcd12345abc6789
^^^^^ |----------|
echo; echo; echo
pattern2=b*9 # everything between 'b' and '9'
echo "var1 = $var1" # Still abcd12345abc6789
echo
echo "pattern2 = $pattern2"
echo "--------------"
echo '${var1%pattern2} =' "${var1%$pattern2}" # abcd12345a
Shortest possible match, strips out last 6 characters abcd12345abc6789
^^^^ |----|
echo '${var1%%pattern2} =' "${var1%%$pattern2}" # a
Longest possible match, strips out last 12 characters abcd12345abc6789
^^^^ |-------------|
Remember, # and ## work from the left end (beginning) of string,
% and %% work from the right end.
echo
exit 0
Example 10-11. Renaming file extensions:
!/bin/bash
rfe.sh: Renaming file extensions.
rfe old_extension new_extension
Example:
To rename all *.gif files in working directory to *.jpg,
rfe gif jpg
E_BADARGS=65
case $# in
0|1) # The vertical bar means "or" in this context.
echo "Usage: basename $0 old_file_suffix new_file_suffix"
exit $E_BADARGS # If 0 or 1 arg, then bail out.
;;
esac
for filename in *.$1
Traverse list of files ending with 1st argument.
do
mv $filename ${filename%$1}$2
Strip off part of filename matching 1st argument,
#+ then append 2nd argument.
done
exit 0
Variable expansion / Substring replacement
These constructs have been adopted from ksh.
${var:pos}
Variable var expanded, starting from offset pos.
${var:pos:len}
Expansion to a max of len characters of variable var, from
offset pos. See Example A-13 for an example of the creative
use of this operator.
${var/Pattern/Replacement}
First match of Pattern, within var replaced with Replacement.
If Replacement is omitted, then the first match of Pattern is
replaced by nothing, that is, deleted.
${var//Pattern/Replacement}
Global replacement. All matches of Pattern, within var
replaced with Replacement.
As above, if Replacement is omitted, then all occurrences of
Pattern are replaced by nothing, that is, deleted.
Example 10-12. Using pattern matching to parse arbitrary
strings
!/bin/bash
var1=abcd-1234-defg
echo "var1 = $var1"
t=${var1#-}
echo "var1 (with everything, up to and including first - stripped out) = $t"
t=${var1#*-} works just the same,
- since # matches the shortest string,
- and * matches everything preceding, including an empty string.
(Thanks, Stephane Chazelas, for pointing this out.)
t=${var1##-}
echo "If var1 contains a "-", returns empty string... var1 = $t"
t=${var1%-}
echo "var1 (with everything from the last - on stripped out) = $t"
echo
path_name=/home/bozo/ideas/thoughts.for.today
echo "path_name = $path_name"
t=${path_name##/*/}
echo "path_name, stripped of prefixes = $t"
Same effect as t=basename $path_name in this particular case.
t=${path_name%/}; t=${t##*/} is a more general solution,
- but still fails sometimes.
If $path_name ends with a newline, then basename $path_name will not work
,
- but the above expression will.
(Thanks, S.C.)
t=${path_name%/.}
Same effect as t=dirname $path_name
echo "path_name, stripped of suffixes = $t"
These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
Removing suffixes, especially when the basename has no suffix,
- but the dirname does, also complicates matters.
(Thanks, S.C.)
echo
t=${path_name:11}
echo "$path_name, with first 11 chars stripped off = $t"
t=${path_name:11:5}
echo "$path_name, with first 11 chars stripped off, length 5 = $t"
echo
t=${path_name/bozo/clown}
echo "$path_name with "bozo" replaced by "clown" = $t"
t=${path_name/today/}
echo "$path_name with "today" deleted = $t"
t=${path_name//o/O}
echo "$path_name with all o's capitalized = $t"
t=${path_name//o/}
echo "$path_name with all o's deleted = $t"
exit 0
${var/#Pattern/Replacement}
If prefix of var matches Pattern, then substitute Replacement
for Pattern.
${var/%Pattern/Replacement}
If suffix of var matches Pattern, then substitute Replacement
for Pattern.
Example 10-13. Matching patterns at prefix or suffix of string
!/bin/bash
var-match.sh:
Demo of pattern replacement at prefix / suffix of string.
v0=abc1234zip1234abc # Original variable.
echo "v0 = $v0" # abc1234zip1234abc
echo
Match at prefix (beginning) of string.
v1=${v0/#abc/ABCDEF} # abc1234zip1234abc
# |-|
echo "v1 = $v1" # ABCDEF1234zip1234abc
# |----|
Match at suffix (end) of string.
v2=${v0/%abc/ABCDEF} # abc1234zip123abc
# |-|
echo "v2 = $v2" # abc1234zip1234ABCDEF
# |----|
echo
Must match at beginning / end of string,
- otherwise no replacement results.
v3=${v0/#123/000} # Matches, but not at beginning.
echo "v3 = $v3" # abc1234zip1234abc
# NO REPLACEMENT.
v4=${v0/%123/000} # Matches, but not at end.
echo "v4 = $v4" # abc1234zip1234abc
# NO REPLACEMENT.
exit 0
${!varprefix*}, ${!varprefix@}
Matches names of all previously declared variables beginning
with varprefix.
This is a variation on indirect reference, but with a * or @.
Bash, version 2.04, adds this feature.
xyz23=whatever
xyz24=
a=${!xyz*} # Expands to names of declared variables
^ ^ ^ + beginning with "xyz".
echo "a = $a" # a = xyz23 xyz24
a=${!xyz@} # Same as above.
echo "a = $a" # a = xyz23 xyz24
echo "---"
abc23=something_else
b=${!abc*}
echo "b = $b" # b = abc23
c=${!b} # Now, the more familiar type of indirect reference.
echo $c # something_else
________________________________________________________________
Chapter 11. Loops and Branches
What needs this iteration, woman?
--Shakespeare, Othello
Operations on code blocks are the key to structured and organized
shell scripts. Looping and branching constructs provide the tools for
accomplishing this.
________________________________________________________________
11.1. Loops
A loop is a block of code that iterates [52] a list of commands as
long as the loop control condition is true.
for loops
for arg in [list]
This is the basic looping construct. It differs significantly
from its C counterpart.
for arg in [list]
do
command(s)...
done
Note
During each pass through the loop, arg takes on the value of each
successive variable in the list.
for arg in "$var1" "$var2" "$var3" ... "$varN"
In pass 1 of the loop, arg = $var1
In pass 2 of the loop, arg = $var2
In pass 3 of the loop, arg = $var3
...
In pass N of the loop, arg = $varN
Arguments in [list] quoted to prevent possible word splitting.
The argument list may contain wild cards.
If do is on same line as for, there needs to be a semicolon
after list.
for arg in [list] ; do
Example 11-1. Simple for loops
!/bin/bash
Listing the planets.
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet # Each planet on a separate line.
done
echo; echo
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
# All planets on same line.
# Entire 'list' enclosed in quotes creates a single variable.
# Why? Whitespace incorporated into the variable.
do
echo $planet
done
echo; echo "Whoops! Pluto is no longer a planet!"
exit 0
Each [list] element may contain multiple parameters. This is
useful when processing parameters in groups. In such cases,
use the set command (see Example 15-16) to force parsing of
each [list] element and assignment of each component to the
positional parameters.
Example 11-2. for loop with two parameters in each [list]
element
!/bin/bash
Planets revisited.
Associate the name of each planet with its distance from the sun.
for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"
do
set -- $planet # Parses variable "planet"
#+ and sets positional parameters.
The "--" prevents nasty surprises if $planet is null or
#+ begins with a dash.
May need to save original positional parameters,
#+ since they get overwritten.
One way of doing this is to use an array,
original_params=("$@")
echo "$1 $2,000,000 miles from the sun"
#-------two tabs---concatenate zeroes onto parameter $2
done
(Thanks, S.C., for additional clarification.)
exit 0
A variable may supply the [list] in a for loop.
Example 11-3. Fileinfo: operating on a file list contained in
a variable
!/bin/bash
fileinfo.sh
FILES="/usr/sbin/accept
/usr/sbin/pwck
/usr/sbin/chroot
/usr/bin/fakefile
/sbin/badblocks
/sbin/ypbind" # List of files you are curious about.
# Threw in a dummy file, /usr/bin/fakefile.
echo
for file in $FILES
do
if [ ! -e "$file" ] # Check if file exists.
then
echo "$file does not exist."; echo
continue # On to next.
fi
ls -l $file | awk '{ print $8 " file size: " $5 }' # Print 2 fields
.
whatis basename $file # File info.
Note that the whatis database needs to have been set up for this to work.
To do this, as root run /usr/bin/makewhatis.
echo
done
exit 0
The [list] in a for loop may be parameterized.
Example 11-4. Operating on a parameterized file list
!/bin/bash
filename="*txt"
for file in $filename
do
echo "Contents of $file"
echo "---"
cat "$file"
echo
done
If the [list] in a for loop contains wild cards (* and ?) used
in filename expansion, then globbing takes place.
Example 11-5. Operating on files with a for loop
!/bin/bash
list-glob.sh: Generating [list] in a for-loop, using "globbing" ...
Globbing = filename expansion.
echo
for file in *
^ Bash performs filename expansion
do
ls -l "$file" # Lists all files in $PWD (current directory).
Recall that the wild card character "*" matches every filename,
#+ however, in "globbing," it doesn't match dot-files.
If the pattern matches no file, it is expanded to itself.
To prevent this, set the nullglob option
#+ (shopt -s nullglob).
Thanks, S.C.
done
echo; echo
for file in [jx]*
do
rm -f $file # Removes only files beginning with "j" or "x" in $PWD.
echo "Removed file "$file"".
done
echo
exit 0
Omitting the in [list] part of a for loop causes the loop to
operate on $@ -- the positional parameters. A particularly
clever illustration of this is Example A-15. See also Example
15-17.
Example 11-6. Missing in [list] in a for loop
!/bin/bash
Invoke this script both with and without arguments,
for a
do
echo -n "$a "
done
The 'in list' missing, therefore the loop operates on '$@'
- (command-line argument list, including whitespace).
echo
exit 0
It is possible to use command substitution to generate the
[list] in a for loop. See also Example 16-54, Example 11-11
and Example 16-48.
Example 11-7. Generating the [list] in a for loop with command
substitution
!/bin/bash
for-loopcmd.sh: for-loop with [list]
- generated by command substitution.
NUMBERS="9 7 3 8 37.53"
for number in echo $NUMBERS # for number in 9 7 3 8 37.53
do
echo -n "$number "
done
echo
exit 0
Here is a somewhat more complex example of using command
substitution to create the [list].
Example 11-8. A grep replacement for binary files
!/bin/bash
bin-grep.sh: Locates matching strings in a binary file.
A "grep" replacement for binary files.
Similar effect to "grep -a"
E_BADARGS=65
E_NOFILE=66
if [ $# -ne 2 ]
then
echo "Usage: basename $0 search_string filename"
exit $E_BADARGS
fi
if [ ! -f "$2" ]
then
echo "File "$2" does not exist."
exit $E_NOFILE
fi
IFS=$'\012' # Per suggestion of Anton Filippov.
# was: IFS="\n"
for word in $( strings "$2" | grep "$1" )
The "strings" command lists strings in binary files.
Output then piped to "grep", which tests for desired string.
do
echo $word
done
As S.C. points out, lines 23 - 30 could be replaced with the simpler
strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'
Try something like "./bin-grep.sh mem /bin/ls"
exit 0
More of the same.
Example 11-9. Listing all users on the system
!/bin/bash
userlist.sh
PASSWORD_FILE=/etc/passwd
n=1 # User number
for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
Field separator = : ^^^^^^
Print first field ^^^^^^^^
Get input from password file /etc/passwd ^^^^^^^^^^^^^^^^^
do
echo "USER #$n = $name"
let "n += 1"
done
USER #1 = root
USER #2 = bin
USER #3 = daemon
...
USER #33 = bozo
exit $?
Discussion:
How is it that an ordinary user, or a script run by same,
- can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.)
Is this a security hole? Why or why not?
Yet another example of the [list] resulting from command
substitution.
Example 11-10. Checking all the binaries in a directory for
authorship
!/bin/bash
findstring.sh:
Find a particular string in the binaries in a specified directory.
directory=/usr/bin/
fstring="Free Software Foundation" # See which files come from the FSF.
for file in $( find $directory -type f -name '*' | sort )
do
strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
In the "sed" expression,
#+ it is necessary to substitute for the normal "/" delimiter
#+ because "/" happens to be one of the characters filtered out.
Failure to do so gives an error message. (Try it.)
done
exit $?
Exercise (easy):
Convert this script to take command-line parameters
- for $directory and $fstring.
A final example of [list] / command substitution, but this
time the "command" is a function.
generate_list ()
{
echo "one two three"
}
for word in $(generate_list) # Let "word" grab output of function.
do
echo "$word"
done
one
two
three
The output of a for loop may be piped to a command or
commands.
Example 11-11. Listing the symbolic links in a directory
!/bin/bash
symlinks.sh: Lists symbolic links in a directory.
directory=${1-pwd}
Defaults to current working directory,
- if not otherwise specified.
Equivalent to code block below.
ARGS=1 # Expect one command-line argument.
if [ $# -ne "$ARGS" ] # If not 1 arg...
then
directory=pwd # current working directory
else
directory=$1
fi
echo "symbolic links in directory "$directory""
for file in "$( find $directory -type l )" # -type l = symbolic links
do
echo "$file"
done | sort # Otherwise file list is unsorted
.
Strictly speaking, a loop isn't really necessary here,
- since the output of the "find" command is expanded into a single word.
However, it's easy to understand and illustrative this way.
As Dominik 'Aeneas' Schnitzer points out,
- failing to quote $( find $directory -type l )
- will choke on filenames with embedded whitespace.
containing whitespace.
exit 0
Jean Helou proposes the following alternative:
echo "symbolic links in directory "$directory""
Backup of the current IFS. One can never be too cautious.
OLDIFS=$IFS
IFS=:
for file in $(find $directory -type l -printf "%p$IFS")
do # ^^^^^^^^^^^^^^^^
echo "$file"
done|sort
And, James "Mike" Conley suggests modifying Helou's code thusly:
OLDIFS=$IFS
IFS='' # Null IFS means no word breaks
for file in $( find $directory -type l )
do
echo $file
done | sort
This works in the "pathological" case of a directory name having
"This also fixes the pathological case of the directory name having
- a colon (or space in earlier example) as well."
The stdout of a loop may be redirected to a file, as this
slight modification to the previous example shows.
Example 11-12. Symbolic links in a directory, saved to a file
!/bin/bash
symlinks.sh: Lists symbolic links in a directory.
OUTFILE=symlinks.list # save-file
directory=${1-pwd}
Defaults to current working directory,
- if not otherwise specified.
echo "symbolic links in directory "$directory"" > "$OUTFILE"
echo "---------------------------" >> "$OUTFILE"
for file in "$( find $directory -type l )" # -type l = symbolic links
do
echo "$file"
done | sort >> "$OUTFILE" # stdout of loop
^^^^^^^^^^^^^ redirected to save file.
echo "Output file = $OUTFILE"
exit $?
There is an alternative syntax to a for loop that will look
very familiar to C programmers. This requires double
parentheses.
Example 11-13. A C-style for loop
!/bin/bash
Multiple ways to count up to 10.
echo
Standard syntax.
for a in 1 2 3 4 5 6 7 8 9 10
do
echo -n "$a "
done
echo; echo
+==========================================+
Using "seq" ...
for a in seq 10
do
echo -n "$a "
done
echo; echo
+==========================================+
Using brace expansion ...
Bash, version 3+.
for a in {1..10}
do
echo -n "$a "
done
echo; echo
+==========================================+
Now, let's do the same, using C-like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and naked "LIMIT"
do
echo -n "$a "
done # A construct borrowed from ksh93.
echo; echo
+=========================================================================+
Let's use the C "comma operator" to increment two variables simultaneously.
for ((a=1, b=1; a <= LIMIT ; a++, b++))
do # The comma concatenates operations.
echo -n "$a-$b "
done
echo; echo
exit 0
See also Example 27-16, Example 27-17, and Example A-6.
---
Now, a for loop used in a "real-life" context.
Example 11-14. Using efax in batch mode
!/bin/bash
Faxing (must have 'efax' package installed).
EXPECTED_ARGS=2
E_BADARGS=85
MODEM_PORT="/dev/ttyS2" # May be different on your machine.
^^^^^ PCMCIA modem card default port.
if [ $# -ne $EXPECTED_ARGS ]
Check for proper number of command-line args.
then
echo "Usage: basename $0 phone# text-file"
exit $E_BADARGS
fi
if [ ! -f "$2" ]
then
echo "File $2 is not a text file."
File is not a regular file, or does not exist.
exit $E_BADARGS
fi
fax make $2 # Create fax-formatted files from text files.
for file in $(ls $2.0*) # Concatenate the converted files.
# Uses wild card (filename "globbing")
#+ in variable list.
do
fil="$fil $file"
done
efax -d "$MODEM_PORT" -t "T$1" $fil # Finally, do the work.
Trying adding -o1 if above line fails.
As S.C. points out, the for-loop can be eliminated with
efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*
- but it's not quite as instructive [grin].
exit $? # Also, efax sends diagnostic messages to stdout.
Note
The keywords do and done delineate the for-loop command block.
However, these may, in certain contexts, be omitted by framing the
command block within curly brackets
for((n=1; n<=10; n++))
No do!
{
echo -n "* $n *"
}
No done!
Outputs:
- 1 ** 2 ** 3 ** 4 ** 5 ** 6 ** 7 ** 8 ** 9 ** 10 *
And, echo $? returns 0, so Bash does not register an error.
echo
But, note that in a classic for-loop: for n in [list] ...
- a terminal semicolon is required.
for n in 1 2 3
{ echo -n "$n "; }
^
Thank you, YongYe, for pointing this out.
while
This construct tests for a condition at the top of a loop, and
keeps looping as long as that condition is true (returns a 0
exit status). In contrast to a for loop, a while loop finds
use in situations where the number of loop repetitions is not
known beforehand.
while [ condition ]
do
command(s)...
done
The bracket construct in a while loop is nothing more than our
old friend, the test brackets used in an if/then test. In
fact, a while loop can legally use the more versatile
double-brackets construct (while [[ condition ]]).
As is the case with for loops, placing the do on the same line
as the condition test requires a semicolon.
while [ condition ] ; do
Note that the test brackets are not mandatory in a while loop.
See, for example, the getopts construct.
Example 11-15. Simple while loop
!/bin/bash
var0=0
LIMIT=10
while [ "$var0" -lt "$LIMIT" ]
^ ^
Spaces, because these are "test-brackets" . . .
do
echo -n "$var0 " # -n suppresses newline.
^ Space, to separate printed out numbers.
var0=expr $var0 + 1 # var0=$(($var0+1)) also works.
# var0=$((var0 + 1)) also works.
# let "var0 += 1" also works.
done # Various other methods also work.
echo
exit 0
Example 11-16. Another while loop
!/bin/bash
echo
# Equivalent to:
while [ "$var1" != "end" ] # while test "$var1" != "end"
do
echo "Input variable #1 (end to exit) "
read var1 # Not 'read $var1' (why?).
echo "variable #1 = $var1" # Need quotes because of "#" . . .
If input is 'end', echoes it here.
Does not test for termination condition until top of loop.
echo
done
exit 0
A while loop may have multiple conditions. Only the final
condition determines when the loop terminates. This
necessitates a slightly different loop syntax, however.
Example 11-17. while loop with multiple conditions
!/bin/bash
var1=unset
previous=$var1
while echo "previous-variable = $previous"
echo
previous=$var1
[ "$var1" != end ] # Keeps track of what $var1 was previously.
# Four conditions on *while*, but only the final one controls loop.
# The *last* exit status is the one that counts.
do
echo "Input variable #1 (end to exit) "
read var1
echo "variable #1 = $var1"
done
Try to figure out how this all works.
It's a wee bit tricky.
exit 0
As with a for loop, a while loop may employ C-style syntax by
using the double-parentheses construct (see also Example 8-5).
Example 11-18. C-style syntax in a while loop
!/bin/bash
wh-loopc.sh: Count to 10 in a "while" loop.
LIMIT=10 # 10 iterations.
a=1
while [ "$a" -le $LIMIT ]
do
echo -n "$a "
let "a+=1"
done # No surprises, so far.
echo; echo
+=================================================================+
Now, we'll repeat with C-like syntax.
((a = 1)) # a=1
Double parentheses permit space when setting a variable, as in C.
while (( a <= LIMIT )) # Double parentheses,
do #+ and no "$" preceding variables.
echo -n "$a "
((a += 1)) # let "a+=1"
Yes, indeed.
Double parentheses permit incrementing a variable with C-like syntax.
done
echo
C and Java programmers can feel right at home in Bash.
exit 0
Inside its test brackets, a while loop can call a function.
t=0
condition ()
{
((t++))
if [ $t -lt 5 ]
then
return 0 # true
else
return 1 # false
fi
}
while condition
^^^^^^^^^
Function call -- four loop iterations.
do
echo "Still going: t = $t"
done
Still going: t = 1
Still going: t = 2
Still going: t = 3
Still going: t = 4
Similar to the if-test construct, a while loop can omit the test
brackets.
while condition
do
command(s) ...
done
By coupling the power of the read command with a while loop,
we get the handy while read construct, useful for reading and
parsing files.
cat $filename | # Supply input from a file.
while read line # As long as there is another line to read ...
do
...
done
=========== Snippet from "sd.sh" example script ========== #
while read value # Read one data point at a time.
do
rt=$(echo "scale=$SC; $rt + $value" | bc)
(( ct++ ))
done
am=$(echo "scale=$SC; $rt / $ct" | bc)
echo $am; return $ct # This function "returns" TWO values!
Caution: This little trick will not work if $ct > 255!
To handle a larger number of data points,
#+ simply comment out the "return $ct" above.
} <"$datafile" # Feed in data file.
Note
A while loop may have its stdin redirected to a file by a < at its
end.
A while loop may have its stdin supplied by a pipe.
until
This construct tests for a condition at the top of a loop, and
keeps looping as long as that condition is false (opposite of
while loop).
until [ condition-is-true ]
do
command(s)...
done
Note that an until loop tests for the terminating condition at
the top of the loop, differing from a similar construct in
some programming languages.
As is the case with for loops, placing the do on the same line
as the condition test requires a semicolon.
until [ condition-is-true ] ; do
Example 11-19. until loop
!/bin/bash
END_CONDITION=end
until [ "$var1" = "$END_CONDITION" ]
Tests condition here, at top of loop.
do
echo "Input variable #1 "
echo "($END_CONDITION to exit)"
read var1
echo "variable #1 = $var1"
echo
done
--- #
As with "for" and "while" loops,
- an "until" loop permits C-like test constructs.
LIMIT=10
var=0
until (( var > LIMIT ))
do # ^^ ^ ^ ^^ No brackets, no $ prefixing variables.
echo -n "$var "
(( var++ ))
done # 0 1 2 3 4 5 6 7 8 9 10
exit 0
How to choose between a for loop or a while loop or until loop? In C,
you would typically use a for loop when the number of loop iterations
is known beforehand. With Bash, however, the situation is fuzzier.
The Bash for loop is more loosely structured and more flexible than
its equivalent in other languages. Therefore, feel free to use
whatever type of loop gets the job done in the simplest way.
________________________________________________________________
11.2. Nested Loops
A nested loop is a loop within a loop, an inner loop within the body
of an outer one. How this works is that the first pass of the outer
loop triggers the inner loop, which executes to completion. Then the
second pass of the outer loop triggers the inner loop again. This
repeats until the outer loop finishes. Of course, a break within
either the inner or outer loop would interrupt this process.
Example 11-20. Nested Loop
!/bin/bash
nested-loop.sh: Nested "for" loops.
outer=1 # Set outer loop counter.
Beginning of outer loop.
for a in 1 2 3 4 5
do
echo "Pass $outer in outer loop."
echo "---------------------"
inner=1 # Reset inner loop counter.
===============================================
Beginning of inner loop.
for b in 1 2 3 4 5
do
echo "Pass $inner in inner loop."
let "inner+=1" # Increment inner loop counter.
done
End of inner loop.
===============================================
let "outer+=1" # Increment outer loop counter.
echo # Space between output blocks in pass of outer loop.
done
End of outer loop.
exit 0
See Example 27-11 for an illustration of nested while loops, and
Example 27-13 to see a while loop nested inside an until loop.
________________________________________________________________
11.3. Loop Control
Tournez cent tours, tournez mille tours,
Tournez souvent et tournez toujours . . .
--Verlaine, "Chevaux de bois"
Commands affecting loop behavior
break, continue
The break and continue loop control commands [53] correspond
exactly to their counterparts in other programming languages.
The break command terminates the loop (breaks out of it),
while continue causes a jump to the next iteration of the
loop, skipping all the remaining commands in that particular
loop cycle.
Example 11-21. Effects of break and continue in a loop
!/bin/bash
LIMIT=19 # Upper limit
echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)."
a=0
while [ $a -le "$LIMIT" ]
do
a=$(($a+1))
if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11.
then
continue # Skip rest of this particular loop iteration.
fi
echo -n "$a " # This will not execute for 3 and 11.
done
Exercise:
Why does the loop print up to 20?
echo; echo
echo Printing Numbers 1 through 20, but something happens after 2.
Same loop, but substituting 'break' for 'continue'.
a=0
while [ "$a" -le "$LIMIT" ]
do
a=$(($a+1))
if [ "$a" -gt 2 ]
then
break # Skip entire rest of loop.
fi
echo -n "$a "
done
echo; echo; echo
exit 0
The break command may optionally take a parameter. A plain
break terminates only the innermost loop in which it is
embedded, but a break N breaks out of N levels of loop.
Example 11-22. Breaking out of multiple loop levels
!/bin/bash
break-levels.sh: Breaking out of loops.
"break N" breaks out of N level loops.
for outerloop in 1 2 3 4 5
do
echo -n "Group $outerloop: "
--------------------------------------------------------
for innerloop in 1 2 3 4 5
do
echo -n "$innerloop "
if [ "$innerloop" -eq 3 ]
then
break # Try break 2 to see what happens.
# ("Breaks" out of both inner and outer loops.)
fi
done
--------------------------------------------------------
echo
done
echo
exit 0
The continue command, similar to break, optionally takes a
parameter. A plain continue cuts short the current iteration
within its loop and begins the next. A continue N terminates
all remaining iterations at its loop level and continues with
the next iteration at the loop, N levels above.
Example 11-23. Continuing at a higher loop level
!/bin/bash
The "continue N" command, continuing at the Nth level loop.
for outer in I II III IV V # outer loop
do
echo; echo -n "Group $outer: "
--------------------------------------------------------------------
for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [[ "$inner" -eq 7 && "$outer" = "III" ]]
then
continue 2 # Continue at loop on 2nd level, that is "outer loop".
# Replace above line with a simple "continue"
# to see normal loop behavior.
fi
echo -n "$inner " # 7 8 9 10 will not echo on "Group III."
done
--------------------------------------------------------------------
done
echo; echo
Exercise:
Come up with a meaningful use for "continue N" in a script.
exit 0
Example 11-24. Using continue N in an actual task
Albert Reiner gives an example of how to use "continue N":
Suppose I have a large number of jobs that need to be run, with
- any data that is to be treated in files of a given name pattern
- in a directory. There are several machines that access
- this directory, and I want to distribute the work over these
Then I usually nohup something like the following on every box:
while true
do
for n in .iso.*
do
[ "$n" = ".iso.opts" ] && continue
beta=${n#.iso.}
[ -r .Iso.$beta ] && continue
[ -r .lock.$beta ] && sleep 10 && continue
lockfile -r0 .lock.$beta || continue
echo -n "$beta: " `date`
run-isotherm $beta
date
ls -alF .Iso.$beta
[ -r .Iso.$beta ] && rm -f .lock.$beta
continue 2
done
break
done
exit 0
The details, in particular the sleep N, are particular to my
- application, but the general pattern is:
while true
do
for job in {pattern}
do
{job already done or running} && continue
{mark job as running, do job, mark job as done}
continue 2
done
break # Or something like `sleep 600' to avoid termination.
done
This way the script will stop only when there are no more jobs to do
- (including jobs that were added during runtime). Through the use
- of appropriate lockfiles it can be run on several machines
- concurrently without duplication of calculations [which run a couple
- of hours in my case, so I really want to avoid this]. Also, as search
- always starts again from the beginning, one can encode priorities in
- the file names. Of course, one could also do this without `continue 2',
- but then one would have to actually check whether or not some job
- was done (so that we should immediately look for the next job) or not
- (in which case we terminate or sleep for a long time before checking
Caution
The continue N construct is difficult to understand and tricky to use
in any meaningful context. It is probably best avoided.
________________________________________________________________
11.4. Testing and Branching
The case and select constructs are technically not loops, since they
do not iterate the execution of a code block. Like loops, however,
they direct program flow according to conditions at the top or bottom
of the block.
Controlling program flow in a code block
case (in) / esac
The case construct is the shell scripting analog to switch in
C/C++. It permits branching to one of a number of code blocks,
depending on condition tests. It serves as a kind of shorthand
for multiple if/then/else statements and is an appropriate
tool for creating menus.
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;
esac
Note
+ Quoting the variables is not mandatory, since word splitting
does not take place.
+ Each test line ends with a right paren ). [54]
+ Each condition block ends with a double semicolon ;;.
+ If a condition tests true, then the associated commands
execute and the case block terminates.
+ The entire case block ends with an esac (case spelled
backwards).
Example 11-25. Using case
!/bin/bash
Testing ranges of characters.
echo; echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[[:lower:]] ) echo "Lowercase letter";;
[[:upper:]] ) echo "Uppercase letter";;
[0-9] ) echo "Digit";;
esac # Allows ranges of characters in [square brackets],
#+ or POSIX ranges in [[double square brackets.
In the first version of this example,
- the tests for lowercase and uppercase characters were
This no longer works in certain locales and/or Linux distros.
POSIX is more portable.
Thanks to Frank Wang for pointing this out.
Exercise:
As the script stands, it accepts a single keystroke, then terminates.
Change the script so it accepts repeated input,
- reports on each keystroke, and terminates only when "X" is hit.
Hint: enclose everything in a "while" loop.
exit 0
Example 11-26. Creating menus using case
!/bin/bash
Crude address database
clear # Clear the screen.
echo " Contact List"
echo " ------- ----"
echo "Choose one of the following persons:"
echo
echo "[E]vans, Roland"
echo "[J]ones, Mildred"
echo "[S]mith, Julie"
echo "[Z]ane, Morris"
echo
read person
case "$person" in
Note variable is quoted.
"E" | "e" )
Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Flash Dr."
echo "Hardscrabble, CO 80753"
echo "(303) 734-9874"
echo "(303) 734-9892 fax"
echo "revans@zzy.net"
echo "Business partner & old friend"
;;
Note double semicolon to terminate each option.
"J" | "j" )
echo
echo "Mildred Jones"
echo "249 E. 7th St., Apt. 19"
echo "New York, NY 10009"
echo "(212) 533-2814"
echo "(212) 533-9972 fax"
echo "milliej@loisaida.com"
echo "Ex-girlfriend"
echo "Birthday: Feb. 11"
;;
Add info for Smith & Zane later.
* )
Default option.
Empty input (hitting RETURN) fits here, too.
echo
echo "Not yet in database."
;;
esac
echo
Exercise:
Change the script so it accepts multiple inputs,
- instead of terminating after displaying just one address.
exit 0
An exceptionally clever use of case involves testing for
command-line parameters.
! /bin/bash
case "$1" in
"") echo "Usage: ${0##*/} "; exit $E_PARAM;;
# No command-line parameters,
# or first parameter empty.
Note that ${0##*/} is ${var##pattern} param substitution.
# Net result is $0.
-*) FILENAME=./$1;; # If filename passed as argument ($1)
#+ starts with a dash,
#+ replace it with ./$1
#+ so further commands don't interpret it
#+ as an option.
- ) FILENAME=$1;; # Otherwise, $1.
esac
Here is a more straightforward example of command-line
parameter handling:
! /bin/bash
while [ $# -gt 0 ]; do # Until you run out of parameters . . .
case "$1" in
-d|--debug)
# "-d" or "--debug" parameter?
DEBUG=1
;;
-c|--conf)
CONFFILE="$2"
shift
if [ ! -f $CONFFILE ]; then
echo "Error: Supplied file doesn't exist!"
exit $E_CONFFILE # File not found error.
fi
;;
esac
shift # Check next set of parameters.
done
From Stefano Falsetto's "Log2Rot" script,
- part of his "rottlog" package.
Used with permission.
Example 11-27. Using command substitution to generate the case
variable
!/bin/bash
case-cmd.sh: Using command substitution to generate a "case" variable.
case $( arch ) in # $( arch ) returns machine architecture.
# Equivalent to 'uname -m' ...
i386 ) echo "80386-based machine";;
i486 ) echo "80486-based machine";;
i586 ) echo "Pentium-based machine";;
i686 ) echo "Pentium2+-based machine";;
- ) echo "Other type of machine";;
esac
exit 0
A case construct can filter strings for globbing patterns.
Example 11-28. Simple string matching
!/bin/bash
match-string.sh: Simple string matching
using a 'case' construct.
match_string ()
{ # Exact string match.
MATCH=0
E_NOMATCH=90
PARAMS=2 # Function requires 2 arguments.
E_BAD_PARAMS=91
[ $# -eq $PARAMS ] || return $E_BAD_PARAMS
case "$1" in
"$2") return $MATCH;;
esac
}
a=one
b=two
c=three
d=two
match_string $a # wrong number of parameters
echo $? # 91
match_string $a $b # no match
echo $? # 90
match_string $b $d # match
echo $? # 0
exit 0
Example 11-29. Checking for alphabetic input
!/bin/bash
isalpha.sh: Using a "case" structure to filter a string.
SUCCESS=0
FAILURE=1 # Was FAILURE=-1,
#+ but Bash no longer allows negative return value.
isalpha () # Tests whether first character of input string is alphabetic.
{
if [ -z "$1" ] # No argument passed?
then
return $FAILURE
fi
case "$1" in
[a-zA-Z]*) return $SUCCESS;; # Begins with a letter?
esac
} # Compare this with "isalpha ()" function in C.
isalpha2 () # Tests whether entire string is alphabetic.
{
[ $# -eq 1 ] || return $FAILURE
case $1 in
[!a-zA-Z]|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}
isdigit () # Tests whether entire string is numerical.
{ # In other words, tests for integer variable.
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!0-9]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}
check_var () # Front-end to isalpha ().
{
if isalpha "$@"
then
echo ""$*" begins with an alpha character."
if isalpha2 "$@"
then # No point in testing if first char is non-alpha.
echo "\"$*\" contains only alpha characters."
else
echo "\"$*\" contains at least one non-alpha character."
fi
else
echo ""$*" begins with a non-alpha character."
# Also "non-alpha" if no argument passed.
fi
echo
}
digit_check () # Front-end to isdigit ().
{
if isdigit "$@"
then
echo ""$*" contains only digits [0 - 9]."
else
echo ""$*" has at least one non-digit character."
fi
echo
}
a=23skidoo
b=H3llo
c=-What?
d=What?
e=$(echo $b) # Command substitution.
f=AbcDef
g=27234
h=27a34
i=27.34
check_var $a
check_var $b
check_var $c
check_var $d
check_var $e
check_var $f
check_var # No argument passed, so what happens?
digit_check $g
digit_check $h
digit_check $i
exit 0 # Script improved by S.C.
Exercise:
Write an 'isfloat ()' function that tests for floating point numbers.
Hint: The function duplicates 'isdigit ()',
- but adds a test for a mandatory decimal point.
select
The select construct, adopted from the Korn Shell, is yet
another tool for building menus.
select variable [in list]
do
command...
break
done
This prompts the user to enter one of the choices presented in
the variable list. Note that select uses the $PS3 prompt (#? )
by default, but this may be changed.
Example 11-30. Creating menus using select
!/bin/bash
PS3='Choose your favorite vegetable: ' # Sets the prompt string.
# Otherwise it defaults to #? .
echo
select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break # What happens if there is no 'break' here?
done
exit
Exercise:
Fix this script to accept user input not specified in
For example, if the user inputs "peas,"
- the script would respond "Sorry. That is not on the menu."
If in list is omitted, then select uses the list of command
line arguments ($@) passed to the script or the function
containing the select construct.
Compare this to the behavior of a
for variable [in list]
construct with the in list omitted.
Example 11-31. Creating menus using select in a function
!/bin/bash
PS3='Choose your favorite vegetable: '
echo
choice_of()
{
select vegetable
[in list] omitted, so 'select' uses arguments passed to function.
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break
done
}
choice_of beans rice carrots radishes rutabaga spinach
$1 $2 $3 $4 $5 $6
passed to choice_of() function
exit 0
See also Example 37-3.
________________________________________________________________
Chapter 12. Command Substitution
Command substitution reassigns the output of a command [55] or even
multiple commands; it literally plugs the command output into another
context. [56]
The classic form of command substitution uses backquotes (...).
Commands within backquotes (backticks) generate command-line text.
script_name=basename $0
echo "The name of this script is $script_name."
The output of commands can be used as arguments to another command,
to set a variable, and even for generating the argument list in a for
loop.
rm cat filename # "filename" contains a list of files to delete.
S. C. points out that "arg list too long" error might result.
Better is xargs rm -- < filename
( -- covers those cases where "filename" begins with a "-" )
textfile_listing=ls *.txt
Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
textfile_listing2=$(ls *.txt) # The alternative form of command substitution
.
echo $textfile_listing2
Same result.
A possible problem with putting a list of files into a single string
is that a newline may creep in.
A safer way to assign a list of files to a parameter is with an array.
shopt -s nullglob # If no match, filename expands to nothing.
textfile_listing=( *.txt )
Thanks, S.C.
Note
Command substitution invokes a subshell.
Caution
Command substitution may result in word splitting.
COMMAND echo a b # 2 args: a and b
COMMAND "echo a b" # 1 arg: "a b"
COMMAND echo # no arg
COMMAND "echo" # one empty arg
Thanks, S.C.
Even when there is no word splitting, command substitution can remove
trailing newlines.
cd "pwd" # This should always work.
However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "pwd" # Error message:
bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD" # Works fine.
old_tty_setting=$(stty -g) # Save old terminal setting.
echo "Hit a key "
stty -icanon -echo # Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress.
stty "$old_tty_setting" # Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variabl
e
Hit any key except RETURN, and the output is "You hit 1 key."
Hit RETURN, and it's "You hit 0 key."
The newline gets eaten in the command substitution.
Code snippet by Stéphane Chazelas.
Caution
Using echo to output an unquoted variable set with command
substitution removes trailing newlines characters from the output of
the reassigned command(s). This can cause unpleasant surprises.
dir_listing=ls -l
echo $dir_listing # unquoted
Expecting a nicely ordered directory listing.
However, what you get is:
total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
The newlines disappeared.
echo "$dir_listing" # quoted
-rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt
-rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh
-rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh
Command substitution even permits setting a variable to the contents
of a file, using either redirection or the cat command.
variable1=<file1 # Set "variable1" to contents of "file1".
variable2=cat file2 # Set "variable2" to contents of "file2".
# This, however, forks a new process,
#+ so the line of code executes slower than the above
version.
Note that the variables may contain embedded whitespace,
- or even (horrors), control characters.
It is not necessary to explicitly assign a variable.
echo " <$0" # Echoes the script itself to stdout.
Excerpts from system file, /etc/rc.d/rc.sysinit
- (on a Red Hat Linux installation)
if [ -f /fsckoptions ]; then
fsckoptions=`cat /fsckoptions`
...
fi
if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
hdmedia=`cat /proc/ide/${disk[$device]}/media`
...
fi
if [ ! -n "uname -r | grep -- "-"" ]; then
ktag="`cat /proc/version`"
...
fi
if [ $usb = "1" ]; then
sleep 5
mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Pr
ot=02"`
kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot
=> 1"`
...
fi
Caution
Do not set a variable to the contents of a long text file unless you
have a very good reason for doing so. Do not set a variable to the
contents of a binary file, even as a joke.
Example 12-1. Stupid script tricks
!/bin/bash
stupid-script-tricks.sh: Don't try this at home, folks.
From "Stupid Script Tricks," Volume I.
exit 99 ### Comment out this line if you dare.
dangerous_variable=cat /boot/vmlinuz # The compressed Linux kernel itself.
echo "string-length of $dangerous_variable = ${#dangerous_variable}"
string-length of $dangerous_variable = 794151
(Newer kernels are bigger.)
Does not give same count as 'wc -c /boot/vmlinuz'.
echo "$dangerous_variable"
Don't try this! It would hang the script.
The document author is aware of no useful applications for
- setting a variable to the contents of a binary file.
exit 0
Notice that a buffer overrun does not occur. This is one instance
where an interpreted language, such as Bash, provides more protection
from programmer mistakes than a compiled language.
Command substitution permits setting a variable to the output of a
loop. The key to this is grabbing the output of an echo command
within the loop.
Example 12-2. Generating a variable from a loop
!/bin/bash
csubloop.sh: Setting a variable to the output of a loop.
variable1=`for i in 1 2 3 4 5
do
echo -n "$i" # The 'echo' command is critical
done` #+ to command substitution here.
echo "variable1 = $variable1" # variable1 = 12345
i=0
variable2=`while [ "$i" -lt 10 ]
do
echo -n "$i" # Again, the necessary 'echo'.
let "i += 1" # Increment.
done`
echo "variable2 = $variable2" # variable2 = 0123456789
Demonstrates that it's possible to embed a loop
- inside a variable declaration.
exit 0
Command substitution makes it possible to extend the toolset
available to Bash. It is simply a matter of writing a program or
script that outputs to stdout (like a well-behaved UNIX tool should)
and assigning that output to a variable.
include <stdio.h>
/* "Hello, world." C program */
int main()
{
printf( "Hello, world.\n" );
return (0);
}
bash$ gcc -o hello hello.c
!/bin/bash
hello.sh
greeting=./hello
echo $greeting
bash$ sh hello.sh
Hello, world.
Note
The $(...) form has superseded backticks for command substitution.
output=$(sed -n /"$1"/p $file) # From "grp.sh" example.
Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)
File_contents2=$(<$file2) # Bash permits this also.
The $(...) form of command substitution treats a double backslash in
a different way than ....
bash$ echo echo \\
bash$ echo $(echo \)
\
The $(...) form of command substitution permits nesting. [57]
word_count=$( wc -w $(echo * | awk '{print $8}') )
Or, for something a bit more elaborate . . .
Example 12-3. Finding anagrams
!/bin/bash
agram2.sh
Example of nested command substitution.
Uses "anagram" utility
- that is part of the author's "yawl" word list package.
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
http://bash.deta.in/yawl-0.3.2.tar.gz
E_NOARGS=86
E_BADARG=87
MINLEN=7
if [ -z "$1" ]
then
echo "Usage $0 LETTERSET"
exit $E_NOARGS # Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
echo "Argument must have at least $MINLEN letters."
exit $E_BADARG
fi
FILTER='.......' # Must have at least 7 letters.
1234567
Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
$( $( nested command sub. ) )
( array assignment )
echo
echo "${#Anagrams[*]} 7+ letter anagrams found"
echo
echo ${Anagrams[0]} # First anagram.
echo ${Anagrams[1]} # Second anagram.
# Etc.
echo "${Anagrams[*]}" # To list all the anagrams in a single line . . .
Look ahead to the Arrays chapter for enlightenment on
See also the agram.sh script for an exercise in anagram finding.
exit $?
Examples of command substitution in shell scripts:
1. Example 11-8
2. Example 11-27
3. Example 9-16
4. Example 16-3
5. Example 16-22
6. Example 16-17
7. Example 16-54
8. Example 11-14
9. Example 11-11
- Example 16-32
- Example 20-8
- Example A-16
- Example 29-3
- Example 16-47
- Example 16-48
- Example 16-49
________________________________________________________________
Chapter 13. Arithmetic Expansion
Arithmetic expansion provides a powerful tool for performing
(integer) arithmetic operations in scripts. Translating a string into
a numerical expression is relatively straightforward using backticks,
double parentheses, or let.
Variations
Arithmetic expansion with backticks (often used in conjunction with
expr)
z=expr $z + 3 # The 'expr' command performs the expansion.
Arithmetic expansion with double parentheses, and using let
The use of backticks (backquotes) in arithmetic expansion has
been superseded by double parentheses -- ((...)) and $((...))
-- and also by the very convenient let construction.
z=$(($z+3))
z=$((z+3)) # Also correct.
# Within double parentheses,
#+ parameter dereferencing
#+ is optional.
$((EXPRESSION)) is arithmetic expansion. # Not to be confused with
#+ command substitution.
You may also use operations within double parentheses without assignment.
n=0
echo "n = $n" # n = 0
(( n += 1 )) # Increment.
(( $n += 1 )) is incorrect!
echo "n = $n" # n = 1
let z=z+3
let "z += 3" # Quotes permit the use of spaces in variable assignment.
# The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion.
Examples of arithmetic expansion in scripts:
1. Example 16-9
2. Example 11-15
3. Example 27-1
4. Example 27-11
5. Example A-16
________________________________________________________________
Chapter 14. Recess Time
This bizarre little intermission gives the reader a chance to relax
and maybe laugh a bit.
Fellow Linux user, greetings! You are reading something which
will bring you luck and good fortune. Just e-mail a copy of
this document to 10 of your friends. Before making the copies,
send a 100-line Bash script to the first person on the list
at the bottom of this letter. Then delete their name and add
yours to the bottom of the list.
Don't break the chain! Make the copies within 48 hours.
Wilfred P. of Brooklyn failed to send out his ten copies and
woke the next morning to find his job description changed
to "COBOL programmer." Howard L. of Newport News sent
out his ten copies and within a month had enough hardware
to build a 100-node Beowulf cluster dedicated to playing
Tuxracer. Amelia V. of Chicago laughed at this letter
and broke the chain. Shortly thereafter, a fire broke out
in her terminal and she now spends her days writing
documentation for MS Windows.
Don't break the chain! Send out your ten copies today!
Courtesy 'NIX "fortune cookies", with some alterations and many
apologies
Part 4. Commands
Mastering the commands on your Linux machine is an indispensable
prelude to writing effective shell scripts.
This section covers the following commands:
* . (See also source)
* ac
* adduser
* agetty
* agrep
* ar
* arch
* at
* autoload
* awk (See also Using awk for math operations)
* badblocks
* banner
* basename
* batch
* bc
* bg
* bind
* bison
* builtin
* bzgrep
* bzip2
* cal
* caller
* cat
* cd
* chattr
* chfn
* chgrp
* chkconfig
* chmod
* chown
* chroot
* cksum
* clear
* clock
* cmp
* col
* colrm
* column
* comm
* command
* compgen
* complete
* compress
* coproc
* cp
* cpio
* cron
* crypt
* csplit
* cu
* cut
* date
* dc
* dd
* debugfs
* declare
* depmod
* df
* dialog
* diff
* diff3
* diffstat
* dig
* dirname
* dirs
* disown
* dmesg
* doexec
* dos2unix
* du
* dump
* dumpe2fs
* e2fsck
* echo
* egrep
* enable
* enscript
* env
* eqn
* eval
* exec
* exit (Related topic: exit status)
* expand
* export
* expr
* factor
* false
* fdformat
* fdisk
* fg
* fgrep
* file
* find
* finger
* flex
* flock
* fmt
* fold
* free
* fsck
* ftp
* fuser
* getfacl
* getopt
* getopts
* gettext
* getty
* gnome-mount
* grep
* groff
* groupmod
* groups (Related topic: the $GROUPS variable)
* gs
* gzip
* halt
* hash
* hdparm
* head
* help
* hexdump
* host
* hostid
* hostname (Related topic: the $HOSTNAME variable)
* hwclock
* iconv
* id (Related topic: the $UID variable)
* ifconfig
* info
* infocmp
* init
* insmod
* install
* ip
* ipcalc
* iptables
* iwconfig
* jobs
* join
* jot
* kill
* killall
* last
* lastcomm
* lastlog
* ldd
* less
* let
* lex
* lid
* ln
* locate
* lockfile
* logger
* logname
* logout
* logrotate
* look
* losetup
* lp
* ls
* lsdev
* lsmod
* lsof
* lspci
* lsusb
* ltrace
* lynx
* lzcat
* lzma
* m4
* mail
* mailstats
* mailto
* make
* MAKEDEV
* man
* mapfile
* mcookie
* md5sum
* merge
* mesg
* mimencode
* mkbootdisk
* mkdir
* mkdosfs
* mke2fs
* mkfifo
* mkisofs
* mknod
* mkswap
* mktemp
* mmencode
* modinfo
* modprobe
* more
* mount
* msgfmt
* mv
* nc
* netconfig
* netstat
* newgrp
* nice
* nl
* nm
* nmap
* nohup
* nslookup
* objdump
* od
* openssl
* passwd
* paste
* patch (Related topic: diff)
* pathchk
* pax
* pgrep
* pidof
* ping
* pkill
* popd
* pr
* printenv
* printf
* procinfo
* ps
* pstree
* ptx
* pushd
* pwd (Related topic: the $PWD variable)
* quota
* rcp
* rdev
* rdist
* read
* readelf
* readlink
* readonly
* reboot
* recode
* renice
* reset
* resize
* restore
* rev
* rlogin
* rm
* rmdir
* rmmod
* route
* rpm
* rpm2cpio
* rsh
* rsync
* runlevel
* run-parts
* rx
* rz
* sar
* scp
* script
* sdiff
* sed
* seq
* service
* set
* setfacl
* setquota
* setserial
* setterm
* sha1sum
* shar
* shopt
* shred
* shutdown
* size
* skill
* sleep
* slocate
* snice
* sort
* source
* sox
* split
* sq
* ssh
* stat
* strace
* strings
* strip
* stty
* su
* sudo
* sum
* suspend
* swapoff
* swapon
* sx
* sync
* sz
* tac
* tail
* tar
* tbl
* tcpdump
* tee
* telinit
* telnet
* Tex
* texexec
* time
* times
* tmpwatch
* top
* touch
* tput
* tr
* traceroute
* true
* tset
* tsort
* tty
* tune2fs
* type
* typeset
* ulimit
* umask
* umount
* uname
* unarc
* unarj
* uncompress
* unexpand
* uniq
* units
* unlzma
* unrar
* unset
* unsq
* unzip
* uptime
* usbmodules
* useradd
* userdel
* usermod
* users
* usleep
* uucp
* uudecode
* uuencode
* uux
* vacation
* vdir
* vmstat
* vrfy
* w
* wait
* wall
* watch
* wc
* wget
* whatis
* whereis
* which
* who
* whoami
* whois
* write
* xargs
* xrandr
* xz
* yacc
* yes
* zcat
* zdiff
* zdump
* zegrep
* zfgrep
* zgrep
* zip
Table of Contents
- Internal Commands and Builtins
15.1. Job Control Commands
- External Filters, Programs and Commands
16.1. Basic Commands
16.2. Complex Commands
16.3. Time / Date Commands
16.4. Text Processing Commands
16.5. File and Archiving Commands
16.6. Communications Commands
16.7. Terminal Control Commands
16.8. Math Commands
16.9. Miscellaneous Commands
- System and Administrative Commands
17.1. Analyzing a System Script
________________________________________________________________
Chapter 15. Internal Commands and Builtins
A builtin is a command contained within the Bash tool set, literally
built in. This is either for performance reasons -- builtins execute
faster than external commands, which usually require forking off [58]
a separate process -- or because a particular builtin needs direct
access to the shell internals.
When a command or the shell itself initiates (or spawns) a new
subprocess to carry out a task, this is called forking. This new
process is the child, and the process that forked it off is the
parent. While the child process is doing its work, the parent process
is still executing.
Note that while a parent process gets the process ID of the child
process, and can thus pass arguments to it, the reverse is not true.
This can create problems that are subtle and hard to track down.
Example 15-1. A script that spawns multiple instances of itself
!/bin/bash
spawn.sh
PIDS=$(pidof sh $0) # Process IDs of the various instances of this script.
P_array=( $PIDS ) # Put them in an array (why?).
echo $PIDS # Show process IDs of parent and child processes.
let "instances = ${#P_array[*]} - 1" # Count elements, less 1.
# Why subtract 1?
echo "$instances instance(s) of this script running."
echo "[Hit Ctl-C to exit.]"; echo
sleep 1 # Wait.
sh $0 # Play it again, Sam.
exit 0 # Not necessary; script will never get to here.
# Why not?
After exiting with a Ctl-C,
- do all the spawned instances of the script die?
If so, why?
Note:
Be careful not to run this script too long.
It will eventually eat up too many system resources.
Is having a script spawn multiple instances of itself
- an advisable scripting technique.
Why or why not?
Generally, a Bash builtin does not fork a subprocess when it executes
within a script. An external system command or filter in a script
usually will fork a subprocess.
A builtin may be a synonym to a system command of the same name, but
Bash reimplements it internally. For example, the Bash echo command
is not the same as /bin/echo, although their behavior is almost
identical.
!/bin/bash
echo "This line uses the "echo" builtin."
/bin/echo "This line uses the /bin/echo system command."
A keyword is a reserved word, token or operator. Keywords have a
special meaning to the shell, and indeed are the building blocks of
the shell's syntax. As examples, for, while, do, and ! are keywords.
Similar to a builtin, a keyword is hard-coded into Bash, but unlike a
builtin, a keyword is not in itself a command, but a subunit of a
command construct. [59]
I/O
echo
prints (to stdout) an expression or variable (see Example
4-1).
echo Hello
echo $a
An echo requires the -e option to print escaped characters.
See Example 5-2.
Normally, each echo command prints a terminal newline, but the
-n option suppresses this.
Note
An echo can be used to feed a sequence of commands down a pipe.
if echo "$VAR" | grep -q txt # if [[ $VAR = txt ]]
then
echo "$VAR contains the substring sequence "txt""
fi
Note
An echo, in combination with command substitution can set a variable.
a=echo "HELLO" | tr A-Z a-z
See also Example 16-22, Example 16-3, Example 16-47, and Example
16-48.
Be aware that echo `command` deletes any linefeeds that the
output of command generates.
The $IFS (internal field separator) variable normally contains
\n (linefeed) as one of its set of whitespace characters. Bash
therefore splits the output of command at linefeeds into
arguments to echo. Then echo outputs these arguments,
separated by spaces.
bash$ ls -l /usr/share/apps/kjezz/sounds
-rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au
-rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au
bash$ echo ls -l /usr/share/apps/kjezz/sounds
total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root ro
ot ...
So, how can we embed a linefeed within an echoed character
string?
Embedding a linefeed?
echo "Why doesn't this string \n split on two lines?"
Doesn't split.
Let's try something else.
echo
echo $"A line of text containing
a linefeed."
Prints as two distinct lines (embedded linefeed).
But, is the "$" variable prefix really necessary?
echo
echo "This string splits
on two lines."
No, the "$" is not needed.
echo
echo "---------------"
echo
echo -n $"Another line of text containing
a linefeed."
Prints as two distinct lines (embedded linefeed).
Even the -n option fails to suppress the linefeed here.
echo
echo
echo "---------------"
echo
echo
However, the following doesn't work as expected.
Why not? Hint: Assignment to a variable.
string1=$"Yet another line of text containing
a linefeed (maybe)."
echo $string1
Yet another line of text containing a linefeed (maybe).
^
Linefeed becomes a space.
Thanks, Steve Parker, for pointing this out.
Note
This command is a shell builtin, and not the same as /bin/echo,
although its behavior is similar.
bash$ type -a echo
echo is a shell builtin
echo is /bin/echo
printf
The printf, formatted print, command is an enhanced echo. It
is a limited variant of the C language printf() library
function, and its syntax is somewhat different.
printf format-string... parameter...
This is the Bash builtin version of the /bin/printf or
/usr/bin/printf command. See the printf manpage (of the system
command) for in-depth coverage.
Caution
Older versions of Bash may not support printf.
Example 15-2. printf in action
!/bin/bash
printf demo
declare -r PI=3.14159265358979 # Read-only variable, i.e., a constant.
declare -r DecimalConstant=31373
Message1="Greetings,"
Message2="Earthling."
echo
printf "Pi to 2 decimal places = %1.2f" $PI
echo
printf "Pi to 9 decimal places = %1.9f" $PI # It even rounds off correctly.
printf "\n" # Prints a line feed,
# Equivalent to 'echo' . . .
printf "Constant = \t%d\n" $DecimalConstant # Inserts tab (\t).
printf "%s %s \n" $Message1 $Message2
echo
==========================================#
Simulation of C function, sprintf().
Loading a variable with a formatted string.
echo
Pi12=$(printf "%1.12f" $PI)
echo "Pi to 12 decimal places = $Pi12" # Roundoff error!
Msg=printf "%s %s \n" $Message1 $Message2
echo $Msg; echo $Msg
As it happens, the 'sprintf' function can now be accessed
- as a loadable module to Bash,
- but this is not portable.
exit 0
Formatting error messages is a useful application of printf
E_BADDIR=85
var=nonexistent_directory
error()
{
printf "$@" >&2
Formats positional params passed, and sends them to stderr.
echo
exit $E_BADDIR
}
cd $var || error $"Can't cd to %s." "$var"
Thanks, S.C.
See also Example 36-17.
read
"Reads" the value of a variable from stdin, that is,
interactively fetches input from the keyboard. The -a option
lets read get array variables (see Example 27-6).
Example 15-3. Variable assignment, using read
!/bin/bash
"Reading" variables.
echo -n "Enter the value of variable 'var1': "
The -n option to echo suppresses newline.
read var1
Note no '$' in front of var1, since it is being set.
echo "var1 = $var1"
echo
A single 'read' statement can set multiple variables.
echo -n "Enter the values of variables 'var2' and 'var3' "
echo =n "(separated by a space or tab): "
read var2 var3
echo "var2 = $var2 var3 = $var3"
If you input only one value,
- the other variable(s) will remain unset (null).
exit 0
A read without an associated variable assigns its input to the
dedicated variable $REPLY.
Example 15-4. What happens when read has no variable
!/bin/bash
read-novar.sh
echo
-------------------------- #
echo -n "Enter a value: "
read var
echo ""var" = "$var""
Everything as expected here.
-------------------------- #
echo
------------------------------------------------------------------- #
echo -n "Enter another value: "
read # No variable supplied for 'read', therefore...
#+ Input to 'read' assigned to default variable, $REPLY.
var="$REPLY"
echo ""var" = "$var""
This is equivalent to the first code block.
------------------------------------------------------------------- #
echo
echo "========================="
echo
This example is similar to the "reply.sh" script.
However, this one shows that $REPLY is available
- even after a 'read' to a variable in the conventional way.
================================================================= #
In some instances, you might wish to discard the first value read.
In such cases, simply ignore the $REPLY variable.
{ # Code block.
read # Line 1, to be discarded.
read line2 # Line 2, saved in variable.
} <$0
echo "Line 2 of this script is:"
echo "$line2" # # read-novar.sh
echo # #!/bin/bash line discarded.
See also the soundcard-on.sh script.
exit 0
Normally, inputting a \ suppresses a newline during input to a
read. The -r option causes an inputted \ to be interpreted
literally.
Example 15-5. Multi-line input to read
!/bin/bash
echo
echo "Enter a string terminated by a \, then press ."
echo "Then, enter a second string (no \ this time), and again press ."
read var1 # The "" suppresses the newline, when reading $var1.
# first line \
# second line
echo "var1 = $var1"
var1 = first line second line
For each line terminated by a ""
- you get a prompt on the next line to continue feeding characters into var1.
echo; echo
echo "Enter another string terminated by a \ , then press ."
read -r var2 # The -r option causes the "" to be read literally.
# first line \
echo "var2 = $var2"
var2 = first line \
Data entry terminates with the first .
echo
exit 0
The read command has some interesting options that permit
echoing a prompt and even reading keystrokes without hitting
ENTER.
Read a keypress without hitting ENTER.
read -s -n1 -p "Hit a key " keypress
echo; echo "Keypress was ""$keypress""."
-s option means do not echo input.
-n N option means accept only N characters of input.
-p option means echo the following prompt before reading input.
Using these options is tricky, since they need to be in the correct order.
The -n option to read also allows detection of the arrow keys
and certain of the other unusual keys.
Example 15-6. Detecting the arrow keys
!/bin/bash
arrow-detect.sh: Detects the arrow keys, and a few more.
Thank you, Sandro Magi, for showing me how.
Character codes generated by the keypresses.
arrowup='[A'
arrowdown='[B'
arrowrt='[C'
arrowleft='[D'
insert='[2'
delete='[3'
SUCCESS=0
OTHER=65
echo -n "Press a key... "
May need to also press ENTER if a key not listed above pressed.
read -n3 key # Read 3 characters.
echo -n "$key" | grep "$arrowup" #Check if character code detected.
if [ "$?" -eq $SUCCESS ]
then
echo "Up-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowdown"
if [ "$?" -eq $SUCCESS ]
then
echo "Down-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowrt"
if [ "$?" -eq $SUCCESS ]
then
echo "Right-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowleft"
if [ "$?" -eq $SUCCESS ]
then
echo "Left-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$insert"
if [ "$?" -eq $SUCCESS ]
then
echo ""Insert" key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$delete"
if [ "$?" -eq $SUCCESS ]
then
echo ""Delete" key pressed."
exit $SUCCESS
fi
echo " Some other key pressed."
exit $OTHER
========================================= #
Mark Alexander came up with a simplified
- version of the above script (Thank you!).
It eliminates the need for grep.
!/bin/bash
uparrow=$'\x1b[A'
downarrow=$'\x1b[B'
leftarrow=$'\x1b[D'
rightarrow=$'\x1b[C'
read -s -n3 -p "Hit an arrow key: " x
case "$x" in
$uparrow)
echo "You pressed up-arrow"
;;
$downarrow)
echo "You pressed down-arrow"
;;
$leftarrow)
echo "You pressed left-arrow"
;;
$rightarrow)
echo "You pressed right-arrow"
;;
esac
exit $?
========================================= #
Antonio Macchi has a simpler alternative.
!/bin/bash
while true
do
read -sn1 a
test "$a" == echo -en "\e" || continue
read -sn1 a
test "$a" == "[" || continue
read -sn1 a
case "$a" in
A) echo "up";;
B) echo "down";;
C) echo "right";;
D) echo "left";;
esac
done
========================================= #
Exercise:
- Add detection of the "Home," "End," "PgUp," and "PgDn" keys.
Note
The -n option to read will not detect the ENTER (newline) key.
The -t option to read permits timed input (see Example 9-4 and
Example A-41).
The -u option takes the file descriptor of the target file.
The read command may also "read" its variable value from a
file redirected to stdin. If the file contains more than one
line, only the first line is assigned to the variable. If read
has more than one parameter, then each of these variables gets
assigned a successive whitespace-delineated string. Caution!
Example 15-7. Using read with file redirection
!/bin/bash
read var1 <data-file
echo "var1 = $var1"
var1 set to the entire first line of the input file "data-file"
read var2 var3 <data-file
echo "var2 = $var2 var3 = $var3"
Note non-intuitive behavior of "read" here.
- Rewinds back to the beginning of input file.
- Each variable is now set to a corresponding string,
separated by whitespace, rather than to an entire line of text.
- The final variable gets the remainder of the line.
- If there are more variables to be set than whitespace-terminated strings
on the first line of the file, then the excess variables remain empty.
echo "------------------------------------------------"
How to resolve the above problem with a loop:
while read line
do
echo "$line"
done <data-file
Thanks, Heiner Steven for pointing this out.
echo "------------------------------------------------"
Use $IFS (Internal Field Separator variable) to split a line of input to
"read", if you do not want the default to be whitespace.
echo "List of all users:"
OIFS=$IFS; IFS=: # /etc/passwd uses ":" for field separator.
while read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
IFS=$OIFS # Restore original $IFS.
This code snippet also by Heiner Steven.
Setting the $IFS variable within the loop itself
- eliminates the need for storing the original $IFS
Thanks, Dim Segebart, for pointing this out.
echo "------------------------------------------------"
echo "List of all users:"
while IFS=: read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
echo
echo "$IFS still $IFS"
exit 0
Note
Piping output to a read, using echo to set variables will fail.
Yet, piping the output of cat seems to work.
cat file1 file2 |
while read line
do
echo $line
done
However, as Bjön Eriksson shows:
Example 15-8. Problems reading from a pipe
!/bin/sh
readpipe.sh
This example contributed by Bjon Eriksson.
shopt -s lastpipe
last="(null)"
cat $0 |
while read line
do
echo "{$line}"
last=$line
done
echo
echo "++++++++++++++++++++++"
printf "\nAll done, last: $last\n" # The output of this line
#+ changes if you uncomment line 5.
# (Bash, version -ge 4.2 required.)
exit 0 # End of code.
# (Partial) output of script follows.
# The 'echo' supplies extra brackets.
./readpipe.sh
{#!/bin/sh}
{last="(null)"}
{cat $0 |}
{while read line}
{do}
{echo "{$line}"}
{last=$line}
{done}
{printf "nAll done, last: $lastn"}
All done, last: (null)
The variable (last) is set within the loop/subshell
but its value does not persist outside the loop.
The gendiff script, usually found in /usr/bin on many Linux distros,
pipes the output of find to a while read construct.
find $1 ( -name "$2" -o -name ".$2" ) -print |
while read f; do
. . .
Tip
It is possible to paste text into the input field of a read (but not
multiple lines!). See Example A-38.
Filesystem
cd
The familiar cd change directory command finds use in scripts
where execution of a command requires being in a specified
directory.
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
[from the previously cited example by Alan Cox]
The -P (physical) option to cd causes it to ignore symbolic
links.
cd - changes to $OLDPWD, the previous working directory.
Caution
The cd command does not function as expected when presented with two
forward slashes.
bash$ cd //
bash$ pwd
//
The output should, of course, be /. This is a problem both from the
command-line and in a script.
pwd
Print Working Directory. This gives the user's (or script's)
current directory (see Example 15-9). The effect is identical
to reading the value of the builtin variable $PWD.
pushd, popd, dirs
This command set is a mechanism for bookmarking working
directories, a means of moving back and forth through
directories in an orderly manner. A pushdown stack is used to
keep track of directory names. Options allow various
manipulations of the directory stack.
pushd dir-name pushes the path dir-name onto the directory
stack (to the top of the stack) and simultaneously changes the
current working directory to dir-name
popd removes (pops) the top directory path name off the
directory stack and simultaneously changes the current working
directory to the directory now at the top of the stack.
dirs lists the contents of the directory stack (compare this
with the $DIRSTACK variable). A successful pushd or popd will
automatically invoke dirs.
Scripts that require various changes to the current working
directory without hard-coding the directory name changes can
make good use of these commands. Note that the implicit
$DIRSTACK array variable, accessible from within a script,
holds the contents of the directory stack.
Example 15-9. Changing the current working directory
!/bin/bash
dir1=/usr/local
dir2=/var/spool
pushd $dir1
Will do an automatic 'dirs' (list directory stack to stdout).
echo "Now in directory pwd." # Uses back-quoted 'pwd'.
Now, do some stuff in directory 'dir1'.
pushd $dir2
echo "Now in directory pwd."
Now, do some stuff in directory 'dir2'.
echo "The top entry in the DIRSTACK array is $DIRSTACK."
popd
echo "Now back in directory pwd."
Now, do some more stuff in directory 'dir1'.
popd
echo "Now back in original working directory pwd."
exit 0
What happens if you don't 'popd' -- then exit the script?
Which directory do you end up in? Why?
Variables
let
The let command carries out arithmetic operations on
variables. [60] In many cases, it functions as a less complex
version of expr.
Example 15-10. Letting let do arithmetic.
!/bin/bash
echo
let a=11 # Same as 'a=11'
let a=a+5 # Equivalent to let "a = a + 5"
# (Double quotes and spaces make it more readable.)
echo "11 + 5 = $a" # 16
let "a <<= 3" # Equivalent to let "a = a << 3"
echo ""$a" (=16) left-shifted 3 places = $a"
# 128
let "a /= 4" # Equivalent to let "a = a / 4"
echo "128 / 4 = $a" # 32
let "a -= 5" # Equivalent to let "a = a - 5"
echo "32 - 5 = $a" # 27
let "a *= 10" # Equivalent to let "a = a * 10"
echo "27 * 10 = $a" # 270
let "a %= 8" # Equivalent to let "a = a % 8"
echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"
# 6
Does "let" permit C-style operators?
Yes, just as the (( ... )) double-parentheses construct does.
let a++ # C-style (post) increment.
echo "6++ = $a" # 6++ = 7
let a-- # C-style decrement.
echo "7-- = $a" # 7-- = 6
Of course, ++a, etc., also allowed . . .
echo
Trinary operator.
Note that $a is 6, see above.
let "t = a<7?7:11" # True
echo $t # 7
let a++
let "t = a<7?7:11" # False
echo $t # 11
exit
Caution
The let command can, in certain contexts, return a surprising exit
status.
Evgeniy Ivanov points out:
var=0
echo $? # 0
# As expected.
let var++
echo $? # 1
# The command was successful, so why isn't $?=0 ???
# Anomaly!
let var++
echo $? # 0
# As expected.
Likewise . . .
let var=0
echo $? # 1
# The command was successful, so why isn't $?=0 ???
However, as Jeff Gorak points out,
- this is part of the design spec for 'let' . . .
"If the last ARG evaluates to 0, let returns 1;
let returns 0 otherwise." ['help let']
eval
eval arg1 [arg2] ... [argN]
Combines the arguments in an expression or list of expressions
and evaluates them. Any variables within the expression are
expanded. The net result is to convert a string into a
command.
Tip
The eval command can be used for code generation from the
command-line or within a script.
bash$ command_string="ps ax"
bash$ process="ps ax"
bash$ eval "$command_string" | grep "$process"
26973 pts/3 R+ 0:00 grep --color ps ax
26974 pts/3 R+ 0:00 ps ax
Each invocation of eval forces a re-evaluation of its
arguments.
a='$b'
b='$c'
c=d
echo $a # $b
# First level.
eval echo $a # $c
# Second level.
eval eval echo $a # d
# Third level.
Thank you, E. Choroba.
Example 15-11. Showing the effect of eval
!/bin/bash
Exercising "eval" ...
y=eval ls -l # Similar to y=ls -l
echo $y #+ but linefeeds removed because "echoed" variable is unquoted
.
echo
echo "$y" # Linefeeds preserved when variable is quoted.
echo; echo
y=eval df # Similar to y=df
echo $y #+ but linefeeds removed.
When LF's not preserved, it may make it easier to parse output,
- using utilities such as "awk".
echo
echo "==========================================================="
echo
eval "seq 3 | sed -e 's/.*/echo var&=ABCDEFGHIJ/'"
var1=ABCDEFGHIJ
var2=ABCDEFGHIJ
var3=ABCDEFGHIJ
echo
echo "==========================================================="
echo
Now, showing how to do something useful with "eval" . . .
(Thank you, E. Choroba!)
version=3.4 # Can we split the version into major and minor
#+ part in one command?
echo "version = $version"
eval major=${version/./;minor=} # Replaces '.' in version by ';minor='
# The substitution yields '3; minor=4'
#+ so eval does minor=4, major=3
echo Major: $major, minor: $minor # Major: 3, minor: 4
Example 15-12. Using eval to select among variables
!/bin/bash
arr-choice.sh
Passing arguments to a function to select
- one particular variable out of a group.
arr0=( 10 11 12 13 14 15 )
arr1=( 20 21 22 23 24 25 )
arr2=( 30 31 32 33 34 35 )
0 1 2 3 4 5 Element number (zero-indexed)
choose_array ()
{
eval array_member=${arr${array_number}[element_number]}
^ ^^^^^^^^^^^^
Using eval to construct the name of a variable,
#+ in this particular case, an array name.
echo "Element $element_number of array $array_number is $array_member"
} # Function can be rewritten to take parameters.
array_number=0 # First array.
element_number=3
choose_array # 13
array_number=2 # Third array.
element_number=4
choose_array # 34
array_number=3 # Null array (arr3 not allocated).
element_number=4
choose_array # (null)
Thank you, Antonio Macchi, for pointing this out.
Example 15-13. Echoing the command-line parameters
!/bin/bash
echo-params.sh
Call this script with a few command-line parameters.
For example:
sh echo-params.sh first second third fourth fifth
params=$# # Number of command-line parameters.
param=1 # Start at first command-line param.
while [ "$param" -le "$params" ]
do
echo -n "Command-line parameter "
echo -n $$param # Gives only the name of variable.
^^^ # $1, $2, $3, etc.
# Why?
# \$ escapes the first "$"
#+ so it echoes literally,
#+ and $param dereferences "$param" . . .
#+ . . . as expected.
echo -n " = "
eval echo $$param # Gives the value of variable.
^^^^ ^^^ # The "eval" forces the evaluation
#+ of \$$
#+ as an indirect variable reference.
(( param ++ )) # On to the next.
done
exit $?
=================================================
$ sh echo-params.sh first second third fourth fifth
Command-line parameter $1 = first
Command-line parameter $2 = second
Command-line parameter $3 = third
Command-line parameter $4 = fourth
Command-line parameter $5 = fifth
Example 15-14. Forcing a log-off
!/bin/bash
Killing ppp to force a log-off.
For dialup connection, of course.
Script should be run as root user.
SERPORT=ttyS3
Depending on the hardware and even the kernel version,
- the modem port on your machine may be different --
- /dev/ttyS1 or /dev/ttyS2.
killppp="eval kill -9 ps ax | awk '/ppp/ { print $1 }'"
-------- process ID of ppp -------
$killppp # This variable is now a command.
The following operations must be done as root user.
chmod 666 /dev/$SERPORT # Restore r+w permissions, or else what?
Since doing a SIGKILL on ppp changed the permissions on the serial port,
- we restore permissions to previous state.
rm /var/lock/LCK..$SERPORT # Remove the serial port lock file. Why?
exit $?
Exercises:
- Have script check whether root user is invoking it.
- Do a check on whether the process to be killed
- is actually running before attempting to kill it.
- Write an alternate version of this script based on 'fuser':
Example 15-15. A version of rot13
!/bin/bash
A version of "rot13" using 'eval'.
Compare to "rot13.sh" example.
setvar_rot_13() # "rot13" scrambling
{
local varname=$1 varvalue=$2
eval $varname='$(echo "$varvalue" | tr a-z n-za-m)'
}
setvar_rot_13 var "foobar" # Run "foobar" through rot13.
echo $var # sbbone
setvar_rot_13 var "$var" # Run "sbbone" through rot13.
# Back to original variable.
echo $var # foobar
This example by Stephane Chazelas.
Modified by document author.
exit 0
Here is another example of using eval to evaluate a complex
expression, this one from an earlier version of YongYe's
Tetris game script.
eval ${1}+="${x} ${y} "
Example A-53 uses eval to convert array elements into a
command list.
The eval command occurs in the older version of indirect
referencing.
eval var=$$var
Tip
The eval command can be used to parameterize brace expansion.
Caution
The eval command can be risky, and normally should be avoided when
there exists a reasonable alternative. An eval $COMMANDS executes the
contents of COMMANDS, which may contain such unpleasant surprises as
rm -rf *. Running an eval on unfamiliar code written by persons
unknown is living dangerously.
set
The set command changes the value of internal script
variables/options. One use for this is to toggle option flags
which help determine the behavior of the script. Another
application for it is to reset the positional parameters that
a script sees as the result of a command (set `command`). The
script can then parse the fields of the command output.
Example 15-16. Using set with positional parameters
!/bin/bash
ex34.sh
Script "set-test"
Invoke this script with three command-line parameters,
for example, "sh ex34.sh one two three".
echo
echo "Positional parameters before set `uname -a` :"
echo "Command-line argument #1 = $1"
echo "Command-line argument #2 = $2"
echo "Command-line argument #3 = $3"
set uname -a # Sets the positional parameters to the output
# of the command `uname -a`
echo
echo +++++
echo $_ # +++++
Flags set in script.
echo $- # hB
Anomalous behavior?
echo
echo "Positional parameters after set `uname -a` :"
$1, $2, $3, etc. reinitialized to result of uname -a
echo "Field #1 of 'uname -a' = $1"
echo "Field #2 of 'uname -a' = $2"
echo "Field #3 of 'uname -a' = $3"
echo ###
echo $_ # ###
echo
exit 0
More fun with positional parameters.
Example 15-17. Reversing the positional parameters
!/bin/bash
revposparams.sh: Reverse positional parameters.
Script by Dan Jacobson, with stylistic revisions by document author.
set a\ b c d\ e;
^ ^ Spaces escaped
^ ^ Spaces not escaped
OIFS=$IFS; IFS=:;
^ Saving old IFS and setting new one.
echo
until [ $# -eq 0 ]
do # Step through positional parameters.
echo "### k0 = "$k"" # Before
k=$1:$k; # Append each pos param to loop variable.
^
echo "### k = "$k"" # After
echo
shift;
done
set $k # Set new positional parameters.
echo -
echo $# # Count of positional parameters.
echo -
echo
for i # Omitting the "in list" sets the variable -- i --
#+ to the positional parameters.
do
echo $i # Display new positional parameters.
done
IFS=$OIFS # Restore IFS.
Question:
Is it necessary to set an new IFS, internal field separator,
- in order for this script to work properly?
What happens if you don't? Try it.
And, why use the new IFS -- a colon -- in line 17,
- to append to the loop variable?
What is the purpose of this?
exit 0
$ ./revposparams.sh
k0 =
k = a b
k0 = a b
k = c a b
k0 = c a b
k = d e c a b
3
d e
c
a b
Invoking set without any options or arguments simply lists all
the environmental and other variables that have been
initialized.
bash$ set
AUTHORCOPY=/home/bozo/posts
BASH=/bin/bash
BASH_VERSION=$'2.05.8(1)-release'
...
XAUTHORITY=/home/bozo/.Xauthority
_=/etc/bashrc
variable22=abc
variable23=xzy
Using set with the -- option explicitly assigns the contents
of a variable to the positional parameters. If no variable
follows the -- it unsets the positional parameters.
Example 15-18. Reassigning the positional parameters
!/bin/bash
variable="one two three four five"
set -- $variable
Sets positional parameters to the contents of "$variable".
first_param=$1
second_param=$2
shift; shift # Shift past first two positional params.
shift 2 also works.
remaining_params="$*"
echo
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
echo "remaining parameters = $remaining_params" # three four five
echo; echo
Again.
set -- $variable
first_param=$1
second_param=$2
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
======================================================
set --
Unsets positional parameters if no variable specified.
first_param=$1
second_param=$2
echo "first parameter = $first_param" # (null value)
echo "second parameter = $second_param" # (null value)
exit 0
See also Example 11-2 and Example 16-56.
unset
The unset command deletes a shell variable, effectively
setting it to null. Note that this command does not affect
positional parameters.
bash$ unset PATH
bash$ echo $PATH
bash$
Example 15-19. "Unsetting" a variable
!/bin/bash
unset.sh: Unsetting a variable.
variable=hello # Initialized.
echo "variable = $variable"
unset variable # Unset.
# In this particular context,
#+ same effect as: variable=
echo "(unset) variable = $variable" # $variable is null.
if [ -z "$variable" ] # Try a string-length test.
then
echo "$variable has zero length."
fi
exit 0
Note
In most contexts, an undeclared variable and one that has been unset
are equivalent. However, the ${parameter:-default} parameter
substitution construct can distinguish between the two.
export
The export [61] command makes available variables to all child
processes of the running script or shell. One important use of
the export command is in startup files, to initialize and make
accessible environmental variables to subsequent user
processes.
Caution
Unfortunately, there is no way to export variables back to the parent
process, to the process that called or invoked the script or shell.
Example 15-20. Using export to pass a variable to an embedded
awk script
!/bin/bash
Yet another version of the "column totaler" script (col-totaler.sh)
- that adds up a specified column (of numbers) in the target file.
This uses the environment to pass a script variable to 'awk' . . .
- and places the awk script in a variable.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: basename $0 filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
===== Same as original script, up to this point =====#
export column_number
Export column number to environment, so it's available for retrieval.
awkscript='{ total += $ENVIRON["column_number"] }
END { print total }'
Yes, a variable can hold an awk script.
Now, run the awk script.
awk "$awkscript" "$filename"
Thanks, Stephane Chazelas.
exit 0
Tip
It is possible to initialize and export variables in the same
operation, as in export var1=xxx.
However, as Greg Keraunen points out, in certain situations this may
have a different effect than setting a variable, then exporting it.
bash$ export var=(a b); echo ${var[0]}
(a b)
bash$ var=(a b); export var; echo ${var[0]}
a
Note
A variable to be exported may require special treatment. See Example
M-2.
declare, typeset
The declare and typeset commands specify and/or restrict
properties of variables.
readonly
Same as declare -r, sets a variable as read-only, or, in
effect, as a constant. Attempts to change the variable fail
with an error message. This is the shell analog of the C
language const type qualifier.
getopts
This powerful tool parses command-line arguments passed to the
script. This is the Bash analog of the getopt external command
and the getopt library function familiar to C programmers. It
permits passing and concatenating multiple options [62] and
associated arguments to a script (for example scriptname -abc
-e /usr/local).
The getopts construct uses two implicit variables. $OPTIND is
the argument pointer (OPTion INDex) and $OPTARG (OPTion
ARGument) the (optional) argument attached to an option. A
colon following the option name in the declaration tags that
option as having an associated argument.
A getopts construct usually comes packaged in a while loop,
which processes the options and arguments one at a time, then
increments the implicit $OPTIND variable to point to the next.
Note
1. The arguments passed from the command-line to the script
must be preceded by a dash (-). It is the prefixed - that
lets getopts recognize command-line arguments as options. In
fact, getopts will not process arguments without the
prefixed -, and will terminate option processing at the
first argument encountered lacking them.
2. The getopts template differs slightly from the standard
while loop, in that it lacks condition brackets.
3. The getopts construct is a highly functional replacement for
the traditional getopt external command.
while getopts ":abcde:fg" Option
Initial declaration.
a, b, c, d, e, f, and g are the options (flags) expected.
The : after option 'e' shows it will have an argument passed with it.
do
case $Option in
a ) # Do something with variable 'a'.
b ) # Do something with variable 'b'.
...
e) # Do something with 'e', and also with $OPTARG,
# which is the associated argument passed with option 'e'.
...
g ) # Do something with variable 'g'.
esac
done
shift $(($OPTIND - 1))
Move argument pointer to next.
All this is not nearly as complicated as it looks .
Example 15-21. Using getopts to read the options/arguments
passed to a script
!/bin/bash
ex33.sh: Exercising getopts and OPTIND
Script modified 10/09/03 at the suggestion of Bill Gradwohl.
Here we observe how 'getopts' processes command-line arguments to script.
The arguments are parsed as "options" (flags) and associated arguments.
Try invoking this script with:
'scriptname -mn'
'scriptname -oq qOption' (qOption can be some arbitrary string.)
'scriptname -qXXX -r'
'scriptname -qr'
'scriptname -q -r'
'scriptname -mnop -mnop' - Unexpected result
(OPTIND is unreliable at stating where an option came from.)
If an option expects an argument ("flag:"), then it will grab
- whatever is next on the command-line.
NO_ARGS=0
E_OPTERROR=85
if [ $# -eq "$NO_ARGS" ] # Script invoked with no command-line args?
then
echo "Usage: basename $0 options (-mnopqrs)"
exit $E_OPTERROR # Exit and explain usage.
# Usage: scriptname -options
# Note: dash (-) necessary
fi
while getopts ":mnopq:rs" Option
do
case $Option in
m ) echo "Scenario #1: option -m- [OPTIND=${OPTIND}]";;
n | o ) echo "Scenario #2: option -$Option- [OPTIND=${OPTIND}]";;
p ) echo "Scenario #3: option -p- [OPTIND=${OPTIND}]";;
q ) echo "Scenario #4: option -q-\
with argument \"$OPTARG\" [OPTIND=${OPTIND}]";;
# Note that option 'q' must have an associated argument,
#+ otherwise it falls through to the default.
r | s ) echo "Scenario #5: option -$Option-";;
* ) echo "Unimplemented option chosen.";; # Default.
esac
done
shift $(($OPTIND - 1))
Decrements the argument pointer so it points to next argument.
$1 now references the first non-option item supplied on the command-line
exit $?
As Bill Gradwohl states,
"The getopts mechanism allows one to specify: scriptname -mnop -mnop
- but there is no reliable way to differentiate what came
- from where by using OPTIND."
There are, however, workarounds.
Script Behavior
source, . (dot command)
This command, when invoked from the command-line, executes a
script. Within a script, a source file-name loads the file
file-name. Sourcing a file (dot-command) imports code into the
script, appending to the script (same effect as the #include
directive in a C program). The net result is the same as if
the "sourced" lines of code were physically present in the
body of the script. This is useful in situations when multiple
scripts use a common data file or function library.
Example 15-22. "Including" a data file
!/bin/bash
Note that this example must be invoked with bash, i.e., bash ex38.sh
. data-file # Load a data file.
Same effect as "source data-file", but more portable.
The file "data-file" must be present in current working directory,
- since it is referred to by its basename.
Now, let's reference some data from that file.
echo "variable1 (from data-file) = $variable1"
echo "variable3 (from data-file) = $variable3"
let "sum = $variable2 + $variable4"
echo "Sum of variable2 + variable4 (from data-file) = $sum"
echo "message1 (from data-file) is "$message1""
Escaped quotes
echo "message2 (from data-file) is "$message2""
print_message This is the message-print function in the data-file.
exit $?
File data-file for Example 15-22, above. Must be present in
same directory.
This is a data file loaded by a script.
Files of this type may contain variables, functions, etc.
It loads with a 'source' or '.' command from a shell script.
Let's initialize some variables.
variable1=23
variable2=474
variable3=5
variable4=97
message1="Greetings from *** line $LINENO *** of the data file!"
message2="Enough for now. Goodbye."
print_message ()
{ # Echoes any message passed to it.
if [ -z "$1" ]
then
return 1 # Error, if argument missing.
fi
echo
until [ -z "$1" ]
do # Step through arguments passed to function.
echo -n "$1" # Echo args one at a time, suppressing line feeds.
echo -n " " # Insert spaces between words.
shift # Next one.
done
echo
return 0
}
If the sourced file is itself an executable script, then it
will run, then return control to the script that called it. A
sourced executable script may use a return for this purpose.
Arguments may be (optionally) passed to the sourced file as
positional parameters.
source $filename $arg1 arg2
It is even possible for a script to source itself, though this
does not seem to have any practical applications.
Example 15-23. A (useless) script that sources itself
!/bin/bash
self-source.sh: a script sourcing itself "recursively."
From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100 # Maximum number of execution passes.
echo -n "$pass_count "
At first execution pass, this just echoes two blank spaces,
- since $pass_count still uninitialized.
let "pass_count += 1"
Assumes the uninitialized variable $pass_count
- can be incremented the first time around.
This works with Bash and pdksh, but
- it relies on non-portable (and possibly dangerous) behavior.
Better would be to initialize $pass_count to 0 before incrementing.
while [ "$pass_count" -le $MAXPASSCNT ]
do
. $0 # Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here. Why?
done
What occurs here is not actually recursion,
- since the script effectively "expands" itself, i.e.,
- generates a new section of code
- with each pass through the 'while' loop',
with each 'source' in line 20.
Of course, the script interprets each newly 'sourced' "#!" line
- as a comment, and not as the start of a new script.
echo
exit 0 # The net effect is counting from 1 to 100.
# Very impressive.
Exercise:
Write a script that uses this trick to actually do something useful.
exit
Unconditionally terminates a script. [63] The exit command may
optionally take an integer argument, which is returned to the
shell as the exit status of the script. It is good practice to
end all but the simplest scripts with an exit 0, indicating a
successful run.
Note
If a script terminates with an exit lacking an argument, the exit
status of the script is the exit status of the last command executed
in the script, not counting the exit. This is equivalent to an exit
$?.
Note
An exit command may also be used to terminate a subshell.
exec
This shell builtin replaces the current process with a
specified command. Normally, when the shell encounters a
command, it forks off a child process to actually execute the
command. Using the exec builtin, the shell does not fork, and
the command exec'ed replaces the shell. When used in a script,
therefore, it forces an exit from the script when the exec'ed
command terminates. [64]
Example 15-24. Effects of exec
!/bin/bash
exec echo "Exiting "$0" at line $LINENO." # Exit from script here.
$LINENO is an internal Bash variable set to the line number it's on.
The following lines never execute.
echo "This echo fails to echo."
exit 99 # This script will not exit here.
# Check exit value after script terminates
#+ with an 'echo $?'.
# It will *not* be 99.
Example 15-25. A script that exec's itself
!/bin/bash
self-exec.sh
Note: Set permissions on this script to 555 or 755,
then call it with ./self-exec.sh or sh ./self-exec.sh.
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl-C to exit ===================="
sleep 1
exec $0 # Spawns another instance of this same script
#+ that replaces the previous one.
echo "This line will never echo!" # Why not?
exit 99 # Will not exit here!
# Exit code will not be 99!
An exec also serves to reassign file descriptors. For example,
exec <zzz-file replaces stdin with the file zzz-file.
Note
The -exec option to find is not the same as the exec shell builtin.
shopt
This command permits changing shell options on the fly (see
Example 25-1 and Example 25-2). It often appears in the Bash
startup files, but also has its uses in scripts. Needs version
2 or later of Bash.
shopt -s cdspell
Allows minor misspelling of directory names with 'cd'
Option -s sets, -u unsets.
cd /hpme # Oops! Mistyped '/home'.
pwd # /home
# The shell corrected the misspelling.
caller
Putting a caller command inside a function echoes to stdout
information about the caller of that function.
!/bin/bash
function1 ()
{
Inside function1 ().
caller 0 # Tell me about it.
}
function1 # Line 9 of script.
9 main test.sh
^ Line number that the function was called from.
^^^^ Invoked from "main" part of script.
^^^^^^^ Name of calling script.
caller 0 # Has no effect because it's not inside a function.
A caller command can also return caller information from a
script sourced within another script. Analogous to a function,
this is a "subroutine call."
You may find this command useful in debugging.
Commands
true
A command that returns a successful (zero) exit status, but
does nothing else.
bash$ true
bash$ echo $?
0
Endless loop
while true # alias for ":"
do
operation-1
operation-2
...
operation-n
Need a way to break out of loop or script will hang.
done
false
A command that returns an unsuccessful exit status, but does
nothing else.
bash$ false
bash$ echo $?
1
Testing "false"
if false
then
echo "false evaluates "true""
else
echo "false evaluates "false""
fi
false evaluates "false"
Looping while "false" (null loop)
while false
do
The following code will not execute.
operation-1
operation-2
...
operation-n
Nothing happens!
done
type [cmd]
Similar to the which external command, type cmd identifies
"cmd." Unlike which, type is a Bash builtin. The useful -a
option to type identifies keywords and builtins, and also
locates system commands with identical names.
bash$ type '['
[ is a shell builtin
bash$ type -a '['
[ is a shell builtin
[ is /usr/bin/[
bash$ type type
type is a shell builtin
The type command can be useful for testing whether a certain
command exists.
hash [cmds]
Records the path name of specified commands -- in the shell
hash table [65] -- so the shell or script will not need to
search the $PATH on subsequent calls to those commands. When
hash is called with no arguments, it simply lists the commands
that have been hashed. The -r option resets the hash table.
bind
The bind builtin displays or modifies readline [66] key
bindings.
help
Gets a short usage summary of a shell builtin. This is the
counterpart to whatis, but for builtins. The display of help
information got a much-needed update in the version 4 release
of Bash.
bash$ help exit
exit: exit [n]
Exit the shell with a status of N. If N is omitted, the exit status
is that of the last command executed.
________________________________________________________________
15.1. Job Control Commands
Certain of the following job control commands take a job identifier
as an argument. See the table at end of the chapter.
jobs
Lists the jobs running in the background, giving the job
number. Not as useful as ps.
Note
It is all too easy to confuse jobs and processes. Certain builtins,
such as kill, disown, and wait accept either a job number or a
process number as an argument. The fg, bg and jobs commands accept
only a job number.
bash$ sleep 100 &
[1] 1384
bash $ jobs
[1]+ Running sleep 100 &
"1" is the job number (jobs are maintained by the current shell).
"1384" is the PID or process ID number (processes are maintained by
the system). To kill this job/process, either a kill %1 or a kill
1384 works.
Thanks, S.C.
disown
Remove job(s) from the shell's table of active jobs.
fg, bg
The fg command switches a job running in the background into
the foreground. The bg command restarts a suspended job, and
runs it in the background. If no job number is specified, then
the fg or bg command acts upon the currently running job.
wait
Suspend script execution until all jobs running in background
have terminated, or until the job number or process ID
specified as an option terminates. Returns the exit status of
waited-for command.
You may use the wait command to prevent a script from exiting
before a background job finishes executing (this would create
a dreaded orphan process).
Example 15-26. Waiting for a process to finish before
proceeding
!/bin/bash
ROOT_UID=0 # Only users with $UID 0 have root privileges.
E_NOTROOT=65
E_NOPARAMS=66
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
"Run along kid, it's past your bedtime."
exit $E_NOTROOT
fi
if [ -z "$1" ]
then
echo "Usage: basename $0 find-string"
exit $E_NOPARAMS
fi
echo "Updating 'locate' database..."
echo "This may take a while."
updatedb /usr & # Must be run as root.
wait
Don't run the rest of the script until 'updatedb' finished.
You want the the database updated before looking up the file name.
locate $1
Without the 'wait' command, in the worse case scenario,
- the script would exit while 'updatedb' was still running,
- leaving it as an orphan process.
exit 0
Optionally, wait can take a job identifier as an argument, for
example, wait%1 or wait $PPID. [67] See the job id table.
Tip
Within a script, running a command in the background with an
ampersand (&) may cause the script to hang until ENTER is hit. This
seems to occur with commands that write to stdout. It can be a major
annoyance.
!/bin/bash
test.sh
ls -l &
echo "Done."
bash$ ./test.sh
Done.
[bozo@localhost test-scripts]$ total 1
-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh
_
As Walter Brameld IV explains it:
As far as I can tell, such scripts don't actually hang. It jus
t
seems that they do because the background command writes text
to
the console after the prompt. The user gets the impression tha
t
the prompt was never displayed. Here's the sequence of events:
1. Script launches background command.
2. Script exits.
3. Shell displays the prompt.
4. Background command continues running and writing text to th
e
console.
5. Background command finishes.
6. User doesn't see a prompt at the bottom of the output, thin
ks script
is hanging.
Placing a wait after the background command seems to remedy this.
!/bin/bash
test.sh
ls -l &
echo "Done."
wait
bash$ ./test.sh
Done.
[bozo@localhost test-scripts]$ total 1
-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh
Redirecting the output of the command to a file or even to
/dev/null also takes care of this problem.
suspend
This has a similar effect to Control-Z, but it suspends the
shell (the shell's parent process should resume it at an
appropriate time).
logout
Exit a login shell, optionally specifying an exit status.
times
Gives statistics on the system time elapsed when executing
commands, in the following form:
0m0.020s 0m0.020s
This capability is of relatively limited value, since it is
not common to profile and benchmark shell scripts.
kill
Forcibly terminate a process by sending it an appropriate
terminate signal (see Example 17-6).
Example 15-27. A script that kills itself
!/bin/bash
self-destruct.sh
kill $$ # Script kills its own process here.
# Recall that "$$" is the script's PID.
echo "This line will not echo."
Instead, the shell sends a "Terminated" message to stdout.
exit 0 # Normal exit? No!
After this script terminates prematurely,
- what exit status does it return?
sh self-destruct.sh
echo $?
143
143 = 128 + 15
TERM signal
Note
kill -l lists all the signals (as does the file
/usr/include/asm/signal.h). A kill -9 is a sure kill, which will
usually terminate a process that stubbornly refuses to die with a
plain kill. Sometimes, a kill -15 works. A zombie process, that is, a
child process that has terminated, but that the parent process has
not (yet) killed, cannot be killed by a logged-on user -- you can't
kill something that is already dead -- but init will generally clean
it up sooner or later.
killall
The killall command kills a running process by name, rather
than by process ID. If there are multiple instances of a
particular command running, then doing a killall on that
command will terminate them all.
Note
This refers to the killall command in /usr/bin, not the killall
script in /etc/rc.d/init.d.
command
The command directive disables aliases and functions for the
command immediately following it.
bash$ command ls
Note
This is one of three shell directives that effect script command
processing. The others are builtin and enable.
builtin
Invoking builtin BUILTIN_COMMAND runs the command
BUILTIN_COMMAND as a shell builtin, temporarily disabling both
functions and external system commands with the same name.
enable
This either enables or disables a shell builtin command. As an
example, enable -n kill disables the shell builtin kill, so
that when Bash subsequently encounters kill, it invokes the
external command /bin/kill.
The -a option to enable lists all the shell builtins,
indicating whether or not they are enabled. The -f filename
option lets enable load a builtin as a shared library (DLL)
module from a properly compiled object file. [68].
autoload
This is a port to Bash of the ksh autoloader. With autoload in
place, a function with an autoload declaration will load from
an external file at its first invocation. [69] This saves
system resources.
Note that autoload is not a part of the core Bash
installation. It needs to be loaded in with enable -f (see
above).
Table 15-1. Job identifiers
Notation Meaning
%N Job number [N]
%S Invocation (command-line) of job begins with string S
%?S Invocation (command-line) of job contains within it string S
%% "current" job (last job stopped in foreground or started in
background)
%+ "current" job (last job stopped in foreground or started in
background)
%- Last job
$! Last background process
________________________________________________________________
Chapter 16. External Filters, Programs and Commands
Standard UNIX commands make shell scripts more versatile. The power
of scripts comes from coupling system commands and shell directives
with simple programming constructs.
________________________________________________________________
16.1. Basic Commands
The first commands a novice learns
ls
The basic file "list" command. It is all too easy to
underestimate the power of this humble command. For example,
using the -R, recursive option, ls provides a tree-like
listing of a directory structure. Other useful options are -S,
sort listing by file size, -t, sort by file modification time,
-v, sort by (numerical) version numbers embedded in the
filenames, [70] -b, show escape characters, and -i, show file
inodes (see Example 16-4).
bash$ ls -l
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
bash$ ls -lv
total 0
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt
Tip
The ls command returns a non-zero exit status when attempting to list
a non-existent file.
bash$ ls abc
ls: abc: No such file or directory
bash$ echo $?
2
Example 16-1. Using ls to create a table of contents for
burning a CDR disk
!/bin/bash
ex40.sh (burn-cd.sh)
Script to automate burning a CDR.
SPEED=10 # May use higher speed if your hardware supports it.
IMAGEFILE=cdimage.iso
CONTENTSFILE=contents
DEVICE=/dev/cdrom For older versions of cdrecord
DEVICE="1,0,0"
DEFAULTDIR=/opt # This is the directory containing the data to be burned.
# Make sure it exists.
# Exercise: Add a test for this.
Uses Joerg Schilling's "cdrecord" package:
http://www.fokus.fhg.de/usr/schilling/cdrecord.html
If this script invoked as an ordinary user, may need to suid cdrecord
- chmod u+s /usr/bin/cdrecord, as root.
Of course, this creates a security hole, though a relatively minor one.
if [ -z "$1" ]
then
IMAGE_DIRECTORY=$DEFAULTDIR
Default directory, if not specified on command-line.
else
IMAGE_DIRECTORY=$1
fi
Create a "table of contents" file.
ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE
The "l" option gives a "long" file listing.
The "R" option makes the listing recursive.
The "F" option marks the file types (directories get a trailing /).
echo "Creating table of contents."
Create an image file preparatory to burning it onto the CDR.
mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image ($IMAGEFILE)."
Burn the CDR.
echo "Burning the disk."
echo "Please be patient, this will take a while."
wodim -v -isosize dev=$DEVICE $IMAGEFILE
In newer Linux distros, the "wodim" utility assumes the
- functionality of "cdrecord."
exitcode=$?
echo "Exit code = $exitcode"
exit $exitcode
cat, tac
cat, an acronym for concatenate, lists a file to stdout. When
combined with redirection (> or >>), it is commonly used to
concatenate files.
Uses of 'cat'
cat filename # Lists the file.
cat file.1 file.2 file.3 > file.123 # Combines three files into one.
The -n option to cat inserts consecutive numbers before all
lines of the target file(s). The -b option numbers only the
non-blank lines. The -v option echoes nonprintable characters,
using ^ notation. The -s option squeezes multiple consecutive
blank lines into a single blank line.
See also Example 16-28 and Example 16-24.
Note
In a pipe, it may be more efficient to redirect the stdin to a file,
rather than to cat the file.
cat filename | tr a-z A-Z
tr a-z A-Z < filename # Same effect, but starts one less process,
#+ and also dispenses with the pipe.
tac, is the inverse of cat, listing a file backwards from its
end.
rev
reverses each line of a file, and outputs to stdout. This does
not have the same effect as tac, as it preserves the order of
the lines, but flips each one around (mirror image).
bash$ cat file1.txt
This is line 1.
This is line 2.
bash$ tac file1.txt
This is line 2.
This is line 1.
bash$ rev file1.txt
.1 enil si sihT
.2 enil si sihT
cp
This is the file copy command. cp file1 file2 copies file1 to
file2, overwriting file2 if it already exists (see Example
16-6).
Tip
Particularly useful are the -a archive flag (for copying an entire
directory tree), the -u update flag (which prevents overwriting
identically-named newer files), and the -r and -R recursive flags.
cp -u source_dir/* dest_dir
"Synchronize" dest_dir to source_dir
- by copying over all newer and not previously existing files.
mv
This is the file move command. It is equivalent to a
combination of cp and rm. It may be used to move multiple
files to a directory, or even to rename a directory. For some
examples of using mv in a script, see Example 10-11 and
Example A-2.
Note
When used in a non-interactive script, mv takes the -f (force) option
to bypass user input.
When a directory is moved to a preexisting directory, it becomes a
subdirectory of the destination directory.
bash$ mv source_directory target_directory
bash$ ls -lF target_directory
total 1
drwxrwxr-x 2 bozo bozo 1024 May 28 19:20 source_directory/
rm
Delete (remove) a file or files. The -f option forces removal
of even readonly files, and is useful for bypassing user input
in a script.
Note
The rm command will, by itself, fail to remove filenames beginning
with a dash. Why? Because rm sees a dash-prefixed filename as an
option.
bash$ rm -badname
rm: invalid option -- b
Try `rm --help' for more information.
One clever workaround is to precede the filename with a " -- " (the
end-of-options flag).
bash$ rm -- -badname
Another method to is to preface the filename to be removed
with a dot-slash .
bash$ rm ./-badname
Warning
When used with the recursive flag -r, this command removes files all
the way down the directory tree from the current directory. A
careless rm -rf * can wipe out a big chunk of a directory structure.
rmdir
Remove directory. The directory must be empty of all files --
including "invisible" dotfiles [71] -- for this command to
succeed.
mkdir
Make directory, creates a new directory. For example, mkdir -p
project/programs/December creates the named directory. The -p
option automatically creates any necessary parent directories.
chmod
Changes the attributes of an existing file or directory (see
Example 15-14).
chmod +x filename
Makes "filename" executable for all users.
chmod u+s filename
Sets "suid" bit on "filename" permissions.
An ordinary user may execute "filename" with same privileges as the file's o
wner.
(This does not apply to shell scripts.)
chmod 644 filename
Makes "filename" readable/writable to owner, readable to others
chmod 444 filename
Makes "filename" read-only for all.
Modifying the file (for example, with a text editor)
- not allowed for a user who does not own the file (except for root),
- and even the file owner must force a file-save
- if she modifies the file.
Same restrictions apply for deleting the file.
chmod 1777 directory-name
Gives everyone read, write, and execute permission in directory,
- however also sets the "sticky bit".
This means that only the owner of the directory,
- owner of the file, and, of course, root
- can delete any particular file in that directory.
chmod 111 directory-name
Gives everyone execute-only permission in a directory.
This means that you can execute and READ the files in that directory
- (execute permission necessarily includes read permission
- because you can't execute a file without being able to read it).
But you can't list the files or search for them with the "find" command.
These restrictions do not apply to root.
chmod 000 directory-name
No permissions at all for that directory.
Can't read, write, or execute files in it.
Can't even list files in it or "cd" to it.
But, you can rename (mv) the directory
- or delete it (rmdir) if it is empty.
You can even symlink to files in the directory,
- but you can't read, write, or execute the symlinks.
These restrictions do not apply to root.
chattr
Change file attributes. This is analogous to chmod above, but
with different options and a different invocation syntax, and
it works only on ext2/ext3 filesystems.
One particularly interesting chattr option is i. A chattr +i
filename marks the file as immutable. The file cannot be
modified, linked to, or deleted, not even by root. This file
attribute can be set or removed only by root. In a similar
fashion, the a option marks the file as append only.
root# chattr +i file1.txt
root# rm file1.txt
rm: remove write-protected regular file `file1.txt'? y
rm: cannot remove `file1.txt': Operation not permitted
If a file has the s (secure) attribute set, then when it is
deleted its block is overwritten with binary zeroes. [72]
If a file has the u (undelete) attribute set, then when it is
deleted, its contents can still be retrieved (undeleted).
If a file has the c (compress) attribute set, then it will
automatically be compressed on writes to disk, and
uncompressed on reads.
Note
The file attributes set with chattr do not show in a file listing (ls
-l).
ln
Creates links to pre-existings files. A "link" is a reference
to a file, an alternate name for it. The ln command permits
referencing the linked file by more than one name and is a
superior alternative to aliasing (see Example 4-6).
The ln creates only a reference, a pointer to the file only a
few bytes in size.
The ln command is most often used with the -s, symbolic or
"soft" link flag. Advantages of using the -s flag are that it
permits linking across file systems or to directories.
The syntax of the command is a bit tricky. For example: ln -s
oldfile newfile links the previously existing oldfile to the
newly created link, newfile.
Caution
If a file named newfile has previously existed, an error message will
result.
Which type of link to use?
As John Macdonald explains it:
Both of these [types of links] provide a certain measure of dual
reference -- if you edit the contents of the file using any name,
your changes will affect both the original name and either a hard or
soft new name. The differences between them occurs when you work at a
higher level. The advantage of a hard link is that the new name is
totally independent of the old name -- if you remove or rename the
old name, that does not affect the hard link, which continues to
point to the data while it would leave a soft link hanging pointing
to the old name which is no longer there. The advantage of a soft
link is that it can refer to a different file system (since it is
just a reference to a file name, not to actual data). And, unlike a
hard link, a symbolic link can refer to a directory.
Links give the ability to invoke a script (or any other type
of executable) with multiple names, and having that script
behave according to how it was invoked.
Example 16-2. Hello or Good-bye
!/bin/bash
hello.sh: Saying "hello" or "goodbye"
Make a link in current working directory ($PWD) to this script:
ln -s hello.sh goodbye
Now, try invoking this script both ways:
./hello.sh
./goodbye
HELLO_CALL=65
GOODBYE_CALL=66
if [ $0 = "./goodbye" ]
then
echo "Good-bye!"
Some other goodbye-type commands, as appropriate.
exit $GOODBYE_CALL
fi
echo "Hello!"
Some other hello-type commands, as appropriate.
exit $HELLO_CALL
man, info
These commands access the manual and information pages on
system commands and installed utilities. When available, the
info pages usually contain more detailed descriptions than do
the man pages.
There have been various attempts at "automating" the writing
of man pages. For a script that makes a tentative first step
in that direction, see Example A-39.
________________________________________________________________
16.2. Complex Commands
Commands for more advanced users
find
-exec COMMAND \;
Carries out COMMAND on each file that find matches. The
command sequence terminates with ; (the ";" is escaped to make
certain the shell passes it to find literally, without
interpreting it as a special character).
bash$ find ~/ -name '*.txt'
/home/bozo/.kde/share/apps/karm/karmdata.txt
/home/bozo/misc/irmeyc.txt
/home/bozo/test-scripts/1.txt
If COMMAND contains {}, then find substitutes the full path
name of the selected file for "{}".
find ~/ -name 'core*' -exec rm {} ;
Removes all core dump files from user's home directory.
find /home/bozo/projects -mtime -1
^ Note minus sign!
Lists all files in /home/bozo/projects directory tree
- that were modified within the last day (current_day - 1).
find /home/bozo/projects -mtime 1
Same as above, but modified exactly one day ago.
mtime = last modification time of the target file
ctime = last status change time (via 'chmod' or otherwise)
atime = last access time
DIR=/home/bozo/junk_files
find "$DIR" -type f -atime +5 -exec rm {} ;
^ ^^
Curly brackets are placeholder for the path name output by "find."
Deletes all files in "/home/bozo/junk_files"
- that have not been accessed in at least 5 days (plus sign ... +5).
"-type filetype", where
f = regular file
d = directory
l = symbolic link, etc.
(The 'find' manpage and info page have complete option listings.)
find /etc -exec grep '[0-9][0-9][.][0-9][0-9][.][0-9][0-9][.][0-9][0-9]' {
} ;
Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
There a few extraneous hits. Can they be filtered out?
Possibly by:
find /etc -type f -exec cat '{}' ; | tr -c '.[:digit:]' '\n' \
| grep '^[^.][^.].[^.][^.].[^.][^.].[^.][^.]$'
[:digit:] is one of the character classes
- introduced with the POSIX 1003.2 standard.
Thanks, Stéphane Chazelas.
Note
The -exec option to find should not be confused with the exec shell
builtin.
Example 16-3. Badname, eliminate file names in current
directory containing bad characters and whitespace.
!/bin/bash
badname.sh
Delete filenames in current directory containing bad characters.
for filename in *
do
badname=echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p
badname=echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p' also works.
Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $
rm $badname 2>/dev/null
^^^^^^^^^^^ Error messages deep-sixed.
done
Now, take care of files containing all manner of whitespace.
find . -name "* *" -exec rm -f {} ;
The path name of the file that find finds replaces the "{}".
The '' ensures that the ';' is interpreted literally, as end of command.
exit 0
Commands below this line will not execute because of exit command.
An alternative to the above script:
find . -name '[+{;"\=?~()<>&|$ ]*' -maxdepth 0 \
-exec rm -f '{}' ;
The "-maxdepth 0" option ensures that find will not search
- subdirectories below $PWD.
(Thanks, S.C.)
Example 16-4. Deleting a file by its inode number
!/bin/bash
idelete.sh: Deleting a file by its inode number.
This is useful when a filename starts with an illegal character,
ARGCOUNT=1 # Filename arg must be passed to script.
E_WRONGARGS=70
E_FILE_NOT_EXIST=71
E_CHANGED_MIND=72
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: basename $0 filename"
exit $E_WRONGARGS
fi
if [ ! -e "$1" ]
then
echo "File ""$1"" does not exist."
exit $E_FILE_NOT_EXIST
fi
inum=ls -i | grep "$1" | awk '{print $1}'
inum = inode (index node) number of file
Every file has an inode, a record that holds its physical address info.
echo; echo -n "Are you absolutely sure you want to delete "$1" (y/n)? "
The '-v' option to 'rm' also asks this.
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
- ) echo "Deleting file "$1".";;
esac
find . -inum $inum -exec rm {} ;
^^
Curly brackets are placeholder
echo "File ""$1"" deleted!"
exit 0
The find command also works without the -exec option.
!/bin/bash
Find suid root files.
A strange suid file might indicate a security hole,
- or even a system intrusion.
directory="/usr/sbin"
Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc.
permissions="+4000" # suid root (dangerous!)
for file in $( find "$directory" -perm "$permissions" )
do
ls -ltF --author "$file"
done
See Example 16-30, Example 3-4, and Example 11-10 for scripts
using find. Its manpage provides more detail on this complex
and powerful command.
xargs
A filter for feeding arguments to a command, and also a tool
for assembling the commands themselves. It breaks a data
stream into small enough chunks for filters and commands to
process. Consider it as a powerful replacement for backquotes.
In situations where command substitution fails with a too many
arguments error, substituting xargs often works. [73]
Normally, xargs reads from stdin or from a pipe, but it can
also be given the output of a file.
The default command for xargs is echo. This means that input
piped to xargs may have linefeeds and other whitespace
characters stripped out.
bash$ ls -l
total 0
-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1
-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2
bash$ ls -l | xargs
total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 J
an...
bash$ find ~/mail -type f | xargs grep "Linux"
./misc:User-Agent: slrn/0.9.8.1 (Linux)
./sent-mail-jul-2005: hosted by the Linux Documentation Project.
./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)
./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article
./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem
. . .
ls | xargs -p -l gzip gzips every file in current directory,
one at a time, prompting before each operation.
Note
Note that xargs processes the arguments passed to it sequentially,
one at a time.
bash$ find /usr/bin | xargs file
/usr/bin: directory
/usr/bin/foomatic-ppd-options: perl script text executable
. . .
Tip
An interesting xargs option is -n NN, which limits to NN the number
of arguments passed.
ls | xargs -n 8 echo lists the files in the current directory in 8
columns.
Tip
Another useful option is -0, in combination with find -print0 or grep
-lZ. This allows handling arguments containing whitespace or quotes.
find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f
grep -rliwZ GUI / | xargs -0 rm -f
Either of the above will remove any file containing "GUI". (Thanks,
S.C.)
Or:
cat /proc/"$pid"/"$OPTION" | xargs -0 echo
Formats output: ^^^^^^^^^^^^^^^
From Han Holl's fixup of "get-commandline.sh"
- script in "/dev and /proc" chapter.
Tip
The -P option to xargs permits running processes in parallel. This
speeds up execution in a machine with a multicore CPU.
!/bin/bash
ls *gif | xargs -t -n1 -P2 gif2png
Converts all the gif images in current directory to png.
Options:
=======
-t Print command to stderr.
-n1 At most 1 argument per command line.
-P2 Run up to 2 processes simultaneously.
Thank you, Roberto Polli, for the inspiration.
Example 16-5. Logfile: Using xargs to monitor system log
!/bin/bash
Generates a log file in current directory
from the tail end of /var/log/messages.
Note: /var/log/messages must be world readable
if this script invoked by an ordinary user.
#root chmod 644 /var/log/messages
LINES=5
( date; uname -a ) >>logfile
Time and machine name
echo ---------------------------------------------------------- >>logfile
tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile
echo >>logfile
echo >>logfile
exit 0
Note:
As Frank Wang points out,
- unmatched quotes (either single or double quotes) in the source file
- may give xargs indigestion.
He suggests the following substitution for line 15:
tail -n $LINES /var/log/messages | tr -d ""'" | xargs | fmt -s >>logfile
Exercise:
Modify this script to track changes in /var/log/messages at intervals
Hint: Use the "watch" command.
As in find, a curly bracket pair serves as a placeholder for
replacement text.
Example 16-6. Copying files in current directory to another
!/bin/bash
copydir.sh
Copy (verbose) all files in current directory ($PWD)
- to directory specified on command-line.
E_NOARGS=85
if [ -z "$1" ] # Exit if no argument given.
then
echo "Usage: basename $0 directory-to-copy-to"
exit $E_NOARGS
fi
ls . | xargs -i -t cp ./{} $1
^^ ^^ ^^
-t is "verbose" (output command-line to stderr) option.
-i is "replace strings" option.
{} is a placeholder for output text.
This is similar to the use of a curly-bracket pair in "find."
List the files in current directory (ls .),
- pass the output of "ls" as arguments to "xargs" (-i -t options),
- then copy (cp) these arguments ({}) to new directory ($1).
The net result is the exact equivalent of
- unless any of the filenames has embedded "whitespace" characters.
exit 0
Example 16-7. Killing processes by name
!/bin/bash
kill-byname.sh: Killing processes by name.
Compare this script with kill-process.sh.
For instance,
- try "./kill-byname.sh xterm" --
- and watch all the xterms on your desktop disappear.
Warning:
This is a fairly dangerous script.
Running it carelessly (especially as root)
- can cause data loss and other undesirable effects.
E_BADARGS=66
if test -z "$1" # No command-line arg supplied?
then
echo "Usage: basename $0 Process(es)_to_kill"
exit $E_BADARGS
fi
PROCESS_NAME="$1"
ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/nul
l
^^ ^^
Notes:
-i is the "replace strings" option to xargs.
The curly brackets are the placeholder for the replacement.
2&>/dev/null suppresses unwanted error messages.
Can grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"?
exit $?
The "killall" command has the same effect as this script,
- but using it is not quite as educational.
Example 16-8. Word frequency analysis using xargs
!/bin/bash
wf2.sh: Crude word frequency analysis on a text file.
Uses 'xargs' to decompose lines of text into single words.
Compare this example to the "wf.sh" script later on.
Check for input file on command-line.
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne "$ARGS" ]
Correct number of arguments passed to script?
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Does file exist?
then
echo "File "$1" does not exist."
exit $E_NOFILE
fi
cat "$1" | xargs -n1 | \
List the file, one word per line.
tr A-Z a-z | \
Shift characters to lowercase.
sed -e 's/.//g' -e 's/,//g' -e 's/ /\
/g' | \
Filter out periods and commas, and
- change space between words to linefeed,
sort | uniq -c | sort -nr
Finally remove duplicates, prefix occurrence count
This does the same job as the "wf.sh" example,
- but a bit more ponderously, and it runs more slowly (why?).
exit $?
expr
All-purpose expression evaluator: Concatenates and evaluates
the arguments according to the operation given (arguments must
be separated by spaces). Operations may be arithmetic,
comparison, string, or logical.
expr 3 + 5
returns 8
expr 5 % 3
returns 2
expr 1 / 0
returns the error message, expr: division by zero
Illegal arithmetic operations not allowed.
expr 5 \* 3
returns 15
The multiplication operator must be escaped when used in
an arithmetic expression with expr.
y=`expr $y + 1`
Increment a variable, with the same effect as let y=y+1
and y=$(($y+1)). This is an example of arithmetic
expansion.
z=`expr substr $string $position $length`
Extract substring of $length characters, starting at
$position.
Example 16-9. Using expr
!/bin/bash
Demonstrating some of the uses of 'expr'
=======================================
echo
Arithmetic Operators
echo "Arithmetic Operators"
echo
a=expr 5 + 3
echo "5 + 3 = $a"
a=expr $a + 1
echo
echo "a + 1 = $a"
echo "(incrementing a variable)"
a=expr 5 % 3
modulo
echo
echo "5 mod 3 = $a"
echo
echo
Logical Operators
Returns 1 if true, 0 if false,
- opposite of normal Bash convention.
echo "Logical Operators"
echo
x=24
y=25
b=expr $x = $y # Test equality.
echo "b = $b" # 0 ( $x -ne $y )
echo
a=3
b=expr $a \> 10
echo 'b=expr $a \> 10, therefore...'
echo "If a > 10, b = 0 (false)"
echo "b = $b" # 0 ( 3 ! -gt 10 )
echo
b=expr $a \< 10
echo "If a < 10, b = 1 (true)"
echo "b = $b" # 1 ( 3 -lt 10 )
echo
Note escaping of operators.
b=expr $a \<= 3
echo "If a <= 3, b = 1 (true)"
echo "b = $b" # 1 ( 3 -le 3 )
There is also a ">=" operator (greater than or equal to).
echo
echo
String Operators
echo "String Operators"
echo
a=1234zipper43231
echo "The string being operated upon is "$a"."
length: length of string
b=expr length $a
echo "Length of "$a" is $b."
index: position of first character in substring
that matches a character in string
b=expr index $a 23
echo "Numerical position of first "2" in "$a" is "$b"."
substr: extract substring, starting position & length specified
b=expr substr $a 2 6
echo "Substring of "$a", starting at position 2,\
and 6 chars long is "$b"."
The default behavior of the 'match' operations is to
- search for the specified match at the BEGINNING of the string.
Using Regular Expressions ...
b=expr match "$a" '[0-9]*' # Numerical count.
echo Number of digits at the beginning of "$a" is $b.
b=expr match "$a" '\([0-9]*\)' # Note that escaped parentheses
== == #+ trigger substring match.
echo "The digits at the beginning of "$a" are "$b"."
echo
exit 0
Important
The : (null) operator can substitute for match. For example, b=`expr
$a : [0-9] is the exact equivalent of b=expr match $a [0-9]` in
the above listing.
!/bin/bash
echo
echo "String operations using "expr $string : " construct"
echo "==================================================="
echo
a=1234zipper5FLIPPER43231
echo "The string being operated upon is "expr "$a" : '\(.*\)'"."
Escaped parentheses grouping operator. == ==
If no escaped parentheses ...
- then 'expr' converts the string operand to an integer.
echo "Length of "$a" is expr "$a" : '.*'." # Length of string
echo "Number of digits at the beginning of "$a" is expr "$a" : '[0-9]*'."
------------------------------------------------------------------------- #
echo
echo "The digits at the beginning of "$a" are expr "$a" : '\([0-9]*\)'."
== ==
echo "The first 7 characters of "$a" are expr "$a" : '\(.......\)'."
===== == ==
Again, escaped parentheses force a substring match.
echo "The last 7 characters of "$a" are expr "$a" : '.*\(.......\)'."
==== end of string operator ^^
(In fact, means skip over one or more of any characters until specified
echo
exit 0
The above script illustrates how expr uses the escaped parentheses --
( ... ) -- grouping operator in tandem with regular expression
parsing to match a substring. Here is a another example, this time
from "real life."
Strip the whitespace from the beginning and end.
LRFDATE=expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'
From Peter Knowles' "booklistgen.sh" script
- for converting files to Sony Librie/PRS-50X format.
(http://booklistgensh.peterknowles.com)
Perl, sed, and awk have far superior string parsing facilities. A
short sed or awk "subroutine" within a script (see Section 36.2) is
an attractive alternative to expr.
See Section 10.1 for more on using expr in string operations.
________________________________________________________________
16.3. Time / Date Commands
Time/date and timing
date
Simply invoked, date prints the date and time to stdout. Where
this command gets interesting is in its formatting and parsing
options.
Example 16-10. Using date
!/bin/bash
Exercising the 'date' command
echo "The number of days since the year's beginning is date +%j."
Needs a leading '+' to invoke formatting.
%j gives day of year.
echo "The number of seconds elapsed since 01/01/1970 is date +%s."
%s yields number of seconds since "UNIX epoch" began,
prefix=temp
suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific.
filename=$prefix.$suffix
echo "Temporary filename = $filename"
It's great for creating "unique and random" temp filenames,
- even better than using $$.
Read the 'date' man page for more formatting options.
exit 0
The -u option gives the UTC (Universal Coordinated Time).
bash$ date
Fri Mar 29 21:07:39 MST 2002
bash$ date -u
Sat Mar 30 04:07:42 UTC 2002
This option facilitates calculating the time between different
dates.
Example 16-11. Date calculations
!/bin/bash
date-calc.sh
Author: Nathan Coulter
Used in ABS Guide with permission (thanks!).
MPHR=60 # Minutes per hour.
HPD=24 # Hours per day.
diff () {
printf '%s' $(( $(date -u -d"$TARGET" +%s) -
$(date -u -d"$CURRENT" +%s)))
%d = day of month.
}
CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z')
TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z')
%F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.
printf '\nIn 2007, %s ' \
"$(date -d"$CURRENT +
$(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"
%B = name of month ^ halfway
printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')"
printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"
printf '\nOn %s at %s, there were\n' \
$(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T)
DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD ))
CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z')
HOURS=$(( $(diff) / $MPHR / $MPHR ))
CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z')
MINUTES=$(( $(diff) / $MPHR ))
CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z')
printf '%s days, %s hours, ' "$DAYS" "$HOURS"
printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)"
printf 'until Christmas Dinner!\n\n'
Exercise:
Rewrite the diff () function to accept passed parameters,
- rather than using global variables.
The date command has quite a number of output options. For
example %N gives the nanosecond portion of the current time.
One interesting use for this is to generate random integers.
date +%N | sed -e 's/000$//' -e 's/^0//'
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Strip off leading and trailing zeroes, if present.
Length of generated integer depends on
- how many zeroes stripped off.
115281032
63408725
394504284
There are many more options (try man date).
date +%j
Echoes day of the year (days elapsed since January 1).
date +%k%M
Echoes hour and minute in 24-hour format, as a single digit string.
The 'TZ' parameter permits overriding the default time zone.
date # Mon Mar 28 21:42:16 MST 2005
TZ=EST date # Mon Mar 28 23:42:16 EST 2005
Thanks, Frank Kannemann and Pete Sjoberg, for the tip.
SixDaysAgo=$(date --date='6 days ago')
OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month!)
OneYearAgo=$(date --date='1 year ago')
See also Example 3-4 and Example A-43.
zdump
Time zone dump: echoes the time in a specified time zone.
bash$ zdump EST
EST Tue Sep 18 22:09:22 2001 EST
time
Outputs verbose timing statistics for executing a command.
time ls -l / gives something like this:
real 0m0.067s
user 0m0.004s
sys 0m0.005s
See also the very similar times command in the previous
section.
Note
As of version 2.0 of Bash, time became a shell reserved word, with
slightly altered behavior in a pipeline.
touch
Utility for updating access/modification times of a file to
current system time or other specified time, but also useful
for creating a new file. The command touch zzz will create a
new file of zero length, named zzz, assuming that zzz did not
previously exist. Time-stamping empty files in this way is
useful for storing date information, for example in keeping
track of modification times on a project.
Note
The touch command is equivalent to : >> newfile or >> newfile (for
ordinary files).
Tip
Before doing a cp -u (copy/update), use touch to update the time
stamp of files you don't wish overwritten.
As an example, if the directory /home/bozo/tax_audit contains the
files spreadsheet-051606.data, spreadsheet-051706.data, and
spreadsheet-051806.data, then doing a touch spreadsheet*.data will
protect these files from being overwritten by files with the same
names during a cp -u /home/bozo/financial_info/spreadsheet*data
/home/bozo/tax_audit.
at
The at job control command executes a given set of commands at
a specified time. Superficially, it resembles cron, however,
at is chiefly useful for one-time execution of a command set.
at 2pm January 15 prompts for a set of commands to execute at
that time. These commands should be shell-script compatible,
since, for all practical purposes, the user is typing in an
executable shell script a line at a time. Input terminates
with a Ctl-D.
Using either the -f option or input redirection (<), at reads
a command list from a file. This file is an executable shell
script, though it should, of course, be non-interactive.
Particularly clever is including the run-parts command in the
file to execute a different set of scripts.
bash$ at 2:30 am Friday < at-jobs.list
job 2 at 2000-10-27 02:30
batch
The batch job control command is similar to at, but it runs a
command list when the system load drops below .8. Like at, it
can read commands from a file with the -f option.
The concept of batch processing dates back to the era of mainframe
computers. It means running a set of commands without user
intervention.
cal
Prints a neatly formatted monthly calendar to stdout. Will do
current year or a large range of past and future years.
sleep
This is the shell equivalent of a wait loop. It pauses for a
specified number of seconds, doing nothing. It can be useful
for timing or in processes running in the background, checking
for a specific event every so often (polling), as in Example
32-6.
sleep 3 # Pauses 3 seconds.
Note
The sleep command defaults to seconds, but minute, hours, or days may
also be specified.
sleep 3 h # Pauses 3 hours!
Note
The watch command may be a better choice than sleep for running
commands at timed intervals.
usleep
Microsleep (the u may be read as the Greek mu, or micro-
prefix). This is the same as sleep, above, but "sleeps" in
microsecond intervals. It can be used for fine-grained timing,
or for polling an ongoing process at very frequent intervals.
usleep 30 # Pauses 30 microseconds.
This command is part of the Red Hat initscripts / rc-scripts
package.
Caution
The usleep command does not provide particularly accurate timing, and
is therefore unsuitable for critical timing loops.
hwclock, clock
The hwclock command accesses or adjusts the machine's hardware
clock. Some options require root privileges. The
/etc/rc.d/rc.sysinit startup file uses hwclock to set the
system time from the hardware clock at bootup.
The clock command is a synonym for hwclock.
________________________________________________________________
16.4. Text Processing Commands
Commands affecting text and text files
sort
File sort utility, often used as a filter in a pipe. This
command sorts a text stream or file forwards or backwards, or
according to various keys or character positions. Using the -m
option, it merges presorted input files. The info page lists
its many capabilities and options. See Example 11-10, Example
11-11, and Example A-8.
tsort
Topological sort, reading in pairs of whitespace-separated
strings and sorting according to input patterns. The original
purpose of tsort was to sort a list of dependencies for an
obsolete version of the ld linker in an "ancient" version of
UNIX.
The results of a tsort will usually differ markedly from those
of the standard sort command, above.
uniq
This filter removes duplicate lines from a sorted file. It is
often seen in a pipe coupled with sort.
cat list-1 list-2 list-3 | sort | uniq > final.list
Concatenates the list files,
sorts them,
removes duplicate lines,
and finally writes the result to an output file.
The useful -c option prefixes each line of the input file with
its number of occurrences.
bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.
bash$ uniq -c testfile
1 This line occurs only once.
2 This line occurs twice.
3 This line occurs three times.
bash$ sort testfile | uniq -c | sort -nr
3 This line occurs three times.
2 This line occurs twice.
1 This line occurs only once.
The sort INPUTFILE | uniq -c | sort -nr command string
produces a frequency of occurrence listing on the INPUTFILE
file (the -nr options to sort cause a reverse numerical sort).
This template finds use in analysis of log files and
dictionary lists, and wherever the lexical structure of a
document needs to be examined.
Example 16-12. Word Frequency Analysis
!/bin/bash
wf.sh: Crude word frequency analysis on a text file.
This is a more efficient version of the "wf2.sh" script.
Check for input file on command-line.
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script?
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Check if file exists.
then
echo "File "$1" does not exist."
exit $E_NOFILE
fi
main ()
sed -e 's/.//g' -e 's/,//g' -e 's/ /\
/g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr
=========================
Frequency of occurrence
Filter out periods and commas, and
- change space between words to linefeed,
- then shift characters to lowercase, and
- finally prefix occurrence count and sort numerically.
Arun Giridhar suggests modifying the above to:
. . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr
This adds a secondary sort key, so instances of
- equal occurrence are sorted alphabetically.
As he explains it:
"This is effectively a radix sort, first on the
- (word or string, optionally case-insensitive)
- and last on the most significant column (frequency)."
As Frank Wang explains, the above is equivalent to
- and the following also works:
exit 0
Exercises:
- Add 'sed' commands to filter out other punctuation,
- Modify the script to also filter out multiple spaces and
bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.
bash$ ./wf.sh testfile
6 this
6 occurs
6 line
3 times
3 three
2 twice
1 only
1 once
expand, unexpand
The expand filter converts tabs to spaces. It is often used in
a pipe.
The unexpand filter converts spaces to tabs. This reverses the
effect of expand.
cut
A tool for extracting fields from files. It is similar to the
print $N command set in awk, but more limited. It may be
simpler to use cut in a script than awk. Particularly
important are the -d (delimiter) and -f (field specifier)
options.
Using cut to obtain a listing of the mounted filesystems:
cut -d ' ' -f1,2 /etc/mtab
Using cut to list the OS and kernel version:
uname -a | cut -d" " -f1,3,11,12
Using cut to extract message headers from an e-mail folder:
bash$ grep '^Subject:' read-messages | cut -c10-80
Re: Linux suitable for mission-critical apps?
MAKE MILLIONS WORKING AT HOME!!!
Spam complaint
Re: Spam complaint
Using cut to parse a file:
List all the users in /etc/passwd.
FILENAME=/etc/passwd
for user in $(cut -d: -f1 $FILENAME)
do
echo $user
done
Thanks, Oleg Philon for suggesting this.
cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{
print $2, $3 }' filename
Note
It is even possible to specify a linefeed as a delimiter. The trick
is to actually embed a linefeed (RETURN) in the command sequence.
bash$ cut -d'
' -f3,7,19 testfile
This is line 3 of testfile.
This is line 7 of testfile.
This is line 19 of testfile.
Thank you, Jaka Kranjc, for pointing this out.
See also Example 16-48.
paste
Tool for merging together different files into a single,
multi-column file. In combination with cut, useful for
creating system log files.
bash$ cat items
alphabet blocks
building blocks
cables
bash$ cat prices
$1.00/dozen
$2.50 ea.
$3.75
bash$ paste items prices
alphabet blocks $1.00/dozen
building blocks $2.50 ea.
cables $3.75
join
Consider this a special-purpose cousin of paste. This powerful
utility allows merging two files in a meaningful fashion,
which essentially creates a simple version of a relational
database.
The join command operates on exactly two files, but pastes
together only those lines with a common tagged field (usually
a numerical label), and writes the result to stdout. The files
to be joined should be sorted according to the tagged field
for the matchups to work properly.
File: 1.data
100 Shoes
200 Laces
300 Socks
File: 2.data
100 $40.00
200 $1.00
300 $2.00
bash$ join 1.data 2.data
File: 1.data 2.data
100 Shoes $40.00
200 Laces $1.00
300 Socks $2.00
Note
The tagged field appears only once in the output.
head
lists the beginning of a file to stdout. The default is 10
lines, but a different number can be specified. The command
has a number of interesting options.
Example 16-13. Which files are scripts?
!/bin/bash
script-detector.sh: Detects scripts within a directory.
TESTCHARS=2 # Test first 2 characters.
SHABANG='#!' # Scripts begin with a "sha-bang."
for file in * # Traverse all the files in current directory.
do
if [[ head -c$TESTCHARS "$file" = "$SHABANG" ]]
head -c2 #!
The '-c' option to "head" outputs a specified
#+ number of characters, rather than lines (the default).
then
echo "File \"$file\" is a script."
else
echo "File \"$file\" is *not* a script."
fi
done
exit 0
Exercises:
- Modify this script to take as an optional argument
- the directory to scan for scripts
- (rather than just the current working directory).
- As it stands, this script gives "false positives" for
- Perl, awk, and other scripting language scripts.
Correct this.
Example 16-14. Generating 10-digit random numbers
!/bin/bash
rnd.sh: Outputs a 10-digit random number
Script by Stephane Chazelas.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
=================================================================== #
Analysis
head:
-c4 option takes first 4 bytes.
od:
-N4 option limits output to 4 bytes.
-tu4 option selects unsigned decimal format for output.
sed:
-n option, in combination with "p" flag to the "s" command,
outputs only matched lines.
The author of this script explains the action of 'sed', as follows.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
----------------------------------> |
Assume output up to "sed" --------> |
is 0000000 1198195154\n
sed begins reading characters: 0000000 1198195154\n.
Here it finds a newline character,
- so it is ready to process the first line (0000000 1198195154).
It looks at its s. The first and only one is
range action
1 s/.* //p
The line number is in the range, so it executes the action:
- tries to substitute the longest string ending with a space in the line
("0000000 ") with nothing (//), and if it succeeds, prints the result
("p" is a flag to the "s" command here, this is different
sed is now ready to continue reading its input. (Note that before
- continuing, if -n option had not been passed, sed would have printed
Now, sed reads the remainder of the characters, and finds the
It is now ready to process its 2nd line (which is also numbered '$' as
It sees it is not matched by any , so its job is done.
In few word this sed commmand means:
"On the first line only, remove any character up to the right-most space,
A better way to do this would have been:
sed -e 's/.* //;q'
Here, two s (could have been written
sed -e 's/.* //' -e q):
range action
nothing (matches line) s/.* //
nothing (matches line) q (quit)
Here, sed only reads its first line of input.
It performs both actions, and prints the line (substituted) before
- quitting (because of the "q" action) since the "-n" option is not passed.
=================================================================== #
An even simpler altenative to the above one-line script would be:
head -c4 /dev/urandom| od -An -tu4
exit
See also Example 16-39.
tail
lists the (tail) end of a file to stdout. The default is 10
lines, but this can be changed with the -n option. Commonly
used to keep track of changes to a system logfile, using the
-f option, which outputs lines appended to the file.
Example 16-15. Using tail to monitor the system log
!/bin/bash
filename=sys.log
cat /dev/null > $filename; echo "Creating / cleaning out file."
Creates the file if it does not already exist,
- and truncates it to zero length if it does.
: > filename and > filename also work.
tail /var/log/messages > $filename
/var/log/messages must have world read permission for this to work.
echo "$filename contains tail end of system log."
exit 0
Tip
To list a specific line of a text file, pipe the output of head to
tail -n 1. For example head -n 8 database.txt | tail -n 1 lists the
8th line of the file database.txt.
To set a variable to a given block of a text file:
var=$(head -n $m $filename | tail -n $n)
filename = name of file
m = from beginning of file, number of lines to end of block
n = number of lines to set variable to (trim from end of block)
Note
Newer implementations of tail deprecate the older tail -$LINES
filename usage. The standard tail -n $LINES filename is correct.
See also Example 16-5, Example 16-39 and Example 32-6.
grep
A multi-purpose file search tool that uses Regular
Expressions. It was originally a command/filter in the
venerable ed line editor: g/re/p -- global - regular
expression - print.
grep pattern [file...]
Search the target file(s) for occurrences of pattern, where
pattern may be literal text or a Regular Expression.
bash$ grep '[rst]ystem.$' osinfo.txt
The GPL governs the distribution of the Linux operating system.
If no target file(s) specified, grep works as a filter on
stdout, as in a pipe.
bash$ ps ax | grep clock
765 tty1 S 0:00 xclock
901 pts/1 S 0:00 grep clock
The -i option causes a case-insensitive search.
The -w option matches only whole words.
The -l option lists only the files in which matches were
found, but not the matching lines.
The -r (recursive) option searches files in the current
working directory and all subdirectories below it.
The -n option lists the matching lines, together with line
numbers.
bash$ grep -n Linux osinfo.txt
2:This is a file containing information about Linux.
6:The GPL governs the distribution of the Linux operating system.
The -v (or --invert-match) option filters out matches.
grep pattern1 *.txt | grep -v pattern2
Matches all lines in "*.txt" files containing "pattern1",
but not "pattern2".
The -c (--count) option gives a numerical count of matches,
rather than actually listing the matches.
grep -c txt .sgml # (number of occurrences of "txt" in ".sgml" files)
grep -cz .
^ dot
means count (-c) zero-separated (-z) items matching "."
that is, non-empty ones (containing at least 1 character).
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 3
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$' # 5
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$' # 9
By default, newline chars (\n) separate items to match.
Note that the -z option is GNU "grep" specific.
Thanks, S.C.
The --color (or --colour) option marks the matching string in
color (on the console or in an xterm window). Since grep
prints out each entire line containing the matching pattern,
this lets you see exactly what is being matched. See also the
-o option, which shows only the matching portion of the
line(s).
Example 16-16. Printing out the From lines in stored e-mail
messages
!/bin/bash
from.sh
Emulates the useful 'from' utility in Solaris, BSD, etc.
Echoes the "From" header line in all messages
- in your e-mail directory.
MAILDIR=~/mail/* # No quoting of variable. Why?
Maybe check if-exists $MAILDIR: if [ -d $MAILDIR ] . . .
GREP_OPTS="-H -A 5 --color" # Show file, plus extra context lines
#+ and display "From" in color.
TARGETSTR="^From" # "From" at beginning of line.
for file in $MAILDIR # No quoting of variable.
do
grep $GREP_OPTS "$TARGETSTR" "$file"
^^^^^^^^^^ # Again, do not quote this variable.
echo
done
exit $?
You might wish to pipe the output of this script to 'more'
- or redirect it to a file . . .
When invoked with more than one target file given, grep
specifies which file contains matches.
bash$ grep Linux osinfo.txt misc.txt
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.
misc.txt:The Linux operating system is steadily gaining in popularity.
Tip
To force grep to show the filename when searching only one target
file, simply give /dev/null as the second file.
bash$ grep Linux osinfo.txt /dev/null
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.
If there is a successful match, grep returns an exit status of
0, which makes it useful in a condition test in a script,
especially in combination with the -q option to suppress
output.
SUCCESS=0 # if grep lookup succeeds
word=Linux
filename=data.file
grep -q "$word" "$filename" # The "-q" option
#+ causes nothing to echo to stdout.
if [ $? -eq $SUCCESS ]
if grep -q "$word" "$filename" can replace lines 5 - 7.
then
echo "$word found in $filename"
else
echo "$word not found in $filename"
fi
Example 32-6 demonstrates how to use grep to search for a word
pattern in a system logfile.
Example 16-17. Emulating grep in a script
!/bin/bash
grp.sh: Rudimentary reimplementation of grep.
E_BADARGS=85
if [ -z "$1" ] # Check for argument to script.
then
echo "Usage: basename $0 pattern"
exit $E_BADARGS
fi
echo
for file in * # Traverse all files in $PWD.
do
output=$(sed -n /"$1"/p $file) # Command substitution.
if [ ! -z "$output" ] # What happens if "$output" is not quoted?
then
echo -n "$file: "
echo "$output"
fi # sed -ne "/$1/s|^|${file}: |p" is equivalent to above.
echo
done
echo
exit 0
Exercises:
- Add newlines to output, if more than one match in any given file.
- Add features.
How can grep search for two (or more) separate patterns? What
if you want grep to display all lines in a file or files that
contain both "pattern1" and "pattern2"?
One method is to pipe the result of grep pattern1 to grep
pattern2.
For example, given the following file:
Filename: tstfile
This is a sample file.
This is an ordinary text file.
This file does not contain any unusual text.
This file is not unusual.
Here is some text.
Now, let's search this file for lines containing both "file"
and "text" . . .
bash$ grep file tstfile
Filename: tstfile
This is a sample file.
This is an ordinary text file.
This file does not contain any unusual text.
This file is not unusual.
bash$ grep file tstfile | grep text
This is an ordinary text file.
This file does not contain any unusual text.
Now, for an interesting recreational use of grep . . .
Example 16-18. Crossword puzzle solver
!/bin/bash
cw-solver.sh
This is actually a wrapper around a one-liner (line 46).
Crossword puzzle and anagramming word game solver.
You know some of the letters in the word you're looking for,
- so you need a list of all valid words
- with the known letters in given positions.
For example: w...i....n
1???5????10
w in position 1, 3 unknowns, i in the 5th, 4 unknowns, n at the end.
(See comments at end of script.)
E_NOPATT=71
DICT=/usr/share/dict/word.lst
^^^^^^^^ Looks for word list here.
ASCII word list, one word per line.
If you happen to need an appropriate list,
- download the author's "yawl" word list package.
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
or
http://bash.deta.in/yawl-0.3.2.tar.gz
if [ -z "$1" ] # If no word pattern specified
then #+ as a command-line argument . . .
echo #+ . . . then . . .
echo "Usage:" #+ Usage message.
echo
echo ""$0" "pattern,""
echo "where "pattern" is in the form"
echo "xxx..x.x..."
echo
echo "The x's represent known letters,"
echo "and the periods are unknown letters (blanks)."
echo "Letters and periods can be in any position."
echo "For example, try: sh cw-solver.sh w...i....n"
echo
exit $E_NOPATT
fi
echo
===============================================
This is where all the work gets done.
grep ^"$1"$ "$DICT" # Yes, only one line!
| |
^ is start-of-word regex anchor.
$ is end-of-word regex anchor.
From Stupid Grep Tricks, vol. 1,
- a book the ABS Guide author may yet get around
- to writing . . . one of these days . . .
===============================================
echo
exit $? # Script terminates here.
If there are too many words generated,
- redirect the output to a file.
$ sh cw-solver.sh w...i....n
wellington
workingman
workingmen
egrep -- extended grep -- is the same as grep -E. This uses a
somewhat different, extended set of Regular Expressions, which
can make the search a bit more flexible. It also allows the
boolean | (or) operator.
bash $ egrep 'matches|Matches' file.txt
Line 1 matches.
Line 3 Matches.
Line 4 contains matches, but also Matches
fgrep -- fast grep -- is the same as grep -F. It does a
literal string search (no Regular Expressions), which
generally speeds things up a bit.
Note
On some Linux distros, egrep and fgrep are symbolic links to, or
aliases for grep, but invoked with the -E and -F options,
respectively.
Example 16-19. Looking up definitions in Webster's 1913
Dictionary
!/bin/bash
dict-lookup.sh
This script looks up definitions in the 1913 Webster's Dictionary.
This Public Domain dictionary is available for download
- from various sites, including
- Project Gutenberg (http://www.gutenberg.org/etext/247).
Convert it from DOS to UNIX format (with only LF at end of line)
- before using it with this script.
Store the file in plain, uncompressed ASCII text.
Set DEFAULT_DICTFILE variable below to path/filename.
E_BADARGS=85
MAXCONTEXTLINES=50 # Maximum number of lines to show.
DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt"
# Default dictionary file pathname.
# Change this as necessary.
Note:
This particular edition of the 1913 Webster's
- begins each entry with an uppercase letter
- (lowercase for the remaining characters).
Only the very first line of an entry begins this way,
- and that's why the search algorithm below works.
if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]
Must at least specify word to look up, and
- it must start with an uppercase letter.
then
echo "Usage: basename $0 Word-to-define [dictionary-file]"
echo
echo "Note: Word to look up must start with capital letter,"
echo "with the rest of the word in lowercase."
echo "--------------------------------------------"
echo "Examples: Abandon, Dictionary, Marking, etc."
exit $E_BADARGS
fi
if [ -z "$2" ] # May specify different dictionary
#+ as an argument to this script.
then
dictfile=$DEFAULT_DICTFILE
else
dictfile="$2"
fi
Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \" "$dictfile")
Definitions in form "Word ..."
And, yes, "fgrep" is fast enough
- to search even a very large text file.
Now, snip out just the definition block.
echo "$Definition" |
sed -n '1,/^[A-Z]/p' |
Print from first line of output
- to the first line of the next entry.
sed '$d' | sed '$d'
Delete last two lines of output
- (blank line and first line of next entry).
exit $?
Exercises:
- Modify the script to accept any type of alphabetic input
- (uppercase, lowercase, mixed case), and convert it
- to an acceptable format for processing.
- Convert the script to a GUI application,
- using something like 'gdialog' or 'zenity' . . .
The script will then no longer take its argument(s)
- Modify the script to parse one of the other available
- Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer.
Note
See also Example A-41 for an example of speedy fgrep lookup on a
large text file.
agrep (approximate grep) extends the capabilities of grep to
approximate matching. The search string may differ by a
specified number of characters from the resulting matches.
This utility is not part of the core Linux distribution.
Tip
To search compressed files, use zgrep, zegrep, or zfgrep. These also
work on non-compressed files, though slower than plain grep, egrep,
fgrep. They are handy for searching through a mixed set of files,
some compressed, some not.
To search bzipped files, use bzgrep.
look
The command look works like grep, but does a lookup on a
"dictionary," a sorted word list. By default, look searches
for a match in /usr/dict/words, but a different dictionary
file may be specified.
Example 16-20. Checking words in a list for validity
!/bin/bash
lookup: Does a dictionary lookup on each word in a data file.
file=words.data # Data file from which to read words to test.
echo
echo "Testing file $file"
echo
while [ "$word" != end ] # Last word in data file.
do # ^^^
read word # From data file, because of redirection at end of loop.
look $word > /dev/null # Don't want to display lines in dictionary file.
Searches for words in the file /usr/share/dict/words
#+ (usually a link to linux.words).
lookup=$? # Exit status of 'look' command.
if [ "$lookup" -eq 0 ]
then
echo "\"$word\" is valid."
else
echo "\"$word\" is invalid."
fi
done <"$file" # Redirects stdin to $file, so "reads" come from there.
echo
exit 0
Code below line will not execute because of "exit" command above.
Stephane Chazelas proposes the following, more concise alternative:
while read word && [[ $word != end ]]
do if look "$word" > /dev/null
then echo ""$word" is valid."
else echo ""$word" is invalid."
fi
done <"$file"
exit 0
sed, awk
Scripting languages especially suited for parsing text files
and command output. May be embedded singly or in combination
in pipes and shell scripts.
sed
Non-interactive "stream editor", permits using many ex
commands in batch mode. It finds many uses in shell scripts.
awk
Programmable file extractor and formatter, good for
manipulating and/or extracting fields (columns) in structured
text files. Its syntax is similar to C.
wc
wc gives a "word count" on a file or I/O stream:
bash $ wc /usr/share/doc/sed-4.1.2/README
13 70 447 README
[13 lines 70 words 447 characters]
wc -w gives only the word count.
wc -l gives only the line count.
wc -c gives only the byte count.
wc -m gives only the character count.
wc -L gives only the length of the longest line.
Using wc to count how many .txt files are in current working
directory:
$ ls *.txt | wc -l
Will work as long as none of the "*.txt" files
- have a linefeed embedded in their name.
Alternative ways of doing this are:
find . -maxdepth 1 -name *.txt -print0 | grep -cz .
(shopt -s nullglob; set -- *.txt; echo $#)
Thanks, S.C.
Using wc to total up the size of all the files whose names
begin with letters in the range d - h
bash$ wc [d-h]* | grep total | awk '{print $3}'
71832
Using wc to count the instances of the word "Linux" in the
main source file for this book.
bash$ grep Linux abs-book.sgml | wc -l
138
See also Example 16-39 and Example 20-8.
Certain commands include some of the functionality of wc as
options.
... | grep foo | wc -l
This frequently used construct can be more concisely rendered.
... | grep -c foo
Just use the "-c" (or "--count") option of grep.
Thanks, S.C.
tr
character translation filter.
Caution
Must use quoting and/or brackets, as appropriate. Quotes prevent the
shell from reinterpreting the special characters in tr command
sequences. Brackets should be quoted to prevent expansion by the
shell.
Either tr "A-Z" "*" <filename or tr A-Z \* <filename changes
all the uppercase letters in filename to asterisks (writes to
stdout). On some systems this may not work, but tr A-Z '[**]'
will.
The -d option deletes a range of characters.
echo "abcdef" # abcdef
echo "abcdef" | tr -d b-d # aef
tr -d 0-9 <filename
Deletes all digits from the file "filename".
The --squeeze-repeats (or -s) option deletes all but the first
instance of a string of consecutive characters. This option is
useful for removing excess whitespace.
bash$ echo "XXXXX" | tr --squeeze-repeats 'X'
X
The -c "complement" option inverts the character set to match.
With this option, tr acts only upon those characters not
matching the specified set.
bash$ echo "acfdeb123" | tr -c b-d +
+c+d+b++++
Note that tr recognizes POSIX character classes. [74]
bash$ echo "abcd2ef1" | tr '[:alpha:]' -
----2--1
Example 16-21. toupper: Transforms a file to all uppercase.
!/bin/bash
Changes a file to all uppercase.
E_BADARGS=85
if [ -z "$1" ] # Standard check for command-line arg.
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
tr a-z A-Z <"$1"
Same effect as above, but using POSIX character set notation:
tr '[:lower:]' '[:upper:]' <"$1"
Thanks, S.C.
Or even . . .
cat "$1" | tr a-z A-Z
Or dozens of other ways . . .
exit 0
Exercise:
Rewrite this script to give the option of changing a file
- to either upper or lowercase.
Hint: Use either the "case" or "select" command.
Example 16-22. lowercase: Changes all filenames in working
directory to lowercase.
!/bin/bash
Changes every filename in working directory to all lowercase.
Inspired by a script of John Dubois,
- which was translated into Bash by Chet Ramey,
- and considerably simplified by the author of the ABS Guide.
for filename in * # Traverse all files in directory.
do
fname=basename $filename
n=echo $fname | tr A-Z a-z # Change name to lowercase.
if [ "$fname" != "$n" ] # Rename only files not already lowercase.
then
mv $fname $n
fi
done
exit $?
Code below this line will not execute because of "exit".
--------------------------------------------------------#
To run it, delete script above line.
The above script will not work on filenames containing blanks or newlines.
Stephane Chazelas therefore suggests the following alternative:
for filename in * # Not necessary to use basename,
# since "*" won't return any file containing "/".
do n=echo "$filename/" | tr '[:upper:]' '[:lower:]'
POSIX char set notation.
Slash added so that trailing newlines are not
removed by command substitution.
Variable substitution:
n=${n%/} # Removes trailing slash, added above, from filename.
[[ $filename == $n ]] || mv "$filename" "$n"
# Checks if filename already lowercase.
done
exit $?
Example 16-23. du: DOS to UNIX text file conversion.
!/bin/bash
Du.sh: DOS to UNIX text file converter.
E_WRONGARGS=85
if [ -z "$1" ]
then
echo "Usage: basename $0 filename-to-convert"
exit $E_WRONGARGS
fi
NEWFILENAME=$1.unx
CR='\015' # Carriage return.
# 015 is octal ASCII code for CR.
# Lines in a DOS text file end in CR-LF.
# Lines in a UNIX text file end in LF only.
tr -d $CR < $1 > $NEWFILENAME
Delete CR's and write to new file.
echo "Original DOS text file is "$1"."
echo "Converted UNIX text file is "$NEWFILENAME"."
exit 0
Exercise:
Change the above script to convert from UNIX to DOS.
Example 16-24. rot13: ultra-weak encryption.
!/bin/bash
rot13.sh: Classic rot13 algorithm,
encryption that might fool a 3-year old
for about 10 minutes.
Usage: ./rot13.sh filename
or ./rot13.sh <filename
or ./rot13.sh and supply keyboard input (stdin)
cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' # "a" goes to "n", "b" to "o" ...
The cat "$@" construct
- permits input either from stdin or from files.
exit 0
Example 16-25. Generating "Crypto-Quote" Puzzles
!/bin/bash
crypto-quote.sh: Encrypt quotes
Will encrypt famous quotes in a simple monoalphabetic substitution.
The result is similar to the "Crypto Quote" puzzles
- seen in the Op Ed pages of the Sunday paper.
key=ETAOINSHRDLUBCFGJMQPVWZYXK
The "key" is nothing more than a scrambled alphabet.
Changing the "key" changes the encryption.
The 'cat "$@"' construction gets input either from stdin or from files.
If using stdin, terminate input with a Control-D.
Otherwise, specify filename as command-line parameter.
cat "$@" | tr "a-z" "A-Z" | tr "A-Z" "$key"
| to uppercase | encrypt
Will work on lowercase, uppercase, or mixed-case quotes.
Passes non-alphabetic characters through unchanged.
Try this script with something like:
"Nothing so needs reforming as other people's habits."
--Mark Twain
Output is:
"CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."
--BEML PZERC
To reverse the encryption:
cat "$@" | tr "$key" "A-Z"
This simple-minded cipher can be broken by an average 12-year old
- using only pencil and paper.
exit 0
Exercise:
Modify the script so that it will either encrypt or decrypt,
- depending on command-line argument(s).
Of course, tr lends itself to code obfuscation.
!/bin/bash
jabh.sh
x="wftedskaebjgdBstbdbsmnjgz"
echo $x | tr "a-z" 'oh, turtleneck Phrase Jar!'
Based on the Wikipedia "Just another Perl hacker" article.
tr variants
The tr utility has two historic variants. The BSD version does not
use brackets (tr a-z A-Z), but the SysV one does (tr '[a-z]'
'[A-Z]'). The GNU version of tr resembles the BSD one.
fold
A filter that wraps lines of input to a specified width. This
is especially useful with the -s option, which breaks lines at
word spaces (see Example 16-26 and Example A-1).
fmt
Simple-minded file formatter, used as a filter in a pipe to
"wrap" long lines of text output.
Example 16-26. Formatted file listing.
!/bin/bash
WIDTH=40 # 40 columns wide.
b=ls /usr/local/bin # Get a file listing...
echo $b | fmt -w $WIDTH
Could also have been done by
echo $b | fold - -s -w $WIDTH
exit 0
See also Example 16-5.
Tip
A powerful alternative to fmt is Kamil Toman's par utility, available
from [http://www.cs.berkeley.edu/~amc/Par/]
http://www.cs.berkeley.edu/~amc/Par/.
col
This deceptively named filter removes reverse line feeds from
an input stream. It also attempts to replace whitespace with
equivalent tabs. The chief use of col is in filtering the
output from certain text processing utilities, such as groff
and tbl.
column
Column formatter. This filter transforms list-type text output
into a "pretty-printed" table by inserting tabs at appropriate
places.
Example 16-27. Using column to format a directory listing
!/bin/bash
colms.sh
A minor modification of the example file in the "column" man page.
(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
; ls -l | sed 1d) | column -t
^^^^^^ ^^
The "sed 1d" in the pipe deletes the first line of output,
- which would be "total N",
- where "N" is the total number of files found by "ls -l".
The -t option to "column" pretty-prints a table.
exit 0
colrm
Column removal filter. This removes columns (characters) from
a file and writes the file, lacking the range of specified
columns, back to stdout. colrm 2 4 <filename removes the
second through fourth characters from each line of the text
file filename.
Caution
If the file contains tabs or nonprintable characters, this may cause
unpredictable behavior. In such cases, consider using expand and
unexpand in a pipe preceding colrm.
nl
Line numbering filter: nl filename lists filename to stdout,
but inserts consecutive numbers at the beginning of each
non-blank line. If filename omitted, operates on stdin.
The output of nl is very similar to cat -b, since, by default
nl does not list blank lines.
Example 16-28. nl: A self-numbering script.
!/bin/bash
line-number.sh
This script echoes itself twice to stdout with its lines numbered.
echo " line number = $LINENO" # 'nl' sees this as line 4
(nl does not number blank lines).
'cat -n' sees it correctly as line #6.
nl basename $0
echo; echo # Now, let's try it with 'cat -n'
cat -n basename $0
The difference is that 'cat -n' numbers the blank lines.
Note that 'nl -ba' will also do so.
exit 0
pr
Print formatting filter. This will paginate files (or stdout)
into sections suitable for hard copy printing or viewing on
screen. Various options permit row and column manipulation,
joining lines, setting margins, numbering lines, adding page
headers, and merging files, among other things. The pr command
combines much of the functionality of nl, paste, fold, column,
and expand.
pr -o 5 --width=65 fileZZZ | more gives a nice paginated
listing to screen of fileZZZ with margins set at 5 and 65.
A particularly useful option is -d, forcing double-spacing
(same effect as sed -G).
gettext
The GNU gettext package is a set of utilities for localizing
and translating the text output of programs into foreign
languages. While originally intended for C programs, it now
supports quite a number of programming and scripting
languages.
The gettext program works on shell scripts. See the info page.
msgfmt
A program for generating binary message catalogs. It is used
for localization.
iconv
A utility for converting file(s) to a different encoding
(character set). Its chief use is for localization.
Convert a string from UTF-8 to UTF-16 and print to the BookList
function write_utf8_string {
STRING=$1
BOOKLIST=$2
echo -n "$STRING" | iconv -f UTF8 -t UTF16 | \
cut -b 3- | tr -d \\n >> "$BOOKLIST"
}
From Peter Knowles' "booklistgen.sh" script
- for converting files to Sony Librie/PRS-50X format.
(http://booklistgensh.peterknowles.com)
recode
Consider this a fancier version of iconv, above. This very
versatile utility for converting a file to a different
encoding scheme. Note that recode is not part of the standard
Linux installation.
TeX, gs
TeX and Postscript are text markup languages used for
preparing copy for printing or formatted video display.
TeX is Donald Knuth's elaborate typsetting system. It is often
convenient to write a shell script encapsulating all the
options and arguments passed to one of these markup languages.
Ghostscript (gs) is a GPL-ed Postscript interpreter.
texexec
Utility for processing TeX and pdf files. Found in /usr/bin on
many Linux distros, it is actually a shell wrapper that calls
Perl to invoke Tex.
texexec --pdfarrange --result=Concatenated.pdf *pdf
Concatenates all the pdf files in the current working directory
- into the merged file, Concatenated.pdf . . .
(The --pdfarrange option repaginates a pdf file. See also --pdfcombine.)
The above command-line could be parameterized and put into a shell script.
enscript
Utility for converting plain text file to PostScript
For example, enscript filename.txt -p filename.ps produces the
PostScript output file filename.ps.
groff, tbl, eqn
Yet another text markup and display formatting language is
groff. This is the enhanced GNU version of the venerable UNIX
roff/troff display and typesetting package. Manpages use
groff.
The tbl table processing utility is considered part of groff,
as its function is to convert table markup into groff
commands.
The eqn equation processing utility is likewise part of groff,
and its function is to convert equation markup into groff
commands.
Example 16-29. manview: Viewing formatted manpages
!/bin/bash
manview.sh: Formats the source of a man page for viewing.
This script is useful when writing man page source.
It lets you look at the intermediate results on the fly
E_WRONGARGS=85
if [ -z "$1" ]
then
echo "Usage: basename $0 filename"
exit $E_WRONGARGS
fi
groff -Tascii -man $1 | less
From the man page for groff.
If the man page includes tables and/or equations,
- then the above code will barf.
The following line can handle such cases.
gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man
Thanks, S.C.
exit $? # See also the "maned.sh" script.
See also Example A-39.
lex, yacc
The lex lexical analyzer produces programs for pattern
matching. This has been replaced by the nonproprietary flex on
Linux systems.
The yacc utility creates a parser based on a set of
specifications. This has been replaced by the nonproprietary
bison on Linux systems.
________________________________________________________________
16.5. File and Archiving Commands
Archiving
tar
The standard UNIX archiving utility. [75] Originally a Tape
ARchiving program, it has developed into a general purpose
package that can handle all manner of archiving with all types
of destination devices, ranging from tape drives to regular
files to even stdout (see Example 3-4). GNU tar has been
patched to accept various compression filters, for example:
tar czvf archive_name.tar.gz *, which recursively archives and
gzips all files in a directory tree except dotfiles in the
current working directory ($PWD). [76]
Some useful tar options:
1. -c create (a new archive)
2. -x extract (files from existing archive)
3. --delete delete (files from existing archive)
Caution
This option will not work on magnetic tape devices.
4. -r append (files to existing archive)
5. -A append (tar files to existing archive)
6. -t list (contents of existing archive)
7. -u update archive
8. -d compare archive with specified filesystem
9. --after-date only process files with a date stamp after
specified date
10. -z gzip the archive
(compress or uncompress, depending on whether combined with
the -c or -x) option
11. -j bzip2 the archive
Caution
It may be difficult to recover data from a corrupted gzipped tar
archive. When archiving important files, make multiple backups.
shar
Shell archiving utility. The text and/or binary files in a
shell archive are concatenated without compression, and the
resultant archive is essentially a shell script, complete with
#!/bin/sh header, containing all the necessary unarchiving
commands, as well as the files themselves. Unprintable binary
characters in the target file(s) are converted to printable
ASCII characters in the output shar file. Shar archives still
show up in Usenet newsgroups, but otherwise shar has been
replaced by tar/gzip. The unshar command unpacks shar
archives.
The mailshar command is a Bash script that uses shar to
concatenate multiple files into a single one for e-mailing.
This script supports compression and uuencoding.
ar
Creation and manipulation utility for archives, mainly used
for binary object file libraries.
rpm
The Red Hat Package Manager, or rpm utility provides a wrapper
for source or binary archives. It includes commands for
installing and checking the integrity of packages, among other
things.
A simple rpm -i package_name.rpm usually suffices to install a
package, though there are many more options available.
Tip
rpm -qf identifies which package a file originates from.
bash$ rpm -qf /bin/ls
coreutils-5.2.1-31
Tip
rpm -qa gives a complete list of all installed rpm packages on a
given system. An rpm -qa package_name lists only the package(s)
corresponding to package_name.
bash$ rpm -qa
redhat-logos-1.1.3-1
glibc-2.2.4-13
cracklib-2.7-12
dosfstools-2.7-1
gdbm-1.8.0-10
ksymoops-2.4.1-1
mktemp-1.5-11
perl-5.6.0-17
reiserfs-utils-3.x.0j-2
...
bash$ rpm -qa docbook-utils
docbook-utils-0.6.9-2
bash$ rpm -qa docbook | grep docbook
docbook-dtd31-sgml-1.0-10
docbook-style-dsssl-1.64-3
docbook-dtd30-sgml-1.0-10
docbook-dtd40-sgml-1.0-11
docbook-utils-pdf-0.6.9-2
docbook-dtd41-sgml-1.0-10
docbook-utils-0.6.9-2
cpio
This specialized archiving copy command (copy input and
output) is rarely seen any more, having been supplanted by
tar/gzip. It still has its uses, such as moving a directory
tree. With an appropriate block size (for copying) specified,
it can be appreciably faster than tar.
Example 16-30. Using cpio to move a directory tree
!/bin/bash
Copying a directory tree using cpio.
Advantages of using 'cpio':
Speed of copying. It's faster than 'tar' with pipes.
Well suited for copying special files (named pipes, etc.)
ARGS=2
E_BADARGS=65
if [ $# -ne "$ARGS" ]
then
echo "Usage: basename $0 source destination"
exit $E_BADARGS
fi
source="$1"
destination="$2"
find "$source" -depth | cpio -admvp "$destination"
^^^^^ ^^^^^
Read the 'find' and 'cpio' info pages to decipher these options.
The above works only relative to $PWD (current directory) . . .
- full pathnames are specified.
Exercise:
Add code to check the exit status ($?) of the 'find | cpio' pipe
- and output appropriate error messages if anything went wrong.
exit $?
rpm2cpio
This command extracts a cpio archive from an rpm one.
Example 16-31. Unpacking an rpm archive
!/bin/bash
de-rpm.sh: Unpack an 'rpm' archive
: ${1?"Usage: basename $0 target-file"}
Must specify 'rpm' archive name as an argument.
TEMPFILE=$$.cpio # Tempfile with "unique" name.
# $$ is process ID of script.
rpm2cpio < $1 > $TEMPFILE # Converts rpm archive into
#+ cpio archive.
cpio --make-directories -F $TEMPFILE -i # Unpacks cpio archive.
rm -f $TEMPFILE # Deletes cpio archive.
exit 0
Exercise:
Add check for whether 1) "target-file" exists and
Hint: Parse output of 'file' command.
pax
The pax portable archive exchange toolkit facilitates periodic
file backups and is designed to be cross-compatible between
various flavors of UNIX. It was designed to replace tar and
cpio.
pax -wf daily_backup.pax ~/linux-server/files
Creates a tar archive of all files in the target directory.
Note that the options to pax must be in the correct order --
- pax -fw has an entirely different effect.
pax -f daily_backup.pax
Lists the files in the archive.
pax -rf daily_backup.pax ~/bsd-server/files
Restores the backed-up files from the Linux machine
Note that pax handles many of the standard archiving and
compression commands.
Compression
gzip
The standard GNU/UNIX compression utility, replacing the
inferior and proprietary compress. The corresponding
decompression command is gunzip, which is the equivalent of
gzip -d.
Note
The -c option sends the output of gzip to stdout. This is useful when
piping to other commands.
The zcat filter decompresses a gzipped file to stdout, as
possible input to a pipe or redirection. This is, in effect, a
cat command that works on compressed files (including files
processed with the older compress utility). The zcat command
is equivalent to gzip -dc.
Caution
On some commercial UNIX systems, zcat is a synonym for uncompress -c,
and will not work on gzipped files.
See also Example 7-7.
bzip2
An alternate compression utility, usually more efficient (but
slower) than gzip, especially on large files. The
corresponding decompression command is bunzip2.
Similar to the zcat command, bzcat decompresses a bzipped2-ed
file to stdout.
Note
Newer versions of tar have been patched with bzip2 support.
compress, uncompress
This is an older, proprietary compression utility found in
commercial UNIX distributions. The more efficient gzip has
largely replaced it. Linux distributions generally include a
compress workalike for compatibility, although gunzip can
unarchive files treated with compress.
Tip
The znew command transforms compressed files into gzipped ones.
sq
Yet another compression (squeeze) utility, a filter that works
only on sorted ASCII word lists. It uses the standard
invocation syntax for a filter, sq < input-file > output-file.
Fast, but not nearly as efficient as gzip. The corresponding
uncompression filter is unsq, invoked like sq.
Tip
The output of sq may be piped to gzip for further compression.
zip, unzip
Cross-platform file archiving and compression utility
compatible with DOS pkzip.exe. "Zipped" archives seem to be a
more common medium of file exchange on the Internet than
"tarballs."
unarc, unarj, unrar
These Linux utilities permit unpacking archives compressed
with the DOS arc.exe, arj.exe, and rar.exe programs.
lzma, unlzma, lzcat
Highly efficient Lempel-Ziv-Markov compression. The syntax of
lzma is similar to that of gzip. The
[http://www.7-zip.org/sdk.html] 7-zip Website has more
information.
xz, unxz, xzcat
A new high-efficiency compression tool, backward compatible
with lzma, and with an invocation syntax similar to gzip. For
more information, see the Wikipedia entry.
File Information
file
A utility for identifying file types. The command file
file-name will return a file specification for file-name, such
as ascii text or data. It references the magic numbers found
in /usr/share/magic, /etc/magic, or /usr/lib/magic, depending
on the Linux/UNIX distribution.
The -f option causes file to run in batch mode, to read from a
designated file a list of filenames to analyze. The -z option,
when used on a compressed target file, forces an attempt to
analyze the uncompressed file type.
bash$ file test.tar.gz
test.tar.gz: gzip compressed data, deflated,
last modified: Sun Sep 16 13:34:51 2001, os: Unix
bash file -z test.tar.gz
test.tar.gz: GNU tar archive (gzip compressed data, deflated,
last modified: Sun Sep 16 13:34:51 2001, os: Unix)
Find sh and Bash scripts in a given directory:
DIRECTORY=/usr/local/bin
KEYWORD=Bourne
Bourne and Bourne-Again shell scripts
file $DIRECTORY/* | fgrep $KEYWORD
Output:
/usr/local/bin/burn-cd: Bourne-Again shell script text executable
/usr/local/bin/burnit: Bourne-Again shell script text executable
/usr/local/bin/cassette.sh: Bourne shell script text executable
/usr/local/bin/copy-cd: Bourne-Again shell script text executable
. . .
Example 16-32. Stripping comments from C program files
!/bin/bash
strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program.
E_NOARGS=0
E_ARGERROR=66
E_WRONG_FILE_TYPE=67
if [ $# -eq "$E_NOARGS" ]
then
echo "Usage: basename $0 C-program-file" >&2 # Error message to stderr.
exit $E_ARGERROR
fi
Test for correct file type.
type=file $1 | awk '{ print $2, $3, $4, $5 }'
"file $1" echoes file type . . .
Then awk removes the first field, the filename . . .
Then the result is fed into the variable "type."
correct_type="ASCII C program text"
if [ "$type" != "$correct_type" ]
then
echo
echo "This script works on C program files only."
echo
exit $E_WRONG_FILE_TYPE
fi
Rather cryptic sed script:
sed '
/^/*/d
/.**//d
' $1
Easy to understand if you take several hours to learn sed fundamentals.
Need to add one more line to the sed script to deal with
- case where line of code has a comment following it on same line.
This is left as a non-trivial exercise.
Also, the above code deletes non-comment lines with a "*/" . . .
exit 0
Code below this line will not execute because of 'exit 0' above.
Stephane Chazelas suggests the following alternative:
usage() {
echo "Usage: basename $0 C-program-file" >&2
exit 1
}
WEIRD=echo -n -e '\377' # or WEIRD=$'\377'
[[ $# -eq 1 ]] || usage
case file "$1" in
"C program text") sed -e "s%/*%${WEIRD}%g;s%*/%${WEIRD}%g" "$1" \
| tr '\377\n' '\n\377' \
| sed -ne 'p;n' \
| tr -d '\n' | tr '\377' '\n';;
*) usage;;
esac
This is still fooled by things like:
printf("/*");
or
/* /* buggy embedded comment */
To handle all special cases (comments in strings, comments in string
- where there is a ", \" ...),
- the only way is to write a C parser (using lex or yacc perhaps?).
exit 0
which
which command gives the full path to "command." This is useful
for finding out whether a particular command or utility is
installed on the system.
$bash which rm
/usr/bin/rm
For an interesting use of this command, see Example 36-16.
whereis
Similar to which, above, whereis command gives the full path
to "command," but also to its manpage.
$bash whereis rm
rm: /bin/rm /usr/share/man/man1/rm.1.bz2
whatis
whatis command looks up "command" in the whatis database. This
is useful for identifying system commands and important
configuration files. Consider it a simplified man command.
$bash whatis whatis
whatis (1) - search the whatis database for complete words
Example 16-33. Exploring /usr/X11R6/bin
!/bin/bash
What are all those mysterious binaries in /usr/X11R6/bin?
DIRECTORY="/usr/X11R6/bin"
Try also "/bin", "/usr/bin", "/usr/local/bin", etc.
for file in $DIRECTORY/*
do
whatis basename $file # Echoes info about the binary.
done
exit 0
Note: For this to work, you must create a "whatis" database
- with /usr/sbin/makewhatis.
You may wish to redirect output of this script, like so:
./what.sh >>whatis.db
or view it a page at a time on stdout,
./what.sh | less
See also Example 11-3.
vdir
Show a detailed directory listing. The effect is similar to ls
-lb.
This is one of the GNU fileutils.
bash$ vdir
total 10
-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo
bash ls -l
total 10
-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo
locate, slocate
The locate command searches for files using a database stored
for just that purpose. The slocate command is the secure
version of locate (which may be aliased to slocate).
$bash locate hickson
/usr/lib/xephem/catalogs/hickson.edb
getfacl, setfacl
These commands retrieve or set the file access control list --
the owner, group, and file permissions.
bash$ getfacl *
file: test1.txt
owner: bozo
group: bozgrp
user::rw-
group::rw-
other::r--
file: test2.txt
owner: bozo
group: bozgrp
user::rw-
group::rw-
other::r--
bash$ setfacl -m u:bozo:rw yearly_budget.csv
bash$ getfacl yearly_budget.csv
file: yearly_budget.csv
owner: accountant
group: budgetgrp
user::rw-
user:bozo:rw-
user:accountant:rw-
group::rw-
mask::rw-
other::r--
readlink
Disclose the file that a symbolic link points to.
bash$ readlink /usr/bin/awk
../../bin/gawk
strings
Use the strings command to find printable strings in a binary
or data file. It will list sequences of printable characters
found in the target file. This might be handy for a quick 'n
dirty examination of a core dump or for looking at an unknown
graphic image file (strings image-file | more might show
something like JFIF, which would identify the file as a jpeg
graphic). In a script, you would probably parse the output of
strings with grep or sed. See Example 11-8 and Example 11-10.
Example 16-34. An "improved" strings command
!/bin/bash
wstrings.sh: "word-strings" (enhanced "strings" command)
This script filters the output of "strings" by checking it
- against a standard word list file.
This effectively eliminates gibberish and noise,
- and outputs only recognized words.
===========================================================
Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne $ARGS ]
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
===========================================================
MINSTRLEN=3 # Minimum string length.
WORDFILE=/usr/share/dict/linux.words # Dictionary file.
May specify a different word list file
- of one-word-per-line format.
For example, the "yawl" word-list package,
http://bash.deta.in/yawl-0.3.2.tar.gz
wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`
Translate output of 'strings' command with multiple passes of 'tr'.
"tr A-Z a-z" converts to lowercase.
"tr '[:space:]'" converts whitespace characters to Z's.
"tr -cs '[:alpha:]' Z" converts non-alphabetic characters to Z's,
- and squeezes multiple consecutive Z's.
"tr -s '\173-\377' Z" converts all characters past 'z' to Z's
- and squeezes multiple consecutive Z's,
- which gets rid of all the weird characters that the previous
- translation failed to deal with.
Finally, "tr Z ' '" converts all those Z's to whitespace,
- which will be seen as word separators in the loop below.
Note the technique of feeding/piping the output of 'tr' back to itself,
- but with different arguments and/or options on each successive pass.
for word in $wlist # Important:
# $wlist must not be quoted here.
# "$wlist" does not work.
# Why not?
do
strlen=${#word} # String length.
if [ "$strlen" -lt "$MINSTRLEN" ] # Skip over short strings.
then
continue
fi
grep -Fw $word "$WORDFILE" # Match whole words only.
^^^ # "Fixed strings" and
#+ "whole words" options.
done
exit $?
Comparison
diff, patch
diff: flexible file comparison utility. It compares the target
files line-by-line sequentially. In some applications, such as
comparing word dictionaries, it may be helpful to filter the
files through sort and uniq before piping them to diff. diff
file-1 file-2 outputs the lines in the files that differ, with
carets showing which file each particular line belongs to.
The --side-by-side option to diff outputs each compared file,
line by line, in separate columns, with non-matching lines
marked. The -c and -u options likewise make the output of the
command easier to interpret.
There are available various fancy frontends for diff, such as
sdiff, wdiff, xdiff, and mgdiff.
Tip
The diff command returns an exit status of 0 if the compared files
are identical, and 1 if they differ (or 2 when binary files are being
compared). This permits use of diff in a test construct within a
shell script (see below).
A common use for diff is generating difference files to be
used with patch The -e option outputs files suitable for ed or
ex scripts.
patch: flexible versioning utility. Given a difference file
generated by diff, patch can upgrade a previous version of a
package to a newer version. It is much more convenient to
distribute a relatively small "diff" file than the entire body
of a newly revised package. Kernel "patches" have become the
preferred method of distributing the frequent releases of the
Linux kernel.
patch -p1 <patch-file
Takes all the changes listed in 'patch-file'
and applies them to the files referenced therein.
This upgrades to a newer version of the package.
Patching the kernel:
cd /usr/src
gzip -cd patchXX.gz | patch -p0
Upgrading kernel source using 'patch'.
From the Linux kernel docs "README",
by anonymous author (Alan Cox?).
Note
The diff command can also recursively compare directories (for the
filenames present).
bash$ diff -r ~/notes1 ~/notes2
Only in /home/bozo/notes1: file02
Only in /home/bozo/notes1: file03
Only in /home/bozo/notes2: file04
Tip
Use zdiff to compare gzipped files.
Tip
Use diffstat to create a histogram (point-distribution graph) of
output from diff.
diff3, merge
An extended version of diff that compares three files at a
time. This command returns an exit value of 0 upon successful
execution, but unfortunately this gives no information about
the results of the comparison.
bash$ diff3 file-1 file-2 file-3
=> ==
1:1c
This is line 1 of "file-1".
2:1c
This is line 1 of "file-2".
3:1c
This is line 1 of "file-3"
The merge (3-way file merge) command is an interesting adjunct
to diff3. Its syntax is merge Mergefile file1 file2. The
result is to output to Mergefile the changes that lead from
file1 to file2. Consider this command a stripped-down version
of patch.
sdiff
Compare and/or edit two files in order to merge them into an
output file. Because of its interactive nature, this command
would find little use in a script.
cmp
The cmp command is a simpler version of diff, above. Whereas
diff reports the differences between two files, cmp merely
shows at what point they differ.
Note
Like diff, cmp returns an exit status of 0 if the compared files are
identical, and 1 if they differ. This permits use in a test construct
within a shell script.
Example 16-35. Using cmp to compare two files within a script.
!/bin/bash
file-comparison.sh
ARGS=2 # Two args to script expected.
E_BADARGS=85
E_UNREADABLE=86
if [ $# -ne "$ARGS" ]
then
echo "Usage: basename $0 file1 file2"
exit $E_BADARGS
fi
if [[ ! -r "$1" || ! -r "$2" ]]
then
echo "Both files to be compared must exist and be readable."
exit $E_UNREADABLE
fi
cmp $1 $2 &> /dev/null
Redirection to /dev/null buries the output of the "cmp" command.
cmp -s $1 $2 has same result ("-s" silent flag to "cmp")
Thank you Anders Gustavsson for pointing this out.
Also works with 'diff', i.e.,
if [ $? -eq 0 ] # Test exit status of "cmp" command.
then
echo "File "$1" is identical to file "$2"."
else
echo "File "$1" differs from file "$2"."
fi
exit 0
Tip
Use zcmp on gzipped files.
comm
Versatile file comparison utility. The files must be sorted
for this to be useful.
comm -options first-file second-file
comm file-1 file-2 outputs three columns:
+ column 1 = lines unique to file-1
+ column 2 = lines unique to file-2
+ column 3 = lines common to both.
The options allow suppressing output of one or more columns.
+ -1 suppresses column 1
+ -2 suppresses column 2
+ -3 suppresses column 3
+ -12 suppresses both columns 1 and 2, etc.
This command is useful for comparing "dictionaries" or word
lists -- sorted text files with one word per line.
Utilities
basename
Strips the path information from a file name, printing only
the file name. The construction basename $0 lets the script
know its name, that is, the name it was invoked by. This can
be used for "usage" messages if, for example a script is
called with missing arguments:
echo "Usage: basename $0 arg1 arg2 ... argn"
dirname
Strips the basename from a filename, printing only the path
information.
Note
basename and dirname can operate on any arbitrary string. The
argument does not need to refer to an existing file, or even be a
filename for that matter (see Example A-7).
Example 16-36. basename and dirname
!/bin/bash
address=/home/bozo/daily-journal.txt
echo "Basename of /home/bozo/daily-journal.txt = basename $address"
echo "Dirname of /home/bozo/daily-journal.txt = dirname $address"
echo
echo "My own home is basename ~/." # basename ~ also works.
echo "The home of my home is dirname ~/." # dirname ~ also works.
exit 0
split, csplit
These are utilities for splitting a file into smaller chunks.
Their usual use is for splitting up large files in order to
back them up on floppies or preparatory to e-mailing or
uploading them.
The csplit command splits a file according to context, the
split occuring where patterns are matched.
Example 16-37. A script that copies itself in sections
!/bin/bash
splitcopy.sh
A script that splits itself into chunks,
- then reassembles the chunks into an exact copy
CHUNKSIZE=4 # Size of first chunk of split files.
OUTPREFIX=xx # csplit prefixes, by default,
#+ files with "xx" ...
csplit "$0" "$CHUNKSIZE"
Some comment lines for padding . . .
Line 15
Line 16
Line 17
Line 18
Line 19
Line 20
cat "$OUTPREFIX"* > "$0.copy" # Concatenate the chunks.
rm "$OUTPREFIX"* # Get rid of the chunks.
exit $?
Encoding and Encryption
sum, cksum, md5sum, sha1sum
These are utilities for generating checksums. A checksum is a
number [77] mathematically calculated from the contents of a
file, for the purpose of checking its integrity. A script
might refer to a list of checksums for security purposes, such
as ensuring that the contents of key system files have not
been altered or corrupted. For security applications, use the
md5sum (message digest 5 checksum) command, or better yet, the
newer sha1sum (Secure Hash Algorithm). [78]
bash$ cksum /boot/vmlinuz
1670054224 804083 /boot/vmlinuz
bash$ echo -n "Top Secret" | cksum
3391003827 10
bash$ md5sum /boot/vmlinuz
0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz
bash$ echo -n "Top Secret" | md5sum
8babc97a6f62a4649716f4df8d61728f -
Note
The cksum command shows the size, in bytes, of its target, whether
file or stdout.
The md5sum and sha1sum commands display a dash when they receive
their input from stdout.
Example 16-38. Checking file integrity
!/bin/bash
file-integrity.sh: Checking whether files in a given directory
have been tampered with.
E_DIR_NOMATCH=80
E_BAD_DBFILE=81
dbfile=File_record.md5
Filename for storing records (database file).
set_up_database ()
{
echo ""$directory"" > "$dbfile"
Write directory name to first line of file.
md5sum "$directory"/* >> "$dbfile"
Append md5 checksums and filenames.
}
check_database ()
{
local n=0
local filename
local checksum
-------------------------------------------
This file check should be unnecessary,
#+ but better safe than sorry.
if [ ! -r "$dbfile" ]
then
echo "Unable to read checksum database file!"
exit $E_BAD_DBFILE
fi
-------------------------------------------
while read record[n]
do
directory_checked="${record[0]}"
if [ "$directory_checked" != "$directory" ]
then
echo "Directories do not match up!"
# Tried to use file for a different directory.
exit $E_DIR_NOMATCH
fi
if [ "$n" -gt 0 ] # Not directory name.
then
filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' )
# md5sum writes records backwards,
#+ checksum first, then filename.
checksum[n]=$( md5sum "${filename[n]}" )
if [ "${record[n]}" = "${checksum[n]}" ]
then
echo "${filename[n]} unchanged."
elif [ "`basename ${filename[n]}`" != "$dbfile" ]
# Skip over checksum database file,
#+ as it will change with each invocation of script.
# ---
# This unfortunately means that when running
#+ this script on $PWD, tampering with the
#+ checksum database file will not be detected.
# Exercise: Fix this.
then
echo "${filename[n]} : CHECKSUM ERROR!"
# File has been changed since last checked.
fi
fi
let "n+=1"
done <"$dbfile" # Read from checksum database file.
}
=================================================== #
main ()
if [ -z "$1" ]
then
directory="$PWD" # If not specified,
else #+ use current working directory.
directory="$1"
fi
clear # Clear screen.
echo " Running file integrity check on $directory"
echo
------------------------------------------------------------------ #
if [ ! -r "$dbfile" ] # Need to create database file?
then
echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo
set_up_database
fi
------------------------------------------------------------------ #
check_database # Do the actual work.
echo
You may wish to redirect the stdout of this script to a file,
- especially if the directory checked has many files in it.
exit 0
For a much more thorough file integrity check,
- consider the "Tripwire" package,
- http://sourceforge.net/projects/tripwire/.
Also see Example A-19, Example 36-16, and Example 10-2 for
creative uses of the md5sum command.
Note
There have been reports that the 128-bit md5sum can be cracked, so
the more secure 160-bit sha1sum is a welcome new addition to the
checksum toolkit.
bash$ md5sum testfile
e181e2c8720c60522c4c4c981108e367 testfile
bash$ sha1sum testfile
5d7425a9c08a66c3177f1e31286fa40986ffc996 testfile
Security consultants have demonstrated that even sha1sum can
be compromised. Fortunately, newer Linux distros include
longer bit-length sha224sum, sha256sum, sha384sum, and
sha512sum commands.
uuencode
This utility encodes binary files (images, sound files,
compressed files, etc.) into ASCII characters, making them
suitable for transmission in the body of an e-mail message or
in a newsgroup posting. This is especially useful where MIME
(multimedia) encoding is not available.
uudecode
This reverses the encoding, decoding uuencoded files back into
the original binaries.
Example 16-39. Uudecoding encoded files
!/bin/bash
Uudecodes all uuencoded files in current working directory.
lines=35 # Allow 35 lines for the header (very generous).
for File in * # Test all the files in $PWD.
do
search1=head -n $lines $File | grep begin | wc -w
search2=tail -n $lines $File | grep end | wc -w
Uuencoded files have a "begin" near the beginning,
#+ and an "end" near the end.
if [ "$search1" -gt 0 ]
then
if [ "$search2" -gt 0 ]
then
echo "uudecoding - $File -"
uudecode $File
fi
fi
done
Note that running this script upon itself fools it
- into thinking it is a uuencoded file,
- because it contains both "begin" and "end".
Exercise:
Modify this script to check each file for a newsgroup header,
- and skip to next if not found.
exit 0
Tip
The fold -s command may be useful (possibly in a pipe) to process
long uudecoded text messages downloaded from Usenet newsgroups.
mimencode, mmencode
The mimencode and mmencode commands process multimedia-encoded
e-mail attachments. Although mail user agents (such as pine or
kmail) normally handle this automatically, these particular
utilities permit manipulating such attachments manually from
the command-line or in batch processing mode by means of a
shell script.
crypt
At one time, this was the standard UNIX file encryption
utility. [79] Politically-motivated government regulations
prohibiting the export of encryption software resulted in the
disappearance of crypt from much of the UNIX world, and it is
still missing from most Linux distributions. Fortunately,
programmers have come up with a number of decent alternatives
to it, among them the author's very own
[ftp://metalab.unc.edu/pub/Linux/utils/file/cruft-0.2.tar.gz]
cruft (see Example A-4).
openssl
This is an Open Source implementation of Secure Sockets Layer
encryption.
To encrypt a file:
openssl aes-128-ecb -salt -in file.txt -out file.encrypted \
-pass pass:my_password
^^^^^^^^^^^ User-selected password.
aes-128-ecb is the encryption method chosen.
To decrypt an openssl-encrypted file:
openssl aes-128-ecb -d -salt -in file.encrypted -out file.txt \
-pass pass:my_password
^^^^^^^^^^^ User-selected password.
Piping openssl to/from tar makes it possible to encrypt an
entire directory tree.
To encrypt a directory:
sourcedir="/home/bozo/testfiles"
encrfile="encr-dir.tar.gz"
password=my_secret_password
tar czvf - "$sourcedir" |
openssl des3 -salt -out "$encrfile" -pass pass:"$password"
^^^^ Uses des3 encryption.
Writes encrypted file "encr-dir.tar.gz" in current working directory.
To decrypt the resulting tarball:
openssl des3 -d -salt -in "$encrfile" -pass pass:"$password" |
tar -xzv
Decrypts and unpacks into current working directory.
Of course, openssl has many other uses, such as obtaining
signed certificates for Web sites. See the info page.
shred
Securely erase a file by overwriting it multiple times with
random bit patterns before deleting it. This command has the
same effect as Example 16-61, but does it in a more thorough
and elegant manner.
This is one of the GNU fileutils.
Caution
Advanced forensic technology may still be able to recover the
contents of a file, even after application of shred.
Miscellaneous
mktemp
Create a temporary file [80] with a "unique" filename. When
invoked from the command-line without additional arguments, it
creates a zero-length file in the /tmp directory.
bash$ mktemp
/tmp/tmp.zzsvql3154
PREFIX=filename
tempfile=mktemp $PREFIX.XXXXXX
^^^^^^ Need at least 6 placeholders
If no filename template supplied,
- "tmp.XXXXXXXXXX" is the default.
echo "tempfile name = $tempfile"
tempfile name = filename.QA2ZpY
or something similar...
Creates a file of that name in the current working directory
- with 600 file permissions.
A "umask 177" is therefore unnecessary,
- but it's good programming practice nevertheless.
make
Utility for building and compiling binary packages. This can
also be used for any set of operations triggered by
incremental changes in source files.
The make command checks a Makefile, a list of file
dependencies and operations to be carried out.
The make utility is, in effect, a powerful scripting language
similar in many ways to Bash, but with the capability of
recognizing dependencies. For in-depth coverage of this useful
tool set, see the GNU software documentation site.
install
Special purpose file copying command, similar to cp, but
capable of setting permissions and attributes of the copied
files. This command seems tailormade for installing software
packages, and as such it shows up frequently in Makefiles (in
the make install : section). It could likewise prove useful in
installation scripts.
dos2unix
This utility, written by Benjamin Lin and collaborators,
converts DOS-formatted text files (lines terminated by CR-LF)
to UNIX format (lines terminated by LF only), and vice-versa.
ptx
The ptx [targetfile] command outputs a permuted index
(cross-reference list) of the targetfile. This may be further
filtered and formatted in a pipe, if necessary.
more, less
Pagers that display a text file or stream to stdout, one
screenful at a time. These may be used to filter the output of
stdout . . . or of a script.
An interesting application of more is to "test drive" a
command sequence, to forestall potentially unpleasant
consequences.
ls /home/bozo | awk '{print "rm -rf " $1}' | more
^^^^
Testing the effect of the following (disastrous) command-line:
ls /home/bozo | awk '{print "rm -rf " $1}' | sh
Hand off to the shell to execute . . . ^^
The less pager has the interesting property of doing a
formatted display of man page source. See Example A-39.
________________________________________________________________
16.6. Communications Commands
Certain of the following commands find use in network data transfer
and analysis, as well as in chasing spammers.
Information and Statistics
host
Searches for information about an Internet host by name or IP
address, using DNS.
bash$ host surfacemail.com
surfacemail.com. has address 202.92.42.236
ipcalc
Displays IP information for a host. With the -h option, ipcalc
does a reverse DNS lookup, finding the name of the host
(server) from the IP address.
bash$ ipcalc -h 202.92.42.236
HOSTNAME=surfacemail.com
nslookup
Do an Internet "name server lookup" on a host by IP address.
This is essentially equivalent to ipcalc -h or dig -x . The
command may be run either interactively or noninteractively,
i.e., from within a script.
The nslookup command has allegedly been "deprecated," but it
is still useful.
bash$ nslookup -sil 66.97.104.180
nslookup kuhleersparnis.ch
Server: 135.116.137.2
Address: 135.116.137.2#53
Non-authoritative answer:
Name: kuhleersparnis.ch
dig
Domain Information Groper. Similar to nslookup, dig does an
Internet name server lookup on a host. May be run from the
command-line or from within a script.
Some interesting options to dig are +time=N for setting a
query timeout to N seconds, +nofail for continuing to query
servers until a reply is received, and -x for doing a reverse
address lookup.
Compare the output of dig -x with ipcalc -h and nslookup.
bash$ dig -x 81.9.6.2
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;2.6.9.81.in-addr.arpa. IN PTR
;; AUTHORITY SECTION:
6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.
2002031705 900 600 86400 3600
;; Query time: 537 msec
;; SERVER: 135.116.137.2#53(135.116.137.2)
;; WHEN: Wed Jun 26 08:35:24 2002
;; MSG SIZE rcvd: 91
Example 16-40. Finding out where to report a spammer
!/bin/bash
spam-lookup.sh: Look up abuse contact to report a spammer.
Thanks, Michael Zick.
Check for command-line arg.
ARGCOUNT=1
E_WRONGARGS=85
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: basename $0 domain-name"
exit $E_WRONGARGS
fi
dig +short $1.contacts.abuse.net -c in -t txt
Also try:
dig +nssearch $1
Tries to find "authoritative name servers" and display SOA records.
The following also works:
whois -h whois.abuse.net $1
^^ ^^^^^^^^^^^^^^^ Specify host.
Can even lookup multiple spammers with this, i.e."
whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .
Exercise:
Expand the functionality of this script
- so that it automatically e-mails a notification
- to the responsible ISP's contact address(es).
Hint: use the "mail" command.
exit $?
spam-lookup.sh chinatietong.com
A known spam domain.
"crnet_mgr@chinatietong.com"
"crnet_tec@chinatietong.com"
"postmaster@chinatietong.com"
For a more elaborate version of this script,
- see the SpamViz home page, http://www.spamviz.net/index.html.
Example 16-41. Analyzing a spam domain
! /bin/bash
is-spammer.sh: Identifying spam domains
$Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $
Above line is RCS ID info.
This is a simplified version of the "is_spammer.bash
- script in the Contributed Scripts appendix.
is-spammer <domain.name>
Uses an external program: 'dig'
Tested with version: 9.2.4rc5
Uses functions.
Uses IFS to parse strings by assignment into arrays.
And even does something useful: checks e-mail blacklists.
Use the domain.name(s) from the text body:
http://www.good_stuff.spammer.biz/just_ignore_everything_else
^^^^^^^^^^^
Or the domain.name(s) from any e-mail address:
Really_Good_Offer@spammer.biz
as the only argument to this script.
(PS: have your Inet connection running)
So, to invoke this script in the above two instances:
is-spammer.sh spammer.biz
Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
No Whitespace == Line Feed:Carriage Return
No_WSP=$'\x0A'$'\x0D'
Field separator for dotted decimal ip addresses
ADR_IFS=${No_WSP}'.'
Get the dns text resource record.
get_txt <error_code> <list_query>
get_txt() {
# Parse $1 by assignment at the dots.
local -a dns
IFS=$ADR_IFS
dns=( $1 )
IFS=$WSP_IFS
if [ "${dns[0]}" == '127' ]
then
# See if there is a reason.
echo $(dig +short $2 -t txt)
fi
}
Get the dns address resource record.
chk_adr <rev_dns> <list_server>
chk_adr() {
local reply
local server
local reason
server=${1}${2}
reply=$( dig +short ${server} )
# If reply might be an error code . . .
if [ ${#reply} -gt 6 ]
then
reason=$(get_txt ${reply} ${server} )
reason=${reason:-${reply}}
fi
echo ${reason:-' not blacklisted.'}
}
Need to get the IP address from the name.
echo 'Get address of: '$1
ip_adr=$(dig +short $1)
dns_reply=${ip_adr:-' no answer '}
echo ' Found address: '${dns_reply}
A valid reply is at least 4 digits plus 3 dots.
if [ ${#ip_adr} -gt 6 ]
then
echo
declare query
# Parse by assignment at the dots.
declare -a dns
IFS=$ADR_IFS
dns=( ${ip_adr} )
IFS=$WSP_IFS
# Reorder octets into dns query order.
rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'
See: http://www.spamhaus.org (Conservative, well maintained)
echo -n 'spamhaus.org says: '
echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')
See: http://ordb.org (Open mail relays)
echo -n ' ordb.org says: '
echo $(chk_adr ${rev_dns} 'relays.ordb.org')
See: http://www.spamcop.net/ (You can report spammers here)
echo -n ' spamcop.net says: '
echo $(chk_adr ${rev_dns} 'bl.spamcop.net')
# other blacklist operations # #
See: http://cbl.abuseat.org.
echo -n ' abuseat.org says: '
echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')
See: http://dsbl.org/usage (Various mail relays)
echo
echo 'Distributed Server Listings'
echo -n ' list.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'list.dsbl.org')
echo -n ' multihop.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')
echo -n 'unconfirmed.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')
else
echo
echo 'Could not use that address.'
fi
exit 0
Exercises:
- Check arguments to script,
and exit with appropriate error message if necessary.
- Check if on-line at invocation of script,
and exit with appropriate error message if necessary.
- Substitute generic variables for "hard-coded" BHL domains.
- Set a time-out for the script using the "+time=" option
to the 'dig' command.
For a much more elaborate version of the above script, see
Example A-28.
traceroute
Trace the route taken by packets sent to a remote host. This
command works within a LAN, WAN, or over the Internet. The
remote host may be specified by an IP address. The output of
this command may be filtered by grep or sed in a pipe.
bash$ traceroute 81.9.6.2
traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms
2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms
3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms *
...
ping
Broadcast an ICMP ECHO_REQUEST packet to another machine,
either on a local or remote network. This is a diagnostic tool
for testing network connections, and it should be used with
caution.
bash$ ping localhost
PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709
usec
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286
usec
--- localhost.localdomain ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms
A successful ping returns an exit status of 0. This can be
tested for in a script.
HNAME=news-15.net # Notorious spammer.
HNAME=$HOST # Debug: test for localhost.
count=2 # Send only two pings.
if [[ ping -c $count "$HNAME" ]]
then
echo ""$HNAME" still up and broadcasting spam your way."
else
echo ""$HNAME" seems to be down. Pity."
fi
whois
Perform a DNS (Domain Name System) lookup. The -h option
permits specifying which particular whois server to query. See
Example 4-6 and Example 16-40.
finger
Retrieve information about users on a network. Optionally,
this command can display a user's ~/.plan, ~/.project, and
~/.forward files, if present.
bash$ finger
Login Name Tty Idle Login Time Office Office Phone
bozo Bozo Bozeman tty1 8 Jun 25 16:59 (:0)
bozo Bozo Bozeman ttyp0 Jun 25 16:59 (:0.0)
bozo Bozo Bozeman ttyp1 Jun 25 17:07 (:0.0)
bash$ finger bozo
Login: bozo Name: Bozo Bozeman
Directory: /home/bozo Shell: /bin/bash
Office: 2355 Clown St., 543-1234
On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle
On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle
On since Fri Aug 31 20:13 (MST) on pts/1
On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle
Mail last read Tue Jul 3 10:08 2007 (MST)
No Plan.
Out of security considerations, many networks disable finger
and its associated daemon. [81]
chfn
Change information disclosed by the finger command.
vrfy
Verify an Internet e-mail address.
This command seems to be missing from newer Linux distros.
Remote Host Access
sx, rx
The sx and rx command set serves to transfer files to and from
a remote host using the xmodem protocol. These are generally
part of a communications package, such as minicom.
sz, rz
The sz and rz command set serves to transfer files to and from
a remote host using the zmodem protocol. Zmodem has certain
advantages over xmodem, such as faster transmission rate and
resumption of interrupted file transfers. Like sx and rx,
these are generally part of a communications package.
ftp
Utility and protocol for uploading / downloading files to or
from a remote host. An ftp session can be automated in a
script (see Example 19-6 and Example A-4).
uucp, uux, cu
uucp: UNIX to UNIX copy. This is a communications package for
transferring files between UNIX servers. A shell script is an
effective way to handle a uucp command sequence.
Since the advent of the Internet and e-mail, uucp seems to
have faded into obscurity, but it still exists and remains
perfectly workable in situations where an Internet connection
is not available or appropriate. The advantage of uucp is that
it is fault-tolerant, so even if there is a service
interruption the copy operation will resume where it left off
when the connection is restored.
---
uux: UNIX to UNIX execute. Execute a command on a remote
system. This command is part of the uucp package.
---
cu: Call Up a remote system and connect as a simple terminal.
It is a sort of dumbed-down version of telnet. This command is
part of the uucp package.
telnet
Utility and protocol for connecting to a remote host.
Caution
The telnet protocol contains security holes and should therefore
probably be avoided. Its use within a shell script is not
recommended.
wget
The wget utility noninteractively retrieves or downloads files
from a Web or ftp site. It works well in a script.
wget -p http://www.xyz23.com/file01.html
The -p or --page-requisite option causes wget to fetch all files
- required to display the specified page.
wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
The -r option recursively follows and retrieves all links
wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2
The -c option lets wget resume an interrupted download.
This works with ftp servers and many HTTP sites.
Example 16-42. Getting a stock quote
!/bin/bash
quote-fetch.sh: Download a stock quote.
E_NOPARAMS=86
if [ -z "$1" ] # Must specify a stock (symbol) to fetch.
then echo "Usage: basename $0 stock-symbol"
exit $E_NOPARAMS
fi
stock_symbol=$1
file_suffix=.html
Fetches an HTML file, so name it appropriately.
URL='http://finance.yahoo.com/q?s='
Yahoo finance board, with stock query suffix.
wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
To look up stuff on http://search.yahoo.com:
URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"
wget -O "$savefilename" "${URL}"
Saves a list of relevant URLs.
exit $?
Exercises:
- Add a test to ensure the user running the script is on-line.
(Hint: parse the output of 'ps -ax' for "ppp" or "connect."
- Modify this script to fetch the local weather report,
- taking the user's zip code as an argument.
See also Example A-30 and Example A-31.
lynx
The lynx Web and file browser can be used inside a script
(with the -dump option) to retrieve a file from a Web or ftp
site noninteractively.
lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE
With the -traversal option, lynx starts at the HTTP URL
specified as an argument, then "crawls" through all links
located on that particular server. Used together with the
-crawl option, outputs page text to a log file.
rlogin
Remote login, initates a session on a remote host. This
command has security issues, so use ssh instead.
rsh
Remote shell, executes command(s) on a remote host. This has
security issues, so use ssh instead.
rcp
Remote copy, copies files between two different networked
machines.
rsync
Remote synchronize, updates (synchronizes) files between two
different networked machines.
bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/
Example 16-43. Updating FC4
!/bin/bash
fc4upd.sh
Script author: Frank Wang.
Slight stylistic modifications by ABS Guide author.
Used in ABS Guide with permission.
Download Fedora Core 4 update from mirror site using rsync.
Should also work for newer Fedora Cores -- 5, 6, . . .
Only download latest package if multiple versions exist,
URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/
URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/
DEST=${1:-/var/www/html/fedora/updates/}
LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt
PID_FILE=/var/run/${0##*/}.pid
E_RETURN=85 # Something unexpected happened.
General rsync options
-r: recursive download
-t: reserve time
-v: verbose
OPTS="-rtv --delete-excluded --delete-after --partial"
rsync include pattern
Leading slash causes absolute path name match.
INCLUDE=(
"/4/i386/kde-i18n-Chinese*"
^ ^
Quoting is necessary to prevent globbing.
)
rsync exclude pattern
Temporarily comment out unwanted pkgs using "#" . . .
EXCLUDE=(
/1
/2
/3
/testing
/4/SRPMS
/4/ppc
/4/x86_64
/4/i386/debug
"/4/i386/kde-i18n-*"
"/4/i386/openoffice.org-langpack-*"
"/4/i386/*i586.rpm"
"/4/i386/GFS-*"
"/4/i386/cman-*"
"/4/i386/dlm-*"
"/4/i386/gnbd-*"
"/4/i386/kernel-smp*"
"/4/i386/kernel-xen*"
"/4/i386/xen-*"
)
init () {
# Let pipe command return possible rsync error, e.g., stalled network.
set -o pipefail # Newly introduced in Bash, version 3.
TMP=${TMPDIR:-/tmp}/${0##*/}.$$ # Store refined download list.
trap "{
rm -f $TMP 2>/dev/null
}" EXIT # Clear temporary file on exit.
}
check_pid () {
Check if process exists.
if [ -s "$PID_FILE" ]; then
echo "PID file exists. Checking ..."
PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
if /bin/ps --pid $PID &>/dev/null; then
echo "Process $PID found. ${0##*/} seems to be running!"
/usr/bin/logger -t ${0##*/} \
"Process $PID found. ${0##*/} seems to be running!"
exit $E_RETURN
fi
echo "Process $PID not found. Start new process . . ."
fi
}
Set overall file update range starting from root or $URL,
- according to above patterns.
set_range () {
include=
exclude=
for p in "${INCLUDE[@]}"; do
include="$include --include \"$p\""
done
for p in "${EXCLUDE[@]}"; do
exclude="$exclude --exclude \"$p\""
done
}
Retrieve and refine rsync update list.
get_list () {
echo $$ > $PID_FILE || {
echo "Can't write to pid file $PID_FILE"
exit $E_RETURN
}
echo -n "Retrieving and refining update list . . ."
# Retrieve list -- 'eval' is needed to run rsync as a single command.
# $3 and $4 is the date and time of file creation.
# $5 is the full package name.
previous=
pre_file=
pre_date=0
eval /bin/nice /usr/bin/rsync \
-r $include $exclude $URL | \
egrep '^dr.x|^-r' | \
awk '{print $3, $4, $5}' | \
sort -k3 | \
{ while read line; do
# Get seconds since epoch, to filter out obsolete pkgs.
cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
# echo $cur_date
# Get file name.
cur_file=$(echo $line | awk '{print $3}')
# echo $cur_file
# Get rpm pkg name from file name, if possible.
if [[ $cur_file == *rpm ]]; then
pkg_name=$(echo $cur_file | sed -r -e \
's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')
else
pkg_name=
fi
# echo $pkg_name
if [ -z "$pkg_name" ]; then # If not a rpm file,
echo $cur_file >> $TMP #+ then append to download list.
elif [ "$pkg_name" != "$previous" ]; then # A new pkg found.
echo $pre_file >> $TMP # Output latest file.
previous=$pkg_name # Save current.
pre_date=$cur_date
pre_file=$cur_file
elif [ "$cur_date" -gt "$pre_date" ]; then
# If same pkg, but newer,
pre_date=$cur_date #+ then update latest pointer.
pre_file=$cur_file
fi
done
echo $pre_file >> $TMP # TMP contains ALL
#+ of refined list now.
# echo "subshell=$BASH_SUBSHELL"
} # Bracket required here to let final "echo $pre_file >> $TMP"
# Remained in the same subshell ( 1 ) with the entire loop.
RET=$? # Get return code of the pipe command.
[ "$RET" -ne 0 ] && {
echo "List retrieving failed with code $RET"
exit $E_RETURN
}
echo "done"; echo
}
Real rsync download part.
get_file () {
echo "Downloading..."
/bin/nice /usr/bin/rsync \
$OPTS \
--filter "merge,+/ $TMP" \
--exclude '*' \
$URL $DEST \
| /usr/bin/tee $LOG
RET=$?
--filter merge,+/ is crucial for the intention.
+ modifier means include and / means absolute path.
Then sorted list in $TMP will contain ascending dir name and
#+ prevent the following --exclude '*' from "shortcutting the circuit."
echo "Done"
rm -f $PID_FILE 2>/dev/null
return $RET
}
Main
init
check_pid
set_range
get_list
get_file
RET=$?
if [ "$RET" -eq 0 ]; then
/usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
else
/usr/bin/logger -t ${0##*/} \
"Fedora update mirrored with failure code: $RET"
fi
exit $RET
See also Example A-32.
Note
Using rcp, rsync, and similar utilities with security implications in
a shell script may not be advisable. Consider, instead, using ssh,
scp, or an expect script.
ssh
Secure shell, logs onto a remote host and executes commands
there. This secure replacement for telnet, rlogin, rcp, and
rsh uses identity authentication and encryption. See its
manpage for details.
Example 16-44. Using ssh
!/bin/bash
remote.bash: Using ssh.
This example by Michael Zick.
Used with permission.
Presumptions:
fd-2 isn't being captured ( '2>/dev/null' ).
ssh/sshd presumes stderr ('2') will display to user.
sshd is running on your machine.
For any 'standard' distribution, it probably is,
- and without any funky ssh-keygen having been done.
Try ssh to your machine from the command-line:
$ ssh $HOSTNAME
Without extra set-up you'll be asked for your password.
enter password
when done, $ exit
Did that work? If so, you're ready for more fun.
Try ssh to your machine as 'root':
$ ssh -l root $HOSTNAME
When asked for password, enter root's, not yours.
Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
Enter 'exit' when done.
The above gives you an interactive shell.
It is possible for sshd to be set up in a 'single command' mode,
- but that is beyond the scope of this example.
The only thing to note is that the following will work in
A basic, write stdout (local) command.
ls -l
Now the same basic command on a remote machine.
Pass a different 'USERNAME' 'HOSTNAME' if desired:
USER=${USERNAME:-$(whoami)}
HOST=${HOSTNAME:-$(hostname)}
Now excute the above command-line on the remote host,
- with all transmissions encrypted.
ssh -l ${USER} ${HOST} " ls -l "
The expected result is a listing of your username's home
- directory on the remote machine.
To see any difference, run this script from somewhere
- other than your home directory.
In other words, the Bash command is passed as a quoted line
- to the remote shell, which executes it on the remote machine.
In this case, sshd does ' bash -c "ls -l" ' on your behalf.
For information on topics such as not having to enter a
- password/passphrase for every command-line, see
exit 0
Caution
Within a loop, ssh may cause unexpected behavior. According to a
[http://groups-beta.google.com/group/comp.unix.shell/msg/dcb446b5fff7
d230] Usenet post in the comp.unix shell archives, ssh inherits the
loop's stdin. To remedy this, pass ssh either the -n or -f option.
Thanks, Jason Bechtel, for pointing this out.
scp
Secure copy, similar in function to rcp, copies files between
two different networked machines, but does so using
authentication, and with a security level similar to ssh.
Local Network
write
This is a utility for terminal-to-terminal communication. It
allows sending lines from your terminal (console or xterm) to
that of another user. The mesg command may, of course, be used
to disable write access to a terminal
Since write is interactive, it would not normally find use in
a script.
netconfig
A command-line utility for configuring a network adapter
(using DHCP). This command is native to Red Hat centric Linux
distros.
Mail
mail
Send or read e-mail messages.
This stripped-down command-line mail client works fine as a
command embedded in a script.
Example 16-45. A script that mails itself
!/bin/sh
self-mailer.sh: Self-mailing script
adr=${1:-whoami} # Default to current user, if not specified.
Typing 'self-mailer.sh wiseguy@superdupergenius.com'
- sends this script to that addressee.
Just 'self-mailer.sh' (no argument) sends the script
- to the person invoking it, for example, bozo@localhost.localdomain.
For more on the ${parameter:-default} construct,
- see the "Parameter Substitution" section
- of the "Variables Revisited" chapter.
============================================================================
cat $0 | mail -s "Script "basename $0" has mailed itself to you." "$adr"
============================================================================
Greetings from the self-mailing script.
A mischievous person has run this script,
- which has caused it to mail itself to you.
Apparently, some people have nothing better
echo "At date, script "basename $0" mailed to "$adr"."
exit 0
Note that the "mailx" command (in "send" mode) may be substituted
- for "mail" ... but with somewhat different options.
mailto
Similar to the mail command, mailto sends e-mail messages from
the command-line or in a script. However, mailto also permits
sending MIME (multimedia) messages.
mailstats
Show mail statistics. This command may be invoked only by
root.
root# mailstats
Statistics from Tue Jan 1 20:32:08 2008
M msgsfr bytes_from msgsto bytes_to msgsrej msgsdis msgsqur Mailer
4 1682 24118K 0 0K 0 0 0 esmtp
9 212 640K 1894 25131K 0 0 0 local
=====================================================================
T 1894 24758K 1894 25131K 0 0 0
C 414 0
vacation
This utility automatically replies to e-mails that the
intended recipient is on vacation and temporarily unavailable.
It runs on a network, in conjunction with sendmail, and is not
applicable to a dial-up POPmail account.
________________________________________________________________
16.7. Terminal Control Commands
Command affecting the console or terminal
tput
Initialize terminal and/or fetch information about it from
terminfo data. Various options permit certain terminal
operations: tput clear is the equivalent of clear; tput reset
is the equivalent of reset.
bash$ tput longname
xterm terminal emulator (X Window System)
Issuing a tput cup X Y moves the cursor to the (X,Y)
coordinates in the current terminal. A clear to erase the
terminal screen would normally precede this.
Some interesting options to tput are:
+ bold, for high-intensity text
+ smul, to underline text in the terminal
+ smso, to render text in reverse
+ sgr0, to reset the terminal parameters (to normal), without
clearing the screen
Example scripts using tput:
1. Example 36-15
2. Example 36-13
3. Example A-44
4. Example A-42
5. Example 27-2
Note that stty offers a more powerful command set for controlling a
terminal.
infocmp
This command prints out extensive information about the current
terminal. It references the terminfo database.
bash$ infocmp
Reconstructed via infocmp from file:
/usr/share/terminfo/r/rxvt
rxvt|rxvt terminal emulator (X Window System),
am, bce, eo, km, mir, msgr, xenl, xon,
colors#8, cols#80, it#8, lines#24, pairs#64,
acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
bel=^G, blink=\E[5m, bold=\E[1m,
civis=\E[?25l,
clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,
...
reset
Reset terminal parameters and clear text screen. As with clear, the
cursor and prompt reappear in the upper lefthand corner of the
terminal.
clear
The clear command simply clears the text screen at the console or in
an xterm. The prompt and cursor reappear at the upper lefthand corner
of the screen or xterm window. This command may be used either at the
command line or in a script. See Example 11-26.
resize
Echoes commands necessary to set $TERM and $TERMCAP to duplicate the
size (dimensions) of the current terminal.
bash$ resize
set noglob;
setenv COLUMNS '80';
setenv LINES '24';
unset noglob;
script
This utility records (saves to a file) all the user keystrokes at the
command-line in a console or an xterm window. This, in effect,
creates a record of a session.
________________________________________________________________
16.8. Math Commands
"Doing the numbers"
factor
Decompose an integer into prime factors.
bash$ factor 27417
27417: 3 13 19 37
Example 16-46. Generating prime numbers
!/bin/bash
primes2.sh
Generating prime numbers the quick-and-easy way,
- without resorting to fancy algorithms.
CEILING=10000 # 1 to 10000
PRIME=0
E_NOTPRIME=
is_prime ()
{
local factors
factors=( $(factor $1) ) # Load output of factor into array.
if [ -z "${factors[2]}" ]
Third element of "factors" array:
- ${factors[2]} is 2nd factor of argument.
If it is blank, then there is no 2nd factor,
- and the argument is therefore prime.
then
return $PRIME # 0
else
return $E_NOTPRIME # null
fi
}
echo
for n in $(seq $CEILING)
do
if is_prime $n
then
printf %5d $n
fi # ^ Five positions per number suffices.
done # For a higher $CEILING, adjust upward, as necessary.
echo
exit
bc
Bash can't handle floating point calculations, and it lacks
operators for certain important mathematical functions.
Fortunately, bc gallops to the rescue.
Not just a versatile, arbitrary precision calculation utility,
bc offers many of the facilities of a programming language. It
has a syntax vaguely resembling C.
Since it is a fairly well-behaved UNIX utility, and may
therefore be used in a pipe, bc comes in handy in scripts.
Here is a simple template for using bc to calculate a script
variable. This uses command substitution.
variable=$(echo "OPTIONS; OPERATIONS" | bc)
Example 16-47. Monthly Payment on a Mortgage
!/bin/bash
monthlypmt.sh: Calculates monthly payment on a mortgage.
This is a modification of code in the
- "mcalc" (mortgage calculator) package,
- Mendel Cooper (yours truly, the ABS Guide author).
http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz
echo
echo "Given the principal, interest rate, and term of a mortgage,"
echo "calculate the monthly payment."
bottom=1.0
echo
echo -n "Enter principal (no commas) "
read principal
echo -n "Enter interest rate (percent) " # If 12%, enter "12", not ".12".
read interest_r
echo -n "Enter term (months) "
read term
interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.
# ^^^^^^^^^^^^^^^^^ Divide by 100.
# "scale" determines how many decimal places.
interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)
top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Standard formula for figuring interest.
echo; echo "Please be patient. This may take a while."
let "months = $term - 1"
====================================================================
for ((x=$months; x > 0; x--))
do
bot=$(echo "scale=9; $interest_rate^$x" | bc)
bottom=$(echo "scale=9; $bottom+$bot" | bc)
bottom = $(($bottom + $bot"))
done
====================================================================
Rick Boivie pointed out a more efficient implementation
- of the above loop, which decreases computation time by 2/3.
for ((x=1; x <= $months; x++))
do
bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc)
done
And then he came up with an even more efficient alternative,
- one that cuts down the run time by about 95%!
bottom=`{
echo "scale=9; bottom=$bottom; interest_rate=$interest_rate"
for ((x=1; x <= $months; x++))
do
echo 'bottom = bottom * interest_rate + 1'
done
echo 'bottom'
} | bc` # Embeds a 'for loop' within command substitution.
On the other hand, Frank Wang suggests:
bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc)
Because . . .
The algorithm behind the loop
- is actually a sum of geometric proportion series.
The sum formula is e0(1-q^n)/(1-q),
- where e0 is the first element and q=e(n+1)/e(n)
- and n is the number of elements.
let "payment = $top/$bottom"
payment=$(echo "scale=2; $top/$bottom" | bc)
Use two decimal places for dollars and cents.
echo
echo "monthly payment = $$payment" # Echo a dollar sign in front of amount.
echo
exit 0
Exercises:
1) Filter input to permit commas in principal amount.
2) Filter input to permit interest to be entered as percent or decimal.
3) If you are really ambitious,
#+ expand this script to print complete amortization tables.
Example 16-48. Base Conversion
!/bin/bash
Shellscript: base.sh - print number to different bases (Bourne Shell)
Author : Heiner Steven (heiner.steven@odn.de)
Date : 07-03-95
Category : Desktop
$Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $
==> Above line is RCS ID info.
Description
Changes
21-03-95 stv fixed error occuring with 0xb as input (0.2)
==> Used in ABS Guide with the script author's permission.
==> Comments added by ABS Guide author.
NOARGS=85
PN=basename "$0" # Program name
VER=echo '$Revision: 1.2 $' | cut -d' ' -f2 # ==> VER=1.2
Usage () {
echo "$PN - print number to different bases, $VER (stv '95)
usage: $PN [number ...]
If no number is given, the numbers are read from standard input.
A number may be
binary (base 2) starting with 0b (i.e. 0b1100)
octal (base 8) starting with 0 (i.e. 014)
hexadecimal (base 16) starting with 0x (i.e. 0xc)
decimal otherwise (i.e. 12)" >&2
exit $NOARGS
} # ==> Prints usage message.
Msg () {
for i # ==> in [list] missing. Why?
do echo "$PN: $i" >&2
done
}
Fatal () { Msg "$@"; exit 66; }
PrintBases () {
# Determine base of the number
for i # ==> in [list] missing...
do # ==> so operates on command-line arg(s).
case "$i" in
0b*) ibase=2;; # binary
0x*|[a-f]*|[A-F]*) ibase=16;; # hexadecimal
0*) ibase=8;; # octal
[1-9]*) ibase=10;; # decimal
*)
Msg "illegal number $i - ignored"
continue;;
esac
# Remove prefix, convert hex digits to uppercase (bc needs this).
number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'`
# ==> Uses ":" as sed separator, rather than "/".
# Convert number to decimal
dec=`echo "ibase=$ibase; $number" | bc` # ==> 'bc' is calculator util
ity.
case "$dec" in
[0-9]*) ;; # number ok
*) continue;; # error: ignore
esac
# Print all conversions in one line.
# ==> 'here document' feeds command list to 'bc'.
echo `bc <<!
obase=16; "hex="; $dec
obase=10; "dec="; $dec
obase=8; "oct="; $dec
obase=2; "bin="; $dec
!
` | sed -e 's: : :g'
done
}
while [ $# -gt 0 ]
==> Is a "while loop" really necessary here,
==>+ since all the cases either break out of the loop
==>+ or terminate the script.
==> (Above comment by Paulo Marcel Coelho Aragao.)
do
case "$1" in
--) shift; break;;
-h) Usage;; # ==> Help message.
-*) Usage;;
*) break;; # First number
esac # ==> Error checking for illegal input might be appropriate.
shift
done
if [ $# -gt 0 ]
then
PrintBases "$@"
else # Read from stdin.
while read line
do
PrintBases $line
done
fi
exit
An alternate method of invoking bc involves using a here
document embedded within a command substitution block. This is
especially appropriate when a script needs to pass a list of
options and commands to bc.
variable=`bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
...or...
variable=$(bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
)
Example 16-49. Invoking bc using a here document
#!/bin/bash
# Invoking 'bc' using command substitution
# in combination with a 'here document'.
var1=`bc << EOF
18.33 * 19.78
EOFecho $var1 # 362.56
$( ... ) notation also works.
v1=23.53
v2=17.881
v3=83.501
v4=171.63
var2=$(bc << EOF
scale = 4
a = ( $v1 + $v2 )
b = ( $v3 * $v4 )
a * b + 15.35
EOF
)
echo $var2 # 593487.8452
var3=$(bc -l << EOF
scale = 9
s ( 1.7 )
EOF
)
Returns the sine of 1.7 radians.
The "-l" option calls the 'bc' math library.
echo $var3 # .991664810
Now, try it in a function...
hypotenuse () # Calculate hypotenuse of a right triangle.
{ # c = sqrt( a^2 + b^2 )
hyp=$(bc -l << EOF
scale = 9
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
Can't directly return floating point values from a Bash function.
But, can echo-and-capture:
echo "$hyp"
}
hyp=$(hypotenuse 3.68 7.31)
echo "hypotenuse = $hyp" # 8.184039344
exit 0
Example 16-50. Calculating PI
!/bin/bash
cannon.sh: Approximating PI by firing cannonballs.
Author: Mendel Cooper
License: Public Domain
Version 2.2, reldate 13oct08.
This is a very simple instance of a "Monte Carlo" simulation:
- a mathematical model of a real-life event,
- using pseudorandom numbers to emulate random chance.
Consider a perfectly square plot of land, 10000 units on a side.
This land has a perfectly circular lake in its center,
- with a diameter of 10000 units.
The plot is actually mostly water, except for land in the four corners.
(Think of it as a square with an inscribed circle.)
We will fire iron cannonballs from an old-style cannon
All the shots impact somewhere on the square,
- either in the lake or on the dry corners.
Since the lake takes up most of the area,
- most of the shots will SPLASH! into the water.
Just a few shots will THUD! into solid ground
- in the four corners of the square.
If we take enough random, unaimed shots at the square,
- Then the ratio of SPLASHES to total shots will approximate
The simplified explanation is that the cannon is actually
- shooting only at the upper right-hand quadrant of the square,
- i.e., Quadrant I of the Cartesian coordinate plane.
Theoretically, the more shots taken, the better the fit.
However, a shell script, as opposed to a compiled language
- with floating-point math built in, requires some compromises.
This decreases the accuracy of the simulation.
DIMENSION=10000 # Length of each side of the plot.
# Also sets ceiling for random integers generated.
MAXSHOTS=1000 # Fire this many shots.
# 10000 or more would be better, but would take too long.
PMULTIPLIER=4.0 # Scaling factor.
declare -r M_PI=3.141592654
# Actual 9-place value of PI, for comparison purposes.
get_random ()
{
SEED=$(head -n 1 /dev/urandom | od -N 1 | awk '{ print $2 }')
RANDOM=$SEED # From "seeding-random.sh"
#+ example script.
let "rnum = $RANDOM % $DIMENSION" # Range less than 10000.
echo $rnum
}
distance= # Declare global variable.
hypotenuse () # Calculate hypotenuse of a right triangle.
{ # From "alt-bc.sh" example.
distance=$(bc -l << EOF
scale = 0
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
Setting "scale" to zero rounds down result to integer value,
- a necessary compromise in this script.
It decreases the accuracy of this simulation.
}
==========================================================
main() {
"Main" code block, mimicking a C-language main() function.
Initialize variables.
shots=0
splashes=0
thuds=0
Pi=0
error=0
while [ "$shots" -lt "$MAXSHOTS" ] # Main loop.
do
xCoord=$(get_random) # Get random X and Y coords.
yCoord=$(get_random)
hypotenuse $xCoord $yCoord # Hypotenuse of
#+ right-triangle = distance.
((shots++))
printf "#%4d " $shots
printf "Xc = %4d " $xCoord
printf "Yc = %4d " $yCoord
printf "Distance = %5d " $distance # Distance from
#+ center of lake
#+ -- the "origin" --
#+ coordinate (0,0).
if [ "$distance" -le "$DIMENSION" ]
then
echo -n "SPLASH! "
((splashes++))
else
echo -n "THUD! "
((thuds++))
fi
Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc)
Multiply ratio by 4.0.
echo -n "PI ~ $Pi"
echo
done
echo
echo "After $shots shots, PI looks like approximately $Pi"
Tends to run a bit high,
- possibly due to round-off error and imperfect randomness of $RANDOM.
But still usually within plus-or-minus 5% . . .
- a pretty fair rough approximation.
error=$(echo "scale=9; $Pi - $M_PI" | bc)
pct_error=$(echo "scale=2; 100.0 * $error / $M_PI" | bc)
echo -n "Deviation from mathematical value of PI = $error"
echo " ($pct_error% error)"
echo
End of "main" code block.
}
==========================================================
exit 0
One might well wonder whether a shell script is appropriate for
- an application as complex and computation-intensive as a simulation.
There are at least two justifications.
- As a proof of concept: to show it can be done.
- To prototype and test the algorithms before rewriting
- it in a compiled high-level language.
See also Example A-37.
dc
The dc (desk calculator) utility is stack-oriented and uses
RPN (Reverse Polish Notation). Like bc, it has much of the
power of a programming language.
Similar to the procedure with bc, echo a command-string to dc.
echo "[Printing a string ... ]P" | dc
The P command prints the string between the preceding brackets.
And now for some simple arithmetic.
echo "7 8 * p" | dc # 56
Pushes 7, then 8 onto the stack,
- multiplies ("*" operator), then prints the result ("p" operator).
Most persons avoid dc, because of its non-intuitive input and
rather cryptic operators. Yet, it has its uses.
Example 16-51. Converting a decimal number to hexadecimal
!/bin/bash
hexconvert.sh: Convert a decimal number to hexadecimal.
E_NOARGS=85 # Command-line arg missing.
BASE=16 # Hexadecimal.
if [ -z "$1" ]
then # Need a command-line argument.
echo "Usage: $0 number"
exit $E_NOARGS
fi # Exercise: add argument validity checking.
hexcvt ()
{
if [ -z "$1" ]
then
echo 0
return # "Return" 0 if no arg passed to function.
fi
echo ""$1" "$BASE" o p" | dc
o sets radix (numerical base) of output.
p prints the top of stack.
For other options: 'man dc' ...
return
}
hexcvt "$1"
exit
Studying the info page for dc is a painful path to
understanding its intricacies. There seems to be a small,
select group of dc wizards who delight in showing off their
mastery of this powerful, but arcane utility.
bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc
Bash
dc <<< 10k5v1+2/p # 1.6180339887
^^^ Feed operations to dc using a Here String.
^^^ Pushes 10 and sets that as the precision (10k).
^^ Pushes 5 and takes its square root
(5v, v = square root).
^^ Pushes 1 and adds it to the running total (1+).
^^ Pushes 2 and divides the running total by that (2/).
^ Pops and prints the result (p)
The result is 1.6180339887 ...
... which happens to be the Pythagorean Golden Ratio, to 10 places.
Example 16-52. Factoring
!/bin/bash
factr.sh: Factor a number
MIN=2 # Will not work for number smaller than this.
E_NOARGS=85
E_TOOSMALL=86
if [ -z $1 ]
then
echo "Usage: $0 number"
exit $E_NOARGS
fi
if [ "$1" -lt "$MIN" ]
then
echo "Number to factor must be $MIN or greater."
exit $E_TOOSMALL
fi
Exercise: Add type checking (to reject non-integer arg).
echo "Factors of $1:"
echo "$1[p]s2[lip/dli%0=1dvsr]s12sid2%0=13sidvsr[dli%0=\
1lrli2+dsi!>.]ds.xd1<2" | dc
Above code written by Michel Charpentier charpov@cs.unh.edu
(as a one-liner, here broken into two lines for display purposes).
Used in ABS Guide with permission (thanks!).
exit
$ sh factr.sh 270138
2
3
11
4093
awk
Yet another way of doing floating point math in a script is
using awk's built-in math functions in a shell wrapper.
Example 16-53. Calculating the hypotenuse of a triangle
!/bin/bash
hypotenuse.sh: Returns the "hypotenuse" of a right triangle.
(square root of sum of squares of the "legs")
ARGS=2 # Script needs sides of triangle passed.
E_BADARGS=85 # Wrong number of arguments.
if [ $# -ne "$ARGS" ] # Test number of arguments to script.
then
echo "Usage: basename $0 side_1 side_2"
exit $E_BADARGS
fi
AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } '
command(s) / parameters passed to awk
Now, pipe the parameters to awk.
echo -n "Hypotenuse of $1 and $2 = "
echo $1 $2 | awk "$AWKSCRIPT"
^^^^^^^^^^^^
An echo-and-pipe is an easy way of passing shell parameters to awk.
exit
Exercise: Rewrite this script using 'bc' rather than awk.
Which method is more intuitive?
________________________________________________________________
16.9. Miscellaneous Commands
Command that fit in no special category
jot, seq
These utilities emit a sequence of integers, with a
user-selectable increment.
The default separator character between each integer is a
newline, but this can be changed with the -s option.
bash$ seq 5
1
2
3
4
5
bash$ seq -s : 5
1:2:3:4:5
Both jot and seq come in handy in a for loop.
Example 16-54. Using seq to generate loop arguments
!/bin/bash
Using "seq"
echo
for a in seq 80 # or for a in $( seq 80 )
Same as for a in 1 2 3 4 5 ... 80 (saves much typing!).
May also use 'jot' (if present on system).
do
echo -n "$a "
done # 1 2 3 4 5 ... 80
Example of using the output of a command to generate
the [list] in a "for" loop.
echo; echo
COUNT=80 # Yes, 'seq' also accepts a replaceable parameter.
for a in seq $COUNT # or for a in $( seq $COUNT )
do
echo -n "$a "
done # 1 2 3 4 5 ... 80
echo; echo
BEGIN=75
END=80
for a in seq $BEGIN $END
Giving "seq" two arguments starts the count at the first one,
- and continues until it reaches the second.
do
echo -n "$a "
done # 75 76 77 78 79 80
echo; echo
BEGIN=45
INTERVAL=5
END=80
for a in seq $BEGIN $INTERVAL $END
Giving "seq" three arguments starts the count at the first one,
- uses the second for a step interval,
- and continues until it reaches the third.
do
echo -n "$a "
done # 45 50 55 60 65 70 75 80
echo; echo
exit 0
A simpler example:
Create a set of 10 files,
- named file.1, file.2 . . . file.10.
COUNT=10
PREFIX=file
for filename in seq $COUNT
do
touch $PREFIX.$filename
Or, can do other operations,
#+ such as rm, grep, etc.
done
Example 16-55. Letter Count"
!/bin/bash
letter-count.sh: Counting letter occurrences in a text file.
Written by Stefano Palmeri.
Used in ABS Guide with permission.
Slightly modified by document author.
MINARGS=2 # Script requires at least two arguments.
E_BADARGS=65
FILE=$1
let LETTERS=$#-1 # How many letters specified (as command-line args).
# (Subtract 1 from number of command-line args.)
show_help(){
echo
echo Usage: `basename $0` file letters
echo Note: `basename $0` arguments are case sensitive.
echo Example: `basename $0` foobar.txt G n U L i N U x.
echo
}
Checks number of arguments.
if [ $# -lt $MINARGS ]; then
echo
echo "Not enough arguments."
echo
show_help
exit $E_BADARGS
fi
Checks if file exists.
if [ ! -f $FILE ]; then
echo "File \"$FILE\" does not exist."
exit $E_BADARGS
fi
Counts letter occurrences .
for n in seq $LETTERS; do
shift
if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then # Checks arg.
echo "$1" -\> `cat $FILE | tr -cd "$1" | wc -c` # Counting.
else
echo "$1 is not a single char."
fi
done
exit $?
This script has exactly the same functionality as letter-count2.sh,
Why?
Note
Somewhat more capable than seq, jot is a classic UNIX utility that is
not normally included in a standard Linux distro. However, the source
rpm is available for download from the
[http://www.mit.edu/afs/athena/system/rhlinux/athena-9.0/free/SRPMS/a
thena-jot-9.0-3.src.rpm] MIT repository.
Unlike seq, jot can generate a sequence of random numbers, using the
-r option.
bash$ jot -r 3 999
1069
1272
1428
getopt
The getopt command parses command-line options preceded by a
dash. This external command corresponds to the getopts Bash
builtin. Using getopt permits handling long options by means
of the -l flag, and this also allows parameter reshuffling.
Example 16-56. Using getopt to parse command-line options
!/bin/bash
Using getopt
Try the following when invoking this script:
sh ex33a.sh -a
sh ex33a.sh -abc
sh ex33a.sh -a -b -c
sh ex33a.sh -d
sh ex33a.sh -dXYZ
sh ex33a.sh -d XYZ
sh ex33a.sh -abcd
sh ex33a.sh -abcdZ
sh ex33a.sh -z
sh ex33a.sh a
Explain the results of each of the above.
E_OPTERR=65
if [ "$#" -eq 0 ]
then # Script needs at least one command-line argument.
echo "Usage $0 -[options a,b,c]"
exit $E_OPTERR
fi
set -- getopt "abcd:" "$@"
Sets positional parameters to command-line arguments.
What happens if you use "$*" instead of "$@"?
while [ ! -z "$1" ]
do
case "$1" in
-a) echo "Option \"a\"";;
-b) echo "Option \"b\"";;
-c) echo "Option \"c\"";;
-d) echo "Option \"d\" $2";;
*) break;;
esac
shift
done
It is usually better to use the 'getopts' builtin in a script.
See "ex33.sh."
exit 0
Note
As Peggy Russell points out:
It is often necessary to include an eval to correctly process
whitespace and quotes.
args=$(getopt -o a:bc:d -- "$@")
eval set -- "$args"
See Example 10-5 for a simplified emulation of getopt.
run-parts
The run-parts command [82] executes all the scripts in a
target directory, sequentially in ASCII-sorted filename order.
Of course, the scripts need to have execute permission.
The cron daemon invokes run-parts to run the scripts in the
/etc/cron.* directories.
yes
In its default behavior the yes command feeds a continuous
string of the character y followed by a line feed to stdout. A
control-C terminates the run. A different output string may be
specified, as in yes different string, which would continually
output different string to stdout.
One might well ask the purpose of this. From the command-line
or in a script, the output of yes can be redirected or piped
into a program expecting user input. In effect, this becomes a
sort of poor man's version of expect.
yes | fsck /dev/hda1 runs fsck non-interactively (careful!).
yes | rm -r dirname has same effect as rm -rf dirname
(careful!).
Warning
Caution advised when piping yes to a potentially dangerous system
command, such as fsck or fdisk. It might have unintended
consequences.
Note
The yes command parses variables, or more accurately, it echoes
parsed variables. For example:
bash$ yes $BASH_VERSION
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
. . .
This particular "feature" may be used to create a very large ASCII
file on the fly:
bash$ yes $PATH > huge_file.txt
Ctl-C
Hit Ctl-C very quickly, or you just might get more than you
bargained for. . . .
The yes command may be emulated in a very simple script
function.
yes ()
{ # Trivial emulation of "yes" ...
local DEFAULT_TEXT="y"
while [ true ] # Endless loop.
do
if [ -z "$1" ]
then
echo "$DEFAULT_TEXT"
else # If argument ...
echo "$1" # ... expand and echo it.
fi
done # The only things missing are the
} #+ --help and --version options.
banner
Prints arguments as a large vertical banner to stdout, using
an ASCII character (default '#'). This may be redirected to a
printer for hardcopy.
Note that banner has been dropped from many Linux distros,
presumably because it is no longer considered useful.
printenv
Show all the environmental variables set for a particular
user.
bash$ printenv | grep HOME
HOME=/home/bozo
lp
The lp and lpr commands send file(s) to the print queue, to be
printed as hard copy. [83] These commands trace the origin of
their names to the line printers of another era. [84]
bash$ lp file1.txt or bash lp <file1.txt
It is often useful to pipe the formatted output from pr to lp.
bash$ pr -options file1.txt | lp
Formatting packages, such as groff and Ghostscript may send
their output directly to lp.
bash$ groff -Tascii file.tr | lp
bash$ gs -options | lp file.ps
Related commands are lpq, for viewing the print queue, and
lprm, for removing jobs from the print queue.
tee
[UNIX borrows an idea from the plumbing trade.]
This is a redirection operator, but with a difference. Like
the plumber's tee, it permits "siphoning off" to a file the
output of a command or commands within a pipe, but without
affecting the result. This is useful for printing an ongoing
process to a file or paper, perhaps to keep track of it for
debugging purposes.
(redirection)
|----> to file
|
==========================|====================
command ---> command ---> |tee ---> command ---> ---> output of pipe
===============================================
cat listfile* | sort | tee check.file | uniq > result.file
^^^^^^^^^^^^^^ ^^^^
The file "check.file" contains the concatenated sorted "listfiles,"
- before the duplicate lines are removed by 'uniq.'
mkfifo
This obscure command creates a named pipe, a temporary
first-in-first-out buffer for transferring data between
processes. [85] Typically, one process writes to the FIFO, and
the other reads from it. See Example A-14.
!/bin/bash
This short script by Omair Eshkenazi.
Used in ABS Guide with permission (thanks!).
mkfifo pipe1 # Yes, pipes can be given names.
mkfifo pipe2 # Hence the designation "named pipe."
(cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2 <pipe1 &
ls -l | tr -s ' ' | cut -d' ' -f3,9- | tee pipe1 |
cut -d' ' -f2 | paste - pipe2
rm -f pipe1
rm -f pipe2
No need to kill background processes when script terminates (why not?).
exit $?
Now, invoke the script and explain the output:
sh mkfifo-example.sh
4830.tar.gz BOZO
pipe1 BOZO
pipe2 BOZO
mkfifo-example.sh BOZO
Mixed.msg BOZO
pathchk
This command checks the validity of a filename. If the
filename exceeds the maximum allowable length (255 characters)
or one or more of the directories in its path is not
searchable, then an error message results.
Unfortunately, pathchk does not return a recognizable error
code, and it is therefore pretty much useless in a script.
Consider instead the file test operators.
dd
Though this somewhat obscure and much feared data duplicator
command originated as a utility for exchanging data on
magnetic tapes between UNIX minicomputers and IBM mainframes,
it still has its uses. The dd command simply copies a file (or
stdin/stdout), but with conversions. Possible conversions
include ASCII/EBCDIC, [86] upper/lower case, swapping of byte
pairs between input and output, and skipping and/or truncating
the head or tail of the input file.
Converting a file to all uppercase:
dd if=$filename conv=ucase > $filename.uppercase
lcase # For lower case conversion
Some basic options to dd are:
+ if=INFILE
INFILE is the source file.
+ of=OUTFILE
OUTFILE is the target file, the file that will have the data
written to it.
+ bs=BLOCKSIZE
This is the size of each block of data being read and
written, usually a power of 2.
+ skip=BLOCKS
How many blocks of data to skip in INFILE before starting to
copy. This is useful when the INFILE has "garbage" or
garbled data in its header or when it is desirable to copy
only a portion of the INFILE.
+ seek=BLOCKS
How many blocks of data to skip in OUTFILE before starting
to copy, leaving blank data at beginning of OUTFILE.
+ count=BLOCKS
Copy only this many blocks of data, rather than the entire
INFILE.
+ conv=CONVERSION
Type of conversion to be applied to INFILE data before
copying operation.
A dd --help lists all the options this powerful utility takes.
Example 16-57. A script that copies itself
!/bin/bash
self-copy.sh
This script copies itself.
file_subscript=copy
dd if=$0 of=$0.$file_subscript 2>/dev/null
Suppress messages from dd: ^^^^^^^^^^^
exit $?
A program whose only output is its own source code
- is called a "quine" per Willard Quine.
Does this script qualify as a quine?
Example 16-58. Exercising dd
!/bin/bash
exercising-dd.sh
Script by Stephane Chazelas.
Somewhat modified by ABS Guide author.
infile=$0 # This script.
outfile=log.txt # Output file left behind.
n=8
p=11
dd if=$infile of=$outfile bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null
Extracts characters n to p (8 to 11) from this script ("bash").
echo -n "hello vertical world" | dd cbs=1 conv=unblock 2> /dev/null
Echoes "hello vertical world" vertically downward.
Why? A newline follows each character dd emits.
exit $?
To demonstrate just how versatile dd is, let's use it to
capture keystrokes.
Example 16-59. Capturing Keystrokes
!/bin/bash
dd-keypress.sh: Capture keystrokes without needing to press ENTER.
keypresses=4 # Number of keypresses to capture.
old_tty_setting=$(stty -g) # Save old terminal settings.
echo "Press $keypresses keys."
stty -icanon -echo # Disable canonical mode.
# Disable local echo.
keys=$(dd bs=1 count=$keypresses 2> /dev/null)
'dd' uses stdin, if "if" (input file) not specified.
stty "$old_tty_setting" # Restore old terminal settings.
echo "You pressed the "$keys" keys."
Thanks, Stephane Chazelas, for showing the way.
exit 0
The dd command can do random access on a data stream.
echo -n . | dd bs=1 seek=4 of=file conv=notrunc
The "conv=notrunc" option means that the output file
Thanks, S.C.
The dd command can copy raw data and disk images to and from
devices, such as floppies and tape drives (Example A-5). A
common use is creating boot floppies.
dd if=kernel-image of=/dev/fd0H1440
Similarly, dd can copy the entire contents of a floppy, even
one formatted with a "foreign" OS, to the hard drive as an
image file.
dd if=/dev/fd0 of=/home/bozo/projects/floppy.img
Likewise, dd can create bootable flash drives and SD cards.
dd if=image.iso of=/dev/sdb
Example 16-60. Preparing a bootable SD card for the Raspberry
Pi
!/bin/bash
rp.sdcard.sh
Preparing an SD card with a bootable image for the Raspberry Pi.
$1 = imagefile name
$2 = sdcard (device file)
Otherwise defaults to the defaults, see below.
DEFAULTbs=4M # Block size, 4 mb default.
DEFAULTif="2013-07-26-wheezy-raspbian.img" # Commonly used distro.
DEFAULTsdcard="/dev/mmcblk0" # May be different. Check!
ROOTUSER_NAME=root # Must run as root!
E_NOTROOT=81
E_NOIMAGE=82
username=$(id -nu) # Who is running this script?
if [ "$username" != "$ROOTUSER_NAME" ]
then
echo "This script must run as root or with root privileges."
exit $E_NOTROOT
fi
if [ -n "$1" ]
then
imagefile="$1"
else
imagefile="$DEFAULTif"
fi
if [ -n "$2" ]
then
sdcard="$2"
else
sdcard="$DEFAULTsdcard"
fi
if [ ! -e $imagefile ]
then
echo "Image file "$imagefile" not found!"
exit $E_NOIMAGE
fi
echo "Last chance to change your mind!"; echo
read -s -n1 -p "Hit a key to write $imagefile to $sdcard [Ctl-c to exit]."
echo; echo
echo "Writing $imagefile to $sdcard ..."
dd bs=$DEFAULTbs if=$imagefile of=$sdcard
exit $?
Exercises:
- Provide additional error checking.
- Have script autodetect device file for SD card (difficult!).
- Have script sutodetect image file (*img) in $PWD.
Other applications of dd include initializing temporary swap
files (Example 31-2) and ramdisks (Example 31-3). It can even
do a low-level copy of an entire hard drive partition,
although this is not necessarily recommended.
People (with presumably nothing better to do with their time)
are constantly thinking of interesting applications of dd.
Example 16-61. Securely deleting a file
!/bin/bash
blot-out.sh: Erase "all" traces of a file.
This script overwrites a target file alternately
- with random bytes, then zeros before finally deleting it.
After that, even examining the raw disk sectors by conventional methods
- will not reveal the original file data.
PASSES=7 # Number of file-shredding passes.
# Increasing this slows script execution,
#+ especially on large target files.
BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size,
#+ otherwise you get weird results.
E_BADARGS=70 # Various error exit codes.
E_NOT_FOUND=71
E_CHANGED_MIND=72
if [ -z "$1" ] # No filename specified.
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
file=$1
if [ ! -e "$file" ]
then
echo "File "$file" not found."
exit $E_NOT_FOUND
fi
echo; echo -n "Are you absolutely sure you want to blot out "$file" (y/n)? "
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
- ) echo "Blotting out file "$file".";;
esac
flength=$(ls -l "$file" | awk '{print $5}') # Field 5 is file length.
pass_count=1
chmod u+w "$file" # Allow overwriting/deleting the file.
echo
while [ "$pass_count" -le "$PASSES" ]
do
echo "Pass #$pass_count"
sync # Flush buffers.
dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength
# Fill with random bytes.
sync # Flush buffers again.
dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength
# Fill with zeros.
sync # Flush buffers yet again.
let "pass_count += 1"
echo
done
rm -f $file # Finally, delete scrambled and shredded file.
sync # Flush buffers a final time.
echo "File "$file" blotted out and deleted."; echo
exit 0
This is a fairly secure, if inefficient and slow method
- of thoroughly "shredding" a file.
The "shred" command, part of the GNU "fileutils" package,
- does the same thing, although more efficiently.
The file cannot not be "undeleted" or retrieved by normal methods.
However . . .
- this simple method would not likely withstand
- sophisticated forensic analysis.
This script may not play well with a journaled file system.
Exercise (difficult): Fix it so it does.
Tom Vier's "wipe" file-deletion package does a much more thorough job
- of file shredding than this simple script.
http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2
For an in-depth analysis on the topic of file deletion and security,
- see Peter Gutmann's paper,
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
See also the dd thread entry in the bibliography.
od
The od, or octal dump filter converts input (or files) to
octal (base-8) or other bases. This is useful for viewing or
processing binary data files or otherwise unreadable system
device files, such as /dev/urandom, and as a filter for binary
data.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
Sample output: 1324725719, 3918166450, 2989231420, etc.
From rnd.sh example script, by Stéphane Chazelas
See also Example 9-16 and Example A-36.
hexdump
Performs a hexadecimal, octal, decimal, or ASCII dump of a
binary file. This command is the rough equivalent of od,
above, but not nearly as useful. May be used to view the
contents of a binary file, in combination with dd and less.
dd if=/bin/ls | hexdump -C | less
The -C option nicely formats the output in tabular form.
objdump
Displays information about an object file or binary executable
in either hexadecimal form or as a disassembled listing (with
the -d option).
bash$ objdump -d /bin/ls
/bin/ls: file format elf32-i386
Disassembly of section .init:
080490bc <.init>:
80490bc: 55 push %ebp
80490bd: 89 e5 mov %esp,%ebp
. . .
mcookie
This command generates a "magic cookie," a 128-bit
(32-character) pseudorandom hexadecimal number, normally used
as an authorization "signature" by the X server. This also
available for use in a script as a "quick 'n dirty" random
number.
random000=$(mcookie)
Of course, a script could use md5sum for the same purpose.
Generate md5 checksum on the script itself.
random001=md5sum $0 | awk '{print $1}'
Uses 'awk' to strip off the filename.
The mcookie command gives yet another way to generate a
"unique" filename.
Example 16-62. Filename generator
!/bin/bash
tempfile-name.sh: temp filename generator
BASE_STR=mcookie # 32-character magic cookie.
POS=11 # Arbitrary position in magic cookie string.
LEN=5 # Get $LEN consecutive characters.
prefix=temp # This is, after all, a "temp" file.
# For more "uniqueness," generate the
#+ filename prefix using the same method
#+ as the suffix, below.
suffix=${BASE_STR:POS:LEN}
# Extract a 5-character string,
#+ starting at position 11.
temp_filename=$prefix.$suffix
# Construct the filename.
echo "Temp filename = "$temp_filename""
sh tempfile-name.sh
Temp filename = temp.e19ea
Compare this method of generating "unique" filenames
- with the 'date' method in ex51.sh.
exit 0
units
This utility converts between different units of measure.
While normally invoked in interactive mode, units may find use
in a script.
Example 16-63. Converting meters to miles
!/bin/bash
unit-conversion.sh
Must have 'units' utility installed.
convert_units () # Takes as arguments the units to convert.
{
cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}')
Strip off everything except the actual conversion factor.
echo "$cf"
}
Unit1=miles
Unit2=meters
cfactor=convert_units $Unit1 $Unit2
quantity=3.73
result=$(echo $quantity*$cfactor | bc)
echo "There are $result $Unit2 in $quantity $Unit1."
What happens if you pass incompatible units,
- such as "acres" and "miles" to the function?
exit 0
Exercise: Edit this script to accept command-line parameters,
with appropriate error checking, of course.
m4
A hidden treasure, m4 is a powerful macro [87] processing
filter, virtually a complete language. Although originally
written as a pre-processor for RatFor, m4 turned out to be
useful as a stand-alone utility. In fact, m4 combines some of
the functionality of eval, tr, and awk, in addition to its
extensive macro expansion facilities.
The April, 2002 issue of Linux Journal has a very nice article
on m4 and its uses.
Example 16-64. Using m4
!/bin/bash
m4.sh: Using the m4 macro processor
Strings
string=abcdA01
echo "len($string)" | m4 # 7
echo "substr($string,4)" | m4 # A01
echo "regexp($string,[0-1][0-1],&Z)" | m4 # 01Z
Arithmetic
var=99
echo "incr($var)" | m4 # 100
echo "eval($var / 3)" | m4 # 33
exit
xmessage
This X-based variant of echo pops up a message/query window on
the desktop.
xmessage Left click to continue -button okay
zenity
The [http://freshmeat.net/projects/zenity] zenity utility is
adept at displaying GTK+ dialog widgets and very suitable for
scripting purposes.
doexec
The doexec command enables passing an arbitrary list of
arguments to a binary executable. In particular, passing
argv[0] (which corresponds to $0 in a script) lets the
executable be invoked by various names, and it can then carry
out different sets of actions, according to the name by which
it was called. What this amounts to is roundabout way of
passing options to an executable.
For example, the /usr/local/bin directory might contain a
binary called "aaa". Invoking doexec /usr/local/bin/aaa list
would list all those files in the current working directory
beginning with an "a", while invoking (the same executable
with) doexec /usr/local/bin/aaa delete would delete those
files.
Note
The various behaviors of the executable must be defined within the
code of the executable itself, analogous to something like the
following in a shell script:
case basename $0 in
"name1" ) do_something;;
"name2" ) do_something_else;;
"name3" ) do_yet_another_thing;;
esac
dialog
The dialog family of tools provide a method of calling
interactive "dialog" boxes from a script. The more elaborate
variations of dialog -- gdialog, Xdialog, and kdialog --
actually invoke X-Windows widgets.
sox
The sox, or "sound exchange" command plays and performs
transformations on sound files. In fact, the /usr/bin/play
executable (now deprecated) is nothing but a shell wrapper for
sox.
For example, sox soundfile.wav soundfile.au changes a WAV
sound file into a (Sun audio format) AU sound file.
Shell scripts are ideally suited for batch-processing sox
operations on sound files. For examples, see the Linux Radio
Timeshift HOWTO and the MP3do Project.
________________________________________________________________
Chapter 17. System and Administrative Commands
The startup and shutdown scripts in /etc/rc.d illustrate the uses
(and usefulness) of many of these comands. These are usually invoked
by root and used for system maintenance or emergency filesystem
repairs. Use with caution, as some of these commands may damage your
system if misused.
Users and Groups
users
Show all logged on users. This is the approximate equivalent
of who -q.
groups
Lists the current user and the groups she belongs to. This
corresponds to the $GROUPS internal variable, but gives the
group names, rather than the numbers.
bash$ groups
bozita cdrom cdwriter audio xgrp
bash$ echo $GROUPS
501
chown, chgrp
The chown command changes the ownership of a file or files.
This command is a useful method that root can use to shift
file ownership from one user to another. An ordinary user may
not change the ownership of files, not even her own files.
[88]
root# chown bozo *.txt
The chgrp command changes the group ownership of a file or
files. You must be owner of the file(s) as well as a member of
the destination group (or root) to use this operation.
chgrp --recursive dunderheads *.data
The "dunderheads" group will now own all the "*.data" files
- all the way down the $PWD directory tree (that's what "recursive" means).
useradd, userdel
The useradd administrative command adds a user account to the
system and creates a home directory for that particular user,
if so specified. The corresponding userdel command removes a
user account from the system [89] and deletes associated
files.
Note
The adduser command is a synonym for useradd and is usually a
symbolic link to it.
usermod
Modify a user account. Changes may be made to the password,
group membership, expiration date, and other attributes of a
given user's account. With this command, a user's password may
be locked, which has the effect of disabling the account.
groupmod
Modify a given group. The group name and/or ID number may be
changed using this command.
id
The id command lists the real and effective user IDs and the
group IDs of the user associated with the current process.
This is the counterpart to the $UID, $EUID, and $GROUPS
internal Bash variables.
bash$ id
uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)
bash$ echo $UID
501
Note
The id command shows the effective IDs only when they differ from the
real ones.
Also see Example 9-5.
lid
The lid (list ID) command shows the group(s) that a given user
belongs to, or alternately, the users belonging to a given
group. May be invoked only by root.
root# lid bozo
bozo(gid=500)
root# lid daemon
bin(gid=1)
daemon(gid=2)
adm(gid=4)
lp(gid=7)
who
Show all users logged on to the system.
bash$ who
bozo tty1 Apr 27 17:45
bozo pts/0 Apr 27 17:46
bozo pts/1 Apr 27 17:47
bozo pts/2 Apr 27 17:49
The -m gives detailed information about only the current user.
Passing any two arguments to who is the equivalent of who -m,
as in who am i or who The Man.
bash$ who -m
localhost.localdomain!bozo pts/2 Apr 27 17:49
whoami is similar to who -m, but only lists the user name.
bash$ whoami
bozo
w
Show all logged on users and the processes belonging to them.
This is an extended version of who. The output of w may be
piped to grep to find a specific user and/or process.
bash$ w | grep startx
bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx
logname
Show current user's login name (as found in /var/run/utmp).
This is a near-equivalent to whoami, above.
bash$ logname
bozo
bash$ whoami
bozo
However . . .
bash$ su
Password: ......
bash# whoami
root
bash# logname
bozo
Note
While logname prints the name of the logged in user, whoami gives the
name of the user attached to the current process. As we have just
seen, sometimes these are not the same.
su
Runs a program or script as a substitute user. su rjones
starts a shell as user rjones. A naked su defaults to root.
See Example A-14.
sudo
Runs a command as root (or another user). This may be used in
a script, thus permitting a regular user to run the script.
!/bin/bash
Some commands.
sudo cp /root/secretfile /home/bozo/secret
Some more commands.
The file /etc/sudoers holds the names of users permitted to
invoke sudo.
passwd
Sets, changes, or manages a user's password.
The passwd command can be used in a script, but probably
should not be.
Example 17-1. Setting a new password
!/bin/bash
setnew-password.sh: For demonstration purposes only.
Not a good idea to actually run this script.
This script must be run as root.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
E_NOSUCHUSER=70
SUCCESS=0
if [ "$UID" -ne "$ROOT_UID" ]
then
echo; echo "Only root can run this script."; echo
exit $E_WRONG_USER
else
echo
echo "You should know better than to run this script, root."
echo "Even root users get the blues... "
echo
fi
username=bozo
NEWPASSWORD=security_violation
Check if bozo lives here.
grep -q "$username" /etc/passwd
if [ $? -ne $SUCCESS ]
then
echo "User $username does not exist."
echo "No password changed."
exit $E_NOSUCHUSER
fi
echo "$NEWPASSWORD" | passwd --stdin "$username"
The '--stdin' option to 'passwd' permits
- getting a new password from stdin (or a pipe).
echo; echo "User $username's password changed!"
Using the 'passwd' command in a script is dangerous.
exit 0
The passwd command's -l, -u, and -d options permit locking,
unlocking, and deleting a user's password. Only root may use
these options.
ac
Show users' logged in time, as read from /var/log/wtmp. This
is one of the GNU accounting utilities.
bash$ ac
total 68.08
last
List last logged in users, as read from /var/log/wtmp. This
command can also show remote logins.
For example, to show the last few times the system rebooted:
bash$ last reboot
reboot system boot 2.6.9-1.667 Fri Feb 4 18:18 (00:02)
reboot system boot 2.6.9-1.667 Fri Feb 4 15:20 (01:27)
reboot system boot 2.6.9-1.667 Fri Feb 4 12:56 (00:49)
reboot system boot 2.6.9-1.667 Thu Feb 3 21:08 (02:17)
. . .
wtmp begins Tue Feb 1 12:50:09 2005
newgrp
Change user's group ID without logging out. This permits
access to the new group's files. Since users may be members of
multiple groups simultaneously, this command finds only
limited use.
Note
Kurt Glaesemann points out that the newgrp command could prove
helpful in setting the default group permissions for files a user
writes. However, the chgrp command might be more convenient for this
purpose.
Terminals
tty
Echoes the name (filename) of the current user's terminal.
Note that each separate xterm window counts as a different
terminal.
bash$ tty
/dev/pts/1
stty
Shows and/or changes terminal settings. This complex command,
used in a script, can control terminal behavior and the way
output displays. See the info page, and study it carefully.
Example 17-2. Setting an erase character
!/bin/bash
erase.sh: Using "stty" to set an erase character when reading input.
echo -n "What is your name? "
read name # Try to backspace
#+ to erase characters of input.
# Problems?
echo "Your name is $name."
stty erase '#' # Set "hashmark" (#) as erase character.
echo -n "What is your name? "
read name # Use # to erase last character typed.
echo "Your name is $name."
exit 0
Even after the script exits, the new key value remains set.
Exercise: How would you reset the erase character to the default value?
Example 17-3. secret password: Turning off terminal echoing
!/bin/bash
secret-pw.sh: secret password
echo
echo -n "Enter password "
read passwd
echo "password is $passwd"
echo -n "If someone had been looking over your shoulder, "
echo "your password would have been compromised."
echo && echo # Two line-feeds in an "and list."
stty -echo # Turns off screen echo.
May also be done with
read -sp passwd
A big Thank You to Leigh James for pointing this out.
echo -n "Enter password again "
read passwd
echo
echo "password is $passwd"
echo
stty echo # Restores screen echo.
exit 0
Do an 'info stty' for more on this useful-but-tricky command.
A creative use of stty is detecting a user keypress (without
hitting ENTER).
Example 17-4. Keypress detection
!/bin/bash
keypress.sh: Detect a user keypress ("hot keys").
echo
old_tty_settings=$(stty -g) # Save old settings (why?).
stty -icanon
Keypress=$(head -c1) # or $(dd bs=1 count=1 2> /dev/null)
# on non-GNU systems
echo
echo "Key pressed was ""$Keypress""."
echo
stty "$old_tty_settings" # Restore old settings.
Thanks, Stephane Chazelas.
exit 0
Also see Example 9-3 and Example A-43.
terminals and modes
Normally, a terminal works in the canonical mode. When a user hits a
key, the resulting character does not immediately go to the program
actually running in this terminal. A buffer local to the terminal
stores keystrokes. When the user hits the ENTER key, this sends all
the stored keystrokes to the program running. There is even a basic
line editor inside the terminal.
bash$ stty -a
speed 9600 baud; rows 36; columns 96; line = 0;
intr = ^C; quit = ^; erase = ^H; kill = ^U; eof = ^D; eol = ; eol2 =
;
start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush
=> ^O;
...
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
Using canonical mode, it is possible to redefine the special keys for
the local terminal line editor.
bash$ cat > filexxx
whaIfoo barhello world
bash$ cat filexxx
hello world
bash$ wc -c < filexxx
12
The process controlling the terminal receives only 12 characters (11
alphabetic ones, plus a newline), although the user hit 26 keys.
In non-canonical ("raw") mode, every key hit (including special
editing keys such as ctl-H) sends a character immediately to the
controlling process.
The Bash prompt disables both icanon and echo, since it replaces the
basic terminal line editor with its own more elaborate one. For
example, when you hit ctl-A at the Bash prompt, there's no ^A echoed
by the terminal, but Bash gets a \1 character, interprets it, and
moves the cursor to the begining of the line.
Stéphane Chazelas
setterm
Set certain terminal attributes. This command writes to its
terminal's stdout a string that changes the behavior of that
terminal.
bash$ setterm -cursor off
bash$
The setterm command can be used within a script to change the
appearance of text written to stdout, although there are
certainly better tools available for this purpose.
setterm -bold on
echo bold hello
setterm -bold off
echo normal hello
tset
Show or initialize terminal settings. This is a less capable
version of stty.
bash$ tset -r
Terminal type is xterm-xfree86.
Kill is control-U (^U).
Interrupt is control-C (^C).
setserial
Set or display serial port parameters. This command must be
run by root and is usually found in a system setup script.
From /etc/pcmcia/serial script:
IRQ=setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'
setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ
getty, agetty
The initialization process for a terminal uses getty or agetty
to set it up for login by a user. These commands are not used
within user shell scripts. Their scripting counterpart is
stty.
mesg
Enables or disables write access to the current user's
terminal. Disabling access would prevent another user on the
network to write to the terminal.
Tip
It can be quite annoying to have a message about ordering pizza
suddenly appear in the middle of the text file you are editing. On a
multi-user network, you might therefore wish to disable write access
to your terminal when you need to avoid interruptions.
wall
This is an acronym for "write all," i.e., sending a message to
all users at every terminal logged into the network. It is
primarily a system administrator's tool, useful, for example,
when warning everyone that the system will shortly go down due
to a problem (see Example 19-1).
bash$ wall System going down for maintenance in 5 minutes!
Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001...
System going down for maintenance in 5 minutes!
Note
If write access to a particular terminal has been disabled with mesg,
then wall cannot send a message to that terminal.
Information and Statistics
uname
Output system specifications (OS, kernel version, etc.) to
stdout. Invoked with the -a option, gives verbose system info
(see Example 16-5). The -s option shows only the OS type.
bash$ uname
Linux
bash$ uname -s
Linux
bash$ uname -a
Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006
i686 i686 i386 GNU/Linux
arch
Show system architecture. Equivalent to uname -m. See Example
11-27.
bash$ arch
i686
bash$ uname -m
i686
lastcomm
Gives information about previous commands, as stored in the
/var/account/pacct file. Command name and user name can be
specified by options. This is one of the GNU accounting
utilities.
lastlog
List the last login time of all system users. This references
the /var/log/lastlog file.
bash$ lastlog
root tty1 Fri Dec 7 18:43:21 -0700 2001
bin Never logged in
daemon Never logged in
...
bozo tty1 Sat Dec 8 21:14:29 -0700 2001
bash$ lastlog | grep root
root tty1 Fri Dec 7 18:43:21 -0700 2001
Caution
This command will fail if the user invoking it does not have read
permission for the /var/log/lastlog file.
lsof
List open files. This command outputs a detailed table of all
currently open files and gives information about their owner,
size, the processes associated with them, and more. Of course,
lsof may be piped to grep and/or awk to parse and analyze its
results.
bash$ lsof
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
init 1 root mem REG 3,5 30748 30303 /sbin/init
init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.
so
init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.
3.so
cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr
...
The lsof command is a useful, if complex administrative tool.
If you are unable to dismount a filesystem and get an error
message that it is still in use, then running lsof helps
determine which files are still open on that filesystem. The
-i option lists open network socket files, and this can help
trace intrusion or hack attempts.
bash$ lsof -an -i tcp
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
firefox 2330 bozo 32u IPv4 9956 TCP 66.0.118.137:57596->67.112.7.104
:http ...
firefox 2330 bozo 38u IPv4 10535 TCP 66.0.118.137:57708->216.79.48.24
:http ...
See Example 30-2 for an effective use of lsof.
strace
System trace: diagnostic and debugging tool for tracing system
calls and signals. This command and ltrace, following, are
useful for diagnosing why a given program or package fails to
run . . . perhaps due to missing libraries or related causes.
bash$ strace df
execve("/bin/df", ["df"], [/* 45 vars */]) = 0
uname({sys="Linux", node="bozo.localdomain", ...}) = 0
brk(0) = 0x804f5e4
...
This is the Linux equivalent of the Solaris truss command.
ltrace
Library trace: diagnostic and debugging tool that traces
library calls invoked by a given command.
bash$ ltrace df
__libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished .
..>:
setlocale(6, "") = "en_US.UTF-8"
bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"
textdomain("coreutils") = "coreutils"
__cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0
getenv("DF_BLOCK_SIZE") = NULL
...
nc
The nc (netcat) utility is a complete toolkit for connecting
to and listening to TCP and UDP ports. It is useful as a
diagnostic and testing tool and as a component in simple
script-based HTTP clients and servers.
bash$ nc localhost.localdomain 25
220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;
Thu, 31 Mar 2005 15:41:35 -0700
A real-life usage example.
Example 17-5. Checking a remote server for identd
! /bin/sh
Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.
Args: target port [port port port ...]
Hose stdout and stderr together.
Advantages: runs slower than ident-scan, giving remote inetd less cause
- for alarm, and only hits the few known daemon ports you specify.
Disadvantages: requires numeric-only port args, the output sleazitude,
- and won't work for r-services when coming from high source ports.
Used in ABS Guide with permission.
E_BADARGS=65 # Need at least two args.
TWO_WINKS=2 # How long to sleep.
THREE_WINKS=3
IDPORT=113 # Authentication "tap ident" port.
RAND1=999
RAND2=31337
TIMEOUT0=9
TIMEOUT1=8
TIMEOUT2=4
case "${2}" in
"" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;
esac
Ping 'em once and see if they are running identd.
nc -z -w $TIMEOUT0 "$1" $IDPORT || \
{ echo "Oops, $1 isn't running identd." ; exit 0 ; }
-z scans for listening daemons.
-w $TIMEOUT = How long to try to connect.
Generate a randomish base port.
RP=expr $$ % $RAND1 + $RAND2
TRG="$1"
shift
while test "$1" ; do
nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &
PROC=$!
sleep $THREE_WINKS
echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1
sleep $TWO_WINKS
Does this look like a lamer script or what . . . ?
ABS Guide author comments: "Ain't really all that bad . . .
kill -HUP $PROC
RP=expr ${RP} + 1
shift
done
exit $?
Notes:
Try commenting out line 30 and running this script
- with "localhost.localdomain 25" as arguments.
For more of Hobbit's 'nc' example scripts,
- look in the documentation:
- the /usr/share/doc/nc-X.XX/scripts directory.
And, of course, there's Dr. Andrew Tridgell's notorious
one-line script in the BitKeeper Affair:
echo clone | nc thunk.org 5000 > e2fsprogs.dat
free
Shows memory and cache usage in tabular form. The output of
this command lends itself to parsing, using grep, awk or Perl.
The procinfo command shows all the information that free does,
and much more.
bash$ free
total used free shared buffers cached
Mem: 30504 28624 1880 15820 1608 16376
-/+ buffers/cache: 10640 19864
Swap: 68540 3128 65412
To show unused RAM memory:
bash$ free | grep Mem | awk '{ print $4 }'
1880
procinfo
Extract and list information and statistics from the /proc
pseudo-filesystem. This gives a very extensive and detailed
listing.
bash$ procinfo | grep Bootup
Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829
lsdev
List devices, that is, show installed hardware.
bash$ lsdev
Device DMA IRQ I/O Ports
cascade 4 2
dma 0080-008f
dma1 0000-001f
dma2 00c0-00df
fpu 00f0-00ff
ide0 14 01f0-01f7 03f6-03f6
...
du
Show (disk) file usage, recursively. Defaults to current
working directory, unless otherwise specified.
bash$ du -ach
1.0k ./wi.sh
1.0k ./tst.sh
1.0k ./random.file
6.0k .
6.0k total
df
Shows filesystem usage in tabular form.
bash$ df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/hda5 273262 92607 166547 36% /
/dev/hda8 222525 123951 87085 59% /home
/dev/hda7 1408796 1075744 261488 80% /usr
dmesg
Lists all system bootup messages to stdout. Handy for
debugging and ascertaining which device drivers were installed
and which system interrupts in use. The output of dmesg may,
of course, be parsed with grep, sed, or awk from within a
script.
bash$ dmesg | grep hda
Kernel command line: ro root=/dev/hda2
hda: IBM-DLGA-23080, ATA DISK drive
hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4
stat
Gives detailed and verbose statistics on a given file (even a
directory or device file) or set of files.
bash$ stat test.cru
File: "test.cru"
Size: 49970 Allocated Blocks: 100 Filetype: Regular File
Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo)
Device: 3,8 Inode: 18185 Links: 1
Access: Sat Jun 2 16:40:24 2001
Modify: Sat Jun 2 16:40:24 2001
Change: Sat Jun 2 16:40:24 2001
If the target file does not exist, stat returns an error
message.
bash$ stat nonexistent-file
nonexistent-file: No such file or directory
In a script, you can use stat to extract information about
files (and filesystems) and set variables accordingly.
!/bin/bash
fileinfo2.sh
Per suggestion of Joël Bourquard and . . .
http://www.linuxquestions.org/questions/showthread.php?t=410766
FILENAME=testfile.txt
file_name=$(stat -c%n "$FILENAME") # Same as "$FILENAME" of course.
file_owner=$(stat -c%U "$FILENAME")
file_size=$(stat -c%s "$FILENAME")
Certainly easier than using "ls -l $FILENAME"
- and then parsing with sed.
file_inode=$(stat -c%i "$FILENAME")
file_type=$(stat -c%F "$FILENAME")
file_access_rights=$(stat -c%A "$FILENAME")
echo "File name: $file_name"
echo "File owner: $file_owner"
echo "File size: $file_size"
echo "File inode: $file_inode"
echo "File type: $file_type"
echo "File access rights: $file_access_rights"
exit 0
sh fileinfo2.sh
File name: testfile.txt
File owner: bozo
File size: 418
File inode: 1730378
File type: regular file
File access rights: -rw-rw-r--
vmstat
Display virtual memory statistics.
bash$ vmstat
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89
uptime
Shows how long the system has been running, along with
associated statistics.
bash$ uptime
10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27
Note
A load average of 1 or less indicates that the system handles
processes immediately. A load average greater than 1 means that
processes are being queued. When the load average gets above 3 (on a
single-core processor), then system performance is significantly
degraded.
hostname
Lists the system's host name. This command sets the host name
in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or
similar). It is equivalent to uname -n, and a counterpart to
the $HOSTNAME internal variable.
bash$ hostname
localhost.localdomain
bash$ echo $HOSTNAME
localhost.localdomain
Similar to the hostname command are the domainname,
dnsdomainname, nisdomainname, and ypdomainname commands. Use
these to display or set the system DNS or NIS/YP domain name.
Various options to hostname also perform these functions.
hostid
Echo a 32-bit hexadecimal numerical identifier for the host
machine.
bash$ hostid
7f0100
Note
This command allegedly fetches a "unique" serial number for a
particular system. Certain product registration procedures use this
number to brand a particular user license. Unfortunately, hostid only
returns the machine network address in hexadecimal, with pairs of
bytes transposed.
The network address of a typical non-networked Linux machine, is
found in /etc/hosts.
bash$ cat /etc/hosts
127.0.0.1 localhost.localdomain localhost
As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0,
which translates in hex to 007f0100, the exact equivalent of what
hostid returns, above. There exist only a few million other Linux
machines with this identical hostid.
sar
Invoking sar (System Activity Reporter) gives a very detailed
rundown on system statistics. The Santa Cruz Operation ("Old"
SCO) released sar as Open Source in June, 1999.
This command is not part of the base Linux distribution, but
may be obtained as part of
the[http://perso.wanadoo.fr/sebastien.godard/] sysstat
utilities package, written by Sebastien Godard.
bash$ sar
Linux 2.4.9 (brooks.seringas.fr) 09/26/03
10:30:00 CPU %user %nice %system %iowait %idle
10:40:00 all 2.21 10.90 65.48 0.00 21.41
10:50:00 all 3.36 0.00 72.36 0.00 24.28
11:00:00 all 1.12 0.00 80.77 0.00 18.11
Average: all 2.23 3.63 72.87 0.00 21.27
14:32:30 LINUX RESTART
15:00:00 CPU %user %nice %system %iowait %idle
15:10:00 all 8.59 2.40 17.47 0.00 71.54
15:20:00 all 4.07 1.00 11.95 0.00 82.98
15:30:00 all 0.79 2.94 7.56 0.00 88.71
Average: all 6.33 1.70 14.71 0.00 77.26
readelf
Show information and statistics about a designated elf binary.
This is part of the binutils package.
bash$ readelf -h /bin/bash
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
. . .
size
The size [/path/to/binary] command gives the segment sizes of
a binary executable or archive file. This is mainly of use to
programmers.
bash$ size /bin/bash
text data bss dec hex filename
495971 22496 17392 535859 82d33 /bin/bash
System Logs
logger
Appends a user-generated message to the system log
(/var/log/messages). You do not have to be root to invoke
logger.
logger Experiencing instability in network connection at 23:10, 05/21.
Now, do a 'tail /var/log/messages'.
By embedding a logger command in a script, it is possible to
write debugging information to /var/log/messages.
logger -t $0 -i Logging at line "$LINENO".
The "-t" option specifies the tag for the logger entry.
The "-i" option records the process ID.
tail /var/log/message
...
Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.
logrotate
This utility manages the system log files, rotating,
compressing, deleting, and/or e-mailing them, as appropriate.
This keeps the /var/log from getting cluttered with old log
files. Usually cron runs logrotate on a daily basis.
Adding an appropriate entry to /etc/logrotate.conf makes it
possible to manage personal log files, as well as system-wide
ones.
Note
Stefano Falsetto has created [http://www.gnu.org/software/rottlog/]
rottlog, which he considers to be an improved version of logrotate.
Job Control
ps
Process Statistics: lists currently executing processes by
owner and PID (process ID). This is usually invoked with ax or
aux options, and may be piped to grep or sed to search for a
specific process (see Example 15-14 and Example 29-3).
bash$ ps ax | grep sendmail
295 ? S 0:00 sendmail: accepting connections on port 25
To display system processes in graphical "tree" format: ps
afjx or ps ax --forest.
pgrep, pkill
Combining the ps command with grep or kill.
bash$ ps a | grep mingetty
2212 tty2 Ss+ 0:00 /sbin/mingetty tty2
2213 tty3 Ss+ 0:00 /sbin/mingetty tty3
2214 tty4 Ss+ 0:00 /sbin/mingetty tty4
2215 tty5 Ss+ 0:00 /sbin/mingetty tty5
2216 tty6 Ss+ 0:00 /sbin/mingetty tty6
4849 pts/2 S+ 0:00 grep mingetty
bash$ pgrep mingetty
2212 mingetty
2213 mingetty
2214 mingetty
2215 mingetty
2216 mingetty
Compare the action of pkill with killall.
pstree
Lists currently executing processes in "tree" format. The -p
option shows the PIDs, as well as the process names.
top
Continuously updated display of most cpu-intensive processes.
The -b option displays in text mode, so that the output may be
parsed or accessed from a script.
bash$ top -b
8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13
45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle
Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff
Swap: 157208K av, 0K used, 157208K free 37244K cach
ed
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top
1 root 8 0 512 512 444 S 0.0 0.6 0:04 init
2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd
...
nice
Run a background job with an altered priority. Priorities run
from 19 (lowest) to -20 (highest). Only root may set the
negative (higher) priorities. Related commands are renice and
snice, which change the priority of a running process or
processes, and skill, which sends a kill signal to a process
or processes.
nohup
Keeps a command running even after user logs off. The command
will run as a foreground process unless followed by &. If you
use nohup within a script, consider coupling it with a wait to
avoid creating an orphan or zombie process.
pidof
Identifies process ID (PID) of a running job. Since job
control commands, such as kill and renice act on the PID of a
process (not its name), it is sometimes necessary to identify
that PID. The pidof command is the approximate counterpart to
the $PPID internal variable.
bash$ pidof xclock
880
Example 17-6. pidof helps kill a process
!/bin/bash
kill-process.sh
NOPROCESS=2
process=xxxyyyzzz # Use nonexistent process.
For demo purposes only...
... don't want to actually kill any actual process with this script.
If, for example, you wanted to use this script to logoff the Internet,
process=pppd
t=pidof $process # Find pid (process id) of $process.
The pid is needed by 'kill' (can't 'kill' by program name).
if [ -z "$t" ] # If process not present, 'pidof' returns null.
then
echo "Process $process was not running."
echo "Nothing killed."
exit $NOPROCESS
fi
kill $t # May need 'kill -9' for stubborn process.
Need a check here to see if process allowed itself to be killed.
Perhaps another " t=pidof $process " or ...
This entire script could be replaced by
kill $(pidof -x process_name)
or
killall process_name
but it would not be as instructive.
exit 0
fuser
Identifies the processes (by PID) that are accessing a given
file, set of files, or directory. May also be invoked with the
-k option, which kills those processes. This has interesting
implications for system security, especially in scripts
preventing unauthorized users from accessing system services.
bash$ fuser -u /usr/bin/vim
/usr/bin/vim: 3207e(bozo)
bash$ fuser -u /dev/null
/dev/null: 3009(bozo) 3010(bozo) 3197(bozo) 3199(bozo)
One important application for fuser is when physically
inserting or removing storage media, such as CD ROM disks or
USB flash drives. Sometimes trying a umount fails with a
device is busy error message. This means that some user(s)
and/or process(es) are accessing the device. An fuser -um
/dev/device_name will clear up the mystery, so you can kill
any relevant processes.
bash$ umount /mnt/usbdrive
umount: /mnt/usbdrive: device is busy
bash$ fuser -um /dev/usbdrive
/mnt/usbdrive: 1772c(bozo)
bash$ kill -9 1772
bash$ umount /mnt/usbdrive
The fuser command, invoked with the -n option identifies the
processes accessing a port. This is especially useful in
combination with nmap.
root# nmap localhost.localdomain
PORT STATE SERVICE
25/tcp open smtp
root# fuser -un tcp 25
25/tcp: 2095(root)
root# ps ax | grep 2095 | grep -v grep
2095 ? Ss 0:00 sendmail: accepting connections
cron
Administrative program scheduler, performing such duties as
cleaning up and deleting system log files and updating the
slocate database. This is the superuser version of at
(although each user may have their own crontab file which can
be changed with the crontab command). It runs as a daemon and
executes scheduled entries from /etc/crontab.
Note
Some flavors of Linux run crond, Matthew Dillon's version of cron.
Process Control and Booting
init
The init command is the parent of all processes. Called in the
final step of a bootup, init determines the runlevel of the
system from /etc/inittab. Invoked by its alias telinit, and by
root only.
telinit
Symlinked to init, this is a means of changing the system
runlevel, usually done for system maintenance or emergency
filesystem repairs. Invoked only by root. This command can be
dangerous -- be certain you understand it well before using!
runlevel
Shows the current and last runlevel, that is, whether the
system is halted (runlevel 0), in single-user mode (1), in
multi-user mode (2 or 3), in X Windows (5), or rebooting (6).
This command accesses the /var/run/utmp file.
halt, shutdown, reboot
Command set to shut the system down, usually just prior to a
power down.
Warning
On some Linux distros, the halt command has 755 permissions, so it
can be invoked by a non-root user. A careless halt in a terminal or a
script may shut down the system!
service
Starts or stops a system service. The startup scripts in
/etc/init.d and /etc/rc.d use this command to start services
at bootup.
root# /sbin/service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
Network
nmap
Network mapper and port scanner. This command scans a server
to locate open ports and the services associated with those
ports. It can also report information about packet filters and
firewalls. This is an important security tool for locking down
a network against hacking attempts.
!/bin/bash
SERVER=$HOST # localhost.localdomain (127.0.0.1).
PORT_NUMBER=25 # SMTP port.
nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open?
grep -w matches whole words only,
exit 0
25/tcp open smtp
ifconfig
Network interface configuration and tuning utility.
bash$ ifconfig -a
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:10 errors:0 dropped:0 overruns:0 frame:0
TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:700 (700.0 b) TX bytes:700 (700.0 b)
The ifconfig command is most often used at bootup to set up
the interfaces, or to shut them down when rebooting.
Code snippets from /etc/rc.d/init.d/network
...
Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0
[ -x /sbin/ifconfig ] || exit 0
...
for i in $interfaces ; do
if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
action "Shutting down interface $i: " ./ifdown $i boot
fi
The GNU-specific "-q" option to "grep" means "quiet", i.e.,
Redirecting output to /dev/null is therefore not strictly necessary.
...
echo "Currently active devices:"
echo /sbin/ifconfig | grep ^[a-z] | awk '{print $1}'
^^^^^ should be quoted to prevent globbing.
The following also work.
echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
echo $(/sbin/ifconfig | sed -e 's/ .*//')
Thanks, S.C., for additional comments.
See also Example 32-6.
netstat
Show current network statistics and information, such as
routing tables and active connections. This utility accesses
information in /proc/net (Chapter 29). See Example 29-4.
netstat -r is equivalent to route.
bash$ netstat
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
Active UNIX domain sockets (w/o servers)
Proto RefCnt Flags Type State I-Node Path
unix 11 [ ] DGRAM 906 /dev/log
unix 3 [ ] STREAM CONNECTED 4514 /tmp/.X11-unix/X0
unix 3 [ ] STREAM CONNECTED 4513
. . .
Note
A netstat -lptu shows sockets that are listening to ports, and the
associated processes. This can be useful for determining whether a
computer has been hacked or compromised.
iwconfig
This is the command set for configuring a wireless network. It
is the wireless equivalent of ifconfig, above.
ip
General purpose utility for setting up, changing, and
analyzing IP (Internet Protocol) networks and attached
devices. This command is part of the iproute2 package.
bash$ ip link show
1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff
3: sit0: mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0
bash$ ip route list
169.254.0.0/16 dev lo scope link
Or, in a script:
!/bin/bash
Script by Juan Nicolas Ruiz
Used with his kind permission.
Setting up (and stopping) a GRE tunnel.
--- start-tunnel.sh ---
LOCAL_IP="192.168.1.17"
REMOTE_IP="10.0.5.33"
OTHER_IFACE="192.168.0.100"
REMOTE_NET="192.168.3.0/24"
/sbin/ip tunnel add netb mode gre remote $REMOTE_IP \
local $LOCAL_IP ttl 255
/sbin/ip addr add $OTHER_IFACE dev netb
/sbin/ip link set netb up
/sbin/ip route add $REMOTE_NET dev netb
exit 0 #############################################
--- stop-tunnel.sh ---
REMOTE_NET="192.168.3.0/24"
/sbin/ip route del $REMOTE_NET dev netb
/sbin/ip link set netb down
/sbin/ip tunnel del netb
exit 0
route
Show info about or make changes to the kernel routing table.
bash$ route
Destination Gateway Genmask Flags MSS Window irtt Iface
pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0
127.0.0.0 * 255.0.0.0 U 40 0 0 lo
default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0
iptables
The iptables command set is a packet filtering tool used
mainly for such security purposes as setting up network
firewalls. This is a complex tool, and a detailed explanation
of its use is beyond the scope of this document. Oskar
Andreasson's tutorial is a reasonable starting point.
See also shutting down iptables and Example 30-2.
chkconfig
Check network and system configuration. This command lists and
manages the network and system services started at bootup in
the /etc/rc?.d directory.
Originally a port from IRIX to Red Hat Linux, chkconfig may
not be part of the core installation of some Linux flavors.
bash$ chkconfig --list
atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off
rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off
...
tcpdump
Network packet "sniffer." This is a tool for analyzing and
troubleshooting traffic on a network by dumping packet headers
that match specified criteria.
Dump ip packet traffic between hosts bozoville and caduceus:
bash$ tcpdump ip host bozoville and caduceus
Of course, the output of tcpdump can be parsed with certain of
the previously discussed text processing utilities.
Filesystem
mount
Mount a filesystem, usually on an external device, such as a
floppy or CDROM. The file /etc/fstab provides a handy listing
of available filesystems, partitions, and devices, including
options, that may be automatically or manually mounted. The
file /etc/mtab shows the currently mounted filesystems and
partitions (including the virtual ones, such as /proc).
mount -a mounts all filesystems and partitions listed in
/etc/fstab, except those with a noauto option. At bootup, a
startup script in /etc/rc.d (rc.sysinit or something similar)
invokes this to get everything mounted.
mount -t iso9660 /dev/cdrom /mnt/cdrom
Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem.
mount /mnt/cdrom
Shortcut, if /mnt/cdrom listed in /etc/fstab
The versatile mount command can even mount an ordinary file on
a block device, and the file will act as if it were a
filesystem. Mount accomplishes that by associating the file
with a loopback device. One application of this is to mount
and examine an ISO9660 filesystem image before burning it onto
a CDR. [90]
Example 17-7. Checking a CD image
As root...
mkdir /mnt/cdtest # Prepare a mount point, if not already there.
mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image.
"-o loop" option equivalent to "losetup /dev/loop0"
cd /mnt/cdtest # Now, check the image.
ls -alR # List the files in the directory tree there.
# And so forth.
umount
Unmount a currently mounted filesystem. Before physically
removing a previously mounted floppy or CDROM disk, the device
must be umounted, else filesystem corruption may result.
umount /mnt/cdrom
You may now press the eject button and safely remove the disk.
Note
The automount utility, if properly installed, can mount and unmount
floppies or CDROM disks as they are accessed or removed. On
"multispindle" laptops with swappable floppy and optical drives, this
can cause problems, however.
gnome-mount
The newer Linux distros have deprecated mount and umount. The
successor, for command-line mounting of removable storage
devices, is gnome-mount. It can take the -d option to mount a
device file by its listing in /dev.
For example, to mount a USB flash drive:
bash$ gnome-mount -d /dev/sda1
gnome-mount 0.4
bash$ df
. . .
/dev/sda1 63584 12034 51550 19% /media/disk
sync
Forces an immediate write of all updated data from buffers to
hard drive (synchronize drive with buffers). While not
strictly necessary, a sync assures the sys admin or user that
the data just changed will survive a sudden power failure. In
the olden days, a sync; sync (twice, just to make absolutely
sure) was a useful precautionary measure before a system
reboot.
At times, you may wish to force an immediate buffer flush, as
when securely deleting a file (see Example 16-61) or when the
lights begin to flicker.
losetup
Sets up and configures loopback devices.
Example 17-8. Creating a filesystem in a file
SIZE=1000000 # 1 meg
head -c $SIZE < /dev/zero > file # Set up file of designated size.
losetup /dev/loop0 file # Set it up as loopback device.
mke2fs /dev/loop0 # Create filesystem.
mount -o loop /dev/loop0 /mnt # Mount it.
Thanks, S.C.
mkswap
Creates a swap partition or file. The swap area must
subsequently be enabled with swapon.
swapon, swapoff
Enable / disable swap partitition or file. These commands
usually take effect at bootup and shutdown.
mke2fs
Create a Linux ext2 filesystem. This command must be invoked
as root.
Example 17-9. Adding a new hard drive
!/bin/bash
Adding a second hard drive to system.
Software configuration. Assumes hardware already mounted.
From an article by the author of the ABS Guide.
In issue #38 of Linux Gazette, http://www.linuxgazette.com.
ROOT_UID=0 # This script must be run as root.
E_NOTROOT=67 # Non-root exit error.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
Use with extreme caution!
If something goes wrong, you may wipe out your current filesystem.
NEWDISK=/dev/hdb # Assumes /dev/hdb vacant. Check!
MOUNTPOINT=/mnt/newdisk # Or choose another mount point.
fdisk $NEWDISK
mke2fs -cv $NEWDISK1 # Check for bad blocks (verbose output).
Note: ^ /dev/hdb1, not /dev/hdb!
mkdir $MOUNTPOINT
chmod 777 $MOUNTPOINT # Makes new drive accessible to all users.
Now, test ...
mount -t ext2 /dev/hdb1 /mnt/newdisk
Try creating a directory.
If it works, umount it, and proceed.
Final step:
Add the following line to /etc/fstab.
/dev/hdb1 /mnt/newdisk ext2 defaults 1 1
exit
See also Example 17-8 and Example 31-3.
mkdosfs
Create a DOS FAT filesystem.
tune2fs
Tune ext2 filesystem. May be used to change filesystem
parameters, such as maximum mount count. This must be invoked
as root.
Warning
This is an extremely dangerous command. Use it at your own risk, as
you may inadvertently destroy your filesystem.
dumpe2fs
Dump (list to stdout) very verbose filesystem info. This must
be invoked as root.
root# dumpe2fs /dev/hda7 | grep 'ount count'
dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09
Mount count: 6
Maximum mount count: 20
hdparm
List or change hard disk parameters. This command must be
invoked as root, and it may be dangerous if misused.
fdisk
Create or change a partition table on a storage device,
usually a hard drive. This command must be invoked as root.
Warning
Use this command with extreme caution. If something goes wrong, you
may destroy an existing filesystem.
fsck, e2fsck, debugfs
Filesystem check, repair, and debug command set.
fsck: a front end for checking a UNIX filesystem (may invoke
other utilities). The actual filesystem type generally
defaults to ext2.
e2fsck: ext2 filesystem checker.
debugfs: ext2 filesystem debugger. One of the uses of this
versatile, but dangerous command is to (attempt to) recover
deleted files. For advanced users only!
Caution
All of these should be invoked as root, and they can damage or
destroy a filesystem if misused.
badblocks
Checks for bad blocks (physical media flaws) on a storage
device. This command finds use when formatting a newly
installed hard drive or testing the integrity of backup media.
[91] As an example, badblocks /dev/fd0 tests a floppy disk.
The badblocks command may be invoked destructively (overwrite
all data) or in non-destructive read-only mode. If root user
owns the device to be tested, as is generally the case, then
root must invoke this command.
lsusb, usbmodules
The lsusb command lists all USB (Universal Serial Bus) buses
and the devices hooked up to them.
The usbmodules command outputs information about the driver
modules for connected USB devices.
bash$ lsusb
Bus 001 Device 001: ID 0000:0000
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 1.00
bDeviceClass 9 Hub
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 8
idVendor 0x0000
idProduct 0x0000
. . .
lspci
Lists pci busses present.
bash$ lspci
00:00.0 Host bridge: Intel Corporation 82845 845
(Brookdale) Chipset Host Bridge (rev 04)
00:01.0 PCI bridge: Intel Corporation 82845 845
(Brookdale) Chipset AGP Bridge (rev 04)
00:1d.0 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #1) (rev 02)
00:1d.1 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #2) (rev 02)
00:1d.2 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #3) (rev 02)
00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 42)
. . .
mkbootdisk
Creates a boot floppy which can be used to bring up the system
if, for example, the MBR (master boot record) becomes
corrupted. Of special interest is the --iso option, which uses
mkisofs to create a bootable ISO9660 filesystem image suitable
for burning a bootable CDR.
The mkbootdisk command is actually a Bash script, written by
Erik Troan, in the /sbin directory.
mkisofs
Creates an ISO9660 filesystem suitable for a CDR image.
chroot
CHange ROOT directory. Normally commands are fetched from
$PATH, relative to /, the default root directory. This changes
the root directory to a different one (and also changes the
working directory to there). This is useful for security
purposes, for instance when the system administrator wishes to
restrict certain users, such as those telnetting in, to a
secured portion of the filesystem (this is sometimes referred
to as confining a guest user to a "chroot jail"). Note that
after a chroot, the execution path for system binaries is no
longer valid.
A chroot /opt would cause references to /usr/bin to be
translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls
would redirect future instances of ls to /aaa/bbb as the base
directory, rather than / as is normally the case. An alias XX
'chroot /aaa/bbb ls' in a user's ~/.bashrc effectively
restricts which portion of the filesystem she may run command
"XX" on.
The chroot command is also handy when running from an
emergency boot floppy (chroot to /dev/fd0), or as an option to
lilo when recovering from a system crash. Other uses include
installation from a different filesystem (an rpm option) or
running a readonly filesystem from a CD ROM. Invoke only as
root, and use with care.
Caution
It might be necessary to copy certain system files to a chrooted
directory, since the normal $PATH can no longer be relied upon.
lockfile
This utility is part of the procmail package
([http://www.procmail.org] www.procmail.org). It creates a
lock file, a semaphore that controls access to a file, device,
or resource.
Definition: A semaphore is a flag or signal. (The usage originated in
railroading, where a colored flag, lantern, or striped movable arm
semaphore indicated whether a particular track was in use and
therefore unavailable for another train.) A UNIX process can check
the appropriate semaphore to determine whether a particular resource
is available/accessible.
The lock file serves as a flag that this particular file,
device, or resource is in use by a process (and is therefore
"busy"). The presence of a lock file permits only restricted
access (or no access) to other processes.
lockfile /home/bozo/lockfiles/$0.lock
Creates a write-protected lockfile prefixed with the name of the script.
lockfile /home/bozo/lockfiles/${0##*/}.lock
A safer version of the above, as pointed out by E. Choroba.
Lock files are used in such applications as protecting system
mail folders from simultaneously being changed by multiple
users, indicating that a modem port is being accessed, and
showing that an instance of Firefox is using its cache.
Scripts may check for the existence of a lock file created by
a certain process to check if that process is running. Note
that if a script attempts to create a lock file that already
exists, the script will likely hang.
Normally, applications create and check for lock files in the
/var/lock directory. [92] A script can test for the presence
of a lock file by something like the following.
appname=xyzip
Application "xyzip" created lock file "/var/lock/xyzip.lock".
if [ -e "/var/lock/$appname.lock" ]
then #+ Prevent other programs & scripts
# from accessing files/resources used by xyzip.
...
flock
Much less useful than the lockfile command is flock. It sets
an "advisory" lock on a file and then executes a command while
the lock is on. This is to prevent any other process from
setting a lock on that file until completion of the specified
command.
flock $0 cat $0 > lockfile__$0
Set a lock on the script the above line appears in,
- while listing the script to stdout.
Note
Unlike lockfile, flock does not automatically create a lock file.
mknod
Creates block or character device files (may be necessary when
installing new hardware on the system). The MAKEDEV utility
has virtually all of the functionality of mknod, and is easier
to use.
MAKEDEV
Utility for creating device files. It must be run as root, and
in the /dev directory. It is a sort of advanced version of
mknod.
tmpwatch
Automatically deletes files which have not been accessed
within a specified period of time. Usually invoked by cron to
remove stale log files.
Backup
dump, restore
The dump command is an elaborate filesystem backup utility,
generally used on larger installations and networks. [93] It
reads raw disk partitions and writes a backup file in a binary
format. Files to be backed up may be saved to a variety of
storage media, including disks and tape drives. The restore
command restores backups made with dump.
fdformat
Perform a low-level format on a floppy disk (/dev/fd0*).
System Resources
ulimit
Sets an upper limit on use of system resources. Usually
invoked with the -f option, which sets a limit on file size
(ulimit -f 1000 limits files to 1 meg maximum). [94] The -t
option limits the coredump size (ulimit -c 0 eliminates
coredumps). Normally, the value of ulimit would be set in
/etc/profile and/or ~/.bash_profile (see Appendix H).
Important
Judicious use of ulimit can protect a system against the dreaded fork
bomb.
!/bin/bash
This script is for illustrative purposes only.
Run it at your own peril -- it WILL freeze your system.
while true # Endless loop.
do
$0 & # This script invokes itself . . .
#+ forks an infinite number of times . . .
#+ until the system freezes up because all resources exhausted.
done # This is the notorious "sorcerer's appentice" scenario.
exit 0 # Will not exit here, because this script will never terminate.
A ulimit -Hu XX (where XX is the user process limit) in /etc/profile
would abort this script when it exceeded the preset limit.
quota
Display user or group disk quotas.
setquota
Set user or group disk quotas from the command-line.
umask
User file creation permissions mask. Limit the default file
attributes for a particular user. All files created by that
user take on the attributes specified by umask. The (octal)
value passed to umask defines the file permissions disabled.
For example, umask 022 ensures that new files will have at
most 755 permissions (777 NAND 022). [95] Of course, the user
may later change the attributes of particular files with
chmod. The usual practice is to set the value of umask in
/etc/profile and/or ~/.bash_profile (see Appendix H).
Example 17-10. Using umask to hide an output file from prying
eyes
!/bin/bash
rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.
Usage: ./rot13a.sh filename
or ./rot13a.sh <filename
or ./rot13a.sh and supply keyboard input (stdin)
umask 177 # File creation mask.
# Files created by this script
#+ will have 600 permissions.
OUTFILE=decrypted.txt # Results output to file "decrypted.txt"
#+ which can only be read/written
# by invoker of script (or root).
cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' > $OUTFILE
^^ Input from stdin or a file. ^^^^^^^^^^ Output redirected to file.
exit 0
rdev
Get info about or make changes to root device, swap space, or
video mode. The functionality of rdev has generally been taken
over by lilo, but rdev remains useful for setting up a ram
disk. This is a dangerous command, if misused.
Modules
lsmod
List installed kernel modules.
bash$ lsmod
Module Size Used by
autofs 9456 2 (autoclean)
opl3 11376 0
serial_cs 5456 0 (unused)
sb 34752 0
uart401 6384 0 [sb]
sound 58368 0 [opl3 sb uart401]
soundlow 464 0 [sound]
soundcore 2800 6 [sb sound]
ds 6448 2 [serial_cs]
i82365 22928 2
pcmcia_core 45984 0 [serial_cs ds i82365]
Note
Doing a cat /proc/modules gives the same information.
insmod
Force installation of a kernel module (use modprobe instead,
when possible). Must be invoked as root.
rmmod
Force unloading of a kernel module. Must be invoked as root.
modprobe
Module loader that is normally invoked automatically in a
startup script. Must be invoked as root.
depmod
Creates module dependency file. Usually invoked from a startup
script.
modinfo
Output information about a loadable module.
bash$ modinfo hid
filename: /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o
description: "USB HID support drivers"
author: "Andreas Gal, Vojtech Pavlik vojtech@suse.cz"
license: "GPL"
Miscellaneous
env
Runs a program or script with certain environmental variables
set or changed (without changing the overall system
environment). The [varname=xxx] permits changing the
environmental variable varname for the duration of the script.
With no options specified, this command lists all the
environmental variable settings. [96]
Note
The first line of a script (the "sha-bang" line) may use env when the
path to the shell or interpreter is unknown.
! /usr/bin/env perl
print "This Perl script will run,\n";
print "even when I don't know where to find Perl.\n";
Good for portable cross-platform scripts,
where the Perl binaries may not be in the expected place.
Thanks, S.C.
Or even ...
!/bin/env bash
Queries the $PATH enviromental variable for the location of bash.
Therefore ...
This script will run where Bash is not in its usual place, in /bin.
...
ldd
Show shared lib dependencies for an executable file.
bash$ ldd /bin/ls
libc.so.6 => /lib/libc.so.6 (0x4000c000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
watch
Run a command repeatedly, at specified time intervals.
The default is two-second intervals, but this may be changed
with the -n option.
watch -n 5 tail /var/log/messages
Shows tail end of system log, /var/log/messages, every five seconds.
Note
Unfortunately, piping the output of watch command to grep does not
work.
strip
Remove the debugging symbolic references from an executable
binary. This decreases its size, but makes debugging it
impossible.
This command often occurs in a Makefile, but rarely in a shell
script.
nm
List symbols in an unstripped compiled binary.
xrandr
Command-line tool for manipulating the root window of the
screen.
Example 17-11. Backlight: changes the brightness of the
(laptop) screen backlight
!/bin/bash
backlight.sh
reldate 02dec2011
A bug in Fedora Core 16/17 messes up the keyboard backlight controls.
This script is a quick-n-dirty workaround, essentially a shell wrapper
- for xrandr. It gives more control than on-screen sliders and widgets.
OUTPUT=$(xrandr | grep LV | awk '{print $1}') # Get display name!
INCR=.05 # For finer-grained control, set INCR to .03 or .02.
old_brightness=$(xrandr --verbose | grep rightness | awk '{ print $2 }')
if [ -z "$1" ]
then
bright=1 # If no command-line arg, set brightness to 1.0 (default).
else
if [ "$1" = "+" ]
then
bright=$(echo "scale=2; $old_brightness + $INCR" | bc) # +.05
else
if [ "$1" = "-" ]
then
bright=$(echo "scale=2; $old_brightness - $INCR" | bc) # -.05
else
if [ "$1" = "#" ] # Echoes current brightness; does not change it.
then
bright=$old_brightness
else
if [[ "$1" = "h" || "$1" = "H" ]]
then
echo
echo "Usage:"
echo "$0 [No args] Sets/resets brightness to default (1.0)."
echo "$0 + Increments brightness by 0.5."
echo "$0 - Decrements brightness by 0.5."
echo "$0 # Echoes current brightness without changing it."
echo "$0 N (number) Sets brightness to N (useful range .7 - 1.2)."
echo "$0 h [H] Echoes this help message."
echo "$0 any-other Gives xrandr usage message."
bright=$old_brightness
else
bright="$1"
fi
fi
fi
fi
fi
xrandr --output "$OUTPUT" --brightness "$bright" # See xrandr manpage.
# As root!
E_CHANGE0=$?
echo "Current brightness = $bright"
exit $E_CHANGE0
=========== Or, alternately . . . ==================== #
!/bin/bash
backlight2.sh
reldate 20jun2012
A bug in Fedora Core 16/17 messes up the keyboard backlight controls.
This is a quick-n-dirty workaround, an alternate to backlight.sh.
target_dir=\
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0
Hardware directory.
actual_brightness=$(cat $target_dir/actual_brightness)
max_brightness=$(cat $target_dir/max_brightness)
Brightness=$target_dir/brightness
let "req_brightness = actual_brightness" # Requested brightness.
if [ "$1" = "-" ]
then # Decrement brightness 1 notch.
let "req_brightness = $actual_brightness - 1"
else
if [ "$1" = "+" ]
then # Increment brightness 1 notch.
let "req_brightness = $actual_brightness + 1"
fi
fi
if [ $req_brightness -gt $max_brightness ]
then
req_brightness=$max_brightness
fi # Do not exceed max. hardware design brightness.
echo
echo "Old brightness = $actual_brightness"
echo "Max brightness = $max_brightness"
echo "Requested brightness = $req_brightness"
echo
=====================================
echo $req_brightness > $Brightness
Must be root for this to take effect.
E_CHANGE1=$? # Successful?
=====================================
if [ "$?" -eq 0 ]
then
echo "Changed brightness!"
else
echo "Failed to change brightness!"
fi
act_brightness=$(cat $Brightness)
echo "Actual brightness = $act_brightness"
scale0=2
sf=100 # Scale factor.
pct=$(echo "scale=$scale0; $act_brightness / $max_brightness * $sf" | bc)
echo "Percentage brightness = $pct%"
exit $E_CHANGE1
rdist
Remote distribution client: synchronizes, clones, or backs up
a file system on a remote server.
________________________________________________________________
17.1. Analyzing a System Script
Using our knowledge of administrative commands, let us examine a
system script. One of the shortest and simplest to understand scripts
is "killall," [97] used to suspend running processes at system
shutdown.
Example 17-12. killall, from /etc/rc.d/init.d
!/bin/sh
--> Comments added by the author of this document marked by "# -->".
--> This is part of the 'rc' script package
--> This particular script seems to be Red Hat / FC specific
--> (may not be present in other distributions).
Bring down all unneeded services that are still running
- (there shouldn't be any, so this is just a sanity check)
for i in /var/lock/subsys/*; do
# --> Standard for/in loop, but since "do" is on same line,
# --> it is necessary to add ";".
# Check if the script is there.
[ ! -f $i ] && continue
# --> This is a clever use of an "and list", equivalent to:
# --> if [ ! -f "$i" ]; then continue
# Get the subsystem name.
subsys=${i#/var/lock/subsys/}
# --> Match variable name, which, in this case, is the file name.
# --> This is the exact equivalent of subsys=`basename $i`.
# --> It gets it from the lock file name
# -->+ (if there is a lock file,
# -->+ that's proof the process has been running).
# --> See the "lockfile" entry, above.
# Bring the subsystem down.
if [ -f /etc/rc.d/init.d/$subsys.init ]; then
/etc/rc.d/init.d/$subsys.init stop
else
/etc/rc.d/init.d/$subsys stop
# --> Suspend running jobs and daemons.
# --> Note that "stop" is a positional parameter,
# -->+ not a shell builtin.
fi
done
That wasn't so bad. Aside from a little fancy footwork with variable
matching, there is no new material there.
Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit
longer than killall, but similar in concept. Make a copy of this
script somewhere in your home directory and experiment with it (do
not run it as root). Do a simulated run with the -vn flags (sh -vn
scriptname). Add extensive comments. Change the commands to echos.
Exercise 2. Look at some of the more complex scripts in
/etc/rc.d/init.d. Try to understand at least portions of them. Follow
the above procedure to analyze them. For some additional insight, you
might also examine the file sysvinitfiles in
/usr/share/doc/initscripts-?.??, which is part of the "initscripts"
documentation.
Part 5. Advanced Topics
At this point, we are ready to delve into certain of the difficult
and unusual aspects of scripting. Along the way, we will attempt to
"push the envelope" in various ways and examine boundary conditions
(what happens when we move into uncharted territory?).
Table of Contents
- Regular Expressions
18.1. A Brief Introduction to Regular Expressions
18.2. Globbing
- Here Documents
19.1. Here Strings
- I/O Redirection
20.1. Using exec
20.2. Redirecting Code Blocks
20.3. Applications
- Subshells
- Restricted Shells
- Process Substitution
- Functions
24.1. Complex Functions and Function Complexities
24.2. Local Variables
24.3. Recursion Without Local Variables
- Aliases
- List Constructs
- Arrays
- Indirect References
- /dev and /proc
29.1. /dev
29.2. /proc
- Network Programming
- Of Zeros and Nulls
- Debugging
- Options
- Gotchas
- Scripting With Style
35.1. Unofficial Shell Scripting Stylesheet
- Miscellany
36.1. Interactive and non-interactive shells and scripts
36.2. Shell Wrappers
36.3. Tests and Comparisons: Alternatives
36.4. Recursion: a script calling itself
36.5. "Colorizing" Scripts
36.6. Optimizations
36.7. Assorted Tips
36.8. Security Issues
36.9. Portability Issues
36.10. Shell Scripting Under Windows
- Bash, versions 2, 3, and 4
37.1. Bash, version 2
37.2. Bash, version 3
37.3. Bash, version 4
________________________________________________________________
Chapter 18. Regular Expressions
. . . the intellectual activity associated with software development
is largely one of gaining insight.
--Stowe Boyd
To fully utilize the power of shell scripting, you need to master
Regular Expressions. Certain commands and utilities commonly used in
scripts, such as grep, expr, sed and awk, interpret and use REs. As
of version 3, Bash has acquired its own RE-match operator: =~.
________________________________________________________________
18.1. A Brief Introduction to Regular Expressions
An expression is a string of characters. Those characters having an
interpretation above and beyond their literal meaning are called
metacharacters. A quote symbol, for example, may denote speech by a
person, ditto, or a meta-meaning [98] for the symbols that follow.
Regular Expressions are sets of characters and/or metacharacters that
match (or specify) patterns.
A Regular Expression contains one or more of the following:
* A character set. These are the characters retaining their literal
meaning. The simplest type of Regular Expression consists only of
a character set, with no metacharacters.
* An anchor. These designate (anchor) the position in the line of
text that the RE is to match. For example, ^, and $ are anchors.
* Modifiers. These expand or narrow (modify) the range of text the
RE is to match. Modifiers include the asterisk, brackets, and the
backslash.
The main uses for Regular Expressions (REs) are text searches and
string manipulation. An RE matches a single character or a set of
characters -- a string or a part of a string.
* The asterisk -- * -- matches any number of repeats of the
character string or RE preceding it, including zero instances.
"1133*" matches 11 + one or more 3's: 113, 1133, 1133333, and so
forth.
* The dot -- . -- matches any one character, except a newline. [99]
"13." matches 13 + at least one of any character (including a
space): 1133, 11333, but not 13 (additional character missing).
See Example 16-18 for a demonstration of dot single-character
matching.
* The caret -- ^ -- matches the beginning of a line, but sometimes,
depending on context, negates the meaning of a set of characters
in an RE.
* The dollar sign -- $ -- at the end of an RE matches the end of a
line.
"XXX$" matches XXX at the end of a line.
"^$" matches blank lines.
* Brackets -- [...] -- enclose a set of characters to match in a
single RE.
"[xyz]" matches any one of the characters x, y, or z.
"[c-n]" matches any one of the characters in the range c to n.
"[B-Pk-y]" matches any one of the characters in the ranges B to P
and k to y.
"[a-z0-9]" matches any single lowercase letter or any digit.
"[^b-d]" matches any character except those in the range b to d.
This is an instance of ^ negating or inverting the meaning of the
following RE (taking on a role similar to ! in a different
context).
Combined sequences of bracketed characters match common word
patterns. "[Yy][Ee][Ss]" matches yes, Yes, YES, yEs, and so
forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches
any Social Security number.
* The backslash -- \ -- escapes a special character, which means
that character gets interpreted literally (and is therefore no
longer special).
A "\$" reverts back to its literal meaning of "$", rather than
its RE meaning of end-of-line. Likewise a "\\" has the literal
meaning of "\".
* Escaped "angle brackets" -- \<...\> -- mark word boundaries.
The angle brackets must be escaped, since otherwise they have
only their literal character meaning.
"\<the\>" matches the word "the," but not the words "them,"
"there," "other," etc.
bash$ cat textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.
This is line 4.
bash$ grep 'the' textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.
bash$ grep '<the>' textfile
This is the only instance of line 2.
The only way to be certain that a particular RE works is to test it.
TEST FILE: tstfile # No match.
# No match.
Run grep "1133*" on this file. # Match.
# No match.
# No match.
This line contains the number 113. # Match.
This line contains the number 13. # No match.
This line contains the number 133. # No match.
This line contains the number 1133. # Match.
This line contains the number 113312. # Match.
This line contains the number 1112. # No match.
This line contains the number 113312312. # Match.
This line contains no numbers at all. # No match.
bash$ grep "1133*" tstfile
Run grep "1133*" on this file. # Match.
This line contains the number 113. # Match.
This line contains the number 1133. # Match.
This line contains the number 113312. # Match.
This line contains the number 113312312. # Match.
* Extended REs. Additional metacharacters added to the basic set.
Used in egrep, awk, and Perl.
* The question mark -- ? -- matches zero or one of the previous RE.
It is generally used for matching single characters.
* The plus -- + -- matches one or more of the previous RE. It
serves a role similar to the *, but does not match zero
occurrences.
GNU versions of sed and awk can use "+",
but it needs to be escaped.
echo a111b | sed -ne '/a1+b/p'
echo a111b | grep 'a1+b'
echo a111b | gawk '/a1+b/'
All of above are equivalent.
Thanks, S.C.
* Escaped "curly brackets" -- \{ \} -- indicate the number of
occurrences of a preceding RE to match.
It is necessary to escape the curly brackets since they have only
their literal character meaning otherwise. This usage is
technically not part of the basic RE set.
"[0-9]\{5\}" matches exactly five digits (characters in the range
of 0 to 9).
Note
Curly brackets are not available as an RE in the "classic" (non-POSIX
compliant) version of awk. However, the GNU extended version of awk,
gawk, has the --re-interval option that permits them (without being
escaped).
bash$ echo 2222 | gawk --re-interval '/2{3}/'
2222
Perl and some egrep versions do not require escaping the curly
brackets.
* Parentheses -- ( ) -- enclose a group of REs. They are useful
with the following "|" operator and in substring extraction using
expr.
* The -- | -- "or" RE operator matches any of a set of alternate
characters.
bash$ egrep 're(a|e)d' misc.txt
People who read seem to be better informed than those who do not.
The clarinet produces sound by the vibration of its reed.
Note
Some versions of sed, ed, and ex support escaped versions of the
extended Regular Expressions described above, as do the GNU
utilities.
* POSIX Character Classes. [:class:]
This is an alternate method of specifying a range of characters
to match.
* [:alnum:] matches alphabetic or numeric characters. This is
equivalent to A-Za-z0-9.
* [:alpha:] matches alphabetic characters. This is equivalent to
A-Za-z.
* [:blank:] matches a space or a tab.
* [:cntrl:] matches control characters.
* [:digit:] matches (decimal) digits. This is equivalent to 0-9.
* [:graph:] (graphic printable characters). Matches characters in
the range of ASCII 33 - 126. This is the same as [:print:],
below, but excluding the space character.
* [:lower:] matches lowercase alphabetic characters. This is
equivalent to a-z.
* [:print:] (printable characters). Matches characters in the range
of ASCII 32 - 126. This is the same as [:graph:], above, but
adding the space character.
* [:space:] matches whitespace characters (space and horizontal
tab).
* [:upper:] matches uppercase alphabetic characters. This is
equivalent to A-Z.
* [:xdigit:] matches hexadecimal digits. This is equivalent to
0-9A-Fa-f.
Important
POSIX character classes generally require quoting or double brackets
([[ ]]).
bash$ grep [[:digit:]] test.file
abc=723
...
if [[ $arow =~ [[:digit:]] ]] # Numerical input?
then # POSIX char class
if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!
...
From ktour.sh example script.
These character classes may even be used with globbing, to a
limited extent.
bash$ ls -l ?[[:digit:]][[:digit:]]?
-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b
POSIX character classes are used in Example 16-21 and Example
16-22.
Sed, awk, and Perl, used as filters in scripts, take REs as arguments
when "sifting" or transforming files or I/O streams. See Example A-12
and Example A-16 for illustrations of this.
The standard reference on this complex topic is Friedl's Mastering
Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives
a very lucid treatment of REs. See the Bibliography for more
information on these books.
________________________________________________________________
18.2. Globbing
Bash itself cannot recognize Regular Expressions. Inside scripts, it
is commands and utilities -- such as sed and awk -- that interpret
RE's.
Bash does carry out filename expansion [100] -- a process known as
globbing -- but this does not use the standard RE set. Instead,
globbing recognizes and expands wild cards. Globbing interprets the
standard wild card characters [101] -- * and ?, character lists in
square brackets, and certain other special characters (such as ^ for
negating the sense of a match). There are important limitations on
wild card characters in globbing, however. Strings containing * will
not match filenames that start with a dot, as, for example, .bashrc.
[102] Likewise, the ? has a different meaning in globbing than as
part of an RE.
bash$ ls -l
total 2
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls -l t?.sh
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
bash$ ls -l [ab]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
bash$ ls -l [a-c]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
bash$ ls -l [^ab]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls -l {b*,c*,est}
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
Bash performs filename expansion on unquoted command-line arguments.
The echo command demonstrates this.
bash$ echo *
a.1 b.1 c.1 t2.sh test1.txt
bash$ echo t*
t2.sh test1.txt
bash$ echo t?.sh
t2.sh
Note
It is possible to modify the way Bash interprets special characters
in globbing. A set -f command disables globbing, and the nocaseglob
and nullglob options to shopt change globbing behavior.
See also Example 11-5.
Caution
Filenames with embedded whitespace can cause globbing to choke. David
Wheeler shows how to avoid many such pitfalls.
IFS="$(printf '\n\t')" # Remove space.
Correct glob use:
Always use for-loop, prefix glob, check if exists file.
for file in ./* ; do # Use ./* ... NEVER bare *
if [ -e "$file" ] ; then # Check whether file exists.
COMMAND ... "$file" ...
fi
done
This example taken from David Wheeler's site, with permission.
________________________________________________________________
Chapter 19. Here Documents
Here and now, boys.
--Aldous Huxley, Island
A here document is a special-purpose code block. It uses a form of
I/O redirection to feed a command list to an interactive program or a
command, such as ftp, cat, or the ex text editor.
COMMAND <<InputComesFromHERE
...
...
...
InputComesFromHERE
A limit string delineates (frames) the command list. The special
symbol << precedes the limit string. This has the effect of
redirecting the output of a command block into the stdin of the
program or command. It is similar to interactive-program <
command-file, where command-file contains
command #1
command #2
...
The here document equivalent looks like this:
interactive-program <<LimitString
command #1
command #2
...
LimitString
Choose a limit string sufficiently unusual that it will not occur
anywhere in the command list and confuse matters.
Note that here documents may sometimes be used to good effect with
non-interactive utilities and commands, such as, for example, wall.
Example 19-1. broadcast: Sends message to everyone logged in
!/bin/bash
wall <<zzz23EndOfMessagezzz23
E-mail your noontime orders for pizza to the system administrator.
(Add an extra dollar for anchovy or mushroom topping.)
Additional message text goes here.
Note: 'wall' prints comment lines.
zzz23EndOfMessagezzz23
Could have been done more efficiently by
wall <message-file
However, embedding the message template in a script
- is a quick-and-dirty one-off solution.
exit
Even such unlikely candidates as the vi text editor lend themselves
to here documents.
Example 19-2. dummyfile: Creates a 2-line dummy file
!/bin/bash
Noninteractive use of 'vi' to edit a file.
Emulates 'sed'.
E_BADARGS=85
if [ -z "$1" ]
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
TARGETFILE=$1
Insert 2 lines in file, then save.
--------Begin here document-----------#
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
----------End here document-----------#
Note that ^[ above is a literal escape
Bram Moolenaar points out that this may not work with 'vim'
- because of possible problems with terminal interaction.
exit
The above script could just as effectively have been implemented with
ex, rather than vi. Here documents containing a list of ex commands
are common enough to form their own category, known as ex scripts.
!/bin/bash
Replace all instances of "Smith" with "Jones"
- in files with a ".txt" filename suffix.
ORIGINAL=Smith
REPLACEMENT=Jones
for word in $(fgrep -l $ORIGINAL *.txt)
do
-------------------------------------
ex $word <<EOF
:%s/$ORIGINAL/$REPLACEMENT/g
:wq
EOF
:%s is the "ex" substitution command.
:wq is write-and-quit.
-------------------------------------
done
Analogous to "ex scripts" are cat scripts.
Example 19-3. Multi-line message using cat
!/bin/bash
'echo' is fine for printing single line messages,
- but somewhat problematic for for message blocks.
A 'cat' here document overcomes this limitation.
cat <<End-of-message
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
End-of-message
Replacing line 7, above, with
- cat > $Newfile <<End-of-message
- writes the output to the file $Newfile, rather than to stdout.
exit 0
Code below disabled, due to "exit 0" above.
S.C. points out that the following also works.
echo "-------------------------------------
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
-------------------------------------"
However, text may not include double quotes unless they are escaped.
The - option to mark a here document limit string (<<-LimitString)
suppresses leading tabs (but not spaces) in the output. This may be
useful in making a script more readable.
Example 19-4. Multi-line message, with tabs suppressed
!/bin/bash
Same as previous example, but...
The - option to a here document <<-
- suppresses leading tabs in the body of the document,
cat <<-ENDOFMESSAGE
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
ENDOFMESSAGE
The output of the script will be flush left.
Leading tab in each line will not show.
Above 5 lines of "message" prefaced by a tab, not spaces.
Spaces not affected by <<- .
Note that this option has no effect on embedded tabs.
exit 0
A here document supports parameter and command substitution. It is
therefore possible to pass different parameters to the body of the
here document, changing its output accordingly.
Example 19-5. Here document with replaceable parameters
!/bin/bash
Another 'cat' here document, using parameter substitution.
Try it with no command-line parameters, ./scriptname
Try it with one command-line parameter, ./scriptname Mortimer
Try it with one two-word quoted command-line parameter,
./scriptname "Mortimer Jones"
CMDLINEPARAM=1 # Expect at least command-line parameter.
if [ $# -ge $CMDLINEPARAM ]
then
NAME=$1 # If more than one command-line param,
#+ then just take the first.
else
NAME="John Doe" # Default, if no command-line parameter.
fi
RESPONDENT="the author of this fine script"
cat <<Endofmessage
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
This comment shows up in the output (why?).
Endofmessage
Note that the blank lines show up in the output.
So does the comment.
exit
This is a useful script containing a here document with parameter
substitution.
Example 19-6. Upload a file pair to Sunsite incoming directory
!/bin/bash
upload.sh
Upload file pair (Filename.lsm, Filename.tar.gz)
- to incoming directory at Sunsite/UNC (ibiblio.org).
Filename.tar.gz is the tarball itself.
Filename.lsm is the descriptor file.
Sunsite requires "lsm" file, otherwise will bounce contributions.
E_ARGERROR=85
if [ -z "$1" ]
then
echo "Usage: basename $0 Filename-to-upload"
exit $E_ARGERROR
fi
Filename=basename $1 # Strips pathname out of file name.
Server="ibiblio.org"
Directory="/incoming/Linux"
These need not be hard-coded into script,
- but may instead be changed to command-line argument.
Password="your.e-mail.address" # Change above to suit.
ftp -n $Server <<End-Of-Session
-n option disables auto-logon
user anonymous "$Password" # If this doesn't work, then try:
# quote user anonymous "$Password"
binary
bell # Ring 'bell' after each file transfer.
cd $Directory
put "$Filename.lsm"
put "$Filename.tar.gz"
bye
End-Of-Session
exit 0
Quoting or escaping the "limit string" at the head of a here document
disables parameter substitution within its body. The reason for this
is that quoting/escaping the limit string effectively escapes the $,
`, and \ special characters, and causes them to be interpreted
literally. (Thank you, Allen Halsey, for pointing this out.)
Example 19-7. Parameter substitution turned off
!/bin/bash
A 'cat' here-document, but with parameter substitution disabled.
NAME="John Doe"
RESPONDENT="the author of this fine script"
cat <<'Endofmessage'
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
Endofmessage
No parameter substitution when the "limit string" is quoted or escaped.
Either of the following at the head of the here document would have
cat <<"Endofmessage"
cat <<\Endofmessage
And, likewise:
cat <<"SpecialCharTest"
Directory listing would follow
if limit string were not quoted.
Arithmetic expansion would take place
if limit string were not quoted.
$((5 + 3))
A a single backslash would echo
if limit string were not quoted.
\\
SpecialCharTest
exit
Disabling parameter substitution permits outputting literal text.
Generating scripts or even program code is one use for this.
Example 19-8. A script that generates another script
#!/bin/bash
# generate-script.sh
# Based on an idea by Albert Reiner.
OUTFILE=generated.sh # Name of the file to generate.
# -----------------------------------------------------------
# 'Here document containing the body of the generated script.
(
cat <<'EOF'
#!/bin/bash
echo "This is a generated shell script."
# Note that since we are inside a subshell,
#+ we can't access variables in the "outside" script.
echo "Generated file will be named: $OUTFILE"
# Above line will not work as normally expected
#+ because parameter expansion has been disabled.
# Instead, the result is literal output.
a=7
b=3
let "c = $a * $b"
echo "c = $c"
exit 0
EOF
) > $OUTFILE
# -----------------------------------------------------------
# Quoting the 'limit string' prevents variable expansion
#+ within the body of the above 'here document.'
# This permits outputting literal strings in the output file.
if [ -f "$OUTFILE" ]
then
chmod 755 $OUTFILE
# Make the generated file executable.
else
echo "Problem in creating file: \"$OUTFILE\""
fi
# This method also works for generating
#+ C programs, Perl programs, Python programs, Makefiles,
#+ and the like.
exit 0
It is possible to set a variable from the output of a here document.
This is actually a devious form of command substitution.
variable=$(cat < EOF
lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted)
Caution
Some utilities will not work inside a here document.
Warning
The closing limit string, on the final line of a here document, must
start in the first character position. There can be no leading
whitespace. Trailing whitespace after the limit string likewise
causes unexpected behavior. The whitespace prevents the limit string
from being recognized. [103]
#!/bin/bash
echo "----------------------------------------------------------------------"
cat <&2
done <<< $(echo ${ArrayVar[*]})
# element0 element1 element2 A B C D
Example 19-13. Prepending a line to a file
#!/bin/bash
# prepend.sh: Add text at beginning of file.
#
# Example contributed by Kenny Stauffer,
#+ and slightly modified by document author.
E_NOSUCHFILE=85
read -p "File: " file # -p arg to 'read' displays prompt.
if [ ! -e "$file" ]
then # Bail out if no such file.
echo "File $file not found."
exit $E_NOSUCHFILE
fi
read -p "Title: " title
cat - $file <<<$title > $file.new
echo "Modified file is $file.new"
exit # Ends script execution.
from 'man bash':
Here Strings
A variant of here documents, the format is:
<<
# Redirect stdout to a file.
# Creates the file if not present, otherwise overwrites it.
ls -lR > dir-tree.list
# Creates a file containing a listing of the directory tree.
: > filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch')
.
# The : serves as a dummy placeholder, producing no output.
> filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch')
.
# (Same result as ": >", above, but this does not work with some shells.
)
COMMAND_OUTPUT >>
# Redirect stdout to a file.
# Creates the file if not present, otherwise appends to it.
# Single-line redirection commands (affect only the line they are on):
# --------------------------------------------------------------------
1>filename
# Redirect stdout to file "filename."
1>>filename
# Redirect and append stdout to file "filename."
2>filename
# Redirect stderr to file "filename."
2>>filename
# Redirect and append stderr to file "filename."
&>filename
# Redirect both stdout and stderr to file "filename."
# This operator is now functional, as of Bash 4, final release.
M>N
# "M" is a file descriptor, which defaults to 1, if not explicitly set.
# "N" is a filename.
# File descriptor "M" is redirect to file "N."
M>&N
# "M" is a file descriptor, which defaults to 1, if not set.
# "N" is another file descriptor.
#=======================================================================
=======
# Redirecting stdout, one line at a time.
LOGFILE=script.log
echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is echoed to stdout, and will not appear in \"$LOGF
ILE\"."
# These redirection commands automatically "reset" after each line.
# Redirecting stderr, one line at a time.
ERRORFILE=script.errors
bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE.
bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE.
bad_command3 # Error message echoed to stderr,
#+ and does not appear in $ERRORFILE.
# These redirection commands also automatically "reset" after each line.
#=======================================================================
2>&1
# Redirects stderr to stdout.
# Error messages get sent to same place as standard output.
>>filename 2>&1
bad_command >>filename 2>&1
# Appends both stdout and stderr to the file "filename" ...
2>&1 | [command(s)]
bad_command 2>&1 | awk '{print $5}' # found
# Sends stderr through a pipe.
# |& was added to Bash 4 as an abbreviation for 2>&1 |.
i>&j
# Redirects file descriptor i to j.
# All output of file pointed to by i gets sent to file pointed to by j.
>&j
# Redirects, by default, file descriptor 1 (stdout) to j.
# All stdout gets sent to file pointed to by j.
0< FILENAME
< FILENAME
# Accept input from a file.
# Companion command to ">", and often used in combination with it.
#
# grep search-word filename
# Open file "filename" for reading and writing,
#+ and assign file descriptor "j" to it.
# If "filename" does not exist, create it.
# If file descriptor "j" is not specified, default to fd 0, stdin.
#
# An application of this is writing at a specified place in a file.
echo 1234567890 > File # Write string to "File".
exec 3<> File # Open "File" and assign fd 3 to it.
read -n 4 <&3 # Read only 4 characters.
echo -n . >&3 # Write a decimal point there.
exec 3>&- # Close fd 3.
cat File # ==> 1234.67890
# Random access, by golly.
|
# Pipe.
# General purpose process and command chaining tool.
# Similar to ">", but more general in effect.
# Useful for chaining commands, scripts, files, and programs together.
cat *.txt | sort | uniq > result-file
# Sorts the output of all the .txt files and deletes duplicate lines,
# finally saves results to "result-file".
Multiple instances of input and output redirection and/or pipes can
be combined in a single command line.
command < input-file > output-file
# Or the equivalent:
< input-file command > output-file # Although this is non-standard.
command1 | command2 | command3 > output-file
See Example 16-31 and Example A-14.
Multiple output streams may be redirected to one file.
ls -yz >> command.log 2>&1
# Capture result of illegal options "yz" in file "command.log."
# Because stderr is redirected to the file,
#+ any error messages will also be there.
# Note, however, that the following does *not* give the same result.
ls -yz 2>&1 >> command.log
# Outputs an error message, but does not write to file.
# More precisely, the command output (in this case, null)
#+ writes to the file, but the error message goes only to stdout.
# If redirecting both stdout and stderr,
#+ the order of the commands makes a difference.
Closing File Descriptors
n<&-
Close input file descriptor n.
0<&-, <&-
Close stdin.
n>&-
Close output file descriptor n.
1>&-, >&-
Close stdout.
Child processes inherit open file descriptors. This is why pipes
work. To prevent an fd from being inherited, close it.
# Redirecting only stderr to a pipe.
exec 3>&1 # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls').
# ^^^^ ^^^^
exec 3>&- # Now close it for the remainder of the
script.
# Thanks, S.C.
For a more detailed introduction to I/O redirection see Appendix F.
________________________________________________________________
20.1. Using exec
An exec filename command redirects stdout to a designated
file. This sends all command output that would normally go to stdout
to that file.
Important
exec N > filename affects the entire script or current shell.
Redirection in the PID of the script or shell from that point on has
changed. However . . .
N > filename affects only the newly-forked process, not the entire
script or shell.
Thank you, Ahmed Darwish, for pointing this out.
Example 20-2. Redirecting stdout using exec
#!/bin/bash
# reassign-stdout.sh
LOGFILE=logfile.txt
exec 6>&1 # Link file descriptor #6 with stdout.
# Saves stdout.
exec > $LOGFILE # stdout replaced with file "logfile.txt".
# ----------------------------------------------------------- #
# All output from commands in this block sent to file $LOGFILE.
echo -n "Logfile: "
date
echo "-------------------------------------"
echo
echo "Output of \"ls -al\" command"
echo
ls -al
echo; echo
echo "Output of \"df\" command"
echo
df
# ----------------------------------------------------------- #
exec 1>&6 6>&- # Restore stdout and close file descriptor #6.
echo
echo "== stdout now restored to default == "
echo
ls -al
echo
exit 0
Example 20-3. Redirecting both stdin and stdout in the same script
with exec
#!/bin/bash
# upperconv.sh
# Converts a specified input file to uppercase.
E_FILE_ACCESS=70
E_WRONG_ARGS=71
if [ ! -r "$1" ] # Is specified input file readable?
then
echo "Can't read from input file!"
echo "Usage: $0 input-file output-file"
exit $E_FILE_ACCESS
fi # Will exit with same error
#+ even if input file ($1) not specified (why?).
if [ -z "$2" ]
then
echo "Need to specify output file."
echo "Usage: $0 input-file output-file"
exit $E_WRONG_ARGS
fi
exec 4<&0
exec < $1 # Will read from input file.
exec 7>&1
exec > $2 # Will write to output file.
# Assumes output file writable (add check?).
# -----------------------------------------------
cat - | tr a-z A-Z # Uppercase conversion.
# ^^^^^ # Reads from stdin.
# ^^^^^^^^^^ # Writes to stdout.
# However, both stdin and stdout were redirected.
# Note that the 'cat' can be omitted.
# -----------------------------------------------
exec 1>&7 7>&- # Restore stout.
exec 0<&4 4<&- # Restore stdin.
# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."
exit 0
I/O redirection is a clever way of avoiding the dreaded inaccessible
variables within a subshell problem.
Example 20-4. Avoiding a subshell
#!/bin/bash
# avoid-subshell.sh
# Suggested by Matthew Walker.
Lines=0
echo
cat myfile.txt | while read line;
do {
echo $line
(( Lines++ )); # Incremented values of this variable
#+ inaccessible outside loop.
# Subshell problem.
}
done
echo "Number of lines read = $Lines" # 0
# Wrong!
echo "------------------------"
exec 3<> myfile.txt
while read line <&3
do {
echo "$line"
(( Lines++ )); # Incremented values of this variable
#+ accessible outside loop.
# No subshell, no problem.
}
done
exec 3>&-
echo "Number of lines read = $Lines" # 8
echo
exit 0
# Lines below not seen by script.
$ cat myfile.txt
Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8.
________________________________________________________________
20.2. Redirecting Code Blocks
Blocks of code, such as while, until, and for loops, even if/then
test blocks can also incorporate redirection of stdin. Even a
function may use this form of redirection (see Example 24-11). The <
operator at the end of the code block accomplishes this.
Example 20-5. Redirected while loop
#!/bin/bash
# redir2.sh
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
#+ Filename=${1:-names.data}
# can replace the above test (parameter substitution).
count=0
echo
while [ "$name" != Smith ] # Why is variable $name in quotes?
do
read name # Reads from $Filename, rather than stdin.
echo $name
let "count += 1"
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
echo; echo "$count names read"; echo
exit 0
# Note that in some older shell scripting languages,
#+ the redirected loop would run as a subshell.
# Therefore, $count would return 0, the initialized value outside the loop.
# Bash and ksh avoid starting a subshell *whenever possible*,
#+ so that this script, for example, runs correctly.
# (Thanks to Heiner Steven for pointing this out.)
# However . . .
# Bash *can* sometimes start a subshell in a PIPED "while-read" loop,
#+ as distinct from a REDIRECTED "while" loop.
abc=hi
echo -e "1\n2\n3" | while read l
do abc="$l"
echo $abc
done
echo $abc
# Thanks, Bruno de Oliveira Schneider, for demonstrating this
#+ with the above snippet of code.
# And, thanks, Brian Onn, for correcting an annotation error.
Example 20-6. Alternate form of redirected while loop
#!/bin/bash
# This is an alternate form of the preceding script.
# Suggested by Heiner Steven
#+ as a workaround in those situations when a redirect loop
#+ runs as a subshell, and therefore variables inside the loop
# +do not keep their values upon loop termination.
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
exec 3<&0 # Save stdin to file descriptor 3.
exec 0<"$Filename" # Redirect standard input.
count=0
echo
while [ "$name" != Smith ]
do
read name # Reads from redirected stdin ($Filename).
echo $name
let "count += 1"
done # Loop reads from file $Filename
#+ because of line 20.
# The original version of this script terminated the "while" loop with
#+ done <"$Filename"
# Exercise:
# Why is this unnecessary?
exec 0<&3 # Restore old stdin.
exec 3<&- # Close temporary fd 3.
echo; echo "$count names read"; echo
exit 0
Example 20-7. Redirected until loop
#!/bin/bash
# Same as previous example, but with "until" loop.
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
# while [ "$name" != Smith ]
until [ "$name" = Smith ] # Change != to =.
do
read name # Reads from $Filename, rather than stdin.
echo $name
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
# Same results as with "while" loop in previous example.
exit 0
Example 20-8. Redirected for loop
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
line_count=`wc $Filename | awk '{ print $1 }'`
# Number of lines in target file.
#
# Very contrived and kludgy, nevertheless shows that
#+ it's possible to redirect stdin within a "for" loop...
#+ if you're clever enough.
#
# More concise is line_count=$(wc -l < "$Filename")
for name in `seq $line_count` # Recall that "seq" prints sequence of numbers.
# while [ "$name" != Smith ] -- more complicated than a "while" loop --
do
read name # Reads from $Filename, rather than stdin.
echo $name
if [ "$name" = Smith ] # Need all this extra baggage here.
then
break
fi
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
exit 0
We can modify the previous example to also redirect the output of the
loop.
Example 20-9. Redirected for loop (both stdin and stdout redirected)
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
Savefile=$Filename.new # Filename to save results in.
FinalName=Jonah # Name to terminate "read" on.
line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target fi
le.
for name in `seq $line_count`
do
read name
echo "$name"
if [ "$name" = "$FinalName" ]
then
break
fi
done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename,
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^ and saves it to backup file.
exit 0
Example 20-10. Redirected if/then test
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
TRUE=1
if [ "$TRUE" ] # if true and if : also work.
then
read name
echo $name
fi <"$Filename"
# ^^^^^^^^^^^^
# Reads only first line of file.
# An "if/then" test has no way of iterating unless embedded in a loop.
exit 0
Example 20-11. Data file names.data for above examples
Aristotle
Arrhenius
Belisarius
Capablanca
Dickens
Euler
Goethe
Hegel
Jonah
Laplace
Maroczy
Purcell
Schmidt
Schopenhauer
Semmelweiss
Smith
Steinmetz
Tukhashevsky
Turing
Venn
Warshawski
Znosko-Borowski
# This is a data file for
#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".
Redirecting the stdout of a code block has the effect of saving its
output to a file. See Example 3-2.
Here documents are a special case of redirected code blocks. That
being the case, it should be possible to feed the output of a here
document into the stdin for a while loop.
# This example by Albert Siersema
# Used with permission (thanks!).
function doesOutput()
# Could be an external command too, of course.
# Here we show you can use a function as well.
{
ls -al *.jpg | awk '{print $5,$9}'
}
nr=0 # We want the while loop to be able to manipulate these and
totalSize=0 #+ to be able to see the changes after the 'while' finished.
while read fileSize fileName ; do
echo "$fileName is $fileSize bytes"
let nr++
totalSize=$((totalSize+fileSize)) # Or: "let totalSize+=fileSize"
done<&7 # This *appends* the date to the file.
# ^^^^^^^ command substitution
# See below.
}
case $LOG_LEVEL in
1) exec 3>&2 4> /dev/null 5> /dev/null;;
2) exec 3>&2 4>&2 5> /dev/null;;
3) exec 3>&2 4>&2 5>&2;;
*) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
esac
FD_LOGVARS=6
if [[ $LOG_VARS ]]
then exec 6>> /var/log/vars.log
else exec 6> /dev/null # Bury output.
fi
FD_LOGEVENTS=7
if [[ $LOG_EVENTS ]]
then
# exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
# Above line fails in versions of Bash more recent than 2.04. Why?
exec 7>> /var/log/event.log # Append to "event.log".
log # Write time and date.
else exec 7> /dev/null # Bury output.
fi
echo "DEBUG3: beginning" >&${FD_DEBUG3}
ls -l >&5 2>&4 # command1 >&5 2>&4
echo "Done" # command2
echo "sending mail" >&${FD_LOGEVENTS}
# Writes "sending mail" to file descriptor #7.
exit 0
________________________________________________________________
Chapter 21. Subshells
Running a shell script launches a new process, a subshell.
Definition: A subshell is a child process launched by a shell (or
shell script).
A subshell is a separate instance of the command processor -- the
shell that gives you the prompt at the console or in an xterm window.
Just as your commands are interpreted at the command-line prompt,
similarly does a script batch-process a list of commands. Each shell
script running is, in effect, a subprocess (child process) of the
parent shell.
A shell script can itself launch subprocesses. These subshells let
the script do parallel processing, in effect executing multiple
subtasks simultaneously.
#!/bin/bash
# subshell-test.sh
(
# Inside parentheses, and therefore a subshell . . .
while [ 1 ] # Endless loop.
do
echo "Subshell running . . ."
done
)
# Script will run forever,
#+ or at least until terminated by a Ctl-C.
exit $? # End of script (but will never get here).
Now, run the script:
sh subshell-test.sh
And, while the script is running, from a different xterm:
ps -ef | grep subshell-test.sh
UID PID PPID C STIME TTY TIME CMD
500 2698 2502 0 14:26 pts/4 00:00:00 sh subshell-test.sh
500 2699 2698 21 14:26 pts/4 00:00:24 sh subshell-test.sh
^^^^
Analysis:
PID 2698, the script, launched PID 2699, the subshell.
Note: The "UID ..." line would be filtered out by the "grep" command,
but is shown here for illustrative purposes.
In general, an external command in a script forks off a subprocess,
[107] whereas a Bash builtin does not. For this reason, builtins
execute more quickly and use fewer system resources than their
external command equivalents.
Command List within Parentheses
( command1; command2; command3; ... )
A command list embedded between parentheses runs as a
subshell.
Variables in a subshell are not visible outside the block of code in
the subshell. They are not accessible to the parent process, to the
shell that launched the subshell. These are, in effect, variables
local to the child process.
Example 21-1. Variable scope in a subshell
#!/bin/bash
# subshell.sh
echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new $BASH_SUBSHELL variable.
echo; echo
outer_variable=Outer
global_variable=
# Define global variable for "storage" of
#+ value of subshell variable.
(
echo "We are inside the subshell."
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner
echo "From inside subshell, \"inner_variable\" = $inner_variable"
echo "From inside subshell, \"outer\" = $outer_variable"
global_variable="$inner_variable" # Will this allow "exporting"
#+ a subshell variable?
)
echo; echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo
if [ -z "$inner_variable" ]
then
echo "inner_variable undefined in main body of shell"
else
echo "inner_variable defined in main body of shell"
fi
echo "From main body of shell, \"inner_variable\" = $inner_variable"
# $inner_variable will show as blank (uninitialized)
#+ because variables defined in a subshell are "local variables".
# Is there a remedy for this?
echo "global_variable = "$global_variable"" # Why doesn't this work?
echo
# =======================================================================
# Additionally ...
echo "-----------------"; echo
var=41 # Global variable.
( let "var+=1"; echo "\$var INSIDE subshell = $var" ) # 42
echo "\$var OUTSIDE subshell = $var" # 41
# Variable operations inside a subshell, even to a GLOBAL variable
#+ do not affect the value of the variable outside the subshell!
exit 0
# Question:
# --------
# Once having exited a subshell,
#+ is there any way to reenter that very same subshell
#+ to modify or access the subshell variables?
See also $BASHPID and Example 34-2.
Definition: The scope of a variable is the context in which it has
meaning, in which it has a value that can be referenced. For example,
the scope of a local variable lies only within the function, block of
code, or subshell within which it is defined, while the scope of a
global variable is the entire script in which it appears.
Note
While the $BASH_SUBSHELL internal variable indicates the nesting
level of a subshell, the $SHLVL variable shows no change within a
subshell.
echo " \$BASH_SUBSHELL outside subshell = $BASH_SUBSHELL" # 0
( echo " \$BASH_SUBSHELL inside subshell = $BASH_SUBSHELL" ) # 1
( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
# ^ ^ *** nested *** ^ ^
echo
echo " \$SHLVL outside subshell = $SHLVL" # 3
( echo " \$SHLVL inside subshell = $SHLVL" ) # 3 (No change!)
Directory changes made in a subshell do not carry over to the parent
shell.
Example 21-2. List User Profiles
#!/bin/bash
# allprofs.sh: Print all user profiles.
# This script written by Heiner Steven, and modified by the document author.
FILE=.bashrc # File containing user profile,
#+ was ".profile" in original script.
for home in `awk -F: '{print $6}' /etc/passwd`
do
[ -d "$home" ] || continue # If no home directory, go to next.
[ -r "$home" ] || continue # If not readable, go to next.
(cd $home; [ -e $FILE ] && less $FILE)
done
# When script terminates, there is no need to 'cd' back to original directory
,
#+ because 'cd $home' takes place in a subshell.
exit 0
A subshell may be used to set up a "dedicated environment" for a
command group.
COMMAND1
COMMAND2
COMMAND3
(
IFS=:
PATH=/bin
unset TERMINFO
set -C
shift 5
COMMAND4
COMMAND5
exit 3 # Only exits the subshell!
)
# The parent shell has not been affected, and the environment is preserved.
COMMAND6
COMMAND7
As seen here, the exit command only terminates the subshell in which
it is running, not the parent shell or script.
One application of such a "dedicated environment" is testing whether
a variable is defined.
if (set -u; : $variable) 2> /dev/null
then
echo "Variable is set."
fi # Variable has been set in current script,
#+ or is an an internal Bash variable,
#+ or is present in environment (has been exported).
# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
# or [[ ${variable-x} != x$variable ]]
# or [[ ${variable+x} = x ]]
# or [[ ${variable-x} != x ]]
Another application is checking for a lock file:
if (set -C; : > lock_file) 2> /dev/null
then
: # lock_file didn't exist: no user running the script
else
echo "Another user is already running that script."
exit 65
fi
# Code snippet by Stéphane Chazelas,
#+ with modifications by Paulo Marcel Coelho Aragao.
+
Processes may execute in parallel within different subshells. This
permits breaking a complex task into subcomponents processed
concurrently.
Example 21-3. Running parallel processes in subshells
(cat list1 list2 list3 | sort | uniq > list123) &
(cat list4 list5 list6 | sort | uniq > list456) &
# Merges and sorts both sets of lists simultaneously.
# Running in background ensures parallel execution.
#
# Same effect as
# cat list1 list2 list3 | sort | uniq > list123 &
# cat list4 list5 list6 | sort | uniq > list456 &
wait # Don't execute the next command until subshells finish.
diff list123 list456
Redirecting I/O to a subshell uses the "|" pipe operator, as in ls
-al | (command).
Note
A code block between curly brackets does not launch a subshell.
{ command1; command2; command3; . . . commandN; }
var1=23
echo "$var1" # 23
{ var1=76; }
echo "$var1" # 76
________________________________________________________________
Chapter 22. Restricted Shells
Disabled commands in restricted shells
. Running a script or portion of a script in restricted mode
disables certain commands that would otherwise be available.
This is a security measure intended to limit the privileges of
the script user and to minimize possible damage from running
the script.
The following commands and actions are disabled:
* Using cd to change the working directory.
* Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV
environmental variables.
* Reading or changing the $SHELLOPTS, shell environmental options.
* Output redirection.
* Invoking commands containing one or more /'s.
* Invoking exec to substitute a different process for the shell.
* Various other commands that would enable monkeying with or
attempting to subvert the script for an unintended purpose.
* Getting out of restricted mode within the script.
Example 22-1. Running a script in restricted mode
#!/bin/bash
# Starting the script with "#!/bin/bash -r"
#+ runs entire script in restricted mode.
echo
echo "Changing directory."
cd /usr/local
echo "Now in `pwd`"
echo "Coming back home."
cd
echo "Now in `pwd`"
echo
# Everything up to here in normal, unrestricted mode.
set -r
# set --restricted has same effect.
echo "==> Now in restricted mode. <=="
echo
echo
echo "Attempting directory change in restricted mode."
cd ..
echo "Still in `pwd`"
echo
echo
echo "\$SHELL = $SHELL"
echo "Attempting to change shell in restricted mode."
SHELL="/bin/ash"
echo
echo "\$SHELL= $SHELL"
echo
echo
echo "Attempting to redirect output in restricted mode."
ls -l /usr/bin > bin.files
ls -l bin.files # Try to list attempted file creation effort.
echo
exit 0
________________________________________________________________
Chapter 23. Process Substitution
Piping the stdout of a command into the stdin of another is a
powerful technique. But, what if you need to pipe the stdout of
multiple commands? This is where process substitution comes in.
Process substitution feeds the output of a process (or processes)
into the stdin of another process.
Template
Command list enclosed within parentheses
>(command_list)
<(command_list)
Process substitution uses /dev/fd/ files to send the
results of the process(es) within parentheses to another
process. [108]
Caution
There is no space between the the "<" or ">" and the parentheses.
Space there would give an error message.
bash$ echo >(true)
/dev/fd/63
bash$ echo <(true)
/dev/fd/63
bash$ echo >(true) <(true)
/dev/fd/63 /dev/fd/62
bash$ wc <(cat /usr/share/dict/linux.words)
483523 483523 4992010 /dev/fd/63
bash$ grep script /usr/share/dict/linux.words | wc
262 262 3601
bash$ wc <(grep script /usr/share/dict/linux.words)
262 262 3601 /dev/fd/63
Note
Bash creates a pipe with two file descriptors, --fIn and fOut--. The
stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a
/dev/fd/fIn argument to echo. On systems lacking /dev/fd/ files,
Bash may use temporary files. (Thanks, S.C.)
Process substitution can compare the output of two different
commands, or even the output of different options to the same
command.
bash$ comm <(ls -l) <(ls -al)
total 12
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh
total 20
drwxrwxrwx 2 bozo bozo 4096 Mar 10 18:10 .
drwx------ 72 bozo bozo 4096 Mar 10 17:58 ..
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh
Process substitution can compare the contents of two directories --
to see which filenames are in one, but not the other.
diff <(ls $first_directory) <(ls $second_directory)
Some other usages and uses of process substitution:
read -a list < <( od -Ad -w24 -t u2 /dev/urandom )
# Read a list of random numbers from /dev/urandom,
#+ process with "od"
#+ and feed into stdin of "read" . . .
# From "insertion-sort.bash" example script.
# Courtesy of JuanJo Ciarlante.
PORT=6881 # bittorrent
# Scan the port to make sure nothing nefarious is going on.
netcat -l $PORT | tee>(md5sum ->mydata-orig.md5) |
gzip | tee>(md5sum - | sed 's/-$/mydata.lz2/'>mydata-gz.md5)>mydata.gz
# Check the decompression:
gzip -d(bzip2 -c > file.tar.bz2) $directory_name
# Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".
#
# Because of the /dev/fd/ system feature,
# the pipe between both commands does not need to be named.
#
# This can be emulated.
#
bzip2 -c < pipe > file.tar.bz2&
tar cf pipe $directory_name
rm pipe
# or
exec 3>&1
tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
exec 3>&-
# Thanks, Stéphane Chazelas
Here is a method of circumventing the problem of an echo piped to a
while-read loop running in a subshell.
Example 23-1. Code block redirection without forking
#!/bin/bash
# wr-ps.bash: while-read loop with process substitution.
# This example contributed by Tomas Pospisek.
# (Heavily edited by the ABS Guide author.)
echo
echo "random input" | while read i
do
global=3D": Not available outside the loop."
# ... because it runs in a subshell.
done
echo "\$global (from outside the subprocess) = $global"
# $global (from outside the subprocess) =
echo; echo "--"; echo
while read i
do
echo $i
global=3D": Available outside the loop."
# ... because it does NOT run in a subshell.
done < <( echo "random input" )
# ^ ^
echo "\$global (using process substitution) = $global"
# Random input
# $global (using process substitution) = 3D: Available outside the loop.
echo; echo "##########"; echo
# And likewise . . .
declare -a inloop
index=0
cat $0 | while read line
do
inloop[$index]="$line"
((index++))
# It runs in a subshell, so ...
done
echo "OUTPUT = "
echo ${inloop[*]} # ... nothing echoes.
echo; echo "--"; echo
declare -a outloop
index=0
while read line
do
outloop[$index]="$line"
((index++))
# It does NOT run in a subshell, so ...
done < <( cat $0 )
echo "OUTPUT = "
echo ${outloop[*]} # ... the entire script echoes.
exit $?
This is a similar example.
Example 23-2. Redirecting the output of process substitution into a
loop.
#!/bin/bash
# psub.bash
# As inspired by Diego Molina (thanks!).
declare -a array0
while read
do
array0[${#array0[@]}]="$REPLY"
done < <( sed -e 's/bash/CRASH-BANG!/' $0 | grep bin | awk '{print $1}' )
# Sets the default 'read' variable, $REPLY, by process substitution,
#+ then copies it into an array.
echo "${array0[@]}"
exit $?
# ====================================== #
bash psub.bash
#!/bin/CRASH-BANG! done #!/bin/CRASH-BANG!
A reader sent in the following interesting example of process
substitution.
# Script fragment taken from SuSE distribution:
# --------------------------------------------------------------#
while read des what mask iface; do
# Some commands ...
done < <(route -n)
# ^ ^ First < is redirection, second is process substitution.
# To test it, let's make it do something.
while read des what mask iface; do
echo $des $what $mask $iface
done < <(route -n)
# Output:
# Kernel IP routing table
# Destination Gateway Genmask Flags Metric Ref Use Iface
# 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
# --------------------------------------------------------------#
# As Stéphane Chazelas points out,
#+ an easier-to-understand equivalent is:
route -n |
while read des what mask iface; do # Variables set from output of pipe.
echo $des $what $mask $iface
done # This yields the same output as above.
# However, as Ulrich Gayer points out . . .
#+ this simplified equivalent uses a subshell for the while loop,
#+ and therefore the variables disappear when the pipe terminates.
# --------------------------------------------------------------#
# However, Filip Moritz comments that there is a subtle difference
#+ between the above two examples, as the following shows.
(
route -n | while read x; do ((y++)); done
echo $y # $y is still unset
while read x; do ((y++)); done < <(route -n)
echo $y # $y has the number of lines of output of route -n
)
More generally spoken
(
: | x=x
# seems to start a subshell like
: | ( x=x )
# while
x=x < <(:)
# does not
)
# This is useful, when parsing csv and the like.
# That is, in effect, what the original SuSE code fragment does.
________________________________________________________________
Chapter 24. Functions
Like "real" programming languages, Bash has functions, though in a
somewhat limited implementation. A function is a subroutine, a code
block that implements a set of operations, a "black box" that
performs a specified task. Wherever there is repetitive code, when a
task repeats with only slight variations in procedure, then consider
using a function.
function function_name {
command...
}
or
function_name () {
command...
}
This second form will cheer the hearts of C programmers (and is more
portable).
As in C, the function's opening bracket may optionally appear on the
second line.
function_name ()
{
command...
}
Note
A function may be "compacted" into a single line.
fun () { echo "This is a function"; echo; }
# ^ ^
In this case, however, a semicolon must follow the final command in
the function.
fun () { echo "This is a function"; echo } # Error!
# ^
fun2 () { echo "Even a single-command function? Yes!"; }
# ^
Functions are called, triggered, simply by invoking their names. A
function call is equivalent to a command.
Example 24-1. Simple functions
#!/bin/bash
# ex59.sh: Exercising functions (simple).
JUST_A_SECOND=1
funky ()
{ # This is about as simple as functions get.
echo "This is a funky function."
echo "Now exiting funky function."
} # Function declaration must precede call.
fun ()
{ # A somewhat more complex function.
i=0
REPEATS=30
echo
echo "And now the fun really begins."
echo
sleep $JUST_A_SECOND # Hey, wait a second!
while [ $i -lt $REPEATS ]
do
echo "----------FUNCTIONS---------->"
echo "<------------ARE-------------"
echo "<------------FUN------------>"
echo
let "i+=1"
done
}
# Now, call the functions.
funky
fun
exit $?
The function definition must precede the first call to it. There is
no method of "declaring" the function, as, for example, in C.
f1
# Will give an error message, since function "f1" not yet defined.
declare -f f1 # This doesn't help either.
f1 # Still an error message.
# However...
f1 ()
{
echo "Calling function \"f2\" from within function \"f1\"."
f2
}
f2 ()
{
echo "Function \"f2\"."
}
f1 # Function "f2" is not actually called until this point,
#+ although it is referenced before its definition.
# This is permissible.
# Thanks, S.C.
Note
Functions may not be empty!
#!/bin/bash
# empty-function.sh
empty ()
{
}
exit 0 # Will not exit here!
# $ sh empty-function.sh
# empty-function.sh: line 6: syntax error near unexpected token `}'
# empty-function.sh: line 6: `}'
# $ echo $?
# 2
# Note that a function containing only comments is empty.
func ()
{
# Comment 1.
# Comment 2.
# This is still an empty function.
# Thank you, Mark Bova, for pointing this out.
}
# Results in same error message as above.
# However ...
not_quite_empty ()
{
illegal_command
} # A script containing this function will *not* bomb
#+ as long as the function is not called.
not_empty ()
{
:
} # Contains a : (null command), and this is okay.
# Thank you, Dominick Geyer and Thiemo Kellner.
It is even possible to nest a function within another function,
although this is not very useful.
f1 ()
{
f2 () # nested
{
echo "Function \"f2\", inside \"f1\"."
}
}
f2 # Gives an error message.
# Even a preceding "declare -f f2" wouldn't help.
echo
f1 # Does nothing, since calling "f1" does not automatically call "f2".
f2 # Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C.
Function declarations can appear in unlikely places, even where a
command would otherwise go.
ls -l | foo() { echo "foo"; } # Permissible, but useless.
if [ "$USER" = bozo ]
then
bozo_greet () # Function definition embedded in an if/then construct.
{
echo "Hello, Bozo."
}
fi
bozo_greet # Works only for Bozo, and other users get an error.
# Something like this might be useful in some contexts.
NO_EXIT=1 # Will enable function definition below.
[[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-
list".
# If $NO_EXIT is 1, declares "exit ()".
# This disables the "exit" builtin by aliasing it to "true".
exit # Invokes "exit ()" function, not "exit" builtin.
# Or, similarly:
filename=file1
[ -f "$filename" ] &&
foo () { rm -f "$filename"; echo "File "$filename" deleted."; } ||
foo () { echo "File "$filename" not found."; touch bar; }
foo
# Thanks, S.C. and Christopher Head
Function names can take strange forms.
_(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; }
# ^^^ No space between function name and parentheses.
# This doesn't always work. Why not?
# Now, let's invoke the function.
_ # __________
# ^^^^^^^^^^ 10 underscores (10 x function name)!
# A "naked" underscore is an acceptable function name.
# In fact, a colon is likewise an acceptable function name.
:(){ echo ":"; }; :
# Of what use is this?
# It's a devious way to obfuscate the code in a script.
See also Example A-56
Note
What happens when different versions of the same function appear in a
script?
# As Yan Chen points out,
# when a function is defined multiple times,
# the final version is what is invoked.
# This is not, however, particularly useful.
func ()
{
echo "First version of func ()."
}
func ()
{
echo "Second version of func ()."
}
func # Second version of func ().
exit $?
# It is even possible to use functions to override
#+ or preempt system commands.
# Of course, this is *not* advisable.
________________________________________________________________
24.1. Complex Functions and Function Complexities
Functions may process arguments passed to them and return an exit
status to the script for further processing.
function_name $arg1 $arg2
The function refers to the passed arguments by position (as if they
were positional parameters), that is, $1, $2, and so forth.
Example 24-2. Function Taking Parameters
#!/bin/bash
# Functions and parameters
DEFAULT=default # Default param value.
func2 () {
if [ -z "$1" ] # Is parameter #1 zero length?
then
echo "-Parameter #1 is zero length.-" # Or no parameter passed.
else
echo "-Parameter #1 is \"$1\".-"
fi
variable=${1-$DEFAULT} # What does
echo "variable = $variable" #+ parameter substitution show?
# ---------------------------
# It distinguishes between
#+ no param and a null param.
if [ "$2" ]
then
echo "-Parameter #2 is \"$2\".-"
fi
return 0
}
echo
echo "Nothing passed."
func2 # Called with no params
echo
echo "Zero-length parameter passed."
func2 "" # Called with zero-length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param" # Called with uninitialized param
echo
echo "One parameter passed."
func2 first # Called with one param
echo
echo "Two parameters passed."
func2 first second # Called with two params
echo
echo "\"\" \"second\" passed."
func2 "" second # Called with zero-length first parameter
echo # and ASCII string as a second one.
exit 0
Important
The shift command works on arguments passed to functions (see Example
36-18).
But, what about command-line arguments passed to the script? Does a
function see them? Well, let's clear up the confusion.
Example 24-3. Functions and command-line args passed to the script
#!/bin/bash
# func-cmdlinearg.sh
# Call this script with a command-line argument,
#+ something like $0 arg1.
func ()
{
echo "$1" # Echoes first arg passed to the function.
} # Does a command-line arg qualify?
echo "First call to function: no arg passed."
echo "See if command-line arg is seen."
func
# No! Command-line arg not seen.
echo "============================================================"
echo
echo "Second call to function: command-line arg passed explicitly."
func $1
# Now it's seen!
exit 0
In contrast to certain other programming languages, shell scripts
normally pass only value parameters to functions. Variable names
(which are actually pointers), if passed as parameters to functions,
will be treated as string literals. Functions interpret their
arguments literally.
Indirect variable references (see Example 37-2) provide a clumsy sort
of mechanism for passing variable pointers to functions.
Example 24-4. Passing an indirect reference to a function
#!/bin/bash
# ind-func.sh: Passing an indirect reference to a function.
echo_var ()
{
echo "$1"
}
message=Hello
Hello=Goodbye
echo_var "$message" # Hello
# Now, let's pass an indirect reference to the function.
echo_var "${!message}" # Goodbye
echo "-------------"
# What happens if we change the contents of "hello" variable?
Hello="Hello, again!"
echo_var "$message" # Hello
echo_var "${!message}" # Hello, again!
exit 0
The next logical question is whether parameters can be dereferenced
after being passed to a function.
Example 24-5. Dereferencing a parameter passed to a function
#!/bin/bash
# dereference.sh
# Dereferencing parameter passed to a function.
# Script by Bruce W. Clare.
dereference ()
{
y=\$"$1" # Name of variable (not value!).
echo $y # $Junk
x=`eval "expr \"$y\" "`
echo $1=$x
eval "$1=\"Some Different Text \"" # Assign new value.
}
Junk="Some Text"
echo $Junk "before" # Some Text before
dereference Junk
echo $Junk "after" # Some Different Text after
exit 0
Example 24-6. Again, dereferencing a parameter passed to a function
#!/bin/bash
# ref-params.sh: Dereferencing a parameter passed to a function.
# (Complex Example)
ITERATIONS=3 # How many times to get input.
icount=1
my_read () {
# Called with my_read varname,
#+ outputs the previous value between brackets as the default value,
#+ then asks for a new value.
local local_var
echo -n "Enter a value "
eval 'echo -n "[$'$1'] "' # Previous value.
# eval echo -n "[\$$1] " # Easier to understand,
#+ but loses trailing space in user prompt.
read local_var
[ -n "$local_var" ] && eval $1=\$local_var
# "And-list": if "local_var" then set "$1" to its value.
}
echo
while [ "$icount" -le "$ITERATIONS" ]
do
my_read var
echo "Entry #$icount = $var"
let "icount += 1"
echo
done
# Thanks to Stephane Chazelas for providing this instructive example.
exit 0
Exit and Return
exit status
Functions return a value, called an exit status. This is
analogous to the exit status returned by a command. The exit
status may be explicitly specified by a return statement,
otherwise it is the exit status of the last command in the
function (0 if successful, and a non-zero error code if not).
This exit status may be used in the script by referencing it
as $?. This mechanism effectively permits script functions to
have a "return value" similar to C functions.
return
Terminates a function. A return command [109] optionally takes
an integer argument, which is returned to the calling script
as the "exit status" of the function, and this exit status is
assigned to the variable $?.
Example 24-7. Maximum of two numbers
#!/bin/bash
# max.sh: Maximum of two integers.
E_PARAM_ERR=250 # If less than 2 params passed to function.
EQUAL=251 # Return value if both params equal.
# Error values out of range of any
#+ params that might be fed to the function.
max2 () # Returns larger of two numbers.
{ # Note: numbers compared must be less than 250.
if [ -z "$2" ]
then
return $E_PARAM_ERR
fi
if [ "$1" -eq "$2" ]
then
return $EQUAL
else
if [ "$1" -gt "$2" ]
then
return $1
else
return $2
fi
fi
}
max2 33 34
return_val=$?
if [ "$return_val" -eq $E_PARAM_ERR ]
then
echo "Need to pass two parameters to the function."
elif [ "$return_val" -eq $EQUAL ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi
exit 0
# Exercise (easy):
# ---------------
# Convert this to an interactive script,
#+ that is, have the script ask for input (two numbers).
Tip
For a function to return a string or array, use a dedicated variable.
count_lines_in_etc_passwd()
{
[[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))
# If /etc/passwd is readable, set REPLY to line count.
# Returns both a parameter value and status information.
# The 'echo' seems unnecessary, but . . .
#+ it removes excess whitespace from the output.
}
if count_lines_in_etc_passwd
then
echo "There are $REPLY lines in /etc/passwd."
else
echo "Cannot count lines in /etc/passwd."
fi
# Thanks, S.C.
Example 24-8. Converting numbers to Roman numerals
#!/bin/bash
# Arabic number to Roman numeral conversion
# Range: 0 - 200
# It's crude, but it works.
# Extending the range and otherwise improving the script is left as an exercis
e.
# Usage: roman number-to-convert
LIMIT=200
E_ARG_ERR=65
E_OUT_OF_RANGE=66
if [ -z "$1" ]
then
echo "Usage: `basename $0` number-to-convert"
exit $E_ARG_ERR
fi
num=$1
if [ "$num" -gt $LIMIT ]
then
echo "Out of range!"
exit $E_OUT_OF_RANGE
fi
to_roman () # Must declare function before first call to it.
{
number=$1
factor=$2
rchar=$3
let "remainder = number - factor"
while [ "$remainder" -ge 0 ]
do
echo -n $rchar
let "number -= factor"
let "remainder = number - factor"
done
return $number
# Exercises:
# ---------
# 1) Explain how this function works.
# Hint: division by successive subtraction.
# 2) Extend to range of the function.
# Hint: use "echo" and command-substitution capture.
}
to_roman $num 100 C
num=$?
to_roman $num 90 LXXXX
num=$?
to_roman $num 50 L
num=$?
to_roman $num 40 XL
num=$?
to_roman $num 10 X
num=$?
to_roman $num 9 IX
num=$?
to_roman $num 5 V
num=$?
to_roman $num 4 IV
num=$?
to_roman $num 1 I
# Successive calls to conversion function!
# Is this really necessary??? Can it be simplified?
echo
exit
See also Example 11-29.
Important
The largest positive integer a function can return is 255. The return
command is closely tied to the concept of exit status, which accounts
for this particular limitation. Fortunately, there are various
workarounds for those situations requiring a large integer return
value from a function.
Example 24-9. Testing large return values in a function
#!/bin/bash
# return-test.sh
# The largest positive value a function can return is 255.
return_test () # Returns whatever passed to it.
{
return $1
}
return_test 27 # o.k.
echo $? # Returns 27.
return_test 255 # Still o.k.
echo $? # Returns 255.
return_test 257 # Error!
echo $? # Returns 1 (return code for miscellaneous error).
# =========================================================
return_test -151896 # Do large negative numbers work?
echo $? # Will this return -151896?
# No! It returns 168.
# Version of Bash before 2.05b permitted
#+ large negative integer return values.
# It happened to be a useful feature.
# Newer versions of Bash unfortunately plug this loophole.
# This may break older scripts.
# Caution!
# =========================================================
exit 0
A workaround for obtaining large integer "return values" is to simply
assign the "return value" to a global variable.
Return_Val= # Global variable to hold oversize return value of function.
alt_return_test ()
{
fvar=$1
Return_Val=$fvar
return # Returns 0 (success).
}
alt_return_test 1
echo $? # 0
echo "return value = $Return_Val" # 1
alt_return_test 256
echo "return value = $Return_Val" # 256
alt_return_test 257
echo "return value = $Return_Val" # 257
alt_return_test 25701
echo "return value = $Return_Val" #25701
A more elegant method is to have the function echo its "return value
to stdout," and then capture it by command substitution. See the
discussion of this in Section 36.7.
Example 24-10. Comparing two large integers
#!/bin/bash
# max2.sh: Maximum of two LARGE integers.
# This is the previous "max.sh" example,
#+ modified to permit comparing large integers.
EQUAL=0 # Return value if both params equal.
E_PARAM_ERR=-99999 # Not enough params passed to function.
# ^^^^^^ Out of range of any params that might be passed.
max2 () # "Returns" larger of two numbers.
{
if [ -z "$2" ]
then
echo $E_PARAM_ERR
return
fi
if [ "$1" -eq "$2" ]
then
echo $EQUAL
return
else
if [ "$1" -gt "$2" ]
then
retval=$1
else
retval=$2
fi
fi
echo $retval # Echoes (to stdout), rather than returning value.
# Why?
}
return_val=$(max2 33001 33997)
# ^^^^ Function name
# ^^^^^ ^^^^^ Params passed
# This is actually a form of command substitution:
#+ treating a function as if it were a command,
#+ and assigning the stdout of the function to the variable "return_val."
# ========================= OUTPUT ========================
if [ "$return_val" -eq "$E_PARAM_ERR" ]
then
echo "Error in parameters passed to comparison function!"
elif [ "$return_val" -eq "$EQUAL" ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi
# =========================================================
exit 0
# Exercises:
# ---------
# 1) Find a more elegant way of testing
#+ the parameters passed to the function.
# 2) Simplify the if/then structure at "OUTPUT."
# 3) Rewrite the script to take input from command-line parameters.
Here is another example of capturing a function "return value."
Understanding it requires some knowledge of awk.
month_length () # Takes month number as an argument.
{ # Returns number of days in month.
monthD="31 28 31 30 31 30 31 31 30 31 30 31" # Declare as local?
echo "$monthD" | awk '{ print $'"${1}"' }' # Tricky.
# ^^^^^^^^^
# Parameter passed to function ($1 -- month number), then to awk.
# Awk sees this as "print $1 . . . print $12" (depending on month number)
# Template for passing a parameter to embedded awk script:
# $'"${script_parameter}"'
# Here's a slightly simpler awk construct:
# echo $monthD | awk -v month=$1 '{print $(month)}'
# Uses the -v awk option, which assigns a variable value
#+ prior to execution of the awk program block.
# Thank you, Rich.
# Needs error checking for correct parameter range (1-12)
#+ and for February in leap year.
}
# ----------------------------------------------
# Usage example:
month=4 # April, for example (4th month).
days_in=$(month_length $month)
echo $days_in # 30
# ----------------------------------------------
See also Example A-7 and Example A-37.
Exercise: Using what we have just learned, extend the previous
Roman numerals example to accept arbitrarily large input.
Redirection
Redirecting the stdin of a function
A function is essentially a code block, which means its stdin
can be redirected (as in Example 3-1).
Example 24-11. Real name from username
#!/bin/bash
# realname.sh
#
# From username, gets "real name" from /etc/passwd.
ARGCOUNT=1 # Expect one arg.
E_WRONGARGS=85
file=/etc/passwd
pattern=$1
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` USERNAME"
exit $E_WRONGARGS
fi
file_excerpt () # Scan file for pattern,
{ #+ then print relevant portion of line.
while read line # "while" does not necessarily need [ condition ]
do
echo "$line" | grep $1 | awk -F":" '{ print $5 }'
# Have awk use ":" delimiter.
done
} <$file # Redirect into function's stdin.
file_excerpt $pattern
# Yes, this entire script could be reduced to
# grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'
# or
# awk -F: '/PATTERN/ {print $5}'
# or
# awk -F: '($1 == "username") { print $5 }' # real name from username
# However, it might not be as instructive.
exit 0
There is an alternate, and perhaps less confusing method of
redirecting a function's stdin. This involves redirecting the
stdin to an embedded bracketed code block within the function.
# Instead of:
Function ()
{
...
} < file
# Try this:
Function ()
{
{
...
} < file
}
# Similarly,
Function () # This works.
{
{
echo $*
} | tr a b
}
Function () # This doesn't work.
{
echo $*
} | tr a b # A nested code block is mandatory here.
# Thanks, S.C.
Note
Emmanuel Rouat's sample bashrc file contains some instructive
examples of functions.
________________________________________________________________
24.2. Local Variables
What makes a variable local?
local variables
A variable declared as local is one that is visible only
within the block of code in which it appears. It has local
scope. In a function, a local variable has meaning only within
that function block. [110]
Example 24-12. Local variable visibility
#!/bin/bash
# ex62.sh: Global and local variables inside a function.
func ()
{
local loc_var=23 # Declared as local variable.
echo # Uses the 'local' builtin.
echo "\"loc_var\" in function = $loc_var"
global_var=999 # Not declared as local.
# Therefore, defaults to global.
echo "\"global_var\" in function = $global_var"
}
func
# Now, to see if local variable "loc_var" exists outside the function.
echo
echo "\"loc_var\" outside function = $loc_var"
# $loc_var outside function =
# No, $loc_var not visible globally.
echo "\"global_var\" outside function = $global_var"
# $global_var outside function = 999
# $global_var is visible globally.
echo
exit 0
# In contrast to C, a Bash variable declared inside a function
#+ is local ONLY if declared as such.
Caution
Before a function is called, all variables declared within the
function are invisible outside the body of the function, not just
those explicitly declared as local.
#!/bin/bash
func ()
{
global_var=37 # Visible only within the function block
#+ before the function has been called.
} # END OF FUNCTION
echo "global_var = $global_var" # global_var =
# Function "func" has not yet been called,
#+ so $global_var is not visible here.
func
echo "global_var = $global_var" # global_var = 37
# Has been set by function call.
Note
As Evgeniy Ivanov points out, when declaring and setting a local
variable in a single command, apparently the order of operations is
to first set the variable, and only afterwards restrict it to local
scope. This is reflected in the return value.
#!/bin/bash
echo "==OUTSIDE Function (global)=="
t=$(exit 1)
echo $? # 1
# As expected.
echo
function0 ()
{
echo "==INSIDE Function=="
echo "Global"
t0=$(exit 1)
echo $? # 1
# As expected.
echo
echo "Local declared & assigned in same command."
local t1=$(exit 1)
echo $? # 0
# Unexpected!
# Apparently, the variable assignment takes place before
#+ the local declaration.
#+ The return value is for the latter.
echo
echo "Local declared, then assigned (separate commands)."
local t2
t2=$(exit 1)
echo $? # 1
# As expected.
}
function0
________________________________________________________________
24.2.1. Local variables and recursion.
Recursion is an interesting and sometimes useful form of
self-reference. Herbert Mayer defines it as ". . . expressing an
algorithm by using a simpler version of that same algorithm . . ."
Consider a definition defined in terms of itself, [111] an expression
implicit in its own expression, [112] a snake swallowing its own
tail, [113] or . . . a function that calls itself. [114]
Example 24-13. Demonstration of a simple recursive function
#!/bin/bash
# recursion-demo.sh
# Demonstration of recursion.
RECURSIONS=9 # How many times to recurse.
r_count=0 # Must be global. Why?
recurse ()
{
var="$1"
while [ "$var" -ge 0 ]
do
echo "Recursion count = "$r_count" +-+ \$var = "$var""
(( var-- )); (( r_count++ ))
recurse "$var" # Function calls itself (recurses)
done #+ until what condition is met?
}
recurse $RECURSIONS
exit $?
Example 24-14. Another simple demonstration
#!/bin/bash
# recursion-def.sh
# A script that defines "recursion" in a rather graphic way.
RECURSIONS=10
r_count=0
sp=" "
define_recursion ()
{
((r_count++))
sp="$sp"" "
echo -n "$sp"
echo "\"The act of recurring ... \"" # Per 1913 Webster's dictionary.
while [ $r_count -le $RECURSIONS ]
do
define_recursion
done
}
echo
echo "Recursion: "
define_recursion
echo
exit $?
Local variables are a useful tool for writing recursive code, but
this practice generally involves a great deal of computational
overhead and is definitely not recommended in a shell script. [115]
Example 24-15. Recursion, using a local variable
#!/bin/bash
# factorial
# ---------
# Does bash permit recursion?
# Well, yes, but...
# It's so slow that you gotta have rocks in your head to try it.
MAX_ARG=5
E_WRONG_ARGS=85
E_RANGE_ERR=86
if [ -z "$1" ]
then
echo "Usage: `basename $0` number"
exit $E_WRONG_ARGS
fi
if [ "$1" -gt $MAX_ARG ]
then
echo "Out of range ($MAX_ARG is maximum)."
# Let's get real now.
# If you want greater range than this,
#+ rewrite it in a Real Programming Language.
exit $E_RANGE_ERR
fi
fact ()
{
local number=$1
# Variable "number" must be declared as local,
#+ otherwise this doesn't work.
if [ "$number" -eq 0 ]
then
factorial=1 # Factorial of 0 = 1.
else
let "decrnum = number - 1"
fact $decrnum # Recursive function call (the function calls itself).
let "factorial = $number * $?"
fi
return $factorial
}
fact $1
echo "Factorial of $1 is $?."
exit 0
Also see Example A-15 for an example of recursion in a script. Be
aware that recursion is resource-intensive and executes slowly, and
is therefore generally not appropriate in a script.
________________________________________________________________
24.3. Recursion Without Local Variables
A function may recursively call itself even without use of local
variables.
Example 24-16. The Fibonacci Sequence
#!/bin/bash
# fibo.sh : Fibonacci sequence (recursive)
# Author: M. Cooper
# License: GPL3
# ----------algorithm--------------
# Fibo(0) = 0
# Fibo(1) = 1
# else
# Fibo(j) = Fibo(j-1) + Fibo(j-2)
# ---------------------------------
MAXTERM=15 # Number of terms (+1) to generate.
MINIDX=2 # If idx is less than 2, then Fibo(idx) = idx.
Fibonacci ()
{
idx=$1 # Doesn't need to be local. Why not?
if [ "$idx" -lt "$MINIDX" ]
then
echo "$idx" # First two terms are 0 1 ... see above.
else
(( --idx )) # j-1
term1=$( Fibonacci $idx ) # Fibo(j-1)
(( --idx )) # j-2
term2=$( Fibonacci $idx ) # Fibo(j-2)
echo $(( term1 + term2 ))
fi
# An ugly, ugly kludge.
# The more elegant implementation of recursive fibo in C
#+ is a straightforward translation of the algorithm in lines 7 - 10.
}
for i in $(seq 0 $MAXTERM)
do # Calculate $MAXTERM+1 terms.
FIBO=$(Fibonacci $i)
echo -n "$FIBO "
done
# 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610
# Takes a while, doesn't it? Recursion in a script is slow.
echo
exit 0
Example 24-17. The Towers of Hanoi
#! /bin/bash
#
# The Towers Of Hanoi
# Bash script
# Copyright (C) 2000 Amit Singh. All Rights Reserved.
# http://hanoi.kernelthread.com
#
# Tested under Bash version 2.05b.0(13)-release.
# Also works under Bash version 3.x.
#
# Used in "Advanced Bash Scripting Guide"
#+ with permission of script author.
# Slightly modified and commented by ABS author.
#=================================================================#
# The Tower of Hanoi is a mathematical puzzle attributed to
#+ Edouard Lucas, a nineteenth-century French mathematician.
#
# There are three vertical posts set in a base.
# The first post has a set of annular rings stacked on it.
# These rings are disks with a hole drilled out of the center,
#+ so they can slip over the posts and rest flat.
# The rings have different diameters, and they stack in ascending
#+ order, according to size.
# The smallest ring is on top, and the largest on the bottom.
#
# The task is to transfer the stack of rings
#+ to one of the other posts.
# You can move only one ring at a time to another post.
# You are permitted to move rings back to the original post.
# You may place a smaller ring atop a larger one,
#+ but *not* vice versa.
# Again, it is forbidden to place a larger ring atop a smaller one.
#
# For a small number of rings, only a few moves are required.
#+ For each additional ring,
#+ the required number of moves approximately doubles,
#+ and the "strategy" becomes increasingly complicated.
#
# For more information, see http://hanoi.kernelthread.com
#+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney.
#
#
# ... ... ...
# | | | | | |
# _|_|_ | | | |
# |_____| | | | |
# |_______| | | | |
# |_________| | | | |
# |___________| | | | |
# | | | | | |
# .--------------------------------------------------------------.
# |**************************************************************|
# #1 #2 #3
#
#=================================================================#
E_NOPARAM=66 # No parameter passed to script.
E_BADPARAM=67 # Illegal number of disks passed to script.
Moves= # Global variable holding number of moves.
# Modification to original script.
dohanoi() { # Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
echo move $2 "-->" $3
((Moves++)) # Modification to original script.
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
case $# in
1) case $(($1>0)) in # Must have at least one disk.
1) # Nested case statement.
dohanoi $1 1 3 2
echo "Total moves = $Moves" # 2^n - 1, where n = # of disks.
exit 0;
;;
*)
echo "$0: illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo " Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
# Exercises:
# ---------
# 1) Would commands beyond this point ever be executed?
# Why not? (Easy)
# 2) Explain the workings of the workings of the "dohanoi" function.
# (Difficult -- see the Dewdney reference, above.)
________________________________________________________________
Chapter 25. Aliases
A Bash alias is essentially nothing more than a keyboard shortcut, an
abbreviation, a means of avoiding typing a long command sequence. If,
for example, we include alias lm="ls -l | more" in the ~/.bashrc
file, then each lm [116] typed at the command-line will automatically
be replaced by a ls -l | more. This can save a great deal of typing
at the command-line and avoid having to remember complex combinations
of commands and options. Setting alias rm="rm -i" (interactive mode
delete) may save a good deal of grief, since it can prevent
inadvertently deleting important files.
In a script, aliases have very limited usefulness. It would be nice
if aliases could assume some of the functionality of the C
preprocessor, such as macro expansion, but unfortunately Bash does
not expand arguments within the alias body. [117] Moreover, a script
fails to expand an alias itself within "compound constructs," such as
if/then statements, loops, and functions. An added limitation is that
an alias will not expand recursively. Almost invariably, whatever we
would like an alias to do could be accomplished much more effectively
with a function.
Example 25-1. Aliases within a script
#!/bin/bash
# alias.sh
shopt -s expand_aliases
# Must set this option, else script will not expand aliases.
# First, some fun.
alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob
Hope."'
Jesse_James
echo; echo; echo;
alias ll="ls -l"
# May use either single (') or double (") quotes to define an alias.
echo "Trying aliased \"ll\":"
ll /usr/X11R6/bin/mk* #* Alias works.
echo
directory=/usr/X11R6/bin/
prefix=mk* # See if wild card causes problems.
echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
echo
alias lll="ls -l $directory$prefix"
echo "Trying aliased \"lll\":"
lll # Long listing of all files in /usr/X11R6/bin stating with mk.
# An alias can handle concatenated variables -- including wild card -- o.k.
TRUE=1
echo
if [ TRUE ]
then
alias rr="ls -l"
echo "Trying aliased \"rr\" within if/then statement:"
rr /usr/X11R6/bin/mk* #* Error message results!
# Aliases not expanded within compound statements.
echo "However, previously expanded alias still recognized:"
ll /usr/X11R6/bin/mk*
fi
echo
count=0
while [ $count -lt 3 ]
do
alias rrr="ls -l"
echo "Trying aliased \"rrr\" within \"while\" loop:"
rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.
# alias.sh: line 57: rrr: command not found
let count+=1
done
echo; echo
alias xyz='cat $0' # Script lists itself.
# Note strong quotes.
xyz
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
#
# However, as Steve Jacobson points out,
#+ the "$0" parameter expands immediately upon declaration of the alias.
exit 0
The unalias command removes a previously set alias.
Example 25-2. unalias: Setting and unsetting an alias
#!/bin/bash
# unalias.sh
shopt -s expand_aliases # Enables alias expansion.
alias llm='ls -al | more'
llm
echo
unalias llm # Unset alias.
llm
# Error message results, since 'llm' no longer recognized.
exit 0
bash$ ./unalias.sh
total 6
drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 .
drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 ..
-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh
./unalias.sh: llm: command not found
________________________________________________________________
Chapter 26. List Constructs
The and list and or list constructs provide a means of processing a
number of commands consecutively. These can effectively replace
complex nested if/then or even case statements.
Chaining together commands
and list
command-1 && command-2 && command-3 && ... command-n
Each command executes in turn, provided that the previous
command has given a return value of true (zero). At the first
false (non-zero) return, the command chain terminates (the
first command returning false is the last one to execute).
An interesting use of a two-condition and list from an early
version of YongYe's Tetris game script:
equation()
{ # core algorithm used for doubling and halving the coordinates
[[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2))
eval ${1}+=\"${x} ${y} \"
}
Example 26-1. Using an and list to test for command-line
arguments
#!/bin/bash
# and list
if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \
# ^^ ^^ ^^
echo "Argument #2 = $2"
then
echo "At least 2 arguments passed to script."
# All the chained commands return true.
else
echo "Fewer than 2 arguments passed to script."
# At least one of the chained commands returns false.
fi
# Note that "if [ ! -z $1 ]" works, but its alleged equivalent,
# "if [ -n $1 ]" does not.
# However, quoting fixes this.
# if "[ -n "$1" ]" works.
# ^ ^ Careful!
# It is always best to QUOTE the variables being tested.
# This accomplishes the same thing, using "pure" if/then statements.
if [ ! -z "$1" ]
then
echo "Argument #1 = $1"
fi
if [ ! -z "$2" ]
then
echo "Argument #2 = $2"
echo "At least 2 arguments passed to script."
else
echo "Fewer than 2 arguments passed to script."
fi
# It's longer and more ponderous than using an "and list".
exit $?
Example 26-2. Another command-line arg test using an and list
#!/bin/bash
ARGS=1 # Number of arguments expected.
E_BADARGS=85 # Exit value if incorrect number of args passed.
test $# -ne $ARGS && \
# ^^^^^^^^^^^^ condition #1
echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS
# ^^
# If condition #1 tests true (wrong number of args passed to script),
#+ then the rest of the line executes, and script terminates.
# Line below executes only if the above test fails.
echo "Correct number of arguments passed to this script."
exit 0
# To check exit value, do a "echo $?" after script termination.
Of course, an and list can also set variables to a default
value.
arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT
# Set $arg1 to command-line arguments, if any.
# But . . . set to DEFAULT if not specified on command-line.
or list
command-1 || command-2 || command-3 || ... command-n
Each command executes in turn for as long as the previous
command returns false. At the first true return, the command
chain terminates (the first command returning true is the last
one to execute). This is obviously the inverse of the "and
list".
Example 26-3. Using or lists in combination with an and list
#!/bin/bash
# delete.sh, a not-so-cunning file deletion utility.
# Usage: delete filename
E_BADARGS=85
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS # No arg? Bail out.
else
file=$1 # Set filename.
fi
[ ! -f "$file" ] && echo "File \"$file\" not found. \
Cowardly refusing to delete a nonexistent file."
# AND LIST, to give error message if file not present.
# Note echo message continuing on to a second line after an escape.
[ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.")
# OR LIST, to delete file if present.
# Note logic inversion above.
# AND LIST executes on true, OR LIST on false.
exit $?
Caution
If the first command in an or list returns true, it will execute.
# ==> The following snippets from the /etc/rc.d/init.d/single
#+==> script by Miquel van Smoorenburg
#+==> illustrate use of "and" and "or" lists.
# ==> "Arrowed" comments added by document author.
[ -x /usr/bin/clear ] && /usr/bin/clear
# ==> If /usr/bin/clear exists, then invoke it.
# ==> Checking for the existence of a command before calling it
#+==> avoids error messages and other awkward consequences.
# ==> . . .
# If they want to run something in single user mode, might as well run it...
for i in /etc/rc1.d/S[0-9][0-9]* ; do
# Check if the script is there.
[ -x "$i" ] || continue
# ==> If corresponding file in $PWD *not* found,
#+==> then "continue" by jumping to the top of the loop.
# Reject backup files and files generated by rpm.
case "$1" in
*.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)
continue;;
esac
[ "$i" = "/etc/rc1.d/S00single" ] && continue
# ==> Set script name, but don't execute it yet.
$i start
done
# ==> . . .
Important
The exit status of an and list or an or list is the exit status of
the last command executed.
Clever combinations of and and or lists are possible, but the logic
may easily become convoluted and require close attention to operator
precedence rules, and possibly extensive debugging.
false && true || echo false # false
# Same result as
( false && true ) || echo false # false
# But NOT
false && ( true || echo false ) # (nothing echoed)
# Note left-to-right grouping and evaluation of statements.
# It's usually best to avoid such complexities.
# Thanks, S.C.
See Example A-7 and Example 7-4 for illustrations of using and / or
list constructs to test variables.
________________________________________________________________
Chapter 27. Arrays
Newer versions of Bash support one-dimensional arrays. Array elements
may be initialized with the variable[xx] notation. Alternatively, a
script may introduce the entire array by an explicit declare -a
variable statement. To dereference (retrieve the contents of) an
array element, use curly bracket notation, that is, ${element[xx]}.
Example 27-1. Simple array usage
#!/bin/bash
area[11]=23
area[13]=37
area[51]=UFOs
# Array members need not be consecutive or contiguous.
# Some members of the array can be left uninitialized.
# Gaps in the array are okay.
# In fact, arrays with sparse data ("sparse arrays")
#+ are useful in spreadsheet-processing software.
echo -n "area[11] = "
echo ${area[11]} # {curly brackets} needed.
echo -n "area[13] = "
echo ${area[13]}
echo "Contents of area[51] are ${area[51]}."
# Contents of uninitialized array variable print blank (null variable).
echo -n "area[43] = "
echo ${area[43]}
echo "(area[43] unassigned)"
echo
# Sum of two array variables assigned to third
area[5]=`expr ${area[11]} + ${area[13]}`
echo "area[5] = area[11] + area[13]"
echo -n "area[5] = "
echo ${area[5]}
area[6]=`expr ${area[11]} + ${area[51]}`
echo "area[6] = area[11] + area[51]"
echo -n "area[6] = "
echo ${area[6]}
# This fails because adding an integer to a string is not permitted.
echo; echo; echo
# -----------------------------------------------------------------
# Another array, "area2".
# Another way of assigning array variables...
# array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
echo -n "area2[0] = "
echo ${area2[0]}
# Aha, zero-based indexing (first element of array is [0], not [1]).
echo -n "area2[1] = "
echo ${area2[1]} # [1] is second element of array.
# -----------------------------------------------------------------
echo; echo; echo
# -----------------------------------------------
# Yet another array, "area3".
# Yet another way of assigning array variables...
# array_name=([xx]=XXX [yy]=YYY ...)
area3=([17]=seventeen [24]=twenty-four)
echo -n "area3[17] = "
echo ${area3[17]}
echo -n "area3[24] = "
echo ${area3[24]}
# -----------------------------------------------
exit 0
As we have seen, a convenient way of initializing an entire array is
the array=( element1 element2 ... elementN ) notation.
base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Using extended brace expansion
#+ to initialize the elements of the array.
# Excerpted from vladz's "base64.sh" script
#+ in the "Contributed Scripts" appendix.
Bash permits array operations on variables, even if the variables are
not explicitly declared as arrays.
string=abcABC123ABCabc
echo ${string[@]} # abcABC123ABCabc
echo ${string[*]} # abcABC123ABCabc
echo ${string[0]} # abcABC123ABCabc
echo ${string[1]} # No output!
# Why?
echo ${#string[@]} # 1
# One element in the array.
# The string itself.
# Thank you, Michael Zick, for pointing this out.
Once again this demonstrates that Bash variables are untyped.
Example 27-2. Formatting a poem
#!/bin/bash
# poem.sh: Pretty-prints one of the ABS Guide author's favorite poems.
# Lines of the poem (single stanza).
Line[1]="I do not know which to prefer,"
Line[2]="The beauty of inflections"
Line[3]="Or the beauty of innuendoes,"
Line[4]="The blackbird whistling"
Line[5]="Or just after."
# Note that quoting permits embedding whitespace.
# Attribution.
Attrib[1]=" Wallace Stevens"
Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""
# This poem is in the Public Domain (copyright expired).
echo
tput bold # Bold print.
for index in 1 2 3 4 5 # Five lines.
do
printf " %s\n" "${Line[index]}"
done
for index in 1 2 # Two attribution lines.
do
printf " %s\n" "${Attrib[index]}"
done
tput sgr0 # Reset terminal.
# See 'tput' docs.
echo
exit 0
# Exercise:
# --------
# Modify this script to pretty-print a poem from a text data file.
Array variables have a syntax all their own, and even standard Bash
commands and operators have special options adapted for array use.
Example 27-3. Various array operations
#!/bin/bash
# array-ops.sh: More fun with arrays.
array=( zero one two three four five )
# Element 0 1 2 3 4 5
echo ${array[0]} # zero
echo ${array:0} # zero
# Parameter expansion of first element,
#+ starting at position # 0 (1st character).
echo ${array:1} # ero
# Parameter expansion of first element,
#+ starting at position # 1 (2nd character).
echo "--------------"
echo ${#array[0]} # 4
# Length of first element of array.
echo ${#array} # 4
# Length of first element of array.
# (Alternate notation)
echo ${#array[1]} # 3
# Length of second element of array.
# Arrays in Bash have zero-based indexing.
echo ${#array[*]} # 6
# Number of elements in array.
echo ${#array[@]} # 6
# Number of elements in array.
echo "--------------"
array2=( [0]="first element" [1]="second element" [3]="fourth element" )
# ^ ^ ^ ^ ^ ^ ^ ^ ^
# Quoting permits embedding whitespace within individual array elements.
echo ${array2[0]} # first element
echo ${array2[1]} # second element
echo ${array2[2]} #
# Skipped in initialization, and therefore null.
echo ${array2[3]} # fourth element
echo ${#array2[0]} # 13 (length of first element)
echo ${#array2[*]} # 3 (number of elements in array)
exit
Many of the standard string operations work on arrays.
Example 27-4. String operations on arrays
#!/bin/bash
# array-strops.sh: String operations on arrays.
# Script by Michael Zick.
# Used in ABS Guide with permission.
# Fixups: 05 May 08, 04 Aug 08.
# In general, any string operation using the ${name ... } notation
#+ can be applied to all string elements in an array,
#+ with the ${name[@] ... } or ${name[*] ...} notation.
arrayZ=( one two three four five five )
echo
# Trailing Substring Extraction
echo ${arrayZ[@]:0} # one two three four five five
# ^ All elements.
echo ${arrayZ[@]:1} # two three four five five
# ^ All elements following element[0].
echo ${arrayZ[@]:1:2} # two three
# ^ Only the two elements after element[0].
echo "---------"
# Substring Removal
# Removes shortest match from front of string(s).
echo ${arrayZ[@]#f*r} # one two three five five
# ^ # Applied to all elements of the array.
# Matches "four" and removes it.
# Longest match from front of string(s)
echo ${arrayZ[@]##t*e} # one two four five five
# ^^ # Applied to all elements of the array.
# Matches "three" and removes it.
# Shortest match from back of string(s)
echo ${arrayZ[@]%h*e} # one two t four five five
# ^ # Applied to all elements of the array.
# Matches "hree" and removes it.
# Longest match from back of string(s)
echo ${arrayZ[@]%%t*e} # one two four five five
# ^^ # Applied to all elements of the array.
# Matches "three" and removes it.
echo "----------------------"
# Substring Replacement
# Replace first occurrence of substring with replacement.
echo ${arrayZ[@]/fiv/XYZ} # one two three four XYZe XYZe
# ^ # Applied to all elements of the array.
# Replace all occurrences of substring.
echo ${arrayZ[@]//iv/YY} # one two three four fYYe fYYe
# Applied to all elements of the array.
# Delete all occurrences of substring.
# Not specifing a replacement defaults to 'delete' ...
echo ${arrayZ[@]//fi/} # one two three four ve ve
# ^^ # Applied to all elements of the array.
# Replace front-end occurrences of substring.
echo ${arrayZ[@]/#fi/XY} # one two three four XYve XYve
# ^ # Applied to all elements of the array.
# Replace back-end occurrences of substring.
echo ${arrayZ[@]/%ve/ZZ} # one two three four fiZZ fiZZ
# ^ # Applied to all elements of the array.
echo ${arrayZ[@]/%o/XX} # one twXX three four five five
# ^ # Why?
echo "-----------------------------"
replacement() {
echo -n "!!!"
}
echo ${arrayZ[@]/%e/$(replacement)}
# ^ ^^^^^^^^^^^^^^
# on!!! two thre!!! four fiv!!! fiv!!!
# The stdout of replacement() is the replacement string.
# Q.E.D: The replacement action is, in effect, an 'assignment.'
echo "------------------------------------"
# Accessing the "for-each":
echo ${arrayZ[@]//*/$(replacement optional_arguments)}
# ^^ ^^^^^^^^^^^^^
# !!! !!! !!! !!! !!! !!!
# Now, if Bash would only pass the matched string
#+ to the function being called . . .
echo
exit 0
# Before reaching for a Big Hammer -- Perl, Python, or all the rest --
# recall:
# $( ... ) is command substitution.
# A function runs as a sub-process.
# A function writes its output (if echo-ed) to stdout.
# Assignment, in conjunction with "echo" and command substitution,
#+ can read a function's stdout.
# The name[@] notation specifies (the equivalent of) a "for-each"
#+ operation.
# Bash is more powerful than you think!
Command substitution can construct the individual elements of an
array.
Example 27-5. Loading the contents of a script into an array
#!/bin/bash
# script-array.sh: Loads this script into an array.
# Inspired by an e-mail from Chris Martin (thanks!).
script_contents=( $(cat "$0") ) # Stores contents of this script ($0)
#+ in an array.
for element in $(seq 0 $((${#script_contents[@]} - 1)))
do # ${#script_contents[@]}
#+ gives number of elements in the array.
#
# Question:
# Why is seq 0 necessary?
# Try changing it to seq 1.
echo -n "${script_contents[$element]}"
# List each field of this script on a single line.
# echo -n "${script_contents[element]}" also works because of ${ ... }.
echo -n " -- " # Use " -- " as a field separator.
done
echo
exit 0
# Exercise:
# --------
# Modify this script so it lists itself
#+ in its original format,
#+ complete with whitespace, line breaks, etc.
In an array context, some Bash builtins have a slightly altered
meaning. For example, unset deletes array elements, or even an entire
array.
Example 27-6. Some special properties of arrays
#!/bin/bash
declare -a colors
# All subsequent commands in this script will treat
#+ the variable "colors" as an array.
echo "Enter your favorite colors (separated from each other by a space)."
read -a colors # Enter at least 3 colors to demonstrate features below.
# Special option to 'read' command,
#+ allowing assignment of elements in an array.
echo
element_count=${#colors[@]}
# Special syntax to extract number of elements in array.
# element_count=${#colors[*]} works also.
#
# The "@" variable allows word splitting within quotes
#+ (extracts variables separated by whitespace).
#
# This corresponds to the behavior of "$@" and "$*"
#+ in positional parameters.
index=0
while [ "$index" -lt "$element_count" ]
do # List all the elements in the array.
echo ${colors[$index]}
# ${colors[index]} also works because it's within ${ ... } brackets.
let "index = $index + 1"
# Or:
# ((index++))
done
# Each array element listed on a separate line.
# If this is not desired, use echo -n "${colors[$index]} "
#
# Doing it with a "for" loop instead:
# for i in "${colors[@]}"
# do
# echo "$i"
# done
# (Thanks, S.C.)
echo
# Again, list all the elements in the array, but using a more elegant method.
echo ${colors[@]} # echo ${colors[*]} also works.
echo
# The "unset" command deletes elements of an array, or entire array.
unset colors[1] # Remove 2nd element of array.
# Same effect as colors[1]=
echo ${colors[@]} # List array again, missing 2nd element.
unset colors # Delete entire array.
# unset colors[*] and
#+ unset colors[@] also work.
echo; echo -n "Colors gone."
echo ${colors[@]} # List array again, now empty.
exit 0
As seen in the previous example, either ${array_name[@]} or
${array_name[*]} refers to all the elements of the array. Similarly,
to get a count of the number of elements in an array, use either
${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length
(number of characters) of ${array_name[0]}, the first element of the
array.
Example 27-7. Of empty arrays and empty elements
#!/bin/bash
# empty-array.sh
# Thanks to Stephane Chazelas for the original example,
#+ and to Michael Zick and Omair Eshkenazi, for extending it.
# And to Nathan Coulter for clarifications and corrections.
# An empty array is not the same as an array with empty elements.
array0=( first second third )
array1=( '' ) # "array1" consists of one empty element.
array2=( ) # No elements . . . "array2" is empty.
array3=( ) # What about this array?
echo
ListArray()
{
echo
echo "Elements in array0: ${array0[@]}"
echo "Elements in array1: ${array1[@]}"
echo "Elements in array2: ${array2[@]}"
echo "Elements in array3: ${array3[@]}"
echo
echo "Length of first element in array0 = ${#array0}"
echo "Length of first element in array1 = ${#array1}"
echo "Length of first element in array2 = ${#array2}"
echo "Length of first element in array3 = ${#array3}"
echo
echo "Number of elements in array0 = ${#array0[*]}" # 3
echo "Number of elements in array1 = ${#array1[*]}" # 1 (Surprise!)
echo "Number of elements in array2 = ${#array2[*]}" # 0
echo "Number of elements in array3 = ${#array3[*]}" # 0
}
# ===================================================================
ListArray
# Try extending those arrays.
# Adding an element to an array.
array0=( "${array0[@]}" "new1" )
array1=( "${array1[@]}" "new1" )
array2=( "${array2[@]}" "new1" )
array3=( "${array3[@]}" "new1" )
ListArray
# or
array0[${#array0[*]}]="new2"
array1[${#array1[*]}]="new2"
array2[${#array2[*]}]="new2"
array3[${#array3[*]}]="new2"
ListArray
# When extended as above, arrays are 'stacks' ...
# Above is the 'push' ...
# The stack 'height' is:
height=${#array2[@]}
echo
echo "Stack height for array2 = $height"
# The 'pop' is:
unset array2[${#array2[@]}-1] # Arrays are zero-based,
height=${#array2[@]} #+ which means first element has index 0.
echo
echo "POP"
echo "New stack height for array2 = $height"
ListArray
# List only 2nd and 3rd elements of array0.
from=1 # Zero-based numbering.
to=2
array3=( ${array0[@]:1:2} )
echo
echo "Elements in array3: ${array3[@]}"
# Works like a string (array of characters).
# Try some other "string" forms.
# Replacement:
array4=( ${array0[@]/second/2nd} )
echo
echo "Elements in array4: ${array4[@]}"
# Replace all matching wildcarded string.
array5=( ${array0[@]//new?/old} )
echo
echo "Elements in array5: ${array5[@]}"
# Just when you are getting the feel for this . . .
array6=( ${array0[@]#*new} )
echo # This one might surprise you.
echo "Elements in array6: ${array6[@]}"
array7=( ${array0[@]#new1} )
echo # After array6 this should not be a surprise.
echo "Elements in array7: ${array7[@]}"
# Which looks a lot like . . .
array8=( ${array0[@]/new1/} )
echo
echo "Elements in array8: ${array8[@]}"
# So what can one say about this?
# The string operations are performed on
#+ each of the elements in var[@] in succession.
# Therefore : Bash supports string vector operations.
# If the result is a zero length string,
#+ that element disappears in the resulting assignment.
# However, if the expansion is in quotes, the null elements remain.
# Michael Zick: Question, are those strings hard or soft quotes?
# Nathan Coulter: There is no such thing as "soft quotes."
#! What's really happening is that
#!+ the pattern matching happens after
#!+ all the other expansions of [word]
#!+ in cases like ${parameter#word}.
zap='new*'
array9=( ${array0[@]/$zap/} )
echo
echo "Number of elements in array9: ${#array9[@]}"
array9=( "${array0[@]/$zap/}" )
echo "Elements in array9: ${array9[@]}"
# This time the null elements remain.
echo "Number of elements in array9: ${#array9[@]}"
# Just when you thought you were still in Kansas . . .
array10=( ${array0[@]#$zap} )
echo
echo "Elements in array10: ${array10[@]}"
# But, the asterisk in zap won't be interpreted if quoted.
array10=( ${array0[@]#"$zap"} )
echo
echo "Elements in array10: ${array10[@]}"
# Well, maybe we _are_ still in Kansas . . .
# (Revisions to above code block by Nathan Coulter.)
# Compare array7 with array10.
# Compare array8 with array9.
# Reiterating: No such thing as soft quotes!
# Nathan Coulter explains:
# Pattern matching of 'word' in ${parameter#word} is done after
#+ parameter expansion and *before* quote removal.
# In the normal case, pattern matching is done *after* quote removal.
exit
The relationship of ${array_name[@]} and ${array_name[*]} is
analogous to that between $@ and $*. This powerful array notation has
a number of uses.
# Copying an array.
array2=( "${array1[@]}" )
# or
array2="${array1[@]}"
#
# However, this fails with "sparse" arrays,
#+ arrays with holes (missing elements) in them,
#+ as Jochen DeSmet points out.
# ------------------------------------------
array1[0]=0
# array1[1] not assigned
array1[2]=2
array2=( "${array1[@]}" ) # Copy it?
echo ${array2[0]} # 0
echo ${array2[2]} # (null), should be 2
# ------------------------------------------
# Adding an element to an array.
array=( "${array[@]}" "new element" )
# or
array[${#array[*]}]="new element"
# Thanks, S.C.
Tip
The array=( element1 element2 ... elementN ) initialization
operation, with the help of command substitution, makes it possible
to load the contents of a text file into an array.
#!/bin/bash
filename=sample_file
# cat sample_file
#
# 1 a b c
# 2 d e fg
declare -a array1
array1=( `cat "$filename"`) # Loads contents
# List file to stdout #+ of $filename into array1.
#
# array1=( `cat "$filename" | tr '\n' ' '`)
# change linefeeds in file to spaces.
# Not necessary because Bash does word splitting,
#+ changing linefeeds to spaces.
echo ${array1[@]} # List the array.
# 1 a b c 2 d e fg
#
# Each whitespace-separated "word" in the file
#+ has been assigned to an element of the array.
element_count=${#array1[*]}
echo $element_count # 8
Clever scripting makes it possible to add array operations.
Example 27-8. Initializing arrays
#! /bin/bash
# array-assign.bash
# Array operations are Bash-specific,
#+ hence the ".bash" in the script name.
# Copyright (c) Michael S. Zick, 2003, All rights reserved.
# License: Unrestricted reuse in any form, for any purpose.
# Version: $ID$
#
# Clarification and additional comments by William Park.
# Based on an example provided by Stephane Chazelas
#+ which appeared in an earlier version of the
#+ Advanced Bash Scripting Guide.
# Output format of the 'times' command:
# User CPU System CPU
# User CPU of dead children System CPU of dead children
# Bash has two versions of assigning all elements of an array
#+ to a new array variable.
# Both drop 'null reference' elements
#+ in Bash versions 2.04 and later.
# An additional array assignment that maintains the relationship of
#+ [subscript]=value for arrays may be added to newer versions.
# Constructs a large array using an internal command,
#+ but anything creating an array of several thousand elements
#+ will do just fine.
declare -a bigOne=( /dev/* ) # All the files in /dev . . .
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of'
echo "Number of elements in array is ${#bigOne[@]}"
# set -vx
echo
echo '- - testing: =( ${array[@]} ) - -'
times
declare -a bigTwo=( ${bigOne[@]} )
# Note parens: ^ ^
times
echo
echo '- - testing: =${array[@]} - -'
times
declare -a bigThree=${bigOne[@]}
# No parentheses this time.
times
# Comparing the numbers shows that the second form, pointed out
#+ by Stephane Chazelas, is faster.
#
# As William Park explains:
#+ The bigTwo array assigned element by element (because of parentheses),
#+ whereas bigThree assigned as a single string.
# So, in essence, you have:
# bigTwo=( [0]="..." [1]="..." [2]="..." ... )
# bigThree=( [0]="... ... ..." )
#
# Verify this by: echo ${bigTwo[0]}
# echo ${bigThree[0]}
# I will continue to use the first form in my example descriptions
#+ because I think it is a better illustration of what is happening.
# The reusable portions of my examples will actual contain
#+ the second form where appropriate because of the speedup.
# MSZ: Sorry about that earlier oversight folks.
# Note:
# ----
# The "declare -a" statements in lines 32 and 44
#+ are not strictly necessary, since it is implicit
#+ in the Array=( ... ) assignment form.
# However, eliminating these declarations slows down
#+ the execution of the following sections of the script.
# Try it, and see.
exit 0
Note
Adding a superfluous declare -a statement to an array declaration may
speed up execution of subsequent operations on the array.
Example 27-9. Copying and concatenating arrays
#! /bin/bash
# CopyArray.sh
#
# This script written by Michael Zick.
# Used here with permission.
# How-To "Pass by Name & Return by Name"
#+ or "Building your own assignment statement".
CpArray_Mac() {
# Assignment Command Statement Builder
echo -n 'eval '
echo -n "$2" # Destination name
echo -n '=( ${'
echo -n "$1" # Source name
echo -n '[@]} )'
# That could all be a single command.
# Matter of style only.
}
declare -f CopyArray # Function "Pointer"
CopyArray=CpArray_Mac # Statement Builder
Hype()
{
# Hype the array named $1.
# (Splice it together with array containing "Really Rocks".)
# Return in array named $2.
local -a TMP
local -a hype=( Really Rocks )
$($CopyArray $1 TMP)
TMP=( ${TMP[@]} ${hype[@]} )
$($CopyArray TMP $2)
}
declare -a before=( Advanced Bash Scripting )
declare -a after
echo "Array Before = ${before[@]}"
Hype before after
echo "Array After = ${after[@]}"
# Too much hype?
echo "What ${after[@]:3:2}?"
declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} )
# ---- substring extraction ----
echo "Array Modest = ${modest[@]}"
# What happened to 'before' ?
echo "Array Before = ${before[@]}"
exit 0
Example 27-10. More on concatenating arrays
#! /bin/bash
# array-append.bash
# Copyright (c) Michael S. Zick, 2003, All rights reserved.
# License: Unrestricted reuse in any form, for any purpose.
# Version: $ID$
#
# Slightly modified in formatting by M.C.
# Array operations are Bash-specific.
# Legacy UNIX /bin/sh lacks equivalents.
# Pipe the output of this script to 'more'
#+ so it doesn't scroll off the terminal.
# Or, redirect output to a file.
declare -a array1=( zero1 one1 two1 )
# Subscript packed.
declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 )
# Subscript sparse -- [1] is not defined.
echo
echo '- Confirm that the array is really subscript sparse. -'
echo "Number of elements: 4" # Hard-coded for illustration.
for (( i = 0 ; i < 4 ; i++ ))
do
echo "Element [$i]: ${array2[$i]}"
done
# See also the more general code example in basics-reviewed.bash.
declare -a dest
# Combine (append) two arrays into a third array.
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator'
echo '- Undefined elements not present, subscripts not maintained. -'
# # The undefined elements do not exist; they are not being dropped.
dest=( ${array1[@]} ${array2[@]} )
# dest=${array1[@]}${array2[@]} # Strange results, possibly a bug.
# Now, list the result.
echo
echo '- - Testing Array Append - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"
done
# Assign an array to a single array element (twice).
dest[0]=${array1[@]}
dest[1]=${array2[@]}
# List the result.
echo
echo '- - Testing modified array - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"
done
# Examine the modified second element.
echo
echo '- - Reassign and list second element - -'
declare -a subArray=${dest[1]}
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
# The assignment of an entire array to a single element
#+ of another array using the '=${ ... }' array assignment
#+ has converted the array being assigned into a string,
#+ with the elements separated by a space (the first character of IFS).
# If the original elements didn't contain whitespace . . .
# If the original array isn't subscript sparse . . .
# Then we could get the original array structure back again.
# Restore from the modified second element.
echo
echo '- - Listing restored element - -'
declare -a subArray=( ${dest[1]} )
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
echo '- - Do not depend on this behavior. - -'
echo '- - This behavior is subject to change - -'
echo '- - in versions of Bash newer than version 2.05b - -'
# MSZ: Sorry about any earlier confusion folks.
exit 0
--
Arrays permit deploying old familiar algorithms as shell scripts.
Whether this is necessarily a good idea is left for the reader to
decide.
Example 27-11. The Bubble Sort
#!/bin/bash
# bubble.sh: Bubble sort, of sorts.
# Recall the algorithm for a bubble sort. In this particular version...
# With each successive pass through the array to be sorted,
#+ compare two adjacent elements, and swap them if out of order.
# At the end of the first pass, the "heaviest" element has sunk to bottom.
# At the end of the second pass, the next "heaviest" one has sunk next to bot
tom.
# And so forth.
# This means that each successive pass needs to traverse less of the array.
# You will therefore notice a speeding up in the printing of the later passes
.
exchange()
{
# Swaps two members of the array.
local temp=${Countries[$1]} # Temporary storage
#+ for element getting swapped out.
Countries[$1]=${Countries[$2]}
Countries[$2]=$temp
return
}
declare -a Countries # Declare array,
#+ optional here since it's initialized below.
# Is it permissable to split an array variable over multiple lines
#+ using an escape (\)?
# Yes.
Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \
Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \
Israel Peru Canada Oman Denmark Wales France Kenya \
Xanadu Qatar Liechtenstein Hungary)
# "Xanadu" is the mythical place where, according to Coleridge,
#+ Kubla Khan did a pleasure dome decree.
clear # Clear the screen to start with.
echo "0: ${Countries[*]}" # List entire array at pass 0.
number_of_elements=${#Countries[@]}
let "comparisons = $number_of_elements - 1"
count=1 # Pass number.
while [ "$comparisons" -gt 0 ] # Beginning of outer loop
do
index=0 # Reset index to start of array after each pass.
while [ "$index" -lt "$comparisons" ] # Beginning of inner loop
do
if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ]
# If out of order...
# Recalling that \> is ASCII comparison operator
#+ within single brackets.
# if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]
#+ also works.
then
exchange $index `expr $index + 1` # Swap.
fi
let "index += 1" # Or, index+=1 on Bash, ver. 3.1 or newer.
done # End of inner loop
# ----------------------------------------------------------------------
# Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative.
#
# for (( last = $number_of_elements - 1 ; last > 0 ; last-- ))
## Fix by C.Y. Hunt ^ (Thanks!)
# do
# for (( i = 0 ; i < last ; i++ ))
# do
# [[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \
# && exchange $i $((i+1))
# done
# done
# ----------------------------------------------------------------------
let "comparisons -= 1" # Since "heaviest" element bubbles to bottom,
#+ we need do one less comparison each pass.
echo
echo "$count: ${Countries[@]}" # Print resultant array at end of each pass.
echo
let "count += 1" # Increment pass count.
done # End of outer loop
# All done.
exit 0
--
Is it possible to nest arrays within arrays?
#!/bin/bash
# "Nested" array.
# Michael Zick provided this example,
#+ with corrections and clarifications by William Park.
AnArray=( $(ls --inode --ignore-backups --almost-all \
--directory --full-time --color=none --time=status \
--sort=time -l ${PWD} ) ) # Commands and options.
# Spaces are significant . . . and don't quote anything in the above.
SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} )
# This array has six elements:
#+ SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]}
# [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} )
#
# Arrays in Bash are (circularly) linked lists
#+ of type string (char *).
# So, this isn't actually a nested array,
#+ but it's functionally similar.
echo "Current directory and date of last status change:"
echo "${SubArray[@]}"
exit 0
--
Embedded arrays in combination with indirect references create some
fascinating possibilities
Example 27-12. Embedded arrays and indirect references
#!/bin/bash
# embedded-arrays.sh
# Embedded arrays and indirect references.
# This script by Dennis Leeuw.
# Used with permission.
# Modified by document author.
ARRAY1=(
VAR1_1=value11
VAR1_2=value12
VAR1_3=value13
)
ARRAY2=(
VARIABLE="test"
STRING="VAR1=value1 VAR2=value2 VAR3=value3"
ARRAY21=${ARRAY1[*]}
) # Embed ARRAY1 within this second array.
function print () {
OLD_IFS="$IFS"
IFS=$'\n' # To print each array element
#+ on a separate line.
TEST1="ARRAY2[*]"
local ${!TEST1} # See what happens if you delete this line.
# Indirect reference.
# This makes the components of $TEST1
#+ accessible to this function.
# Let's see what we've got so far.
echo
echo "\$TEST1 = $TEST1" # Just the name of the variable.
echo; echo
echo "{\$TEST1} = ${!TEST1}" # Contents of the variable.
# That's what an indirect
#+ reference does.
echo
echo "-------------------------------------------"; echo
echo
# Print variable
echo "Variable VARIABLE: $VARIABLE"
# Print a string element
IFS="$OLD_IFS"
TEST2="STRING[*]"
local ${!TEST2} # Indirect reference (as above).
echo "String element VAR2: $VAR2 from STRING"
# Print an array element
TEST2="ARRAY21[*]"
local ${!TEST2} # Indirect reference (as above).
echo "Array element VAR1_1: $VAR1_1 from ARRAY21"
}
print
echo
exit 0
# As the author of the script notes,
#+ "you can easily expand it to create named-hashes in bash."
# (Difficult) exercise for the reader: implement this.
--
Arrays enable implementing a shell script version of the Sieve of
Eratosthenes. Of course, a resource-intensive application of this
nature should really be written in a compiled language, such as C. It
runs excruciatingly slowly as a script.
Example 27-13. The Sieve of Eratosthenes
#!/bin/bash
# sieve.sh (ex68.sh)
# Sieve of Eratosthenes
# Ancient algorithm for finding prime numbers.
# This runs a couple of orders of magnitude slower
#+ than the equivalent program written in C.
LOWER_LIMIT=1 # Starting with 1.
UPPER_LIMIT=1000 # Up to 1000.
# (You may set this higher . . . if you have time on your hands.)
PRIME=1
NON_PRIME=0
let SPLIT=UPPER_LIMIT/2
# Optimization:
# Need to test numbers only halfway to upper limit. Why?
declare -a Primes
# Primes[] is an array.
initialize ()
{
# Initialize the array.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
Primes[i]=$PRIME
let "i += 1"
done
# Assume all array members guilty (prime)
#+ until proven innocent.
}
print_primes ()
{
# Print out the members of the Primes[] array tagged as prime.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
then
printf "%8d" $i
# 8 spaces per number gives nice, even columns.
fi
let "i += 1"
done
}
sift () # Sift out the non-primes.
{
let i=$LOWER_LIMIT+1
# Let's start with 2.
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
# Don't bother sieving numbers already sieved (tagged as non-prime).
then
t=$i
while [ "$t" -le "$UPPER_LIMIT" ]
do
let "t += $i "
Primes[t]=$NON_PRIME
# Tag as non-prime all multiples.
done
fi
let "i += 1"
done
}
# ==============================================
# main ()
# Invoke the functions sequentially.
initialize
sift
print_primes
# This is what they call structured programming.
# ==============================================
echo
exit 0
# -------------------------------------------------------- #
# Code below line will not execute, because of 'exit.'
# This improved version of the Sieve, by Stephane Chazelas,
#+ executes somewhat faster.
# Must invoke with command-line argument (limit of primes).
UPPER_LIMIT=$1 # From command-line.
let SPLIT=UPPER_LIMIT/2 # Halfway to max number.
Primes=( '' $(seq $UPPER_LIMIT) )
i=1
until (( ( i += 1 ) > SPLIT )) # Need check only halfway.
do
if [[ -n ${Primes[i]} ]]
then
t=$i
until (( ( t += i ) > UPPER_LIMIT ))
do
Primes[t]=
done
fi
done
echo ${Primes[*]}
exit $?
Example 27-14. The Sieve of Eratosthenes, Optimized
#!/bin/bash
# Optimized Sieve of Eratosthenes
# Script by Jared Martin, with very minor changes by ABS Guide author.
# Used in ABS Guide with permission (thanks!).
# Based on script in Advanced Bash Scripting Guide.
# http://tldp.org/LDP/abs/html/arrays.html#PRIMES0 (ex68.sh).
# http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf (reference)
# Check results against http://primes.utm.edu/lists/small/1000.txt
# Necessary but not sufficient would be, e.g.,
# (($(sieve 7919 | wc -w) == 1000)) && echo "7919 is the 1000th prime"
UPPER_LIMIT=${1:?"Need an upper limit of primes to search."}
Primes=( '' $(seq ${UPPER_LIMIT}) )
typeset -i i t
Primes[i=1]='' # 1 is not a prime.
until (( ( i += 1 ) > (${UPPER_LIMIT}/i) )) # Need check only ith-way.
do # Why?
if ((${Primes[t=i*(i-1), i]}))
# Obscure, but instructive, use of arithmetic expansion in subscript.
then
until (( ( t += i ) > ${UPPER_LIMIT} ))
do Primes[t]=; done
fi
done
# echo ${Primes[*]}
echo # Change to original script for pretty-printing (80-col. display).
printf "%8d" ${Primes[*]}
echo; echo
exit $?
Compare these array-based prime number generators with alternatives
that do not use arrays, Example A-15, and Example 16-46.
--
Arrays lend themselves, to some extent, to emulating data structures
for which Bash has no native support.
Example 27-15. Emulating a push-down stack
#!/bin/bash
# stack.sh: push-down stack simulation
# Similar to the CPU stack, a push-down stack stores data items
#+ sequentially, but releases them in reverse order, last-in first-out.
BP=100 # Base Pointer of stack array.
# Begin at element 100.
SP=$BP # Stack Pointer.
# Initialize it to "base" (bottom) of stack.
Data= # Contents of stack location.
# Must use global variable,
#+ because of limitation on function return range.
# 100 Base pointer <-- Base Pointer
# 99 First data item
# 98 Second data item
# ... More data
# Last data item <-- Stack pointer
declare -a stack
push() # Push item on stack.
{
if [ -z "$1" ] # Nothing to push?
then
return
fi
let "SP -= 1" # Bump stack pointer.
stack[$SP]=$1
return
}
pop() # Pop item off stack.
{
Data= # Empty out data item.
if [ "$SP" -eq "$BP" ] # Stack empty?
then
return
fi # This also keeps SP from getting past 100,
#+ i.e., prevents a runaway stack.
Data=${stack[$SP]}
let "SP += 1" # Bump stack pointer.
return
}
status_report() # Find out what's happening.
{
echo "-------------------------------------"
echo "REPORT"
echo "Stack Pointer = $SP"
echo "Just popped \""$Data"\" off the stack."
echo "-------------------------------------"
echo
}
# =======================================================
# Now, for some fun.
echo
# See if you can pop anything off empty stack.
pop
status_report
echo
push garbage
pop
status_report # Garbage in, garbage out.
value1=23; push $value1
value2=skidoo; push $value2
value3=LAST; push $value3
pop # LAST
status_report
pop # skidoo
status_report
pop # 23
status_report # Last-in, first-out!
# Notice how the stack pointer decrements with each push,
#+ and increments with each pop.
echo
exit 0
# =======================================================
# Exercises:
# ---------
# 1) Modify the "push()" function to permit pushing
# + multiple element on the stack with a single function call.
# 2) Modify the "pop()" function to permit popping
# + multiple element from the stack with a single function call.
# 3) Add error checking to the critical functions.
# That is, return an error code, depending on
# + successful or unsuccessful completion of the operation,
# + and take appropriate action.
# 4) Using this script as a starting point,
# + write a stack-based 4-function calculator.
--
Fancy manipulation of array "subscripts" may require intermediate
variables. For projects involving this, again consider using a more
powerful programming language, such as Perl or C.
Example 27-16. Complex array application: Exploring a weird
mathematical series
#!/bin/bash
# Douglas Hofstadter's notorious "Q-series":
# Q(1) = Q(2) = 1
# Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2
# This is a "chaotic" integer series with strange
#+ and unpredictable behavior.
# The first 20 terms of the series are:
# 1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12
# See Hofstadter's book, _Goedel, Escher, Bach: An Eternal Golden Braid_,
#+ p. 137, ff.
LIMIT=100 # Number of terms to calculate.
LINEWIDTH=20 # Number of terms printed per line.
Q[1]=1 # First two terms of series are 1.
Q[2]=1
echo
echo "Q-series [$LIMIT terms]:"
echo -n "${Q[1]} " # Output first two terms.
echo -n "${Q[2]} "
for ((n=3; n <= $LIMIT; n++)) # C-like loop expression.
do # Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]] for n>2
# Need to break the expression into intermediate terms,
#+ since Bash doesn't handle complex array arithmetic very well.
let "n1 = $n - 1" # n-1
let "n2 = $n - 2" # n-2
t0=`expr $n - ${Q[n1]}` # n - Q[n-1]
t1=`expr $n - ${Q[n2]}` # n - Q[n-2]
T0=${Q[t0]} # Q[n - Q[n-1]]
T1=${Q[t1]} # Q[n - Q[n-2]]
Q[n]=`expr $T0 + $T1` # Q[n - Q[n-1]] + Q[n - Q[n-2]]
echo -n "${Q[n]} "
if [ `expr $n % $LINEWIDTH` -eq 0 ] # Format output.
then # ^ modulo
echo # Break lines into neat chunks.
fi
done
echo
exit 0
# This is an iterative implementation of the Q-series.
# The more intuitive recursive implementation is left as an exercise.
# Warning: calculating this series recursively takes a VERY long time
#+ via a script. C/C++ would be orders of magnitude faster.
--
Bash supports only one-dimensional arrays, though a little trickery
permits simulating multi-dimensional ones.
Example 27-17. Simulating a two-dimensional array, then tilting it
#!/bin/bash
# twodim.sh: Simulating a two-dimensional array.
# A one-dimensional array consists of a single row.
# A two-dimensional array stores rows sequentially.
Rows=5
Columns=5
# 5 X 5 Array.
declare -a alpha # char alpha [Rows] [Columns];
# Unnecessary declaration. Why?
load_alpha ()
{
local rc=0
local index
for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y
do # Use different symbols if you like.
local row=`expr $rc / $Columns`
local column=`expr $rc % $Rows`
let "index = $row * $Rows + $column"
alpha[$index]=$i
# alpha[$row][$column]
let "rc += 1"
done
# Simpler would be
#+ declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y )
#+ but this somehow lacks the "flavor" of a two-dimensional array.
}
print_alpha ()
{
local row=0
local index
echo
while [ "$row" -lt "$Rows" ] # Print out in "row major" order:
do #+ columns vary,
#+ while row (outer loop) remains the same.
local column=0
echo -n " " # Lines up "square" array with rotated one.
while [ "$column" -lt "$Columns" ]
do
let "index = $row * $Rows + $column"
echo -n "${alpha[index]} " # alpha[$row][$column]
let "column += 1"
done
let "row += 1"
echo
done
# The simpler equivalent is
# echo ${alpha[*]} | xargs -n $Columns
echo
}
filter () # Filter out negative array indices.
{
echo -n " " # Provides the tilt.
# Explain how.
if [[ "$1" -ge 0 && "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]]
then
let "index = $1 * $Rows + $2"
# Now, print it rotated.
echo -n " ${alpha[index]}"
# alpha[$row][$column]
fi
}
rotate () # Rotate the array 45 degrees --
{ #+ "balance" it on its lower lefthand corner.
local row
local column
for (( row = Rows; row > -Rows; row-- ))
do # Step through the array backwards. Why?
for (( column = 0; column < Columns; column++ ))
do
if [ "$row" -ge 0 ]
then
let "t1 = $column - $row"
let "t2 = $column"
else
let "t1 = $column"
let "t2 = $column + $row"
fi
filter $t1 $t2 # Filter out negative array indices.
# What happens if you don't do this?
done
echo; echo
done
# Array rotation inspired by examples (pp. 143-146) in
#+ "Advanced C Programming on the IBM PC," by Herbert Mayer
#+ (see bibliography).
# This just goes to show that much of what can be done in C
#+ can also be done in shell scripting.
}
#--------------- Now, let the show begin. ------------#
load_alpha # Load the array.
print_alpha # Print it out.
rotate # Rotate it 45 degrees counterclockwise.
#-----------------------------------------------------#
exit 0
# This is a rather contrived, not to mention inelegant simulation.
# Exercises:
# ---------
# 1) Rewrite the array loading and printing functions
# in a more intuitive and less kludgy fashion.
#
# 2) Figure out how the array rotation functions work.
# Hint: think about the implications of backwards-indexing an array.
#
# 3) Rewrite this script to handle a non-square array,
# such as a 6 X 4 one.
# Try to minimize "distortion" when the array is rotated.
A two-dimensional array is essentially equivalent to a
one-dimensional one, but with additional addressing modes for
referencing and manipulating the individual elements by row and
column position.
For an even more elaborate example of simulating a two-dimensional
array, see Example A-10.
--
For more interesting scripts using arrays, see:
* Example 12-3
* Example 16-46
* Example A-22
* Example A-44
* Example A-41
* Example A-42
________________________________________________________________
Chapter 28. Indirect References
We have seen that referencing a variable, $var, fetches its value.
But, what about the value of a value? What about $$var?
The actual notation is \$$var, usually preceded by an eval (and
sometimes an echo). This is called an indirect reference.
Example 28-1. Indirect Variable References
#!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.
# First, let's fool around a little.
var=23
echo "\$var = $var" # $var = 23
# So far, everything as expected. But ...
echo "\$\$var = $$var" # $$var = 4570var
# Not useful ...
# \$\$ expanded to PID of the script
# -- refer to the entry on the $$ variable --
#+ and "var" is echoed as plain text.
# (Thank you, Jakob Bohm, for pointing this out.)
echo "\\\$\$var = \$$var" # \$$var = $23
# As expected. The first $ is escaped and pasted on to
#+ the value of var ($var = 23 ).
# Meaningful, but still not useful.
# Now, let's start over and do it the right way.
# ============================================== #
a=letter_of_alphabet # Variable "a" holds the name of another variable.
letter_of_alphabet=z
echo
# Direct reference.
echo "a = $a" # a = letter_of_alphabet
# Indirect reference.
eval a=\$$a
# ^^^ Forcing an eval(uation), and ...
# ^ Escaping the first $ ...
# ------------------------------------------------------------------------
# The 'eval' forces an update of $a, sets it to the updated value of \$$a.
# So, we see why 'eval' so often shows up in indirect reference notation.
# ------------------------------------------------------------------------
echo "Now a = $a" # Now a = z
echo
# Now, let's try changing the second-order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24
echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24
# In this simple case, the following also works (why?).
# eval t=\$$t; echo "\"t\" = $t"
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$NEW_VAL
echo "Changing value of \"table_cell_3\" to $NEW_VAL."
echo "\"table_cell_3\" now $table_cell_3"
echo -n "dereferenced \"t\" now "; eval echo \$$t
# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3
)
echo
# (Thanks, Stephane Chazelas, for clearing up the above behavior.)
# A more straightforward method is the ${!t} notation, discussed in the
#+ "Bash, version 2" section.
# See also ex78.sh.
exit 0
Indirect referencing in Bash is a multi-step process. First, take the
name of a variable: varname. Then, reference it: $varname. Then,
reference the reference: $$varname. Then, escape the first $:
\$$varname. Finally, force a reevaluation of the expression and
assign it: eval newvar=\$$varname.
Of what practical use is indirect referencing of variables? It gives
Bash a little of the functionality of pointers in C, for instance, in
table lookup. And, it also has some other very interesting
applications. . . .
Nils Radtke shows how to build "dynamic" variable names and evaluate
their contents. This can be useful when sourcing configuration files.
#!/bin/bash
# ---------------------------------------------
# This could be "sourced" from a separate file.
isdnMyProviderRemoteNet=172.16.0.100
isdnYourProviderRemoteNet=10.0.0.10
isdnOnlineService="MyProvider"
# ---------------------------------------------
remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
echo "$remoteNet" # 172.16.0.100
# ================================================================
# And, it gets even better.
# Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:
chkMirrorArchs () {
arch="$1";
if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
then
return 0;
else
return 1;
fi;
}
getSparc="true"
unset getIa64
chkMirrorArchs sparc
echo $? # 0
# True
chkMirrorArchs Ia64
echo $? # 1
# False
# Notes:
# -----
# Even the to-be-substituted variable name part is built explicitly.
# The parameters to the chkMirrorArchs calls are all lower case.
# The variable name is composed of two parts: "get" and "Sparc" . . .
Example 28-2. Passing an indirect reference to awk
#!/bin/bash
# Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
# This one uses indirect references.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1 # Name of file to operate on.
column_number=$2 # Which column to total up.
#===== Same as original script, up to this point =====#
# A multi-line awk script is invoked by
# awk "
# ...
# ...
# ...
# "
# Begin awk script.
# -------------------------------------------------
awk "
{ total += \$${column_number} # Indirect reference
}
END {
print total
}
" "$filename"
# Note that awk doesn't need an eval preceding \$$.
# -------------------------------------------------
# End awk script.
# Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit $?
Caution
This method of indirect referencing is a bit tricky. If the second
order variable changes its value, then the first order variable must
be properly dereferenced (as in the above example). Fortunately, the
${!variable} notation introduced with version 2 of Bash (see Example
37-2 and Example A-22) makes indirect referencing more intuitive.
Bash does not support pointer arithmetic, and this severely limits
the usefulness of indirect referencing. In fact, indirect referencing
in a scripting language is, at best, something of an afterthought.
________________________________________________________________
Chapter 29. /dev and /proc
A Linux or UNIX filesystem typically has the /dev and /proc
special-purpose directories.
________________________________________________________________
29.1. /dev
The /dev directory contains entries for the physical devices that may
or may not be present in the hardware. [118] Appropriately enough,
these are called device files. As an example, the hard drive
partitions containing the mounted filesystem(s) have entries in /dev,
as df shows.
bash$ df
Filesystem 1k-blocks Used Available Use%
Mounted on
/dev/hda6 495876 222748 247527 48% /
/dev/hda1 50755 3887 44248 9% /boot
/dev/hda8 367013 13262 334803 4% /home
/dev/hda5 1714416 1123624 503704 70% /usr
Among other things, the /dev directory contains loopback devices,
such as /dev/loop0. A loopback device is a gimmick that allows an
ordinary file to be accessed as if it were a block device. [119] This
permits mounting an entire filesystem within a single large file. See
Example 17-8 and Example 17-7.
A few of the pseudo-devices in /dev have other specialized uses, such
as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive
partition), /dev/udp (User Datagram Packet port), and /dev/tcp.
For instance:
To manually mount a USB flash drive, append the following line to
/etc/fstab. [120]
/dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0
(See also Example A-23.)
Checking whether a disk is in the CD-burner (soft-linked to
/dev/hdc):
head -1 /dev/hdc
# head: cannot open '/dev/hdc' for reading: No medium found
# (No disc in the drive.)
# head: error reading '/dev/hdc': Input/output error
# (There is a disk in the drive, but it can't be read;
#+ possibly it's an unrecorded CDR blank.)
# Stream of characters and assorted gibberish
# (There is a pre-recorded disk in the drive,
#+ and this is raw output -- a stream of ASCII and binary data.)
# Here we see the wisdom of using 'head' to limit the output
#+ to manageable proportions, rather than 'cat' or something similar.
# Now, it's just a matter of checking/parsing the output and taking
#+ appropriate action.
When executing a command on a /dev/tcp/$host/$port pseudo-device
file, Bash opens a TCP connection to the associated socket.
A socket is a communications node associated with a specific I/O
port. (This is analogous to a hardware socket, or receptacle, for a
connecting cable.) It permits data transfer between hardware devices
on the same machine, between machines on the same network, between
machines across different networks, and, of course, between machines
at different locations on the Internet.
The following examples assume an active Internet connection.
Getting the time from nist.gov:
bash$ cat /dev/tcp/www.net.cn/80
bash$ echo -e "GET / HTTP/1.0\n" >&5
bash$ cat <&5
[Thanks, Mark and Mihai Maties.]
Example 29-1. Using /dev/tcp for troubleshooting
#!/bin/bash
# dev-tcp.sh: /dev/tcp redirection to check Internet connection.
# Script by Troy Engel.
# Used with permission.
TCP_HOST=news-15.net # A known spam-friendly ISP.
TCP_PORT=80 # Port 80 is http.
# Try to connect. (Somewhat similar to a 'ping' . . .)
echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}
MYEXIT=$?
: <From the bash reference:
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer
port number or service name, Bash attempts to open a TCP connection to the
corresponding socket.
EXPLANATION
if [ "X$MYEXIT" = "X0" ]; then
echo "Connection successful. Exit code: $MYEXIT"
else
echo "Connection unsuccessful. Exit code: $MYEXIT"
fi
exit $MYEXIT
Example 29-2. Playing music
#!/bin/bash
# music.sh
# Music without external files
# Author: Antonio Macchi
# Used in ABS Guide with permission.
# /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte),
#+ 1 channel (mono)
duration=2000 # If 8000 bytes = 1 second, then 2000 = 1/4 second.
volume=$'\xc0' # Max volume = \xff (or \x00).
mute=$'\x80' # No volume = \x80 (the middle).
function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz ::
{ #+ 8000 fps / 440 = 16 :: A = 16 bytes per second)
for t in `seq 0 $duration`
do
test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute
done
}
e=`mknote 49`
g=`mknote 41`
a=`mknote 36`
b=`mknote 32`
c=`mknote 30`
cis=`mknote 29`
d=`mknote 27`
e2=`mknote 24`
n=`mknote 32767`
# European notation.
echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \
$n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp
# dsp = Digital Signal Processor
exit # A "bonny" example of an elegant shell script!
________________________________________________________________
29.2. /proc
The /proc directory is actually a pseudo-filesystem. The files in
/proc mirror currently running system and kernel processes and
contain information and statistics about them.
bash$ cat /proc/devices
Character devices:
1 mem
2 pty
3 ttyp
4 ttyS
5 cua
7 vcs
10 misc
14 sound
29 fb
36 netlink
128 ptm
136 pts
162 raw
254 pcmcia
Block devices:
1 ramdisk
2 fd
3 ide0
9 md
bash$ cat /proc/interrupts
CPU0
0: 84505 XT-PIC timer
1: 3375 XT-PIC keyboard
2: 0 XT-PIC cascade
5: 1 XT-PIC soundblaster
8: 1 XT-PIC rtc
12: 4231 XT-PIC PS/2 Mouse
14: 109373 XT-PIC ide0
NMI: 0
ERR: 0
bash$ cat /proc/partitions
major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse run
ning use aveq
3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 1
11550 644030
3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140
3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0
3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210
...
bash$ cat /proc/loadavg
0.13 0.42 0.27 2/44 1119
bash$ cat /proc/apm
1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?
bash$ cat /proc/acpi/battery/BAT0/info
present: yes
design capacity: 43200 mWh
last full capacity: 36640 mWh
battery technology: rechargeable
design voltage: 10800 mV
design capacity warning: 1832 mWh
design capacity low: 200 mWh
capacity granularity 1: 1 mWh
capacity granularity 2: 1 mWh
model number: IBM-02K6897
serial number: 1133
battery type: LION
OEM info: Panasonic
bash$ fgrep Mem /proc/meminfo
MemTotal: 515216 kB
MemFree: 266248 kB
Shell scripts may extract data from certain of the files in /proc.
[121]
FS=iso # ISO filesystem support in kernel?
grep $FS /proc/filesystems # iso9660
kernel_version=$( awk '{ print $3 }' /proc/version )
CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )
if [ "$CPU" = "Pentium(R)" ]
then
run_some_commands
...
else
run_other_commands
...
fi
cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )
# Current operating speed (in MHz) of the cpu on your machine.
# On a laptop this may vary, depending on use of battery
#+ or AC power.
#!/bin/bash
# get-commandline.sh
# Get the command-line parameters of a process.
OPTION=cmdline
# Identify PID.
pid=$( echo $(pidof "$1") | awk '{ print $1 }' )
# Get only first ^^^^^^^^^^^^^^^^^^ of multiple instances.
echo
echo "Process ID of (first instance of) "$1" = $pid"
echo -n "Command-line arguments: "
cat /proc/"$pid"/"$OPTION" | xargs -0 echo
# Formats output: ^^^^^^^^^^^^^^^
# (Thanks, Han Holl, for the fixup!)
echo; echo
# For example:
# sh get-commandline.sh xterm
+
devfile="/proc/bus/usb/devices"
text="Spd"
USB1="Spd=12"
USB2="Spd=480"
bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')
# ^^^^ Stop after first match.
if [ "$bus_speed" = "$USB1" ]
then
echo "USB 1.1 port found."
# Do something appropriate for USB 1.1.
fi
Note
It is even possible to control certain peripherals with commands sent
to the /proc directory.
root# echo on > /proc/acpi/ibm/light
This turns on the Thinklight in certain models of IBM/Lenovo
Thinkpads. (May not work on all Linux distros.)
Of course, caution is advised when writing to /proc.
The /proc directory contains subdirectories with unusual numerical
names. Every one of these names maps to the process ID of a currently
running process. Within each of these subdirectories, there are a
number of files that hold useful information about the corresponding
process. The stat and status files keep running statistics on the
process, the cmdline file holds the command-line arguments the
process was invoked with, and the exe file is a symbolic link to the
complete path name of the invoking process. There are a few more such
files, but these seem to be the most interesting from a scripting
standpoint.
Example 29-3. Finding the process associated with a PID
#!/bin/bash
# pid-identifier.sh:
# Gives complete path name to process associated with pid.
ARGNO=1 # Number of arguments the script expects.
E_WRONGARGS=65
E_BADPID=66
E_NOSUCHPROCESS=67
E_NOPERMISSION=68
PROCFILE=exe
if [ $# -ne $ARGNO ]
then
echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr.
exit $E_WRONGARGS
fi
pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
# Checks for pid in "ps" listing, field #1.
# Then makes sure it is the actual process, not the process invoked by this sc
ript.
# The last "grep $1" filters out this possibility.
#
# pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
# also works, as Teemu Huovila, points out.
if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-lengt
h string,
then #+ no running process corresponds to the pid given.
echo "No such process running."
exit $E_NOSUCHPROCESS
fi
# Alternatively:
# if ! ps $1 > /dev/null 2>&1
# then # no running process corresponds to the pid given.
# echo "No such process running."
# exit $E_NOSUCHPROCESS
# fi
# To simplify the entire process, use "pidof".
if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission.
then
echo "Process $1 running, but..."
echo "Can't get read permission on /proc/$1/$PROCFILE."
exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.
fi
# The last two tests may be replaced by:
# if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
# this will test whether it is possible
# to send a signal to the process.
# then echo "PID doesn't exist or you're not its owner" >&2
# exit $E_BADPID
# fi
exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
# Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
#
# /proc/pid-number/exe is a symbolic link
#+ to the complete path name of the invoking process.
if [ -e "$exe_file" ] # If /proc/pid-number/exe exists,
then #+ then the corresponding process exists.
echo "Process #$1 invoked by $exe_file."
else
echo "No such process running."
fi
# This elaborate script can *almost* be replaced by
# ps ax | grep $1 | awk '{ print $5 }'
# However, this will not work...
#+ because the fifth field of 'ps' is argv[0] of the process,
#+ not the executable file path.
#
# However, either of the following would work.
# find /proc/$1/exe -printf '%l\n'
# lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
# Additional commentary by Stephane Chazelas.
exit 0
Example 29-4. On-line connect status
#!/bin/bash
# connect-stat.sh
# Note that this script may need modification
#+ to work with a wireless connection.
PROCNAME=pppd # ppp daemon
PROCFILENAME=status # Where to look.
NOTCONNECTED=85
INTERVAL=2 # Update every 2 seconds.
pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME |
awk '{ print $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.
#
# However, as Oleg Philon points out,
#+ this could have been considerably simplified by using "pidof".
# pidno=$( pidof $PROCNAME )
#
# Moral of the story:
#+ When a command sequence gets too complex, look for a shortcut.
if [ -z "$pidno" ] # If no pid, then process is not running.
then
echo "Not connected."
# exit $NOTCONNECTED
else
echo "Connected."; echo
fi
while [ true ] # Endless loop, script can be improved here.
do
if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
# While process running, then "status" file exists.
then
echo "Disconnected."
# exit $NOTCONNECTED
fi
netstat -s | grep "packets received" # Get some connect statistics.
netstat -s | grep "packets delivered"
sleep $INTERVAL
echo; echo
done
exit 0
# As it stands, this script must be terminated with a Control-C.
# Exercises:
# ---------
# Improve the script so it exits on a "q" keystroke.
# Make the script more user-friendly in other ways.
# Fix the script to work with wireless/DSL connections.
Warning
In general, it is dangerous to write to the files in /proc, as this
can corrupt the filesystem or crash the machine.
________________________________________________________________
Chapter 30. Network Programming
The Net's a cross between an elephant and a white elephant sale: it
never forgets, and it's always crap.
--Nemo
A Linux system has quite a number of tools for accessing,
manipulating, and troubleshooting network connections. We can
incorporate some of these tools into scripts -- scripts that expand
our knowledge of networking, useful scripts that can facilitate the
administration of a network.
Here is a simple CGI script that demonstrates connecting to a remote
server.
Example 30-1. Print the server environment
#!/bin/bash
# test-cgi.sh
# by Michael Zick
# Used with permission
# May have to change the location for your site.
# (At the ISP's servers, Bash may not be in the usual place.)
# Other places: /usr/bin or /usr/local/bin
# Might even try it without any path in sha-bang.
# Disable filename globbing.
set -f
# Header tells browser what to expect.
echo Content-type: text/plain
echo
echo CGI/1.0 test script report:
echo
echo environment settings:
set
echo
echo whereis bash?
whereis bash
echo
echo who are we?
echo ${BASH_VERSINFO[*]}
echo
echo argc is $#. argv is "$*".
echo
# CGI/1.0 expected environment variables.
echo SERVER_SOFTWARE = $SERVER_SOFTWARE
echo SERVER_NAME = $SERVER_NAME
echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE
echo SERVER_PROTOCOL = $SERVER_PROTOCOL
echo SERVER_PORT = $SERVER_PORT
echo REQUEST_METHOD = $REQUEST_METHOD
echo HTTP_ACCEPT = "$HTTP_ACCEPT"
echo PATH_INFO = "$PATH_INFO"
echo PATH_TRANSLATED = "$PATH_TRANSLATED"
echo SCRIPT_NAME = "$SCRIPT_NAME"
echo QUERY_STRING = "$QUERY_STRING"
echo REMOTE_HOST = $REMOTE_HOST
echo REMOTE_ADDR = $REMOTE_ADDR
echo REMOTE_USER = $REMOTE_USER
echo AUTH_TYPE = $AUTH_TYPE
echo CONTENT_TYPE = $CONTENT_TYPE
echo CONTENT_LENGTH = $CONTENT_LENGTH
exit 0
# Here document to give short instructions.
:<<-'_test_CGI_'
1) Drop this in your http://domain.name/cgi-bin directory.
2) Then, open http://domain.name/cgi-bin/test-cgi.sh.
_test_CGI_
For security purposes, it may be helpful to identify the IP addresses
a computer is accessing.
Example 30-2. IP addresses
#!/bin/bash
# ip-addresses.sh
# List the IP addresses your computer is connected to.
# Inspired by Greg Bledsoe's ddos.sh script,
# Linux Journal, 09 March 2011.
# URL:
# http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos
# Greg licensed his script under the GPL2,
#+ and as a derivative, this script is likewise GPL2.
connection_type=TCP # Also try UDP.
field=2 # Which field of the output we're interested in.
no_match=LISTEN # Filter out records containing this. Why?
lsof_args=-ni # -i lists Internet-associated files.
# -n preserves numerical IP addresses.
# What happens without the -n option? Try it.
router="[0-9][0-9][0-9][0-9][0-9]->"
# Delete the router info.
lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |
awk '{print $9}' | cut -d : -f $field | sort | uniq |
sed s/"^$router"//
# Bledsoe's script assigns the output of a filtered IP list,
# (similar to lines 19-22, above) to a variable.
# He checks for multiple connections to a single IP address,
# then uses:
#
# iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset
#
# ... within a 60-second delay loop to bounce packets from DDOS attacks.
# Exercise:
# --------
# Use the 'iptables' command to extend this script
#+ to reject connection attempts from well-known spammer IP domains.
More examples of network programming:
1. Getting the time from nist.gov
2. Downloading a URL
3. A GRE tunnel
4. Checking if an Internet server is up
5. Example 16-41
6. Example A-28
7. Example A-29
8. Example 29-1
See also the networking commands in the System and Administrative
Commands chapter and the communications commands in the External
Filters, Programs and Commands chapter.
________________________________________________________________
Chapter 31. Of Zeros and Nulls
Faultily faultless, icily regular, splendidly null
Dead perfection; no more.
--Alfred Lord Tennyson
/dev/zero ... /dev/null
Uses of /dev/null
Think of /dev/null as a black hole. It is essentially the
equivalent of a write-only file. Everything written to it
disappears. Attempts to read or output from it result in
nothing. All the same, /dev/null can be quite useful from both
the command-line and in scripts.
Suppressing stdout.
cat $filename >/dev/null
# Contents of the file will not list to stdout.
Suppressing stderr (from Example 16-3).
rm $badname 2>/dev/null
# So error messages [stderr] deep-sixed.
Suppressing output from both stdout and stderr.
cat $filename 2>/dev/null >/dev/null
# If "$filename" does not exist, there will be no error message output.
# If "$filename" does exist, the contents of the file will not list to stdout.
# Therefore, no output at all will result from the above line of code.
#
# This can be useful in situations where the return code from a command
#+ needs to be tested, but no output is desired.
#
# cat $filename &>/dev/null
# also works, as Baris Cicek points out.
Deleting contents of a file, but preserving the file itself,
with all attendant permissions (from Example 2-1 and Example
2-3):
cat /dev/null > /var/log/messages
# : > /var/log/messages has same effect, but does not spawn a new process.
cat /dev/null > /var/log/wtmp
Automatically emptying the contents of a logfile (especially
good for dealing with those nasty "cookies" sent by commercial
Web sites):
Example 31-1. Hiding the cookie jar
# Obsolete Netscape browser.
# Same principle applies to newer browsers.
if [ -f ~/.netscape/cookies ] # Remove, if exists.
then
rm -f ~/.netscape/cookies
fi
ln -s /dev/null ~/.netscape/cookies
# All cookies now get sent to a black hole, rather than saved to disk.
Uses of /dev/zero
Like /dev/null, /dev/zero is a pseudo-device file, but it
actually produces a stream of nulls (binary zeros, not the
ASCII kind). Output written to /dev/zero disappears, and it is
fairly difficult to actually read the nulls emitted there,
though it can be done with od or a hex editor. The chief use
of /dev/zero is creating an initialized dummy file of
predetermined length intended as a temporary swap file.
Example 31-2. Setting up a swapfile using /dev/zero
#!/bin/bash
# Creating a swap file.
# A swap file provides a temporary storage cache
#+ which helps speed up certain filesystem operations.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=85 # Not root?
FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0
# This script must be run as root.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo; echo "You must be root to run this script."; echo
exit $E_WRONG_USER
fi
blocks=${1:-$MINBLOCKS} # Set to default of 40 blocks,
#+ if nothing specified on command-line.
# This is the equivalent of the command block below.
# --------------------------------------------------
# if [ -n "$1" ]
# then
# blocks=$1
# else
# blocks=$MINBLOCKS
# fi
# --------------------------------------------------
if [ "$blocks" -lt $MINBLOCKS ]
then
blocks=$MINBLOCKS # Must be at least 40 blocks long.
fi
######################################################################
echo "Creating swap file of size $blocks blocks (KB)."
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.
mkswap $FILE $blocks # Designate it a swap file.
swapon $FILE # Activate swap file.
retcode=$? # Everything worked?
# Note that if one or more of these commands fails,
#+ then it could cause nasty problems.
######################################################################
# Exercise:
# Rewrite the above block of code so that if it does not execute
#+ successfully, then:
# 1) an error message is echoed to stderr,
# 2) all temporary files are cleaned up, and
# 3) the script exits in an orderly fashion with an
#+ appropriate error code.
echo "Swap file created and activated."
exit $retcode
Another application of /dev/zero is to "zero out" a file of a
designated size for a special purpose, such as mounting a
filesystem on a loopback device (see Example 17-8) or
"securely" deleting a file (see Example 16-61).
Example 31-3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ which acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown,
#+ less RAM available to system.
#
# Of what use is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk,
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root.
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk # Create with mkdir /mnt/ramdisk.
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id -nu`
if [ "$username" != "$ROOTUSER_NAME" ]
then
echo "Must be root to run \"`basename $0`\"."
exit $E_NON_ROOT_USER
fi
if [ ! -d "$MOUNTPT" ] # Test whether mount point already there,
then #+ so no error if this script is run
mkdir $MOUNTPT #+ multiple times.
fi
##############################################################################
dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device.
# Why is this necessary?
mke2fs $DEVICE # Create an ext2 filesystem on it.
mount $DEVICE $MOUNTPT # Mount it.
chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk.
# However, must be root to unmount it.
##############################################################################
# Need to test whether above commands succeed. Could cause problems otherwise.
# Exercise: modify this script to make it safer.
echo "\"$MOUNTPT\" now available for use."
# The ramdisk is now accessible for storing files, even by an ordinary user.
# Caution, the ramdisk is volatile, and its contents will disappear
#+ on reboot or power loss.
# Copy anything you want saved to a regular directory.
# After reboot, run this script to again set up ramdisk.
# Remounting /mnt/ramdisk without the other steps will not work.
# Suitably modified, this script can by invoked in /etc/rc.d/rc.local,
#+ to set up ramdisk automatically at bootup.
# That may be appropriate on, for example, a database server.
exit 0
In addition to all the above, /dev/zero is needed by ELF
(Executable and Linking Format) UNIX/Linux binaries.
________________________________________________________________
Chapter 32. Debugging
Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are, by
definition, not smart enough to debug it.
--Brian Kernighan
The Bash shell contains no built-in debugger, and only bare-bones
debugging-specific commands and constructs. Syntax errors or outright
typos in the script generate cryptic error messages that are often of
no help in debugging a non-functional script.
Example 32-1. A buggy script
#!/bin/bash
# ex74.sh
# This is a buggy script.
# Where, oh where is the error?
a=37
if [$a -gt 27 ]
then
echo $a
fi
exit $? # 0! Why?
Output from script:
./ex74.sh: [37: command not found
What's wrong with the above script? Hint: after the if.
Example 32-2. Missing keyword
#!/bin/bash
# missing-keyword.sh
# What error message will this script generate? And why?
for a in 1 2 3
do
echo "$a"
# done # Required keyword 'done' commented out in line 8.
exit 0 # Will not exit here!
# === #
# From command line, after script terminates:
echo $? # 2
Output from script:
missing-keyword.sh: line 10: syntax error: unexpected end of file
Note that the error message does not necessarily reference the line
in which the error occurs, but the line where the Bash interpreter
finally becomes aware of the error.
Error messages may disregard comment lines in a script when reporting
the line number of a syntax error.
What if the script executes, but does not work as expected? This is
the all too familiar logic error.
Example 32-3. test24: another buggy script
#!/bin/bash
# This script is supposed to delete all filenames in current directory
#+ containing embedded spaces.
# It doesn't work.
# Why not?
badname=`ls | grep ' '`
# Try this:
# echo "$badname"
rm "$badname"
exit 0
Try to find out what's wrong with Example 32-3 by uncommenting the
echo "$badname" line. Echo statements are useful for seeing whether
what you expect is actually what you get.
In this particular case, rm "$badname" will not give the desired
results because $badname should not be quoted. Placing it in quotes
ensures that rm has only one argument (it will match only one
filename). A partial fix is to remove to quotes from $badname and to
reset $IFS to contain only a newline, IFS=$'\n'. However, there are
simpler ways of going about it.
# Correct methods of deleting filenames containing spaces.
rm *\ *
rm *" "*
rm *' '*
# Thank you. S.C.
Summarizing the symptoms of a buggy script,
1. It bombs with a "syntax error" message, or
2. It runs, but does not work as expected (logic error).
3. It runs, works as expected, but has nasty side effects (logic
bomb).
Tools for debugging non-working scripts include
1. Inserting echo statements at critical points in the script to
trace the variables, and otherwise give a snapshot of what is
going on.
Tip
Even better is an echo that echoes only when debug is on.
### debecho (debug-echo), by Stefano Falsetto ###
### Will echo passed parameters only if DEBUG is set to a value. ###
debecho () {
if [ ! -z "$DEBUG" ]; then
echo "$1" >&2
# ^^^ to stderr
fi
}
DEBUG=on
Whatever=whatnot
debecho $Whatever # whatnot
DEBUG=
Whatever=notwhat
debecho $Whatever # (Will not echo.)
2. Using the tee filter to check processes or data flows at critical
points.
3. Setting option flags -n -v -x
sh -n scriptname checks for syntax errors without actually
running the script. This is the equivalent of inserting set -n or
set -o noexec into the script. Note that certain types of syntax
errors can slip past this check.
sh -v scriptname echoes each command before executing it. This is
the equivalent of inserting set -v or set -o verbose in the
script.
The -n and -v flags work well together. sh -nv scriptname gives a
verbose syntax check.
sh -x scriptname echoes the result each command, but in an
abbreviated manner. This is the equivalent of inserting set -x or
set -o xtrace in the script.
Inserting set -u or set -o nounset in the script runs it, but
gives an unbound variable error message and aborts the script.
set -u # Or set -o nounset
# Setting a variable to null will not trigger the error/abort.
# unset_var=
echo $unset_var # Unset (and undeclared) variable.
echo "Should not echo!"
# sh t2.sh
# t2.sh: line 6: unset_var: unbound variable
4. Using an "assert" function to test a variable or condition at
critical points in a script. (This is an idea borrowed from C.)
Example 32-4. Testing a condition with an assert
#!/bin/bash
# assert.sh
#######################################################################
assert () # If condition false,
{ #+ exit from script
#+ with appropriate error message.
E_PARAM_ERR=98
E_ASSERT_FAILED=99
if [ -z "$2" ] # Not enough parameters passed
then #+ to assert() function.
return $E_PARAM_ERR # No damage done.
fi
lineno=$2
if [ ! $1 ]
then
echo "Assertion failed: \"$1\""
echo "File \"$0\", line $lineno" # Give name of file and line number.
exit $E_ASSERT_FAILED
# else
# return
# and continue executing the script.
fi
} # Insert a similar assert() function into a script you need to debug.
#######################################################################
a=5
b=4
condition="$a -lt $b" # Error message and exit from script.
# Try setting "condition" to something else
#+ and see what happens.
assert "$condition" $LINENO
# The remainder of the script executes only if the "assert" does not fail.
# Some commands.
# Some more commands . . .
echo "This statement echoes only if the \"assert\" does not fail."
# . . .
# More commands . . .
exit $?
5. Using the $LINENO variable and the caller builtin.
6. Trapping at exit.
The exit command in a script triggers a signal 0, terminating the
process, that is, the script itself. [122] It is often useful to
trap the exit, forcing a "printout" of variables, for example.
The trap must be the first command in the script.
Trapping signals
trap
Specifies an action on receipt of a signal; also useful for
debugging.
A signal is a message sent to a process, either by the kernel or
another process, telling it to take some specified action (usually to
terminate). For example, hitting a Control-C sends a user interrupt,
an INT signal, to a running program.
A simple instance:
trap '' 2
# Ignore interrupt 2 (Control-C), with no action specified.
trap 'echo "Control-C disabled."' 2
# Message when Control-C pressed.
Example 32-5. Trapping at exit
#!/bin/bash
# Hunting variables with a trap.
trap 'echo Variable Listing --- a = $a b = $b' EXIT
# EXIT is the name of the signal generated upon exit from a script.
#
# The command specified by the "trap" doesn't execute until
#+ the appropriate signal is sent.
echo "This prints before the \"trap\" --"
echo "even though the script sees the \"trap\" first."
echo
a=39
b=36
exit 0
# Note that commenting out the 'exit' command makes no difference,
#+ since the script exits in any case after running out of commands.
Example 32-6. Cleaning up after Control-C
#!/bin/bash
# logon.sh: A quick 'n dirty script to check whether you are on-line yet.
umask 177 # Make sure temp files are not world readable.
TRUE=1
LOGFILE=/var/log/messages
# Note that $LOGFILE must be readable
#+ (as root, chmod 644 /var/log/messages).
TEMPFILE=temp.$$
# Create a "unique" temp file name, using process id of the script.
# Using 'mktemp' is an alternative.
# For example:
# TEMPFILE=`mktemp temp.XXXXXX`
KEYWORD=address
# At logon, the line "remote IP address xxx.xxx.xxx.xxx"
# appended to /var/log/messages.
ONLINE=22
USER_INTERRUPT=13
CHECK_LINES=100
# How many lines in log file to check.
trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT
# Cleans up the temp file if script interrupted by control-c.
echo
while [ $TRUE ] #Endless loop.
do
tail -n $CHECK_LINES $LOGFILE> $TEMPFILE
# Saves last 100 lines of system log file as temp file.
# Necessary, since newer kernels generate many log messages at log on.
search=`grep $KEYWORD $TEMPFILE`
# Checks for presence of the "IP address" phrase,
#+ indicating a successful logon.
if [ ! -z "$search" ] # Quotes necessary because of possible spaces.
then
echo "On-line"
rm -f $TEMPFILE # Clean up temp file.
exit $ONLINE
else
echo -n "." # The -n option to echo suppresses newline,
#+ so you get continuous rows of dots.
fi
sleep 1
done
# Note: if you change the KEYWORD variable to "Exit",
#+ this script can be used while on-line
#+ to check for an unexpected logoff.
# Exercise: Change the script, per the above note,
# and prettify it.
exit 0
# Nick Drage suggests an alternate method:
while true
do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0
echo -n "." # Prints dots (.....) until connected.
sleep 2
done
# Problem: Hitting Control-C to terminate this process may be insufficient.
#+ (Dots may keep on echoing.)
# Exercise: Fix this.
# Stephane Chazelas has yet another alternative:
CHECK_INTERVAL=1
while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD"
do echo -n .
sleep $CHECK_INTERVAL
done
echo "On-line"
# Exercise: Discuss the relative strengths and weaknesses
# of each of these various approaches.
Example 32-7. A Simple Implementation of a Progress Bar
#! /bin/bash
# progress-bar2.sh
# Author: Graham Ewart (with reformatting by ABS Guide author).
# Used in ABS Guide with permission (thanks!).
# Invoke this script with bash. It doesn't work with sh.
interval=1
long_interval=10
{
trap "exit" SIGUSR1
sleep $interval; sleep $interval
while true
do
echo -n '.' # Use dots.
sleep $interval
done; } & # Start a progress bar as a background process.
pid=$!
trap "echo !; kill -USR1 $pid; wait $pid" EXIT # To handle ^C.
echo -n 'Long-running process '
sleep $long_interval
echo ' Finished!'
kill -USR1 $pid
wait $pid # Stop the progress bar.
trap EXIT
exit $?
Note
The DEBUG argument to trap causes a specified action to execute after
every command in a script. This permits tracing variables, for
example.
Example 32-8. Tracing a variable
#!/bin/bash
trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
# Echoes the value of $variable after every command.
variable=29; line=$LINENO
echo " Just initialized \$variable to $variable in line number $line."
let "variable *= 3"; line=$LINENO
echo " Just multiplied \$variable by 3 in line number $line."
exit 0
# The "trap 'command1 . . . command2 . . .' DEBUG" construct is
#+ more appropriate in the context of a complex script,
#+ where inserting multiple "echo $variable" statements might be
#+ awkward and time-consuming.
# Thanks, Stephane Chazelas for the pointer.
Output of script:
VARIABLE-TRACE> $variable = ""
VARIABLE-TRACE> $variable = "29"
Just initialized $variable to 29.
VARIABLE-TRACE> $variable = "29"
VARIABLE-TRACE> $variable = "87"
Just multiplied $variable by 3.
VARIABLE-TRACE> $variable = "87"
Of course, the trap command has other uses aside from debugging, such
as disabling certain keystrokes within a script (see Example A-43).
Example 32-9. Running multiple processes (on an SMP box)
#!/bin/bash
# parent.sh
# Running multiple processes on an SMP box.
# Author: Tedman Eng
# This is the first of two scripts,
#+ both of which must be present in the current working directory.
LIMIT=$1 # Total number of process to start
NUMPROC=4 # Number of concurrent threads (forks?)
PROCID=1 # Starting Process ID
echo "My PID is $$"
function start_thread() {
if [ $PROCID -le $LIMIT ] ; then
./child.sh $PROCID&
let "PROCID++"
else
echo "Limit reached."
wait
exit
fi
}
while [ "$NUMPROC" -gt 0 ]; do
start_thread;
let "NUMPROC--"
done
while true
do
trap "start_thread" SIGRTMIN
done
exit 0
# ======== Second script follows ========
#!/bin/bash
# child.sh
# Running multiple processes on an SMP box.
# This script is called by parent.sh.
# Author: Tedman Eng
temp=$RANDOM
index=$1
shift
let "temp %= 5"
let "temp += 4"
echo "Starting $index Time:$temp" "$@"
sleep ${temp}
echo "Ending $index"
kill -s SIGRTMIN $PPID
exit 0
# ======================= SCRIPT AUTHOR'S NOTES ======================= #
# It's not completely bug free.
# I ran it with limit = 500 and after the first few hundred iterations,
#+ one of the concurrent threads disappeared!
# Not sure if this is collisions from trap signals or something else.
# Once the trap is received, there's a brief moment while executing the
#+ trap handler but before the next trap is set. During this time, it may
#+ be possible to miss a trap signal, thus miss spawning a child process.
# No doubt someone may spot the bug and will be writing
#+ . . . in the future.
# ===================================================================== #
# ----------------------------------------------------------------------#
#################################################################
# The following is the original script written by Vernia Damiano.
# Unfortunately, it doesn't work properly.
#################################################################
#!/bin/bash
# Must call script with at least one integer parameter
#+ (number of concurrent processes).
# All other parameters are passed through to the processes started.
INDICE=8 # Total number of process to start
TEMPO=5 # Maximum sleep time per process
E_BADARGS=65 # No arg(s) passed to script.
if [ $# -eq 0 ] # Check for at least one argument passed to script.
then
echo "Usage: `basename $0` number_of_processes [passed params]"
exit $E_BADARGS
fi
NUMPROC=$1 # Number of concurrent process
shift
PARAMETRI=( "$@" ) # Parameters of each process
function avvia() {
local temp
local index
temp=$RANDOM
index=$1
shift
let "temp %= $TEMPO"
let "temp += 1"
echo "Starting $index Time:$temp" "$@"
sleep ${temp}
echo "Ending $index"
kill -s SIGRTMIN $$
}
function parti() {
if [ $INDICE -gt 0 ] ; then
avvia $INDICE "${PARAMETRI[@]}" &
let "INDICE--"
else
trap : SIGRTMIN
fi
}
trap parti SIGRTMIN
while [ "$NUMPROC" -gt 0 ]; do
parti;
let "NUMPROC--"
done
wait
trap - SIGRTMIN
exit $?
: <|)
-D (none) List double-quoted strings prefixed by $, but do not
execute commands in script
-a allexport Export all defined variables
-b notify Notify when jobs running in background terminate (not of
much use in a script)
-c ... (none) Read commands from ...
checkjobs Informs user of any open jobs upon shell exit. Introduced
in version 4 of Bash, and still "experimental." Usage: shopt -s
checkjobs (Caution: may hang!)
-e errexit Abort script at first error, when a command exits with
non-zero status (except in until or while loops, if-tests, list
constructs)
-f noglob Filename expansion (globbing) disabled
globstar globbing star-match Enables the ** globbing operator
(version 4+ of Bash). Usage: shopt -s globstar
-i interactive Script runs in interactive mode
-n noexec Read commands in script, but do not execute them (syntax
check)
-o Option-Name (none) Invoke the Option-Name option
-o posix POSIX Change the behavior of Bash, or invoked script, to
conform to POSIX standard.
-o pipefail pipe failure Causes a pipeline to return the exit status
of the last command in the pipe that returned a non-zero return
value.
-p privileged Script runs as "suid" (caution!)
-r restricted Script runs in restricted mode (see Chapter 22).
-s stdin Read commands from stdin
-t (none) Exit after first command
-u nounset Attempt to use undefined variable outputs error message,
and forces an exit
-v verbose Print each command to stdout before executing it
-x xtrace Similar to -v, but expands commands
- (none) End of options flag. All other arguments are positional
parameters.
-- (none) Unset positional parameters. If arguments given (-- arg1
arg2), positional parameters set to arguments.
________________________________________________________________
Chapter 34. Gotchas
Turandot: Gli enigmi sono tre, la morte una!
Caleph: No, no! Gli enigmi sono tre, una la vita!
--Puccini
Here are some (non-recommended!) scripting practices that will bring
excitement into an otherwise dull life.
* Assigning reserved words or characters to variable names.
case=value0 # Causes problems.
23skidoo=value1 # Also problems.
# Variable names starting with a digit are reserved by the shell.
# Try _23skidoo=value1. Starting variables with an underscore is okay.
# However . . . using just an underscore will not work.
_=25
echo $_ # $_ is a special variable set to last arg of last command.
# But . . . _ is a valid function name!
xyz((!*=value2 # Causes severe problems.
# As of version 3 of Bash, periods are not allowed within variable names.
* Using a hyphen or other reserved characters in a variable name
(or function name).
var-1=23
# Use 'var_1' instead.
function-whatever () # Error
# Use 'function_whatever ()' instead.
# As of version 3 of Bash, periods are not allowed within function names.
function.whatever () # Error
# Use 'functionWhatever ()' instead.
* Using the same name for a variable and a function. This can make
a script difficult to understand.
do_something ()
{
echo "This function does something with \"$1\"."
}
do_something=do_something
do_something do_something
# All this is legal, but highly confusing.
* Using whitespace inappropriately. In contrast to other
programming languages, Bash can be quite finicky about
whitespace.
var1 = 23 # 'var1=23' is correct.
# On line above, Bash attempts to execute command "var1"
# with the arguments "=" and "23".
let c = $a - $b # Instead: let c=$a-$b or let "c = $a - $b"
if [ $a -le 5] # if [ $a -le 5 ] is correct.
# ^^ if [ "$a" -le 5 ] is even better.
# [[ $a -le 5 ]] also works.
* Not terminating with a semicolon the final command in a code
block within curly brackets.
{ ls -l; df; echo "Done." }
# bash: syntax error: unexpected end of file
{ ls -l; df; echo "Done."; }
# ^ ### Final command needs semicolon.
* Assuming uninitialized variables (variables before a value is
assigned to them) are "zeroed out". An uninitialized variable has
a value of null, not zero.
#!/bin/bash
echo "uninitialized_var = $uninitialized_var"
# uninitialized_var =
# However . . .
# if $BASH_VERSION >= 4.2; then
if [[ ! -v uninitialized_var ]]
then
uninitialized_var=0 # Initialize it to zero!
fi
* Mixing up = and -eq in a test. Remember, = is for comparing
literal variables and -eq for integers.
if [ "$a" = 273 ] # Is $a an integer or string?
if [ "$a" -eq 273 ] # If $a is an integer.
# Sometimes you can interchange -eq and = without adverse consequences.
# However . . .
a=273.0 # Not an integer.
if [ "$a" = 273 ]
then
echo "Comparison works."
else
echo "Comparison does not work."
fi # Comparison does not work.
# Same with a=" 273" and a="0273".
# Likewise, problems trying to use "-eq" with non-integer values.
if [ "$a" -eq 273.0 ]
then
echo "a = $a"
fi # Aborts with an error message.
# test.sh: [: 273.0: integer expression expected
* Misusing string comparison operators.
Example 34-1. Numerical and string comparison are not equivalent
#!/bin/bash
# bad-op.sh: Trying to use a string comparison on integers.
echo
number=1
# The following while-loop has two errors:
#+ one blatant, and the other subtle.
while [ "$number" < 5 ] # Wrong! Should be: while [ "$number" -lt 5 ]
do
echo -n "$number "
let "number += 1"
done
# Attempt to run this bombs with the error message:
#+ bad-op.sh: line 10: 5: No such file or directory
# Within single brackets, "<" must be escaped,
#+ and even then, it's still wrong for comparing integers.
echo "---------------------"
while [ "$number" \< 5 ] # 1 2 3 4
do #
echo -n "$number " # It *seems* to work, but . . .
let "number += 1" #+ it actually does an ASCII comparison,
done #+ rather than a numerical one.
echo; echo "---------------------"
# This can cause problems. For example:
lesser=5
greater=105
if [ "$greater" \< "$lesser" ]
then
echo "$greater is less than $lesser"
fi # 105 is less than 5
# In fact, "105" actually is less than "5"
#+ in a string comparison (ASCII sort order).
echo
exit 0
* Attempting to use let to set string variables.
let "a = hello, you"
echo "$a" # 0
* Sometimes variables within "test" brackets ([ ]) need to be
quoted (double quotes). Failure to do so may cause unexpected
behavior. See Example 7-6, Example 20-5, and Example 9-6.
* Quoting a variable containing whitespace prevents splitting.
Sometimes this produces unintended consequences.
* Commands issued from a script may fail to execute because the
script owner lacks execute permission for them. If a user cannot
invoke a command from the command-line, then putting it into a
script will likewise fail. Try changing the attributes of the
command in question, perhaps even setting the suid bit (as root,
of course).
* Attempting to use - as a redirection operator (which it is not)
will usually result in an unpleasant surprise.
command1 2> - | command2
# Trying to redirect error output of command1 into a pipe . . .
# . . . will not work.
command1 2>& - | command2 # Also futile.
Thanks, S.C.
* Using Bash version 2+ functionality may cause a bailout with
error messages. Older Linux machines may have version 1.XX of
Bash as the default installation.
#!/bin/bash
minimum_version=2
# Since Chet Ramey is constantly adding features to Bash,
# you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate.
E_BAD_VERSION=80
if [ "$BASH_VERSION" \< "$minimum_version" ]
then
echo "This script works only with Bash, version $minimum or greater."
echo "Upgrade strongly recommended."
exit $E_BAD_VERSION
fi
...
* Using Bash-specific functionality in a Bourne shell script
(#!/bin/sh) on a non-Linux machine may cause unexpected behavior.
A Linux system usually aliases sh to bash, but this does not
necessarily hold true for a generic UNIX machine.
* Using undocumented features in Bash turns out to be a dangerous
practice. In previous releases of this book there were several
scripts that depended on the "feature" that, although the maximum
value of an exit or return value was 255, that limit did not
apply to negative integers. Unfortunately, in version 2.05b and
later, that loophole disappeared. See Example 24-9.
* In certain contexts, a misleading exit status may be returned.
This may occur when setting a local variable within a function or
when assigning an arithmetic value to a variable.
* The exit status of an arithmetic expression is not equivalent to
an error code.
var=1 && ((--var)) && echo $var
# ^^^^^^^^^ Here the and-list terminates with exit status 1.
# $var doesn't echo!
echo $? # 1
* A script with DOS-type newlines (\r\n) will fail to execute,
since #!/bin/bash\r\n is not recognized, not the same as the
expected #!/bin/bash\n. The fix is to convert the script to
UNIX-style newlines.
#!/bin/bash
echo "Here"
unix2dos $0 # Script changes itself to DOS format.
chmod 755 $0 # Change back to execute permission.
# The 'unix2dos' command removes execute permission.
./$0 # Script tries to run itself again.
# But it won't work as a DOS file.
echo "There"
exit 0
* A shell script headed by #!/bin/sh will not run in full
Bash-compatibility mode. Some Bash-specific functions might be
disabled. Scripts that need complete access to all the
Bash-specific extensions should start with #!/bin/bash.
* Putting whitespace in front of the terminating limit string of a
here document will cause unexpected behavior in a script.
* Putting more than one echo statement in a function whose output
is captured.
add2 ()
{
echo "Whatever ... " # Delete this line!
let "retval = $1 + $2"
echo $retval
}
num1=12
num2=43
echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"
# Sum of 12 and 43 = Whatever ...
# 55
# The "echoes" concatenate.
This will not work.
* A script may not export variables back to its parent process, the
shell, or to the environment. Just as we learned in biology, a
child process can inherit from a parent, but not vice versa.
WHATEVER=/home/bozo
export WHATEVER
exit 0
bash$ echo $WHATEVER
bash$
Sure enough, back at the command prompt, $WHATEVER remains unset.
* Setting and manipulating variables in a subshell, then attempting
to use those same variables outside the scope of the subshell
will result an unpleasant surprise.
Example 34-2. Subshell Pitfalls
#!/bin/bash
# Pitfalls of variables in a subshell.
outer_variable=outer
echo
echo "outer_variable = $outer_variable"
echo
(
# Begin subshell
echo "outer_variable inside subshell = $outer_variable"
inner_variable=inner # Set
echo "inner_variable inside subshell = $inner_variable"
outer_variable=inner # Will value change globally?
echo "outer_variable inside subshell = $outer_variable"
# Will 'exporting' make a difference?
# export inner_variable
# export outer_variable
# Try it and see.
# End subshell
)
echo
echo "inner_variable outside subshell = $inner_variable" # Unset.
echo "outer_variable outside subshell = $outer_variable" # Unchanged.
echo
exit 0
# What happens if you uncomment lines 19 and 20?
# Does it make a difference?
* Piping echo output to a read may produce unexpected results. In
this scenario, the read acts as if it were running in a subshell.
Instead, use the set command (as in Example 15-18).
Example 34-3. Piping the output of echo to a read
#!/bin/bash
# badread.sh:
# Attempting to use 'echo and 'read'
#+ to assign variables non-interactively.
# shopt -s lastpipe
a=aaa
b=bbb
c=ccc
echo "one two three" | read a b c
# Try to reassign a, b, and c.
echo
echo "a = $a" # a = aaa
echo "b = $b" # b = bbb
echo "c = $c" # c = ccc
# Reassignment failed.
### However . . .
## Uncommenting line 6:
# shopt -s lastpipe
##+ fixes the problem!
### This is a new feature in Bash, version 4.2.
# ------------------------------
# Try the following alternative.
var=`echo "one two three"`
set -- $var
a=$1; b=$2; c=$3
echo "-------"
echo "a = $a" # a = one
echo "b = $b" # b = two
echo "c = $c" # c = three
# Reassignment succeeded.
# ------------------------------
# Note also that an echo to a 'read' works within a subshell.
# However, the value of the variable changes *only* within the subshell.
a=aaa # Starting all over again.
b=bbb
c=ccc
echo; echo
echo "one two three" | ( read a b c;
echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" )
# a = one
# b = two
# c = three
echo "-----------------"
echo "Outside subshell: "
echo "a = $a" # a = aaa
echo "b = $b" # b = bbb
echo "c = $c" # c = ccc
echo
exit 0
In fact, as Anthony Richardson points out, piping to any loop can
cause a similar problem.
# Loop piping troubles.
# This example by Anthony Richardson,
#+ with addendum by Wilbert Berendsen.
foundone=false
find $HOME -type f -atime +30 -size 100k |
while true
do
read f
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."
foundone=true
# ------------------------------------
echo "Subshell level = $BASH_SUBSHELL"
# Subshell level = 1
# Yes, we're inside a subshell.
# ------------------------------------
done
# foundone will always be false here since it is
#+ set to true inside a subshell
if [ $foundone = false ]
then
echo "No files need archiving."
fi
# =====================Now, here is the correct way:=================
foundone=false
for f in $(find $HOME -type f -atime +30 -size 100k) # No pipe here.
do
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."
foundone=true
done
if [ $foundone = false ]
then
echo "No files need archiving."
fi
# ==================And here is another alternative==================
# Places the part of the script that reads the variables
#+ within a code block, so they share the same subshell.
# Thank you, W.B.
find $HOME -type f -atime +30 -size 100k | {
foundone=false
while read f
do
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."
foundone=true
done
if ! $foundone
then
echo "No files need archiving."
fi
}
A lookalike problem occurs when trying to write the stdout of a
tail -f piped to grep.
tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log
# The "error.log" file will not have anything written to it.
# As Samuli Kaipiainen points out, this results from grep
#+ buffering its output.
# The fix is to add the "--line-buffered" parameter to grep.
* Using "suid" commands within scripts is risky, as it may
compromise system security. [123]
* Using shell scripts for CGI programming may be problematic. Shell
script variables are not "typesafe," and this can cause
undesirable behavior as far as CGI is concerned. Moreover, it is
difficult to "cracker-proof" shell scripts.
* Bash does not handle the double slash (//) string correctly.
* Bash scripts written for Linux or BSD systems may need fixups to
run on a commercial UNIX machine. Such scripts often employ the
GNU set of commands and filters, which have greater functionality
than their generic UNIX counterparts. This is particularly true
of such text processing utilites as tr.
* Sadly, updates to Bash itself have broken older scripts that used
to work perfectly fine. Let us recall how risky it is to use
undocumented Bash features.
Danger is near thee --
Beware, beware, beware, beware.
Many brave hearts are asleep in the deep.
So beware --
Beware.
--A.J. Lamb and H.W. Petrie
________________________________________________________________
Chapter 35. Scripting With Style
Get into the habit of writing shell scripts in a structured and
systematic manner. Even on-the-fly and "written on the back of an
envelope" scripts will benefit if you take a few minutes to plan and
organize your thoughts before sitting down and coding.
Herewith are a few stylistic guidelines. This is not (necessarily)
intended as an Official Shell Scripting Stylesheet.
________________________________________________________________
35.1. Unofficial Shell Scripting Stylesheet
* Comment your code. This makes it easier for others to understand
(and appreciate), and easier for you to maintain.
PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
# It made perfect sense when you wrote it last year,
#+ but now it's a complete mystery.
# (From Antek Sawicki's "pw.sh" script.)
Add descriptive headers to your scripts and functions.
#!/bin/bash
#************************************************#
# xyz.sh #
# written by Bozo Bozeman #
# July 05, 2001 #
# #
# Clean up project files. #
#************************************************#
E_BADDIR=85 # No such directory.
projectdir=/home/bozo/projects # Directory to clean up.
# --------------------------------------------------------- #
# cleanup_pfiles () #
# Removes all files in designated directory. #
# Parameter: $target_directory #
# Returns: 0 on success, $E_BADDIR if something went wrong. #
# --------------------------------------------------------- #
cleanup_pfiles ()
{
if [ ! -d "$1" ] # Test if target directory exists.
then
echo "$1 is not a directory."
return $E_BADDIR
fi
rm -f "$1"/*
return 0 # Success.
}
cleanup_pfiles $projectdir
exit $?
* Avoid using "magic numbers," [124] that is, "hard-wired" literal
constants. Use meaningful variable names instead. This makes the
script easier to understand and permits making changes and
updates without breaking the application.
if [ -f /var/log/messages ]
then
...
fi
# A year later, you decide to change the script to check /var/log/syslog.
# It is now necessary to manually change the script, instance by instance,
#+ and hope nothing breaks.
# A better way:
LOGFILE=/var/log/messages # Only line that needs to be changed.
if [ -f "$LOGFILE" ]
then
...
fi
* Choose descriptive names for variables and functions.
fl=`ls -al $dirname` # Cryptic.
file_listing=`ls -al $dirname` # Better.
MAXVAL=10 # All caps used for a script constant.
while [ "$index" -le "$MAXVAL" ]
...
E_NOTFOUND=95 # Uppercase for an errorcode,
#+ and name prefixed with E_.
if [ ! -e "$filename" ]
then
echo "File $filename not found."
exit $E_NOTFOUND
fi
MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental
export MAIL_DIRECTORY #+ variable.
GetAnswer () # Mixed case works well for a
{ #+ function name, especially
prompt=$1 #+ when it improves legibility.
echo -n $prompt
read answer
return $answer
}
GetAnswer "What is your favorite number? "
favorite_number=$?
echo $favorite_number
_uservariable=23 # Permissible, but not recommended.
# It's better for user-defined variables not to start with an underscore.
# Leave that for system variables.
* Use exit codes in a systematic and meaningful way.
E_WRONG_ARGS=95
...
...
exit $E_WRONG_ARGS
See also Appendix E.
Ender suggests using the exit codes in /usr/include/sysexits.h in
shell scripts, though these are primarily intended for C and C++
programming.
* Use standardized parameter flags for script invocation. Ender
proposes the following set of flags.
-a All: Return all information (including hidden file info).
-b Brief: Short version, usually for other scripts.
-c Copy, concatenate, etc.
-d Daily: Use information from the whole day, and not merely
information for a specific instance/user.
-e Extended/Elaborate: (often does not include hidden file info).
-h Help: Verbose usage w/descs, aux info, discussion, help.
See also -V.
-l Log output of script.
-m Manual: Launch man-page for base command.
-n Numbers: Numerical data only.
-r Recursive: All files in a directory (and/or all sub-dirs).
-s Setup & File Maintenance: Config files for this script.
-u Usage: List of invocation flags for the script.
-v Verbose: Human readable output, more or less formatted.
-V Version / License / Copy(right|left) / Contribs (email too).
See also Section G.1.
* Break complex scripts into simpler modules. Use functions where
appropriate. See Example 37-4.
* Don't use a complex construct where a simpler one will do.
COMMAND
if [ $? -eq 0 ]
...
# Redundant and non-intuitive.
if COMMAND
...
# More concise (if perhaps not quite as legible).
... reading the UNIX source code to the Bourne shell (/bin/sh). I was
shocked at how much simple algorithms could be made cryptic, and
therefore useless, by a poor choice of code style. I asked myself,
"Could someone be proud of this code?"
--Landon Noll
________________________________________________________________
Chapter 36. Miscellany
Nobody really knows what the Bourne shell's grammar is. Even
examination of the source code is little help.
--Tom Duff
________________________________________________________________
36.1. Interactive and non-interactive shells and scripts
An interactive shell reads commands from user input on a tty. Among
other things, such a shell reads startup files on activation,
displays a prompt, and enables job control by default. The user can
interact with the shell.
A shell running a script is always a non-interactive shell. All the
same, the script can still access its tty. It is even possible to
emulate an interactive shell in a script.
#!/bin/bash
MY_PROMPT='$ '
while :
do
echo -n "$MY_PROMPT"
read line
eval "$line"
done
exit 0
# This example script, and much of the above explanation supplied by
# Stéphane Chazelas (thanks again).
Let us consider an interactive script to be one that requires input
from the user, usually with read statements (see Example 15-3). "Real
life" is actually a bit messier than that. For now, assume an
interactive script is bound to a tty, a script that a user has
invoked from the console or an xterm.
Init and startup scripts are necessarily non-interactive, since they
must run without human intervention. Many administrative and system
maintenance scripts are likewise non-interactive. Unvarying
repetitive tasks cry out for automation by non-interactive scripts.
Non-interactive scripts can run in the background, but interactive
ones hang, waiting for input that never comes. Handle that difficulty
by having an expect script or embedded here document feed input to an
interactive script running as a background job. In the simplest case,
redirect a file to supply input to a read statement (read variable
> $LOGFILE
# Now, do it.
exec $OPERATION "$@"
# It's necessary to do the logging before the operation.
# Why?
Example 36-4. A shell wrapper around an awk script
#!/bin/bash
# pr-ascii.sh: Prints a table of ASCII characters.
START=33 # Range of printable ASCII characters (decimal).
END=127 # Will not work for unprintable characters (> 127).
echo " Decimal Hex Character" # Header.
echo " ------- --- ---------"
for ((i=START; i<=END; i++))
do
echo $i | awk '{printf(" %3d %2x %c\n", $1, $1, $1)}'
# The Bash printf builtin will not work in this context:
# printf "%c" "$i"
done
exit 0
# Decimal Hex Character
# ------- --- ---------
# 33 21 !
# 34 22 "
# 35 23 #
# 36 24 $
#
# . . .
#
# 122 7a z
# 123 7b {
# 124 7c |
# 125 7d }
# Redirect the output of this script to a file
#+ or pipe it to "more": sh pr-asc.sh | more
Example 36-5. A shell wrapper around another awk script
#!/bin/bash
# Adds up a specified column (of numbers) in the target file.
# Floating-point (decimal) numbers okay, because awk can handle them.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
# Passing shell variables to the awk part of the script is a bit tricky.
# One method is to strong-quote the Bash-script variable
#+ within the awk script.
# $'$BASH_SCRIPT_VAR'
# ^ ^
# This is done in the embedded awk script below.
# See the awk documentation for more details.
# A multi-line awk script is here invoked by
# awk '
# ...
# ...
# ...
# '
# Begin awk script.
# -----------------------------
awk '
{ total += $'"${column_number}"'
}
END {
print total
}
' "$filename"
# -----------------------------
# End awk script.
# It may not be safe to pass shell variables to an embedded awk script,
#+ so Stephane Chazelas proposes the following alternative:
# ---------------------------------------
# awk -v column_number="$column_number" '
# { total += $column_number
# }
# END {
# print total
# }' "$filename"
# ---------------------------------------
exit 0
For those scripts needing a single do-it-all tool, a Swiss army
knife, there is Perl. Perl combines the capabilities of sed and awk,
and throws in a large subset of C, to boot. It is modular and
contains support for everything ranging from object-oriented
programming up to and including the kitchen sink. Short Perl scripts
lend themselves to embedding within shell scripts, and there may be
some substance to the claim that Perl can totally replace shell
scripting (though the author of the ABS Guide remains skeptical).
Example 36-6. Perl embedded in a Bash script
#!/bin/bash
# Shell commands may precede the Perl script.
echo "This precedes the embedded Perl script within \"$0\"."
echo "==============================================================="
perl -e 'print "This line prints from an embedded Perl script.\n";'
# Like sed, Perl also uses the "-e" option.
echo "==============================================================="
echo "However, the script may also contain shell and system commands."
exit 0
It is even possible to combine a Bash script and Perl script within
the same file. Depending on how the script is invoked, either the
Bash part or the Perl part will execute.
Example 36-7. Bash and Perl scripts combined
#!/bin/bash
# bashandperl.sh
echo "Greetings from the Bash part of the script, $0."
# More Bash commands may follow here.
exit
# End of Bash part of the script.
# =======================================================
#!/usr/bin/perl
# This part of the script must be invoked with
# perl -x bashandperl.sh
print "Greetings from the Perl part of the script, $0.\n";
# Perl doesn't seem to like "echo" ...
# More Perl commands may follow here.
# End of Perl part of the script.
bash$ bash bashandperl.sh
Greetings from the Bash part of the script.
bash$ perl -x bashandperl.sh
Greetings from the Perl part of the script.
It is, of course, possible to embed even more exotic scripting
languages within shell wrappers. Python, for example ...
Example 36-8. Python embedded in a Bash script
#!/bin/bash
# ex56py.sh
# Shell commands may precede the Python script.
echo "This precedes the embedded Python script within \"$0.\""
echo "==============================================================="
python -c 'print "This line prints from an embedded Python script.\n";'
# Unlike sed and perl, Python uses the "-c" option.
python -c 'k = raw_input( "Hit a key to exit to outer script. " )'
echo "==============================================================="
echo "However, the script may also contain shell and system commands."
exit 0
Wrapping a script around mplayer and the Google's translation server,
you can create something that talks back to you.
Example 36-9. A script that speaks
#!/bin/bash
# Courtesy of:
# http://elinux.org/RPi_Text_to_Speech_(Speech_Synthesis)
# You must be on-line for this script to work,
#+ so you can access the Google translation server.
# Of course, mplayer must be present on your computer.
speak()
{
local IFS=+
# Invoke mplayer, then connect to Google translation server.
/usr/bin/mplayer -ao alsa -really-quiet -noconsolecontrols \
"http://translate.google.com/translate_tts?tl=en&q="$*""
# Google translates, but can also speak.
}
LINES=4
spk=$(tail -$LINES $0) # Tail end of same script!
speak "$spk"
exit
# Browns. Nice talking to you.
One interesting example of a complex shell wrapper is Martin
Matusiak's undvd script, which provides an easy-to-use command-line
interface to the complex
[http://www.mplayerhq.hu/DOCS/HTML/en/mencoder.html] mencoder
utility. Another example is Itzchak Rehberg's
[http://projects.izzysoft.de/trac/ext3undel] Ext3Undel, a set of
scripts to recover deleted file on an ext3 filesystem.
________________________________________________________________
36.3. Tests and Comparisons: Alternatives
For tests, the [[ ]] construct may be more appropriate than [ ].
Likewise, arithmetic comparisons might benefit from the (( ))
construct.
a=8
# All of the comparisons below are equivalent.
test "$a" -lt 16 && echo "yes, $a < 16" # "and list"
/bin/test "$a" -lt 16 && echo "yes, $a < 16"
[ "$a" -lt 16 ] && echo "yes, $a < 16"
[[ $a -lt 16 ]] && echo "yes, $a < 16" # Quoting variables within
(( a < 16 )) && echo "yes, $a < 16" # [[ ]] and (( )) not necessar
y.
city="New York"
# Again, all of the comparisons below are equivalent.
test "$city" \< Paris && echo "Yes, Paris is greater than $city"
# Greater ASCII order.
/bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city"
[ "$city" \< Paris ] && echo "Yes, Paris is greater than $city"
[[ $city < Paris ]] && echo "Yes, Paris is greater than $city"
# Need not quote $city.
# Thank you, S.C.
________________________________________________________________
36.4. Recursion: a script calling itself
Can a script recursively call itself? Indeed.
Example 36-10. A (useless) script that recursively calls itself
#!/bin/bash
# recurse.sh
# Can a script recursively call itself?
# Yes, but is this of any practical use?
# (See the following.)
RANGE=10
MAXVAL=9
i=$RANDOM
let "i %= $RANGE" # Generate a random number between 0 and $RANGE - 1.
if [ "$i" -lt "$MAXVAL" ]
then
echo "i = $i"
./$0 # Script recursively spawns a new instance of itself.
fi # Each child script does the same, until
#+ a generated $i equals $MAXVAL.
# Using a "while" loop instead of an "if/then" test causes problems.
# Explain why.
exit 0
# Note:
# ----
# This script must have execute permission for it to work properly.
# This is the case even if it is invoked by an "sh" command.
# Explain why.
Example 36-11. A (useful) script that recursively calls itself
#!/bin/bash
# pb.sh: phone book
# Written by Rick Boivie, and used with permission.
# Modifications by ABS Guide author.
MINARGS=1 # Script needs at least one argument.
DATAFILE=./phonebook
# A data file in current working directory
#+ named "phonebook" must exist.
PROGNAME=$0
E_NOARGS=70 # No arguments error.
if [ $# -lt $MINARGS ]; then
echo "Usage: "$PROGNAME" data-to-look-up"
exit $E_NOARGS
fi
if [ $# -eq $MINARGS ]; then
grep $1 "$DATAFILE"
# 'grep' prints an error message if $DATAFILE not present.
else
( shift; "$PROGNAME" $* ) | grep $1
# Script recursively calls itself.
fi
exit 0 # Script exits here.
# Therefore, it's o.k. to put
#+ non-hashmarked comments and data after this point.
# ------------------------------------------------------------------------
Sample "phonebook" datafile:
John Doe 1555 Main St., Baltimore, MD 21228 (410) 222-3333
Mary Moe 9899 Jones Blvd., Warren, NH 03787 (603) 898-3232
Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
Zoe Zenobia 4481 N. Baker St., San Francisco, SF 94338 (415) 501-1631
# ------------------------------------------------------------------------
$bash pb.sh Roe
Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
$bash pb.sh Roe Sam
Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
# When more than one argument is passed to this script,
#+ it prints *only* the line(s) containing all the arguments.
Example 36-12. Another (useful) script that recursively calls itself
#!/bin/bash
# usrmnt.sh, written by Anthony Richardson
# Used in ABS Guide with permission.
# usage: usrmnt.sh
# description: mount device, invoking user must be listed in the
# MNTUSERS group in the /etc/sudoers file.
# ----------------------------------------------------------
# This is a usermount script that reruns itself using sudo.
# A user with the proper permissions only has to type
# usermount /dev/fd0 /mnt/floppy
# instead of
# sudo usermount /dev/fd0 /mnt/floppy
# I use this same technique for all of my
#+ sudo scripts, because I find it convenient.
# ----------------------------------------------------------
# If SUDO_COMMAND variable is not set we are not being run through
#+ sudo, so rerun ourselves. Pass the user's real and group id . . .
if [ -z "$SUDO_COMMAND" ]
then
mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*
exit 0
fi
# We will only get here if we are being run by sudo.
/bin/mount $* -o uid=$mntusr,gid=$grpusr
exit 0
# Additional notes (from the author of this script):
# -------------------------------------------------
# 1) Linux allows the "users" option in the /etc/fstab
# file so that any user can mount removable media.
# But, on a server, I like to allow only a few
# individuals access to removable media.
# I find using sudo gives me more control.
# 2) I also find sudo to be more convenient than
# accomplishing this task through groups.
# 3) This method gives anyone with proper permissions
# root access to the mount command, so be careful
# about who you allow access.
# You can get finer control over which access can be mounted
# by using this same technique in separate mntfloppy, mntcdrom,
# and mntsamba scripts.
Caution
Too many levels of recursion can exhaust the script's stack space,
causing a segfault.
________________________________________________________________
36.5. "Colorizing" Scripts
The ANSI [126] escape sequences set screen attributes, such as bold
text, and color of foreground and background. DOS batch files
commonly used ANSI escape codes for color output, and so can Bash
scripts.
Example 36-13. A "colorized" address database
#!/bin/bash
# ex30a.sh: "Colorized" version of ex30.sh.
# Crude address database
clear # Clear the screen.
echo -n " "
echo -e '\E[37;44m'"\033[1mContact List\033[0m"
# White on blue background
echo; echo
echo -e "\033[1mChoose one of the following persons:\033[0m"
# Bold
tput sgr0 # Reset attributes.
echo "(Enter only the first letter of name.)"
echo
echo -en '\E[47;34m'"\033[1mE\033[0m" # Blue
tput sgr0 # Reset colors to "normal."
echo "vans, Roland" # "[E]vans, Roland"
echo -en '\E[47;35m'"\033[1mJ\033[0m" # Magenta
tput sgr0
echo "ambalaya, Mildred"
echo -en '\E[47;32m'"\033[1mS\033[0m" # Green
tput sgr0
echo "mith, Julie"
echo -en '\E[47;31m'"\033[1mZ\033[0m" # Red
tput sgr0
echo "ane, Morris"
echo
read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Flash Dr."
echo "Hardscrabble, CO 80753"
echo "(303) 734-9874"
echo "(303) 734-9892 fax"
echo "revans@zzy.net"
echo "Business partner & old friend"
;;
"J" | "j" )
echo
echo "Mildred Jambalaya"
echo "249 E. 7th St., Apt. 19"
echo "New York, NY 10009"
echo "(212) 533-2814"
echo "(212) 533-9972 fax"
echo "milliej@loisaida.com"
echo "Girlfriend"
echo "Birthday: Feb. 11"
;;
# Add info for Smith & Zane later.
* )
# Default option.
# Empty input (hitting RETURN) fits here, too.
echo
echo "Not yet in database."
;;
esac
tput sgr0 # Reset colors to "normal."
echo
exit 0
Example 36-14. Drawing a box
#!/bin/bash
# Draw-box.sh: Drawing a box using ASCII characters.
# Script by Stefano Palmeri, with minor editing by document author.
# Minor edits suggested by Jim Angstadt.
# Used in the ABS Guide with permission.
######################################################################
### draw_box function doc ###
# The "draw_box" function lets the user
#+ draw a box in a terminal.
#
# Usage: draw_box ROW COLUMN HEIGHT WIDTH [COLOR]
# ROW and COLUMN represent the position
#+ of the upper left angle of the box you're going to draw.
# ROW and COLUMN must be greater than 0
#+ and less than current terminal dimension.
# HEIGHT is the number of rows of the box, and must be > 0.
# HEIGHT + ROW must be <= than current terminal height.
# WIDTH is the number of columns of the box and must be > 0.
# WIDTH + COLUMN must be <= than current terminal width.
#
# E.g.: If your terminal dimension is 20x80,
# draw_box 2 3 10 45 is good
# draw_box 2 3 19 45 has bad HEIGHT value (19+2 > 20)
# draw_box 2 3 18 78 has bad WIDTH value (78+3 > 80)
#
# COLOR is the color of the box frame.
# This is the 5th argument and is optional.
# 0=black 1=red 2=green 3=tan 4=blue 5=purple 6=cyan 7=white.
# If you pass the function bad arguments,
#+ it will just exit with code 65,
#+ and no messages will be printed on stderr.
#
# Clear the terminal before you start to draw a box.
# The clear command is not contained within the function.
# This allows the user to draw multiple boxes, even overlapping ones.
### end of draw_box function doc ###
######################################################################
draw_box(){
#=============#
HORZ="-"
VERT="|"
CORNER_CHAR="+"
MINARGS=4
E_BADARGS=65
#=============#
if [ $# -lt "$MINARGS" ]; then # If args are less than 4, exit.
exit $E_BADARGS
fi
# Looking for non digit chars in arguments.
# Probably it could be done better (exercise for the reader?).
if echo $@ | tr -d [:blank:] | tr -d [:digit:] | grep . &> /dev/null; then
exit $E_BADARGS
fi
BOX_HEIGHT=`expr $3 - 1` # -1 correction needed because angle char "+"
BOX_WIDTH=`expr $4 - 1` #+ is a part of both box height and width.
T_ROWS=`tput lines` # Define current terminal dimension
T_COLS=`tput cols` #+ in rows and columns.
if [ $1 -lt 1 ] || [ $1 -gt $T_ROWS ]; then # Start checking if arguments
exit $E_BADARGS #+ are correct.
fi
if [ $2 -lt 1 ] || [ $2 -gt $T_COLS ]; then
exit $E_BADARGS
fi
if [ `expr $1 + $BOX_HEIGHT + 1` -gt $T_ROWS ]; then
exit $E_BADARGS
fi
if [ `expr $2 + $BOX_WIDTH + 1` -gt $T_COLS ]; then
exit $E_BADARGS
fi
if [ $3 -lt 1 ] || [ $4 -lt 1 ]; then
exit $E_BADARGS
fi # End checking arguments.
plot_char(){ # Function within a function.
echo -e "\E[${1};${2}H"$3
}
echo -ne "\E[3${5}m" # Set box frame color, if defined.
# start drawing the box
count=1 # Draw vertical lines using
for (( r=$1; count<=$BOX_HEIGHT; r++)); do #+ plot_char function.
plot_char $r $2 $VERT
let count=count+1
done
count=1
c=`expr $2 + $BOX_WIDTH`
for (( r=$1; count<=$BOX_HEIGHT; r++)); do
plot_char $r $c $VERT
let count=count+1
done
count=1 # Draw horizontal lines using
for (( c=$2; count<=$BOX_WIDTH; c++)); do #+ plot_char function.
plot_char $1 $c $HORZ
let count=count+1
done
count=1
r=`expr $1 + $BOX_HEIGHT`
for (( c=$2; count<=$BOX_WIDTH; c++)); do
plot_char $r $c $HORZ
let count=count+1
done
plot_char $1 $2 $CORNER_CHAR # Draw box angles.
plot_char $1 `expr $2 + $BOX_WIDTH` $CORNER_CHAR
plot_char `expr $1 + $BOX_HEIGHT` $2 $CORNER_CHAR
plot_char `expr $1 + $BOX_HEIGHT` `expr $2 + $BOX_WIDTH` $CORNER_CHAR
echo -ne "\E[0m" # Restore old colors.
P_ROWS=`expr $T_ROWS - 1` # Put the prompt at bottom of the terminal.
echo -e "\E[${P_ROWS};1H"
}
# Now, let's try drawing a box.
clear # Clear the terminal.
R=2 # Row
C=3 # Column
H=10 # Height
W=45 # Width
col=1 # Color (red)
draw_box $R $C $H $W $col # Draw the box.
exit 0
# Exercise:
# --------
# Add the option of printing text within the drawn box.
The simplest, and perhaps most useful ANSI escape sequence is bold
text, \033[1m ... \033[0m. The \033 represents an escape, the "[1"
turns on the bold attribute, while the "[0" switches it off. The "m"
terminates each term of the escape sequence.
bash$ echo -e "\033[1mThis is bold text.\033[0m"
A similar escape sequence switches on the underline attribute (on an
rxvt and an aterm).
bash$ echo -e "\033[4mThis is underlined text.\033[0m"
Note
With an echo, the -e option enables the escape sequences.
Other escape sequences change the text and/or background color.
bash$ echo -e '\E[34;47mThis prints in blue.'; tput sgr0
bash$ echo -e '\E[33;44m'"yellow text on blue background"; tput sgr0
bash$ echo -e '\E[1;33;44m'"BOLD yellow text on blue background"; tput sgr0
Note
It's usually advisable to set the bold attribute for light-colored
foreground text.
The tput sgr0 restores the terminal settings to normal. Omitting this
lets all subsequent output from that particular terminal remain blue.
Note
Since tput sgr0 fails to restore terminal settings under certain
circumstances, echo -ne \E[0m may be a better choice.
Use the following template for writing colored text on a colored
background.
echo -e '\E[COLOR1;COLOR2mSome text goes here.'
The "\E[" begins the escape sequence. The semicolon-separated numbers
"COLOR1" and "COLOR2" specify a foreground and a background color,
according to the table below. (The order of the numbers does not
matter, since the foreground and background numbers fall in
non-overlapping ranges.) The "m" terminates the escape sequence, and
the text begins immediately after that.
Note also that single quotes enclose the remainder of the command
sequence following the echo -e.
The numbers in the following table work for an rxvt terminal. Results
may vary for other terminal emulators.
Table 36-1. Numbers representing colors in Escape Sequences
Color Foreground Background
black 30 40
red 31 41
green 32 42
yellow 33 43
blue 34 44
magenta 35 45
cyan 36 46
white 37 47
Example 36-15. Echoing colored text
#!/bin/bash
# color-echo.sh: Echoing text messages in color.
# Modify this script for your own purposes.
# It's easier than hand-coding color.
black='\E[30;47m'
red='\E[31;47m'
green='\E[32;47m'
yellow='\E[33;47m'
blue='\E[34;47m'
magenta='\E[35;47m'
cyan='\E[36;47m'
white='\E[37;47m'
alias Reset="tput sgr0" # Reset text attributes to normal
#+ without clearing screen.
cecho () # Color-echo.
# Argument $1 = message
# Argument $2 = color
{
local default_msg="No message passed."
# Doesn't really need to be a local variable.
message=${1:-$default_msg} # Defaults to default message.
color=${2:-$black} # Defaults to black, if not specified.
echo -e "$color"
echo "$message"
Reset # Reset to normal.
return
}
# Now, let's try it out.
# ----------------------------------------------------
cecho "Feeling blue..." $blue
cecho "Magenta looks more like purple." $magenta
cecho "Green with envy." $green
cecho "Seeing red?" $red
cecho "Cyan, more familiarly known as aqua." $cyan
cecho "No color passed (defaults to black)."
# Missing $color argument.
cecho "\"Empty\" color passed (defaults to black)." ""
# Empty $color argument.
cecho
# Missing $message and $color arguments.
cecho "" ""
# Empty $message and $color arguments.
# ----------------------------------------------------
echo
exit 0
# Exercises:
# ---------
# 1) Add the "bold" attribute to the 'cecho ()' function.
# 2) Add options for colored backgrounds.
Example 36-16. A "horserace" game
#!/bin/bash
# horserace.sh: Very simple horserace simulation.
# Author: Stefano Palmeri
# Used with permission.
################################################################
# Goals of the script:
# playing with escape sequences and terminal colors.
#
# Exercise:
# Edit the script to make it run less randomly,
#+ set up a fake betting shop . . .
# Um . . . um . . . it's starting to remind me of a movie . . .
#
# The script gives each horse a random handicap.
# The odds are calculated upon horse handicap
#+ and are expressed in European(?) style.
# E.g., odds=3.75 means that if you bet $1 and win,
#+ you receive $3.75.
#
# The script has been tested with a GNU/Linux OS,
#+ using xterm and rxvt, and konsole.
# On a machine with an AMD 900 MHz processor,
#+ the average race time is 75 seconds.
# On faster computers the race time would be lower.
# So, if you want more suspense, reset the USLEEP_ARG variable.
#
# Script by Stefano Palmeri.
################################################################
E_RUNERR=65
# Check if md5sum and bc are installed.
if ! which bc &> /dev/null; then
echo bc is not installed.
echo "Can\'t run . . . "
exit $E_RUNERR
fi
if ! which md5sum &> /dev/null; then
echo md5sum is not installed.
echo "Can\'t run . . . "
exit $E_RUNERR
fi
# Set the following variable to slow down script execution.
# It will be passed as the argument for usleep (man usleep)
#+ and is expressed in microseconds (500000 = half a second).
USLEEP_ARG=0
# Clean up the temp directory, restore terminal cursor and
#+ terminal colors -- if script interrupted by Ctl-C.
trap 'echo -en "\E[?25h"; echo -en "\E[0m"; stty echo;\
tput cup 20 0; rm -fr $HORSE_RACE_TMP_DIR' TERM EXIT
# See the chapter on debugging for an explanation of 'trap.'
# Set a unique (paranoid) name for the temp directory the script needs.
HORSE_RACE_TMP_DIR=$HOME/.horserace-`date +%s`-`head -c10 /dev/urandom \
| md5sum | head -c30`
# Create the temp directory and move right in.
mkdir $HORSE_RACE_TMP_DIR
cd $HORSE_RACE_TMP_DIR
# This function moves the cursor to line $1 column $2 and then prints $3.
# E.g.: "move_and_echo 5 10 linux" is equivalent to
#+ "tput cup 4 9; echo linux", but with one command instead of two.
# Note: "tput cup" defines 0 0 the upper left angle of the terminal,
#+ echo defines 1 1 the upper left angle of the terminal.
move_and_echo() {
echo -ne "\E[${1};${2}H""$3"
}
# Function to generate a pseudo-random number between 1 and 9.
random_1_9 ()
{
head -c10 /dev/urandom | md5sum | tr -d [a-z] | tr -d 0 | cut -c1
}
# Two functions that simulate "movement," when drawing the horses.
draw_horse_one() {
echo -n " "//$MOVE_HORSE//
}
draw_horse_two(){
echo -n " "\\\\$MOVE_HORSE\\\\
}
# Define current terminal dimension.
N_COLS=`tput cols`
N_LINES=`tput lines`
# Need at least a 20-LINES X 80-COLUMNS terminal. Check it.
if [ $N_COLS -lt 80 ] || [ $N_LINES -lt 20 ]; then
echo "`basename $0` needs a 80-cols X 20-lines terminal."
echo "Your terminal is ${N_COLS}-cols X ${N_LINES}-lines."
exit $E_RUNERR
fi
# Start drawing the race field.
# Need a string of 80 chars. See below.
BLANK80=`seq -s "" 100 | head -c80`
clear
# Set foreground and background colors to white.
echo -ne '\E[37;47m'
# Move the cursor on the upper left angle of the terminal.
tput cup 0 0
# Draw six white lines.
for n in `seq 5`; do
echo $BLANK80 # Use the 80 chars string to colorize the terminal.
done
# Sets foreground color to black.
echo -ne '\E[30m'
move_and_echo 3 1 "START 1"
move_and_echo 3 75 FINISH
move_and_echo 1 5 "|"
move_and_echo 1 80 "|"
move_and_echo 2 5 "|"
move_and_echo 2 80 "|"
move_and_echo 4 5 "| 2"
move_and_echo 4 80 "|"
move_and_echo 5 5 "V 3"
move_and_echo 5 80 "V"
# Set foreground color to red.
echo -ne '\E[31m'
# Some ASCII art.
move_and_echo 1 8 "..@@@..@@@@@...@@@@@.@...@..@@@@..."
move_and_echo 2 8 ".@...@...@.......@...@...@.@......."
move_and_echo 3 8 ".@@@@@...@.......@...@@@@@.@@@@...."
move_and_echo 4 8 ".@...@...@.......@...@...@.@......."
move_and_echo 5 8 ".@...@...@.......@...@...@..@@@@..."
move_and_echo 1 43 "@@@@...@@@...@@@@..@@@@..@@@@."
move_and_echo 2 43 "@...@.@...@.@.....@.....@....."
move_and_echo 3 43 "@@@@..@@@@@.@.....@@@@...@@@.."
move_and_echo 4 43 "@..@..@...@.@.....@.........@."
move_and_echo 5 43 "@...@.@...@..@@@@..@@@@.@@@@.."
# Set foreground and background colors to green.
echo -ne '\E[32;42m'
# Draw eleven green lines.
tput cup 5 0
for n in `seq 11`; do
echo $BLANK80
done
# Set foreground color to black.
echo -ne '\E[30m'
tput cup 5 0
# Draw the fences.
echo "++++++++++++++++++++++++++++++++++++++\
++++++++++++++++++++++++++++++++++++++++++"
tput cup 15 0
echo "++++++++++++++++++++++++++++++++++++++\
++++++++++++++++++++++++++++++++++++++++++"
# Set foreground and background colors to white.
echo -ne '\E[37;47m'
# Draw three white lines.
for n in `seq 3`; do
echo $BLANK80
done
# Set foreground color to black.
echo -ne '\E[30m'
# Create 9 files to stores handicaps.
for n in `seq 10 7 68`; do
touch $n
done
# Set the first type of "horse" the script will draw.
HORSE_TYPE=2
# Create position-file and odds-file for every "horse".
#+ In these files, store the current position of the horse,
#+ the type and the odds.
for HN in `seq 9`; do
touch horse_${HN}_position
touch odds_${HN}
echo \-1 > horse_${HN}_position
echo $HORSE_TYPE >> horse_${HN}_position
# Define a random handicap for horse.
HANDICAP=`random_1_9`
# Check if the random_1_9 function returned a good value.
while ! echo $HANDICAP | grep [1-9] &> /dev/null; do
HANDICAP=`random_1_9`
done
# Define last handicap position for horse.
LHP=`expr $HANDICAP \* 7 + 3`
for FILE in `seq 10 7 $LHP`; do
echo $HN >> $FILE
done
# Calculate odds.
case $HANDICAP in
1) ODDS=`echo $HANDICAP \* 0.25 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
2 | 3) ODDS=`echo $HANDICAP \* 0.40 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
4 | 5 | 6) ODDS=`echo $HANDICAP \* 0.55 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
7 | 8) ODDS=`echo $HANDICAP \* 0.75 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
9) ODDS=`echo $HANDICAP \* 0.90 + 1.25 | bc`
echo $ODDS > odds_${HN}
esac
done
# Print odds.
print_odds() {
tput cup 6 0
echo -ne '\E[30;42m'
for HN in `seq 9`; do
echo "#$HN odds->" `cat odds_${HN}`
done
}
# Draw the horses at starting line.
draw_horses() {
tput cup 6 0
echo -ne '\E[30;42m'
for HN in `seq 9`; do
echo /\\$HN/\\" "
done
}
print_odds
echo -ne '\E[47m'
# Wait for a enter key press to start the race.
# The escape sequence '\E[?25l' disables the cursor.
tput cup 17 0
echo -e '\E[?25l'Press [enter] key to start the race...
read -s
# Disable normal echoing in the terminal.
# This avoids key presses that might "contaminate" the screen
#+ during the race.
stty -echo
# --------------------------------------------------------
# Start the race.
draw_horses
echo -ne '\E[37;47m'
move_and_echo 18 1 $BLANK80
echo -ne '\E[30m'
move_and_echo 18 1 Starting...
sleep 1
# Set the column of the finish line.
WINNING_POS=74
# Define the time the race started.
START_TIME=`date +%s`
# COL variable needed by following "while" construct.
COL=0
while [ $COL -lt $WINNING_POS ]; do
MOVE_HORSE=0
# Check if the random_1_9 function has returned a good value.
while ! echo $MOVE_HORSE | grep [1-9] &> /dev/null; do
MOVE_HORSE=`random_1_9`
done
# Define old type and position of the "randomized horse".
HORSE_TYPE=`cat horse_${MOVE_HORSE}_position | tail -n 1`
COL=$(expr `cat horse_${MOVE_HORSE}_position | head -n 1`)
ADD_POS=1
# Check if the current position is an handicap position.
if seq 10 7 68 | grep -w $COL &> /dev/null; then
if grep -w $MOVE_HORSE $COL &> /dev/null; then
ADD_POS=0
grep -v -w $MOVE_HORSE $COL > ${COL}_new
rm -f $COL
mv -f ${COL}_new $COL
else ADD_POS=1
fi
else ADD_POS=1
fi
COL=`expr $COL + $ADD_POS`
echo $COL > horse_${MOVE_HORSE}_position # Store new position.
# Choose the type of horse to draw.
case $HORSE_TYPE in
1) HORSE_TYPE=2; DRAW_HORSE=draw_horse_two
;;
2) HORSE_TYPE=1; DRAW_HORSE=draw_horse_one
esac
echo $HORSE_TYPE >> horse_${MOVE_HORSE}_position
# Store current type.
# Set foreground color to black and background to green.
echo -ne '\E[30;42m'
# Move the cursor to new horse position.
tput cup `expr $MOVE_HORSE + 5` \
`cat horse_${MOVE_HORSE}_position | head -n 1`
# Draw the horse.
$DRAW_HORSE
usleep $USLEEP_ARG
# When all horses have gone beyond field line 15, reprint odds.
touch fieldline15
if [ $COL = 15 ]; then
echo $MOVE_HORSE >> fieldline15
fi
if [ `wc -l fieldline15 | cut -f1 -d " "` = 9 ]; then
print_odds
: > fieldline15
fi
# Define the leading horse.
HIGHEST_POS=`cat *position | sort -n | tail -1`
# Set background color to white.
echo -ne '\E[47m'
tput cup 17 0
echo -n Current leader: `grep -w $HIGHEST_POS *position | cut -c7`\
" "
done
# Define the time the race finished.
FINISH_TIME=`date +%s`
# Set background color to green and enable blinking text.
echo -ne '\E[30;42m'
echo -en '\E[5m'
# Make the winning horse blink.
tput cup `expr $MOVE_HORSE + 5` \
$DRAW_HORSE
Disable blinking text.
echo -en '\E[25m'
Set foreground and background color to white.
echo -ne '\E[37;47m'
move_and_echo 18 1 $BLANK80
Set foreground color to black.
echo -ne '\E[30m'
Make winner blink.
tput cup 17 0
echo -e "\E[5mWINNER: $MOVE_HORSE\E[25m"" Odds: cat odds_${MOVE_HORSE}"\
" Race time: expr $FINISH_TIME - $START_TIME secs"
Restore cursor and old colors.
echo -en "\E[?25h"
echo -en "\E[0m"
Restore echoing.
stty echo
Remove race temp directory.
rm -rf $HORSE_RACE_TMP_DIR
tput cup 19 0
exit 0
See also Example A-21, Example A-44, Example A-52, and Example A-40.
Caution
There is, however, a major problem with all this. ANSI escape
sequences are emphatically non-portable. What works fine on some
terminal emulators (or the console) may work differently, or not at
all, on others. A "colorized" script that looks stunning on the
script author's machine may produce unreadable output on someone
else's. This somewhat compromises the usefulness of colorizing
scripts, and possibly relegates this technique to the status of a
gimmick. Colorized scripts are probably inappropriate in a commercial
setting, i.e., your supervisor might disapprove.
Alister's [http://code.google.com/p/ansi-color/] ansi-color utility
(based on Moshe Jacobson's color utility considerably simplifies
using ANSI escape sequences. It substitutes a clean and logical
syntax for the clumsy constructs just discussed.
Henry/teikedvl has likewise created a utility
([http://scriptechocolor.sourceforge.net/]
http://scriptechocolor.sourceforge.net/) to simplify creation of
colorized scripts.
________________________________________________________________
36.6. Optimizations
Most shell scripts are quick 'n dirty solutions to non-complex
problems. As such, optimizing them for speed is not much of an issue.
Consider the case, though, where a script carries out an important
task, does it well, but runs too slowly. Rewriting it in a compiled
language may not be a palatable option. The simplest fix would be to
rewrite the parts of the script that slow it down. Is it possible to
apply principles of code optimization even to a lowly shell script?
Check the loops in the script. Time consumed by repetitive operations
adds up quickly. If at all possible, remove time-consuming operations
from within loops.
Use builtin commands in preference to system commands. Builtins
execute faster and usually do not launch a subshell when invoked.
Avoid unnecessary commands, particularly in a pipe.
cat "$file" | grep "$word"
grep "$word" "$file"
The above command-lines have an identical effect,
- but the second runs faster since it launches one fewer subprocess.
The cat command seems especially prone to overuse in scripts.
Disabling certain Bash options can speed up scripts.
As Erik Brandsberg points out:
If you don't need Unicode support, you can get potentially a 2x or
more improvement in speed by simply setting the LC_ALL variable.
export LC_ALL=C
[specifies the locale as ANSI C,
thereby disabling Unicode support]
[In an example script ...]
Without [Unicode support]:
erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
live2.pcap > out.txt
real 0m20.483s
user 1m34.470s
sys 0m12.869s
With [Unicode support]:
erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
live2.pcap > out.txt
real 0m50.232s
user 3m51.118s
sys 0m11.221s
A large part of the overhead that is optimized is, I believe,
regex match using [[ string =~ REGEX ]],
but it may help with other portions of the code as well.
I hadn't [seen it] mentioned that this optimization helped
with Bash, but I had seen it helped with "grep,"
so why not try?
Note
Certain operators, notably expr, are very inefficient and might be
replaced by double parentheses arithmetic expansion. See Example
A-59.
Math tests
math via $(( ))
real 0m0.294s
user 0m0.288s
sys 0m0.008s
math via expr:
real 1m17.879s # Much slower!
user 0m3.600s
sys 0m8.765s
math via let:
real 0m0.364s
user 0m0.372s
sys 0m0.000s
Condition testing constructs in scripts deserve close scrutiny.
Substitute case for if-then constructs and combine tests when
possible, to minimize script execution time. Again, refer to Example
A-59.
Test using "case" construct:
real 0m0.329s
user 0m0.320s
sys 0m0.000s
Test with if [], no quotes:
real 0m0.438s
user 0m0.432s
sys 0m0.008s
Test with if [], quotes:
real 0m0.476s
user 0m0.452s
sys 0m0.024s
Test with if [], using -eq:
real 0m0.457s
user 0m0.456s
sys 0m0.000s
Note
Erik Brandsberg recommends using associative arrays in preference to
conventional numeric-indexed arrays in most cases. When overwriting
values in a numeric array, there is a significant performance penalty
vs. associative arrays. Running a test script confirms this. See
Example A-60.
Assignment tests
Assigning a simple variable
real 0m0.418s
user 0m0.416s
sys 0m0.004s
Assigning a numeric index array entry
real 0m0.582s
user 0m0.564s
sys 0m0.016s
Overwriting a numeric index array entry
real 0m21.931s
user 0m21.913s
sys 0m0.016s
Linear reading of numeric index array
real 0m0.422s
user 0m0.416s
sys 0m0.004s
Assigning an associative array entry
real 0m1.800s
user 0m1.796s
sys 0m0.004s
Overwriting an associative array entry
real 0m1.798s
user 0m1.784s
sys 0m0.012s
Linear reading an associative array entry
real 0m0.420s
user 0m0.420s
sys 0m0.000s
Assigning a random number to a simple variable
real 0m0.402s
user 0m0.388s
sys 0m0.016s
Assigning a sparse numeric index array entry randomly into 64k cells
real 0m12.678s
user 0m12.649s
sys 0m0.028s
Reading sparse numeric index array entry
real 0m0.087s
user 0m0.084s
sys 0m0.000s
Assigning a sparse associative array entry randomly into 64k cells
real 0m0.698s
user 0m0.696s
sys 0m0.004s
Reading sparse associative index array entry
real 0m0.083s
user 0m0.084s
sys 0m0.000s
Use the time and times tools to profile computation-intensive
commands. Consider rewriting time-critical code sections in C, or
even in assembler.
Try to minimize file I/O. Bash is not particularly efficient at
handling files, so consider using more appropriate tools for this
within the script, such as awk or Perl.
Write your scripts in a modular and coherent form, [127] so they can
be reorganized and tightened up as necessary. Some of the
optimization techniques applicable to high-level languages may work
for scripts, but others, such as loop unrolling, are mostly
irrelevant. Above all, use common sense.
For an excellent demonstration of how optimization can dramatically
reduce the execution time of a script, see Example 16-47.
________________________________________________________________
36.7. Assorted Tips
36.7.1. Ideas for more powerful scripts
* You have a problem that you want to solve by writing a Bash
script. Unfortunately, you don't know quite where to start. One
method is to plunge right in and code those parts of the script
that come easily, and write the hard parts as pseudo-code.
!/bin/bash
ARGCOUNT=1 # Need name as argument.
E_WRONGARGS=65
if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
Can't figure out how to code this . . .
- . . . so write it in pseudo-code.
then
echo "Usage: name-of-script name"
^^^^^^^^^^^^^^ More pseudo-code.
exit $E_WRONGARGS
fi
. . .
exit 0
Later on, substitute working code for the pseudo-code.
Line 6 becomes:
if [ $# -ne "$ARGCOUNT" ]
Line 12 becomes:
echo "Usage: basename $0 name"
For an example of using pseudo-code, see the Square Root
exercise.
* To keep a record of which user scripts have run during a
particular session or over a number of sessions, add the
following lines to each script you want to keep track of. This
will keep a continuing file record of the script names and
invocation times.
Append (>>) following to end of each script tracked.
whoami>> $SAVE_FILE # User invoking the script.
echo $0>> $SAVE_FILE # Script name.
date>> $SAVE_FILE # Date and time.
echo>> $SAVE_FILE # Blank line as separator.
Of course, SAVE_FILE defined and exported as environmental variable in ~/.b
ashrc
- (something like ~/.scripts-run)
* The >> operator appends lines to a file. What if you wish to
prepend a line to an existing file, that is, to paste it in at
the beginning?
file=data.txt
title="This is the title line of data text file"
echo $title | cat - $file >$file.new
"cat -" concatenates stdout to $file.
End result is
- to write a new file with $title appended at beginning.
This is a simplified variant of the Example 19-13 script given
earlier. And, of course, sed can also do this.
* A shell script may act as an embedded command inside another
shell script, a Tcl or wish script, or even a Makefile. It can be
invoked as an external shell command in a C program using the
system() call, i.e., system("script_name");.
* Setting a variable to the contents of an embedded sed or awk
script increases the readability of the surrounding shell
wrapper. See Example A-1 and Example 15-20.
* Put together files containing your favorite and most useful
definitions and functions. As necessary, "include" one or more of
these "library files" in scripts with either the dot (.) or
source command.
SCRIPT LIBRARY
Note:
No "#!" here.
No "live code" either.
Useful variable definitions
ROOT_UID=0 # Root has $UID 0.
E_NOTROOT=101 # Not root user error.
MAXRETVAL=255 # Maximum (positive) return value of a function.
SUCCESS=0
FAILURE=-1
Functions
Usage () # "Usage:" message.
{
if [ -z "$1" ] # No arg passed.
then
msg=filename
else
msg=$@
fi
echo "Usage: basename $0 "$msg""
}
Check_if_root () # Check if root running script.
{ # From "ex39.sh" example.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
}
CreateTempfileName () # Creates a "unique" temp filename.
{ # From "ex51.sh" example.
prefix=temp
suffix=eval date +%s
Tempfilename=$prefix.$suffix
}
isalpha2 () # Tests whether entire string is alphabetic.
{ # From "isalpha.sh" example.
[ $# -eq 1 ] || return $FAILURE
case $1 in
[!a-zA-Z]|"") return $FAILURE;;
*) return $SUCCESS;;
esac # Thanks, S.C.
}
abs () # Absolute value.
{ # Caution: Max return value = 255.
E_ARGERR=-999999
if [ -z "$1" ] # Need arg passed.
then
return $E_ARGERR # Obvious error value returned.
fi
if [ "$1" -ge 0 ] # If non-negative,
then #
absval=$1 # stays as-is.
else # Otherwise,
let "absval = (( 0 - $1 ))" # change sign.
fi
return $absval
}
tolower () # Converts string(s) passed as argument(s)
{ #+ to lowercase.
if [ -z "$1" ] # If no argument(s) passed,
then #+ send error message
echo "(null)" #+ (C-style void-pointer error message)
return #+ and return from function.
fi
echo "$@" | tr A-Z a-z
Translate all passed arguments ($@).
return
Use command substitution to set a variable to function output.
For example:
oldvar="A seT of miXed-caSe LEtTerS"
newvar=tolower "$oldvar"
echo "$newvar" # a set of mixed-case letters
Exercise: Rewrite this function to change lowercase passed argument(s)
to uppercase ... toupper() [easy].
}
* Use special-purpose comment headers to increase clarity and
legibility in scripts.
Caution.
rm -rf *.zzy ## The "-rf" options to "rm" are very dangerous,
##+ especially with wild cards.
This is line 1
- and this is the final line.
o List item.
Another point of view.
while [ "$var1" != "end" ] #> while test "$var1" != "end"
* Dotan Barak contributes template code for a progress bar in a
script.
Example 36-17. A Progress Bar
!/bin/bash
progress-bar.sh
Author: Dotan Barak (very minor revisions by ABS Guide author).
Used in ABS Guide with permission (thanks!).
BAR_WIDTH=50
BAR_CHAR_START="["
BAR_CHAR_END="]"
BAR_CHAR_EMPTY="."
BAR_CHAR_FULL="="
BRACKET_CHARS=2
LIMIT=100
print_progress_bar()
{
# Calculate how many characters will be full.
let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"
# Calculate how many characters will be empty.
let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"
# Prepare the bar.
bar_line="${BAR_CHAR_START}"
for ((j=0; j<full_limit; j++)); do
bar_line="${bar_line}${BAR_CHAR_FULL}"
done
for ((j=0; j<empty_limit; j++)); do
bar_line="${bar_line}${BAR_CHAR_EMPTY}"
done
bar_line="${bar_line}${BAR_CHAR_END}"
printf "%3d%% %s" $2 ${bar_line}
}
Here is a sample of code that uses it.
MAX_PERCENT=100
for ((i=0; i<=MAX_PERCENT; i++)); do
#
usleep 10000
# ... Or run some other commands ...
#
print_progress_bar ${BAR_WIDTH} ${i}
echo -en "\r"
done
echo ""
exit
* A particularly clever use of if-test constructs is for comment
blocks.
!/bin/bash
COMMENT_BLOCK=
Try setting the above variable to some value
- for an unpleasant surprise.
if [ $COMMENT_BLOCK ]; then
Comment block --
=> ===============================
This is a comment line.
This is another comment line.
This is yet another comment line.
=> ===============================
echo "This will not echo."
Comment blocks are error-free! Whee!
fi
echo "No more comments, please."
exit 0
Compare this with using here documents to comment out code
blocks.
* Using the $? exit status variable, a script may test if a
parameter contains only digits, so it can be treated as an
integer.
!/bin/bash
SUCCESS=0
E_BADINPUT=85
test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null
An integer is either equal to 0 or not equal to 0.
2>/dev/null suppresses error message.
if [ $? -ne "$SUCCESS" ]
then
echo "Usage: basename $0 integer-input"
exit $E_BADINPUT
fi
let "sum = $1 + 25" # Would give error if $1 not integer.
echo "Sum = $sum"
Any variable, not just a command-line parameter, can be tested this way.
exit 0
* The 0 - 255 range for function return values is a severe
limitation. Global variables and other workarounds are often
problematic. An alternative method for a function to communicate
a value back to the main body of the script is to have the
function write to stdout (usually with echo) the "return value,"
and assign this to a variable. This is actually a variant of
command substitution.
Example 36-18. Return value trickery
!/bin/bash
multiplication.sh
multiply () # Multiplies params passed.
{ # Will accept a variable number of args.
local product=1
until [ -z "$1" ] # Until uses up arguments passed...
do
let "product *= $1"
shift
done
echo $product # Will not echo to stdout,
} #+ since this will be assigned to a variable.
mult1=15383; mult2=25211
val1=multiply $mult1 $mult2
Assigns stdout (echo) of function to the variable val1.
echo "$mult1 X $mult2 = $val1" # 387820813
mult1=25; mult2=5; mult3=20
val2=multiply $mult1 $mult2 $mult3
echo "$mult1 X $mult2 X $mult3 = $val2" # 2500
mult1=188; mult2=37; mult3=25; mult4=47
val3=multiply $mult1 $mult2 $mult3 $mult4
echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300
exit 0
The same technique also works for alphanumeric strings. This
means that a function can "return" a non-numeric value.
capitalize_ichar () # Capitalizes initial character
{ #+ of argument string(s) passed.
string0="$@" # Accepts multiple arguments.
firstchar=${string0:0:1} # First character.
string1=${string0:1} # Rest of string(s).
FirstChar=echo "$firstchar" | tr a-z A-Z
# Capitalize first character.
echo "$FirstChar$string1" # Output to stdout.
}
newstring=`capitalize_ichar "every sentence should start with a capital letter
."`
echo "$newstring" # Every sentence should start with a capital letter
.
It is even possible for a function to "return" multiple values
with this method.
Example 36-19. Even more return value trickery
!/bin/bash
sum-product.sh
A function may "return" more than one value.
sum_and_product () # Calculates both sum and product of passed args.
{
echo $(( $1 + $2 )) $(( $1 * $2 ))
Echoes to stdout each calculated value, separated by space.
}
echo
echo "Enter first number "
read first
echo
echo "Enter second number "
read second
echo
retval=sum_and_product $first $second # Assigns output of function.
sum=echo "$retval" | awk '{print $1}' # Assigns first field.
product=echo "$retval" | awk '{print $2}' # Assigns second field.
echo "$first + $second = $sum"
echo "$first * $second = $product"
echo
exit 0
Caution
There can be only one echo statement in the function for this to
work. If you alter the previous example:
sum_and_product ()
{
echo "This is the sum_and_product function." # This messes things up!
echo $(( $1 + $2 )) $(( $1 * $2 ))
}
...
retval=sum_and_product $first $second # Assigns output of function.
Now, this will not work correctly.
* Next in our bag of tricks are techniques for passing an array to
a function, then "returning" an array back to the main body of
the script.
Passing an array involves loading the space-separated elements of
the array into a variable with command substitution. Getting an
array back as the "return value" from a function uses the
previously mentioned strategem of echoing the array in the
function, then invoking command substitution and the ( ... )
operator to assign it to an array.
Example 36-20. Passing and returning arrays
!/bin/bash
array-function.sh: Passing an array to a function and ...
"returning" an array from a function
Pass_Array ()
{
local passed_array # Local variable!
passed_array=( echo "$1" )
echo "${passed_array[@]}"
List all the elements of the new array
#+ declared and set within the function.
}
original_array=( element1 element2 element3 element4 element5 )
echo
echo "original_array = ${original_array[@]}"
List all elements of original array.
This is the trick that permits passing an array to a function.
argument=echo ${original_array[@]}
Pack a variable
- with all the space-separated elements of the original array.
Attempting to just pass the array itself will not work.
This is the trick that allows grabbing an array as a "return value".
returned_array=( Pass_Array "$argument" )
Assign 'echoed' output of function to array variable.
echo "returned_array = ${returned_array[@]}"
echo "============================================================="
Now, try it again,
- attempting to access (list) the array from outside the function.
Pass_Array "$argument"
The function itself lists the array, but ...
- accessing the array from outside the function is forbidden.
echo "Passed array (within function) = ${passed_array[@]}"
NULL VALUE since the array is a variable local to the function.
echo
And here is an even more explicit example:
ret_array ()
{
for element in {11..20}
do
echo "$element " # Echo individual elements
done #+ of what will be assembled into an array.
}
arr=( $(ret_array) ) # Assemble into array.
echo "Capturing array "arr" from function ret_array () ..."
echo "Third element of array "arr" is ${arr[2]}." # 13 (zero-indexed)
echo -n "Entire array is: "
echo ${arr[@]} # 11 12 13 14 15 16 17 18 19 20
echo
exit 0
Nathan Coulter points out that passing arrays with elements containing
- whitespace breaks this example.
For a more elaborate example of passing arrays to functions, see
Example A-10.
* Using the double-parentheses construct, it is possible to use
C-style syntax for setting and incrementing/decrementing
variables and in for and while loops. See Example 11-13 and
Example 11-18.
* Setting the path and umask at the beginning of a script makes it
more portable -- more likely to run on a "foreign" machine whose
user may have bollixed up the $PATH and umask.
!/bin/bash
PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
umask 022 # Files that the script creates will have 755 permission.
Thanks to Ian D. Allen, for this tip.
* A useful scripting technique is to repeatedly feed the output of
a filter (by piping) back to the same filter, but with a
different set of arguments and/or options. Especially suitable
for this are tr and grep.
From "wstrings.sh" example.
wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`
Example 36-21. Fun with anagrams
!/bin/bash
agram.sh: Playing games with anagrams.
Find anagrams of...
LETTERSET=etaoinshrdlu
FILTER='.......' # How many letters minimum?
1234567
anagram "$LETTERSET" | # Find all anagrams of the letterset...
grep "$FILTER" | # With at least 7 letters,
grep '^is' | # starting with 'is'
grep -v 's$' | # no plurals
grep -v 'ed$' # no past tense verbs
Possible to add many combinations of conditions and filters.
Uses "anagram" utility
- that is part of the author's "yawl" word list package.
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
http://bash.deta.in/yawl-0.3.2.tar.gz
exit 0 # End of code.
bash$ sh agram.sh
islander
isolate
isolead
isotheral
Exercises:
Modify this script to take the LETTERSET as a command-line parameter.
Parameterize the filters in lines 11 - 13 (as with $FILTER),
- so that they can be specified by passing arguments to a function.
For a slightly different approach to anagramming,
- see the agram2.sh script.
See also Example 29-4, Example 16-25, and Example A-9.
* Use "anonymous here documents" to comment out blocks of code, to
save having to individually comment out each line with a #. See
Example 19-11.
* Running a script on a machine that relies on a command that might
not be installed is dangerous. Use whatis to avoid potential
problems with this.
CMD=command1 # First choice.
PlanB=command2 # Fallback option.
command_test=$(whatis "$CMD" | grep 'nothing appropriate')
If 'command1' not found on system , 'whatis' will return
- "command1: nothing appropriate."
A safer alternative is:
command_test=$(whereis "$CMD" | grep /)
But then the sense of the following test would have to be reversed,
- since the $command_test variable holds content only if
- the $CMD exists on the system.
(Thanks, bojster.)
if [[ -z "$command_test" ]] # Check whether command present.
then
$CMD option1 option2 # Run command1 with options.
else # Otherwise,
$PlanB #+ run command2.
fi
* An if-grep test may not return expected results in an error case,
when text is output to stderr, rather that stdout.
if ls -l nonexistent_filename | grep -q 'No such file or directory'
then echo "File "nonexistent_filename" does not exist."
fi
Redirecting stderr to stdout fixes this.
if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
^^^^
then echo "File "nonexistent_filename" does not exist."
fi
Thanks, Chris Martin, for pointing this out.
* If you absolutely must access a subshell variable outside the
subshell, here's a way to do it.
TMPFILE=tmpfile # Create a temp file to store the variable.
( # Inside the subshell ...
inner_variable=Inner
echo $inner_variable
echo $inner_variable >>$TMPFILE # Append to temp file.
)
# Outside the subshell ...
echo; echo "-----"; echo
echo $inner_variable # Null, as expected.
echo "-----"; echo
Now ...
read inner_variable <$TMPFILE # Read back shell variable.
rm -f "$TMPFILE" # Get rid of temp file.
echo "$inner_variable" # It's an ugly kludge, but it works.
* The run-parts command is handy for running a set of command
scripts in a particular sequence, especially in combination with
cron or at.
* For doing multiple revisions on a complex script, use the rcs
Revision Control System package.
Among other benefits of this is automatically updated ID header
tags. The co command in rcs does a parameter replacement of
certain reserved key words, for example, replacing # $Id$ in a
script with something like:
$Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $
________________________________________________________________
36.7.2. Widgets
It would be nice to be able to invoke X-Windows widgets from a shell
script. There happen to exist several packages that purport to do so,
namely Xscript, Xmenu, and widtools. The first two of these no longer
seem to be maintained. Fortunately, it is still possible to obtain
widtools
[http://www.batse.msfc.nasa.gov/~mallozzi/home/software/xforms/src/wi
dtools-2.0.tgz] here.
Caution
The widtools (widget tools) package requires the XForms library to be
installed. Additionally, the Makefile needs some judicious editing
before the package will build on a typical Linux system. Finally,
three of the six widgets offered do not work (and, in fact,
segfault).
The dialog family of tools offers a method of calling "dialog"
widgets from a shell script. The original dialog utility works in a
text console, but its successors, gdialog, Xdialog, and kdialog use
X-Windows-based widget sets.
Example 36-22. Widgets invoked from a shell script
!/bin/bash
dialog.sh: Using 'gdialog' widgets.
Must have 'gdialog' installed on your system to run this script.
Or, you can replace all instance of 'gdialog' below with 'kdialog' ...
Version 1.1 (corrected 04/05/05)
This script was inspired by the following article.
"Scripting for X Productivity," by Marco Fioretti,
LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.
Thank you, all you good people at LJ.
Input error in dialog box.
E_INPUT=85
Dimensions of display, input widgets.
HEIGHT=50
WIDTH=60
Output file name (constructed out of script name).
OUTFILE=$0.output
Display this script in a text widget.
gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH
Now, we'll try saving input in a file.
echo -n "VARIABLE=" > $OUTFILE
gdialog --title "User Input" --inputbox "Enter variable, please:" \
$HEIGHT $WIDTH 2>> $OUTFILE
if [ "$?" -eq 0 ]
It's good practice to check exit status.
then
echo "Executed "dialog box" without errors."
else
echo "Error(s) in "dialog box" execution."
# Or, clicked on "Cancel", instead of "OK" button.
rm $OUTFILE
exit $E_INPUT
fi
Now, we'll retrieve and display the saved variable.
. $OUTFILE # 'Source' the saved file.
echo "The variable input in the "input box" was: "$VARIABLE""
rm $OUTFILE # Clean up by removing the temp file.
# Some applications may need to retain this file.
exit $?
Exercise: Rewrite this script using the 'zenity' widget set.
The xmessage command is a simple method of popping up a message/query
window. For example:
xmessage Fatal error in script! -button exit
The latest entry in the widget sweepstakes is zenity. This utility
pops up GTK+ dialog widgets-and-windows, and it works very nicely
within a script.
get_info ()
{
zenity --entry # Pops up query window . . .
#+ and prints user entry to stdout.
# Also try the --calendar and --scale options.
}
answer=$( get_info ) # Capture stdout in $answer variable.
echo "User entered: "$answer""
For other methods of scripting with widgets, try Tk or wish (Tcl
derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk
extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl
(Perl with Gtk extensions), or PyQt (Python with Qt extensions).
________________________________________________________________
36.8. Security Issues
36.8.1. Infected Shell Scripts
A brief warning about script security is indicated. A shell script
may contain a worm, trojan, or even a virus. For that reason, never
run as root a script (or permit it to be inserted into the system
startup scripts in /etc/rc.d) unless you have obtained said script
from a trusted source or you have carefully analyzed it to make
certain it does nothing harmful.
Various researchers at Bell Labs and other sites, including M.
Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the
implications of shell script viruses. They conclude that it is all
too easy for even a novice, a "script kiddie," to write one. [128]
Here is yet another reason to learn scripting. Being able to look at
and understand scripts may protect your system from being compromised
by a rogue script.
________________________________________________________________
36.8.2. Hiding Shell Script Source
For security purposes, it may be necessary to render a script
unreadable. If only there were a utility to create a stripped binary
executable from a script. Francisco Rosales' shc -- generic shell
script compiler does exactly that.
Unfortunately, according to
[http://www.linuxjournal.com/article/8256] an article in the October,
2005 Linux Journal, the binary can, in at least some cases, be
decrypted to recover the original script source. Still, this could be
a useful method of keeping scripts secure from all but the most
skilled hackers.
________________________________________________________________
36.8.3. Writing Secure Shell Scripts
Dan Stromberg suggests the following guidelines for writing
(relatively) secure shell scripts.
* Don't put secret data in environment variables.
* Don't pass secret data in an external command's arguments (pass
them in via a pipe or redirection instead).
* Set your $PATH carefully. Don't just trust whatever path you
inherit from the caller if your script is running as root. In
fact, whenever you use an environment variable inherited from the
caller, think about what could happen if the caller put something
misleading in the variable, e.g., if the caller set $HOME to
/etc.
________________________________________________________________
36.9. Portability Issues
It is easier to port a shell than a shell script.
--Larry Wall
This book deals specifically with Bash scripting on a GNU/Linux
system. All the same, users of sh and ksh will find much of value
here.
As it happens, many of the various shells and scripting languages
seem to be converging toward the POSIX 1003.2 standard. Invoking Bash
with the --posix option or inserting a set -o posix at the head of a
script causes Bash to conform very closely to this standard. Another
alternative is to use a #!/bin/sh sha-bang header in the script,
rather than #!/bin/bash. [129] Note that /bin/sh is a link to
/bin/bash in Linux and certain other flavors of UNIX, and a script
invoked this way disables extended Bash functionality.
Most Bash scripts will run as-is under ksh, and vice-versa, since
Chet Ramey has been busily porting ksh features to the latest
versions of Bash.
On a commercial UNIX machine, scripts using GNU-specific features of
standard commands may not work. This has become less of a problem in
the last few years, as the GNU utilities have pretty much displaced
their proprietary counterparts even on "big-iron" UNIX. Caldera's
release of the source to many of the original UNIX utilities has
accelerated the trend.
Bash has certain features that the traditional Bourne shell lacks.
Among these are:
* Certain extended invocation options
* Command substitution using $( ) notation
* Brace expansion
* Certain array operations, and associative arrays
* The double brackets extended test construct
* The double-parentheses arithmetic-evaluation construct
* Certain string manipulation operations
* Process substitution
* A Regular Expression matching operator
* Bash-specific builtins
* Coprocesses
See the Bash F.A.Q. for a complete listing.
________________________________________________________________
36.9.1. A Test Suite
Let us illustrate some of the incompatibilities between Bash and the
classic Bourne shell. Download and install the "Heirloom Bourne
Shell" and run the following script, first using Bash, then the
classic sh.
Example 36-23. Test Suite
!/bin/bash
test-suite.sh
A partial Bash compatibility test suite.
Run this on your version of Bash, or some other shell.
default_option=FAIL # Tests below will fail unless . . .
echo
echo -n "Testing "
sleep 1; echo -n ". "
sleep 1; echo -n ". "
sleep 1; echo ". "
echo
Double brackets
String="Double brackets supported?"
echo -n "Double brackets test: "
if [[ "$String" = "Double brackets supported?" ]]
then
echo "PASS"
else
echo "FAIL"
fi
Double brackets and regex matching
String="Regex matching supported?"
echo -n "Regex matching: "
if [[ "$String" =~ R.....matching* ]]
then
echo "PASS"
else
echo "FAIL"
fi
Arrays
test_arr=$default_option # FAIL
Array=( If supports arrays will print PASS )
test_arr=${Array[5]}
echo "Array test: $test_arr"
Command Substitution
csub_test ()
{
echo "PASS"
}
test_csub=$default_option # FAIL
test_csub=$(csub_test)
echo "Command substitution test: $test_csub"
echo
Completing this script is an exercise for the reader.
Add to the above similar tests for double parentheses,
- brace expansion, process substitution, etc.
exit $?
________________________________________________________________
36.10. Shell Scripting Under Windows
Even users running that other OS can run UNIX-like shell scripts, and
therefore benefit from many of the lessons of this book. The
[http://sourceware.cygnus.com/cygwin/] Cygwin package from Cygnus and
the [http://www.mkssoftware.com/] MKS utilities from Mortice Kern
Associates add shell scripting capabilities to Windows.
Another alternative is
[http://www2.research.att.com/~gsf/download/uwin/uwin.html] UWIN,
written by David Korn of AT&T, of Korn Shell fame.
In 2006, Microsoft released the Windows Powershell®, which contains
limited Bash-like command-line scripting capabilities.
________________________________________________________________
Chapter 37. Bash, versions 2, 3, and 4
37.1. Bash, version 2
The current version of Bash, the one you have running on your
machine, is most likely version 2.xx.yy, 3.xx.yy, or 4.xx.yy.
bash$ echo $BASH_VERSION
3.2.25(1)-release
The version 2 update of the classic Bash scripting language added
array variables, string and parameter expansion, and a better method
of indirect variable references, among other features.
Example 37-1. String expansion
!/bin/bash
String expansion.
Introduced with version 2 of Bash.
Strings of the form $'xxx'
- have the standard escaped characters interpreted.
echo $'Ringing bell 3 times \a \a \a'
# May only ring once with certain terminals.
# Or ...
# May not ring at all, depending on terminal settings.
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
echo $'\102\141\163\150'
# B a s h
# Octal equivalent of characters.
exit
Example 37-2. Indirect variable references - the new way
!/bin/bash
Indirect variable referencing.
This has a few of the attributes of references in C++.
a=letter_of_alphabet
letter_of_alphabet=z
echo "a = $a" # Direct reference.
echo "Now a = ${!a}" # Indirect reference.
The ${!variable} notation is more intuitive than the old
echo
t=table_cell_3
table_cell_3=24
echo "t = ${!t}" # t = 24
table_cell_3=387
echo "Value of t changed to ${!t}" # 387
No 'eval' necessary.
This is useful for referencing members of an array or table,
- or for simulating a multi-dimensional array.
An indexing option (analogous to pointer arithmetic)
- would have been nice. Sigh.
exit 0
See also, ind-ref.sh example.
Example 37-3. Simple database application, using indirect variable
referencing
!/bin/bash
resistor-inventory.sh
Simple database / table-lookup application.
============================================================== #
Data
B1723_value=470 # Ohms
B1723_powerdissip=.25 # Watts
B1723_colorcode="yellow-violet-brown" # Color bands
B1723_loc=173 # Where they are
B1723_inventory=78 # How many
B1724_value=1000
B1724_powerdissip=.25
B1724_colorcode="brown-black-red"
B1724_loc=24N
B1724_inventory=243
B1725_value=10000
B1725_powerdissip=.125
B1725_colorcode="brown-black-orange"
B1725_loc=24N
B1725_inventory=89
============================================================== #
echo
PS3='Enter catalog number: '
echo
select catalog_number in "B1723" "B1724" "B1725"
do
Inv=${catalog_number}_inventory
Val=${catalog_number}_value
Pdissip=${catalog_number}_powerdissip
Loc=${catalog_number}_loc
Ccode=${catalog_number}_colorcode
echo
echo "Catalog number $catalog_number:"
Now, retrieve value, using indirect referencing.
echo "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt]\
resistors in stock." # ^ ^
As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).
echo "These are located in bin # ${!Loc}."
echo "Their color code is "${!Ccode}"."
break
done
echo; echo
Exercises:
- Rewrite this script to read its data from an external file.
- Rewrite this script to use arrays,
- rather than indirect variable referencing.
Which method is more straightforward and intuitive?
Which method is easier to code?
Notes:
Shell scripts are inappropriate for anything except the most simple
- database applications, and even then it involves workarounds and kludges.
Much better is to use a language with native support for data structures,
- such as C++ or Java (or even Perl).
exit 0
Example 37-4. Using arrays and other miscellaneous trickery to deal
four random hands from a deck of cards
!/bin/bash
cards.sh
Deals four random hands from a deck of cards.
UNPICKED=0
PICKED=1
DUPE_CARD=99
LOWER_LIMIT=0
UPPER_LIMIT=51
CARDS_IN_SUIT=13
CARDS=52
declare -a Deck
declare -a Suits
declare -a Cards
It would have been easier to implement and more intuitive
- with a single, 3-dimensional array.
Perhaps a future version of Bash will support multidimensional arrays.
initialize_Deck ()
{
i=$LOWER_LIMIT
until [ "$i" -gt $UPPER_LIMIT ]
do
Deck[i]=$UNPICKED # Set each card of "Deck" as unpicked.
let "i += 1"
done
echo
}
initialize_Suits ()
{
Suits[0]=C #Clubs
Suits[1]=D #Diamonds
Suits[2]=H #Hearts
Suits[3]=S #Spades
}
initialize_Cards ()
{
Cards=(2 3 4 5 6 7 8 9 10 J Q K A)
Alternate method of initializing an array.
}
pick_a_card ()
{
card_number=$RANDOM
let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards.
if [ "${Deck[card_number]}" -eq $UNPICKED ]
then
Deck[card_number]=$PICKED
return $card_number
else
return $DUPE_CARD
fi
}
parse_card ()
{
number=$1
let "suit_number = number / CARDS_IN_SUIT"
suit=${Suits[suit_number]}
echo -n "$suit-"
let "card_no = number % CARDS_IN_SUIT"
Card=${Cards[card_no]}
printf %-4s $Card
Print cards in neat columns.
}
seed_random () # Seed random number generator.
{ # What happens if you don't do this?
seed=eval date +%s
let "seed %= 32766"
RANDOM=$seed
} # Consider other methods of seeding the random number generator.
deal_cards ()
{
echo
cards_picked=0
while [ "$cards_picked" -le $UPPER_LIMIT ]
do
pick_a_card
t=$?
if [ "$t" -ne $DUPE_CARD ]
then
parse_card $t
u=$cards_picked+1
# Change back to 1-based indexing, temporarily. Why?
let "u %= $CARDS_IN_SUIT"
if [ "$u" -eq 0 ] # Nested if/then condition test.
then
echo
echo
fi # Each hand set apart with a blank line.
let "cards_picked += 1"
fi
done
echo
return 0
}
Structured programming:
Entire program logic modularized in functions.
===============
seed_random
initialize_Deck
initialize_Suits
initialize_Cards
deal_cards
===============
exit
Exercise 1:
Add comments to thoroughly document this script.
Exercise 2:
Add a routine (function) to print out each hand sorted in suits.
You may add other bells and whistles if you like.
Exercise 3:
Simplify and streamline the logic of the script.
________________________________________________________________
37.2. Bash, version 3
On July 27, 2004, Chet Ramey released version 3 of Bash. This update
fixed quite a number of bugs and added new features.
Some of the more important added features:
* A new, more generalized {a..z} brace expansion operator.
!/bin/bash
for i in {1..10}
Simpler and more straightforward than
do
echo -n "$i "
done
echo
1 2 3 4 5 6 7 8 9 10
Or just . . .
echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z
echo {e..m} # e f g h i j k l m
echo {z..a} # z y x w v u t s r q p o n m l k j i h g f e d c b a
# Works backwards, too.
echo {25..30} # 25 26 27 28 29 30
echo {3..-2} # 3 2 1 0 -1 -2
echo {X..d} # X Y Z [ ] ^ _ ` a b c d
# Shows (some of) the ASCII characters between Z and a,
#+ but don't rely on this type of behavior because . . .
echo {]..a} # {]..a}
# Why?
You can tack on prefixes and suffixes.
echo "Number #"{1..4}, "..."
# Number #1, Number #2, Number #3, Number #4, ...
You can concatenate brace-expansion sets.
echo {1..3}{x..z}" +" "..."
# 1x + 1y + 1z + 2x + 2y + 2z + 3x + 3y + 3z + ...
# Generates an algebraic expression.
# This could be used to find permutations.
You can nest brace-expansion sets.
echo {{a..c},{1..3}}
# a b c 1 2 3
# The "comma operator" splices together strings.
########## ######### ############ ########### ######### ###############
Unfortunately, brace expansion does not lend itself to parameterization.
var1=1
var2=5
echo {$var1..$var2} # {1..5}
Yet, as Emiliano G. points out, using "eval" overcomes this limitation.
start=0
end=10
for index in $(eval echo {$start..$end})
do
echo -n "$index " # 0 1 2 3 4 5 6 7 8 9 10
done
echo
* The ${!array[@]} operator, which expands to all the indices of a
given array.
!/bin/bash
Array=(element-zero element-one element-two element-three)
echo ${Array[0]} # element-zero
# First element of array.
echo ${!Array[@]} # 0 1 2 3
# All the indices of Array.
for i in ${!Array[@]}
do
echo ${Array[i]} # element-zero
# element-one
# element-two
# element-three
#
# All the elements in Array.
done
* The =~ Regular Expression matching operator within a double
brackets test expression. (Perl has a similar operator.)
!/bin/bash
variable="This is a fine mess."
echo "$variable"
Regex matching with =~ operator within [[ double brackets ]].
if [[ "$variable" =~ T.........fines ]]
NOTE: As of version 3.2 of Bash, expression to match no longer quoted.
then
echo "match found"
# match found
fi
Or, more usefully:
!/bin/bash
input=$1
if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]
^ NOTE: Quoting not necessary, as of version 3.2 of Bash.
NNN-NN-NNNN (where each N is a digit).
then
echo "Social Security number."
Process SSN.
else
echo "Not a Social Security number!"
Or, ask for corrected input.
fi
For additional examples of using the =~ operator, see Example
A-29, Example 19-14, Example A-35, and Example A-24.
* The new set -o pipefail option is useful for debugging pipes. If
this option is set, then the exit status of a pipe is the exit
status of the last command in the pipe to fail (return a non-zero
value), rather than the actual final command in the pipe.
See Example 16-43.
Caution
The update to version 3 of Bash breaks a few scripts that worked
under earlier versions. Test critical legacy scripts to make sure
they still work!
As it happens, a couple of the scripts in the Advanced Bash Scripting
Guide had to be fixed up (see Example 9-4, for instance).
________________________________________________________________
37.2.1. Bash, version 3.1
The version 3.1 update of Bash introduces a number of bugfixes and a
few minor changes.
* The += operator is now permitted in in places where previously
only the = assignment operator was recognized.
a=1
echo $a # 1
a+=5 # Won't work under versions of Bash earlier than 3.1.
echo $a # 15
a+=Hello
echo $a # 15Hello
Here, += functions as a string concatenation operator. Note that
its behavior in this particular context is different than within
a let construct.
a=1
echo $a # 1
let a+=5 # Integer arithmetic, rather than string concatenation.
echo $a # 6
let a+=Hello # Doesn't "add" anything to a.
echo $a # 6
Jeffrey Haemer points out that this concatenation operator can be
quite useful. In this instance, we append a directory to the
$PATH.
bash$ echo $PATH
/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games
bash$ PATH+=:/opt/bin
bash$ echo $PATH
/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games:/opt/bin
________________________________________________________________
37.2.2. Bash, version 3.2
This is pretty much a bugfix update.
* In global parameter substitutions, the pattern no longer anchors
at the start of the string.
* The --wordexp option disables process substitution.
* The =~ Regular Expression match operator no longer requires
quoting of the pattern within [[ ... ]].
Caution
In fact, quoting in this context is not advisable as it may cause
regex evaluation to fail. Chet Ramey states in the Bash FAQ that
quoting explicitly disables regex evaluation. See also the
[https://bugs.launchpad.net/ubuntu-website/+bug/109931] Ubuntu Bug
List and
[http://en.wikinerds.org/index.php/Bash_syntax_and_semantics]
Wikinerds on Bash syntax.
Setting shopt -s compat31 in a script causes reversion to the
original behavior.
________________________________________________________________
37.3. Bash, version 4
Chet Ramey announced Version 4 of Bash on the 20th of February, 2009.
This release has a number of significant new features, as well as
some important bugfixes.
Among the new goodies:
* Associative arrays. [130]
An associative array can be thought of as a set of two linked arrays
-- one holding the data, and the other the keys that index the
individual elements of the data array.
Example 37-5. A simple address database
!/bin/bash4
fetch_address.sh
declare -A address
-A option declares associative array.
address[Charles]="414 W. 10th Ave., Baltimore, MD 21236"
address[John]="202 E. 3rd St., New York, NY 10009"
address[Wilma]="1854 Vermont Ave, Los Angeles, CA 90023"
echo "Charles's address is ${address[Charles]}."
Charles's address is 414 W. 10th Ave., Baltimore, MD 21236.
echo "Wilma's address is ${address[Wilma]}."
Wilma's address is 1854 Vermont Ave, Los Angeles, CA 90023.
echo "John's address is ${address[John]}."
John's address is 202 E. 3rd St., New York, NY 10009.
echo
echo "${!address[*]}" # The array indices ...
Charles John Wilma
Example 37-6. A somewhat more elaborate address database
!/bin/bash4
fetch_address-2.sh
A more elaborate version of fetch_address.sh.
SUCCESS=0
E_DB=99 # Error code for missing entry.
declare -A address
-A option declares associative array.
store_address ()
{
address[$1]="$2"
return $?
}
fetch_address ()
{
if [[ -z "${address[$1]}" ]]
then
echo "$1's address is not in database."
return $E_DB
fi
echo "$1's address is ${address[$1]}."
return $?
}
store_address "Lucas Fayne" "414 W. 13th Ave., Baltimore, MD 21236"
store_address "Arvid Boyce" "202 E. 3rd St., New York, NY 10009"
store_address "Velma Winston" "1854 Vermont Ave, Los Angeles, CA 90023"
Exercise:
Rewrite the above store_address calls to read data from a file,
- then assign field 1 to name, field 2 to address in the array.
Each line in the file would have a format corresponding to the above.
Use a while-read loop to read from file, sed or awk to parse the fields.
fetch_address "Lucas Fayne"
Lucas Fayne's address is 414 W. 13th Ave., Baltimore, MD 21236.
fetch_address "Velma Winston"
Velma Winston's address is 1854 Vermont Ave, Los Angeles, CA 90023.
fetch_address "Arvid Boyce"
Arvid Boyce's address is 202 E. 3rd St., New York, NY 10009.
fetch_address "Bozo Bozeman"
Bozo Bozeman's address is not in database.
exit $? # In this case, exit code = 99, since that is function return.
See Example A-53 for an interesting usage of an associative
array.
Caution
Elements of the index array may include embedded space characters, or
even leading and/or trailing space characters. However, index array
elements containing only whitespace are not permitted.
address[ ]="Blank" # Error!
* Enhancements to the case construct: the ;;& and ;& terminators.
Example 37-7. Testing characters
!/bin/bash4
test_char ()
{
case "$1" in
[[:print:]] ) echo "$1 is a printable character.";;& # |
# The ;;& terminator continues to the next pattern test. |
[[:alnum:]] ) echo "$1 is an alpha/numeric character.";;& # v
[[:alpha:]] ) echo "$1 is an alphabetic character.";;& # v
[[:lower:]] ) echo "$1 is a lowercase alphabetic character.";;&
[[:digit:]] ) echo "$1 is an numeric character.";& # |
# The ;& terminator executes the next statement ... # |
%%%@@@@@ ) echo "********************************";; # v
^^^^^^^^ ... even with a dummy pattern.
esac
}
echo
test_char 3
3 is a printable character.
3 is an alpha/numeric character.
3 is an numeric character.
echo
test_char m
m is a printable character.
m is an alpha/numeric character.
m is an alphabetic character.
m is a lowercase alphabetic character.
echo
test_char /
/ is a printable character.
echo
The ;;& terminator can save complex if/then conditions.
The ;& is somewhat less useful.
* The new coproc builtin enables two parallel processes to
communicate and interact. As Chet Ramey states in the Bash FAQ
[131] , ver. 4.01:
There is a new 'coproc' reserved word that specifies a coproce
ss:
an asynchronous command run with two pipes connected to the cr
eating
shell. Coprocs can be named. The input and output file descrip
tors
and the PID of the coprocess are available to the calling shel
l in
variables with coproc-specific names.
George Dimitriu explains,
"... coproc ... is a feature used in Bash process substitution
,
which now is made publicly available."
This means it can be explicitly invoked in a script, rather th
an
just being a behind-the-scenes mechanism used by Bash.
Coprocesses use file descriptors. File descriptors enable
processes and pipes to communicate.
!/bin/bash4
A coprocess communicates with a while-read loop.
coproc { cat mx_data.txt; sleep 2; }
^^^^^^^
Try running this without "sleep 2" and see what happens.
while read -u ${COPROC[0]} line # ${COPROC[0]} is the
do #+ file descriptor of the coprocess.
echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/'
done
kill $COPROC_PID # No longer need the coprocess,
#+ so kill its PID.
But, be careful!
!/bin/bash4
echo; echo
a=aaa
b=bbb
c=ccc
coproc echo "one two three"
while read -u ${COPROC[0]} a b c; # Note that this loop
do #+ runs in a subshell.
echo "Inside while-read loop: ";
echo "a = $a"; echo "b = $b"; echo "c = $c"
echo "coproc file descriptor: ${COPROC[0]}"
done
a = one
b = two
c = three
So far, so good, but ...
echo "-----------------"
echo "Outside while-read loop: "
echo "a = $a" # a =
echo "b = $b" # b =
echo "c = $c" # c =
echo "coproc file descriptor: ${COPROC[0]}"
echo
The coproc is still running, but ...
- it still doesn't enable the parent process
- to "inherit" variables from the child process, the while-read loop.
Compare this to the "badread.sh" script.
Caution
The coprocess is asynchronous, and this might cause a problem. It may
terminate before another process has finished communicating with it.
!/bin/bash4
coproc cpname { for i in {0..10}; do echo "index = $i"; done; }
^^^^^^ This is a named coprocess.
read -u ${cpname[0]}
echo $REPLY # index = 0
echo ${COPROC[0]} #+ No output ... the coprocess timed out
after the first loop iteration.
However, George Dimitriu has a partial fix.
coproc cpname { for i in {0..10}; do echo "index = $i"; done; sleep 1;
echo hi > myo; cat - >> myo; }
^^^^^ This is a named coprocess.
echo "I am main"$'\04' >&${cpname[1]}
myfd=${cpname[0]}
echo myfd=$myfd
while read -u $myfd
do
echo $REPLY;
done
echo $cpname_PID
Run this with and without the commented-out while-loop, and it is
- apparent that each process, the executing shell and the coprocess,
- waits for the other to finish writing in its own write-enabled pipe.
* The new mapfile builtin makes it possible to load an array with
the contents of a text file without using a loop or command
substitution.
!/bin/bash4
mapfile Arr1 < $0
Same result as Arr1=( $(cat $0) )
echo "${Arr1[@]}" # Copies this entire script out to stdout.
echo "--"; echo
But, not the same as read -a !!!
read -a Arr2 < $0
echo "${Arr2[@]}" # Reads only first line of script into the array.
exit
* The read builtin got a minor facelift. The -t timeout option now
accepts (decimal) fractional values [132] and the -i option
permits preloading the edit buffer. [133] Unfortunately, these
enhancements are still a work in progress and not (yet) usable in
scripts.
* Parameter substitution gets case-modification operators.
!/bin/bash4
var=veryMixedUpVariable
echo ${var} # veryMixedUpVariable
echo ${var^} # VeryMixedUpVariable
echo ${var^^} # VERYMIXEDUPVARIABLE
** All chars --> uppercase.
echo ${var,} # veryMixedUpVariable
echo ${var,,} # verymixedupvariable
** All chars --> lowercase.
* The declare builtin now accepts the -l lowercase and -c
capitalize options.
!/bin/bash4
declare -l var1 # Will change to lowercase
var1=MixedCaseVARIABLE
echo "$var1" # mixedcasevariable
Same effect as echo $var1 | tr A-Z a-z
declare -c var2 # Changes only initial char to uppercase.
var2=originally_lowercase
echo "$var2" # Originally_lowercase
NOT the same effect as echo $var2 | tr a-z A-Z
* Brace expansion has more options.
Increment/decrement, specified in the final term within braces.
!/bin/bash4
echo {40..60..2}
40 42 44 46 48 50 52 54 56 58 60
All the even numbers, between 40 and 60.
echo {60..40..2}
60 58 56 54 52 50 48 46 44 42 40
All the even numbers, between 40 and 60, counting backwards.
In effect, a decrement.
echo {60..40..-2}
The same output. The minus sign is not necessary.
But, what about letters and symbols?
echo {X..d}
X Y Z [ ] ^ _ ` a b c d
Does not echo the \ which escapes a space.
Zero-padding, specified in the first term within braces, prefixes
each term in the output with the same number of zeroes.
bash4$ echo {010..15}
010 011 012 013 014 015
bash4$ echo {000..10}
000 001 002 003 004 005 006 007 008 009 010
* Substring extraction on positional parameters now starts with $0
as the zero-index. (This corrects an inconsistency in the
treatment of positional parameters.)
!/bin/bash
show-params.bash
Requires version 4+ of Bash.
Invoke this scripts with at least one positional parameter.
E_BADPARAMS=99
if [ -z "$1" ]
then
echo "Usage $0 param1 ..."
exit $E_BADPARAMS
fi
echo ${@:0}
bash3 show-params.bash4 one two three
one two three
bash4 show-params.bash4 one two three
show-params.bash4 one two three
$0 $1 $2 $3
* The new ** globbing operator matches filenames and directories
recursively.
!/bin/bash4
filelist.bash4
shopt -s globstar # Must enable globstar, otherwise ** doesn't work.
# The globstar shell option is new to version 4 of Bash.
echo "Using *"; echo
for filename in *
do
echo "$filename"
done # Lists only files in current directory ($PWD).
echo; echo "--------------"; echo
echo "Using **"
for filename in **
do
echo "$filename"
done # Lists complete file tree, recursively.
exit
Using *
allmyfiles
filelist.bash4
Using **
allmyfiles
allmyfiles/file.index.txt
allmyfiles/my_music
allmyfiles/my_music/me-singing-60s-folksongs.ogg
allmyfiles/my_music/me-singing-opera.ogg
allmyfiles/my_music/piano-lesson.1.ogg
allmyfiles/my_pictures
allmyfiles/my_pictures/at-beach-with-Jade.png
allmyfiles/my_pictures/picnic-with-Melissa.png
filelist.bash4
* The new $BASHPID internal variable.
* There is a new builtin error-handling function named
command_not_found_handle.
!/bin/bash4
command_not_found_handle ()
{ # Accepts implicit parameters.
echo "The following command is not valid: ""$1"""
echo "With the following argument(s): ""$2"" ""$3""" # $4, $5 ...
} # $1, $2, etc. are not explicitly passed to the function.
bad_command arg1 arg2
The following command is not valid: "bad_command"
With the following argument(s): "arg1" "arg2"
Editorial comment
Associative arrays? Coprocesses? Whatever happened to the lean and
mean Bash we have come to know and love? Could it be suffering from
(horrors!) "feature creep"? Or perhaps even Korn shell envy?
Note to Chet Ramey: Please add only essential features in future Bash
releases -- perhaps for-each loops and support for multi-dimensional
arrays. [134] Most Bash users won't need, won't use, and likely won't
greatly appreciate complex "features" like built-in debuggers, Perl
interfaces, and bolt-on rocket boosters.
________________________________________________________________
37.3.1. Bash, version 4.1
Version 4.1 of Bash, released in May, 2010, was primarily a bugfix
update.
* The printf command now accepts a -v option for setting array
indices.
* Within double brackets, the > and < string comparison operators
now conform to the locale. Since the locale setting may affect
the sorting order of string expressions, this has side-effects on
comparison tests within [[ ... ]] expressions.
* The read builtin now takes a -N option (read -N chars), which
causes the read to terminate after chars characters.
Example 37-8. Reading N characters
!/bin/bash
Requires Bash version -ge 4.1 ...
num_chars=61
read -N $num_chars var < $0 # Read first 61 characters of script!
echo "$var"
exit
Output of Script #######
!/bin/bash
Requires Bash version -ge 4.1 ...
num_chars=61
* Here documents embedded in $( ... ) command substitution
constructs may terminate with a simple ).
Example 37-9. Using a here document to set a variable
!/bin/bash
here-commsub.sh
Requires Bash version -ge 4.1 ...
multi_line_var=$( cat <<ENDxxx
This is line 1 of the variable
This is line 2 of the variable
This is line 3 of the variable
ENDxxx)
Rather than what Bash 4.0 requires:
- that the terminating limit string and
- the terminating close-parenthesis be on separate lines.
ENDxxx
)
echo "$multi_line_var"
Bash still emits a warning, though.
warning: here-document at line 10 delimited
- by end-of-file (wanted `ENDxxx')
________________________________________________________________
37.3.2. Bash, version 4.2
Version 4.2 of Bash, released in February, 2011, contains a number of
new features and enhancements, in addition to bugfixes.
* Bash now supports the the \u and \U Unicode escape.
Unicode is a cross-platform standard for encoding into numerical
values letters and graphic symbols. This permits representing and
displaying characters in foreign alphabets and unusual fonts.
echo -e '\u2630' # Horizontal triple bar character.
Equivalent to the more roundabout:
echo -e "\xE2\x98\xB0"
# Recognized by earlier Bash versions.
echo -e '\u220F' # PI (Greek letter and mathematical symbol)
echo -e '\u0416' # Capital "ZHE" (Cyrillic letter)
echo -e '\u2708' # Airplane (Dingbat font) symbol
echo -e '\u2622' # Radioactivity trefoil
echo -e "The amplifier circuit requires a 100 \u2126 pull-up resistor."
unicode_var='\u2640'
echo -e $unicode_var # Female symbol
printf "$unicode_var \n" # Female symbol, with newline
And for something a bit more elaborate . . .
We can store Unicode symbols in an associative array,
- then retrieve them by name.
Run this in a gnome-terminal or a terminal with a large, bold font
declare -A symbol # Associative array.
symbol[script_E]='\u2130'
symbol[script_F]='\u2131'
symbol[script_J]='\u2110'
symbol[script_M]='\u2133'
symbol[Rx]='\u211E'
symbol[TEL]='\u2121'
symbol[FAX]='\u213B'
symbol[care_of]='\u2105'
symbol[account]='\u2100'
symbol[trademark]='\u2122'
echo -ne "${symbol[script_E]} "
echo -ne "${symbol[script_F]} "
echo -ne "${symbol[script_J]} "
echo -ne "${symbol[script_M]} "
echo -ne "${symbol[Rx]} "
echo -ne "${symbol[TEL]} "
echo -ne "${symbol[FAX]} "
echo -ne "${symbol[care_of]} "
echo -ne "${symbol[account]} "
echo -ne "${symbol[trademark]} "
echo
Note
The above example uses the $' ... ' string-expansion construct.
* When the lastpipe shell option is set, the last command in a pipe
doesn't run in a subshell.
Example 37-10. Piping input to a read
!/bin/bash
lastpipe-option.sh
line='' # Null value.
echo "$line = "$line"" # $line =
echo
shopt -s lastpipe # Error on Bash version -lt 4.2.
echo "Exit status of attempting to set "lastpipe" option is $?"
1 if Bash version -lt 4.2, 0 otherwise.
echo
head -1 $0 | read line # Pipe the first line of the script to read.
^^^^^^^^^ Not in a subshell!!!
echo "$line = "$line""
Older Bash releases $line =
Bash version 4.2 $line = #!/bin/bash
This option offers possible "fixups" for these example scripts:
Example 34-3 and Example 15-8.
* Negative array indices permit counting backwards from the end of
an array.
Example 37-11. Negative array indices
!/bin/bash
neg-array.sh
Requires Bash, version -ge 4.2.
array=( zero one two three four five ) # Six-element array.
0 1 2 3 4 5
-6 -5 -4 -3 -2 -1
Negative array indices now permitted.
echo ${array[-1]} # five
echo ${array[-2]} # four
...
echo ${array[-6]} # zero
Negative array indices count backward from the last element+1.
But, you cannot index past the beginning of the array.
echo ${array[-7]} # array: bad array subscript
So, what is this new feature good for?
echo "The last element in the array is "${array[-1]}""
Which is quite a bit more straightforward than:
echo "The last element in the array is "${array[${#array[*]}-1]}""
echo
And ...
index=0
let "neg_element_count = 0 - ${#array[*]}"
Number of elements, converted to a negative number.
while [ $index -gt $neg_element_count ]; do
((index--)); echo -n "${array[index]} "
done # Lists the elements in the array, backwards.
# We have just simulated the "tac" command on this array.
echo
See also neg-offset.sh.
* Substring extraction uses a negative length parameter to specify
an offset from the end of the target string.
Example 37-12. Negative parameter in string-extraction construct
!/bin/bash
Bash, version -ge 4.2
Negative length-index in substring extraction.
Important: It changes the interpretation of this construct!
stringZ=abcABC123ABCabc
echo ${stringZ} # abcABC123ABCabc
Position within string: 0123456789.....
echo ${stringZ:2:3} # cAB
Count 2 chars forward from string beginning, and extract 3 chars.
${string:position:length}
So far, nothing new, but now ...
# abcABC123ABCabc
Position within string: 0123....6543210
echo ${stringZ:3:-6} # ABC123
^
Index 3 chars forward from beginning and 6 chars backward from end,
- and extract everything in between.
${string:offset-from-front:offset-from-end}
When the "length" parameter is negative,
- it serves as an offset-from-end parameter.
See also neg-array.sh.
________________________________________________________________
Chapter 38. Endnotes
38.1. Author's Note
doce ut discas
(Teach, that you yourself may learn.)
How did I come to write a scripting book? It's a strange tale. It
seems that a few years back I needed to learn shell scripting -- and
what better way to do that than to read a good book on the subject? I
was looking to buy a tutorial and reference covering all aspects of
the subject. I was looking for a book that would take difficult
concepts, turn them inside out, and explain them in excruciating
detail, with well-commented examples. [135] In fact, I was looking
for this very book, or something very much like it. Unfortunately, it
didn't exist, and if I wanted it, I'd have to write it. And so, here
we are, folks.
That reminds me of the apocryphal story about a mad professor. Crazy
as a loon, the fellow was. At the sight of a book, any book -- at the
library, at a bookstore, anywhere -- he would become totally obsessed
with the idea that he could have written it, should have written it
-- and done a better job of it to boot. He would thereupon rush home
and proceed to do just that, write a book with the very same title.
When he died some years later, he allegedly had several thousand
books to his credit, probably putting even Asimov to shame. The books
might not have been any good, who knows, but does that really matter?
Here's a fellow who lived his dream, even if he was obsessed by it,
driven by it . . . and somehow I can't help admiring the old coot.
________________________________________________________________
38.2. About the Author
Who is this guy anyhow?
The author claims no credentials or special qualifications, [136]
other than a compulsion to write. [137]
This book is somewhat of a departure from his other major work, HOW-2
Meet Women: The Shy Man's Guide to Relationships. He has also written
the Software-Building HOWTO. Of late, he has been trying his (heavy)
hand at fiction: Dave Dawson Over Berlin (First Installment) Dave
Dawson Over Berlin (Second Installment) and Dave Dawson Over Berlin
(Third Installment) . He also has a few Instructables
([http://www.instructables.com/id/Arduino-Morse-Code-Shield/] here,
[http://www.instructables.com/id/Haywired-Hackduino/] here,
[http://www.instructables.com/id/Arduino-DIY-SD-Card-Logging-Shield/]
here,
[http://www.instructables.com/id/Binguino-An-Arduino-based-Bingo-Numb
er-Generato/] here,
[http://www.instructables.com/id/The-Raspberry-Pi-Lapdock-Connection/
] here,
[http://www.instructables.com/id/The-Raspberry-Pi-Arduino-Connection/
] here, and
[http://www.instructables.com/id/Switchable-Dual-Voltage-33v5v-Hacdui
no/] here to his (dis)credit.
A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has
emitted a few software truffles, including the
[http://ibiblio.org/pub/Linux/utils/file/cruft-0.2.tar.gz] cruft
one-time pad encryption utility, the
[http://ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz] mcalc
mortgage calculator, the
[http://ibiblio.org/pub/Linux/games/amusements/judge-1.0.tar.gz]
judge Scrabble® adjudicator, the
[http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz] yawl word
gaming list package, and the [http://bash.deta.in/qky.README.html]
Quacky anagramming gaming package. He got off to a rather shaky start
in the computer game -- programming FORTRAN IV on a CDC 3800 (on
paper coding pads, with occasional forays on a keypunch machine and a
Friden Flexowriter) -- and is not the least bit nostalgic for those
days.
Living in an out-of-the-way community with wife and orange tabby, he
cherishes human frailty, especially his own. [138]
________________________________________________________________
38.3. Where to Go For Help
[mailto:thegrendel.abs@gmail.com] The author is no longer supporting
or updating this document. He will not answer questions about this
book or about general scripting topics.
If you need assistance with a schoolwork assignment, read the
pertinent sections of this and other reference works. Do your best to
solve the problem using your own wits and resources. Please do not
waste the author's time. You will get neither help nor sympathy.
[139]
Likewise, kindly refrain from annoying the author with solicitations,
offers of employment, or "business opportunities." He is doing just
fine, and requires neither help nor sympathy, thank you.
Please note that the author will not answer scripting questions for
Sun/Solaris/Oracle or Apple systems. The endarkened execs and the
arachnoid corporate attorneys of those particular outfits have been
using litigation in a predatory manner and/or as a weapon against the
Open Source Community. Any Solaris or Apple users needing scripting
help will therefore kindly direct their concerns to corporate
customer service.
... sophisticated in mechanism but possibly agile operating under
noises being extremely suppressed ...
--CI-300 printer manual
________________________________________________________________
38.4. Tools Used to Produce This Book
38.4.1. Hardware
A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running
Red Hat 7.1/7.3. Sure, it's slow and has a funky keyboard, but it
beats the heck out of a No. 2 pencil and a Big Chief tablet.
Update: upgraded to a 770Z Thinkpad (P2-366, 192 meg RAM) running
FC3. Anyone feel like donating a later-model laptop to a starving
writer ?
Update: upgraded to a T61 Thinkpad running Mandriva 2011. No longer
starving , but not too proud to accept donations.
________________________________________________________________
38.4.2. Software and Printware
i. Bram Moolenaar's powerful SGML-aware [http://www.vim.org] vim
text editor.
ii. [http://www.netfolder.com/DSSSL/] OpenJade, a DSSSL rendering
engine for converting SGML documents into other formats.
iii. Norman Walsh's DSSSL stylesheets.
iv. DocBook, The Definitive Guide, by Norman Walsh and Leonard
Muellner (O'Reilly, ISBN 1-56592-580-7). This is still the
standard reference for anyone attempting to write a document in
Docbook SGML format.
________________________________________________________________
38.5. Credits
Community participation made this project possible. The author
gratefully acknowledges that writing this book would have been
unthinkable without help and feedback from all you people out there.
[mailto:feloy@free.fr] Philippe Martin translated the first version
(0.1) of this document into DocBook/SGML. While not on the job at a
small French company as a software developer, he enjoys working on
GNU/Linux documentation and software, reading literature, playing
music, and, for his peace of mind, making merry with friends. You may
run across him somewhere in France or in the Basque Country, or you
can email him at [mailto:feloy@free.fr] feloy@free.fr.
Philippe Martin also pointed out that positional parameters past $9
are possible using {bracket} notation. (See Example 4-5).
Stéphane Chazelas sent a long list of corrections, additions, and
example scripts. More than a contributor, he had, in effect, for a
while taken on the role of co-editor for this document. Merci
beaucoup!
Paulo Marcel Coelho Aragao offered many corrections, both major and
minor, and contributed quite a number of helpful suggestions.
I would like to especially thank Patrick Callahan, Mike Novak, and
Pal Domokos for catching bugs, pointing out ambiguities, and for
suggesting clarifications and changes in the preliminary version
(0.1) of this document. Their lively discussion of shell scripting
and general documentation issues inspired me to try to make this
document more readable.
I'm grateful to Jim Van Zandt for pointing out errors and omissions
in version 0.2 of this document. He also contributed an instructive
example script.
Many thanks to [mailto:mikaku@fiwix.org] Jordi Sanfeliu for giving
permission to use his fine tree script (Example A-16), and to Rick
Boivie for revising it.
Likewise, thanks to [mailto:charpov@cs.unh.edu] Michel Charpentier
for permission to use his dc factoring script (Example 16-52).
Kudos to [mailto:friedman@prep.ai.mit.edu] Noah Friedman for
permission to use his string function script (Example A-18).
Emmanuel Rouat suggested corrections and additions on command
substitution, aliases, and path management. He also contributed a
very nice sample .bashrc file (Appendix M).
[mailto:heiner.steven@odn.de] Heiner Steven kindly gave permission to
use his base conversion script, Example 16-48. He also made a number
of corrections and many helpful suggestions. Special thanks.
Rick Boivie contributed the delightfully recursive pb.sh script
(Example 36-11), revised the tree.sh script (Example A-16), and
suggested performance improvements for the monthlypmt.sh script
(Example 16-47).
Florian Wisser enlightened me on some of the fine points of testing
strings (see Example 7-6), and on other matters.
Oleg Philon sent suggestions concerning cut and pidof.
Michael Zick extended the empty array example to demonstrate some
surprising array properties. He also contributed the isspammer
scripts (Example 16-41 and Example A-28).
Marc-Jano Knopp sent corrections and clarifications on DOS batch
files.
Hyun Jin Cha found several typos in the document in the process of
doing a Korean translation. Thanks for pointing these out.
Andreas Abraham sent in a long list of typographical errors and other
corrections. Special thanks!
Others contributing scripts, making helpful suggestions, and pointing
out errors were Gabor Kiss, Leopold Toetsch, Peter Tillier, Marcus
Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell,
Jess Thrysoee, Adam Lazur, Bram Moolenaar, Baris Cicek, Greg
Keraunen, Keith Matthews, Sandro Magi, Albert Reiner, Dim Segebart,
Rory Winston, Lee Bigelow, Wayne Pollock, "jipe," "bojster," "nyal,"
"Hobbit," "Ender," "Little Monster" (Alexis), "Mark," "Patsie,"
"vladz," Peggy Russell, Emilio Conti, Ian. D. Allen, Hans-Joerg
Diers, Arun Giridhar, Dennis Leeuw, Dan Jacobson, Aurelio Marinho
Jargas, Edward Scholtz, Jean Helou, Chris Martin, Lee Maschmeyer,
Bruno Haible, Wilbert Berendsen, Sebastien Godard, Bjön Eriksson,
John MacDonald, John Lange, Joshua Tschida, Troy Engel, Manfred
Schwarb, Amit Singh, Bill Gradwohl, E. Choroba, David Lombard, Jason
Parker, Steve Parker, Bruce W. Clare, William Park, Vernia Damiano,
Mihai Maties, Mark Alexander, Jeremy Impson, Ken Fuchs, Jared Martin,
Frank Wang, Sylvain Fourmanoit, Matthew Sage, Matthew Walker, Kenny
Stauffer, Filip Moritz, Andrzej Stefanski, Daniel Albers, Jeffrey
Haemer, Stefano Palmeri, Nils Radtke, Sigurd Solaas, Serghey Rodin,
Jeroen Domburg, Alfredo Pironti, Phil Braham, Bruno de Oliveira
Schneider, Stefano Falsetto, Chris Morgan, Walter Dnes, Linc
Fessenden, Michael Iatrou, Pharis Monalo, Jesse Gough, Fabian Kreutz,
Mark Norman, Harald Koenig, Dan Stromberg, Peter Knowles, Francisco
Lobo, Mariusz Gniazdowski, Sebastian Arming, Chetankumar Phulpagare,
Benno Schulenberg, Tedman Eng, Jochen DeSmet, Juan Nicolas Ruiz,
Oliver Beckstein, Achmed Darwish, Dotan Barak, Richard Neill, Albert
Siersema, Omair Eshkenazi, Geoff Lee, Graham Ewart, JuanJo Ciarlante,
Cliff Bamford, Nathan Coulter, Ramses Rodriguez Martinez, Evgeniy
Ivanov, Craig Barnes, George Dimitriu, Kevin LeBlanc, Antonio Macchi,
Tomas Pospisek, David Wheeler, Erik Brandsberg, YongYe, Andreas
Kühne, Pádraig Brady, Joseph Steinhauser, and David Lawyer (himself
an author of four HOWTOs).
My gratitude to Chet Ramey and Brian Fox for writing Bash, and
building into it elegant and powerful scripting capabilities rivaling
those of ksh.
Very special thanks to the hard-working volunteers at the Linux
Documentation Project. The LDP hosts a repository of Linux knowledge
and lore, and has, to a great extent, enabled the publication of this
book.
Thanks and appreciation to IBM, Red Hat, Google, the
[http://www.fsf.org] Free Software Foundation, and all the good
people fighting the good fight to keep Open Source software free and
open.
Belated thanks to my fourth grade teacher, Miss Spencer, for
emotional support and for convincing me that maybe, just maybe I
wasn't a total loss.
Thanks most of all to my wife, Anita, for her encouragement,
inspiration, and emotional support.
________________________________________________________________
38.6. Disclaimer
(This is a variant of the standard [http://www.tldp.org] LDP
disclaimer.)
No liability for the contents of this document can be accepted. Use
the concepts, examples and information at your own risk. There may be
errors, omissions, and inaccuracies that could cause you to lose
data, harm your system, or induce involuntary electrocution, so
proceed with appropriate caution. The author takes no responsibility
for any damages, incidental or otherwise.
As it happens, it is highly unlikely that either you or your system
will suffer ill effects, aside from uncontrollable hiccups. In fact,
the raison d'etre of this book is to enable its readers to analyze
shell scripts and determine whether they have unanticipated
consequences.
________________________________________________________________
Bibliography
Those who do not understand UNIX are condemned to reinvent it,
poorly.
--Henry Spencer
Edited by Peter Denning, Computers Under Attack: Intruders, Worms,
and Viruses, ACM Press, 1990, 0-201-53067-8.
This compendium contains a couple of articles on shell script
viruses.
Ken Burtch, [http://www.samspublishing.com/title/0672326426] Linux
Shell Scripting with Bash, 1st edition, Sams Publishing (Pearson),
2004, 0672326426.
Covers much of the same material as the ABS Guide, though in a
different style.
Daniel Goldman, Definitive Guide to Sed, 1st edition, 2013.
This ebook is an excellent introduction to sed. Rather than being a
conversion from a printed volume, it was specifically designed and
formatted for viewing on an ebook reader. Well-written, informative,
and useful as a reference as well as a tutorial. Highly recommended.
Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly
and Associates, 1997, 1-156592-225-5.
Unfolding the full power of shell scripting requires at least a
passing familiarity with sed and awk. This is the classic tutorial.
It includes an excellent introduction to Regular Expressions.
Recommended.
Jeffrey Friedl, Mastering Regular Expressions, O'Reilly and
Associates, 2002, 0-596-00289-0.
Still the best all-around reference on Regular Expressions.
Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly
and Associates, 2002, 0-596-00343-9.
This excellent manual provides a decent introduction to shell
scripting from a sys admin point of view. It includes comprehensive
explanations of the startup and initialization scripts in a UNIX
system.
Stephen Kochan and Patrick Wood, Unix Shell Programming, Hayden,
1990, 067248448X.
Still considered a standard reference, though somewhat dated, and a
bit "wooden" stylistically speaking. [140] In fact, this book was the
ABS Guide author's first exposure to UNIX shell scripting, lo these
many years ago.
Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox
Press, 1996, 1874416680.
Surprisingly good in-depth coverage of various programming languages
available for Linux, including a fairly strong chapter on shell
scripting.
Herbert Mayer, Advanced C Programming on the IBM PC, Windcrest Books,
1989, 0830693637.
Excellent coverage of algorithms and general programming practices.
Highly recommended, but unfortunately out of print.
David Medinets, Unix Shell Programming Tools, McGraw-Hill, 1999,
0070397333.
Pretty good treatment of shell scripting, with examples, and a short
intro to Tcl and Perl.
Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd
edition, O'Reilly and Associates, 1998, 1-56592-347-2.
This is a valiant effort at a decent shell primer, but sadly
deficient in its coverage of writing scripts and lacking sufficient
examples.
Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991,
093573922X.
A very handy pocket reference, despite lacking coverage of
Bash-specific features.
Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 3rd
edition, O'Reilly and Associates, Random House, 2002, 0-596-00330-7.
Contains a couple of sections of very informative in-depth articles
on shell programming, but falls short of being a self-teaching
manual. It reproduces much of the Regular Expressions tutorial from
the Dougherty and Robbins book, above. The comprehensive coverage of
UNIX commands makes this book worthy of a place on your bookshelf.
Clifford Pickover, Computers, Pattern, Chaos, and Beauty, St.
Martin's Press, 1990, 0-312-04123-3.
A treasure trove of ideas and recipes for computer-based exploration
of mathematical oddities.
George Polya, How To Solve It, Princeton University Press, 1973,
0-691-02356-5.
The classic tutorial on problem-solving methods (algorithms), with
special emphasis on how to teach them.
Chet Ramey and Brian Fox,
[http://www.network-theory.co.uk/bash/manual/] The GNU Bash Reference
Manual, Network Theory Ltd, 2003, 0-9541617-7-7.
This manual is the definitive reference for GNU Bash. The authors of
this manual, Chet Ramey and Brian Fox, are the original developers of
GNU Bash. For each copy sold, the publisher donates $1 to the Free
Software Foundation.
Arnold Robbins, Bash Reference Card, SSC, 1998, 1-58731-010-5.
Excellent Bash pocket reference (don't leave home without it,
especially if you're a sysadmin). A bargain at $4.95, but
unfortunately no longer available for free download.
Arnold Robbins, Effective Awk Programming, Free Software Foundation /
O'Reilly and Associates, 2000, 1-882114-26-4.
The absolute best awk tutorial and reference. The free electronic
version of this book is part of the awk documentation, and printed
copies of the latest version are available from O'Reilly and
Associates.
This book served as an inspiration for the author of the ABS Guide.
Bill Rosenblatt, Learning the Korn Shell, O'Reilly and Associates,
1993, 1-56592-054-6.
This well-written book contains some excellent pointers on shell
scripting in general.
Paul Sheer, LINUX: Rute User's Tutorial and Exposition, 1st edition,
, 2002, 0-13-033351-4.
Very detailed and readable introduction to Linux system
administration.
The book is available in print, or
[http://burks.brighton.ac.uk/burks/linux/rute/rute.htm] on-line.
Ellen Siever and the staff of O'Reilly and Associates, Linux in a
Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1-56592-585-8.
The all-around best Linux command reference. It even has a Bash
section.
Dave Taylor, Wicked Cool Shell Scripts: 101 Scripts for Linux, Mac OS
X, and Unix Systems, 1st edition, No Starch Press, 2004,
1-59327-012-7.
Pretty much what the title promises . . .
The UNIX CD Bookshelf, 3rd edition, O'Reilly and Associates, 2003,
0-596-00392-7.
An array of seven UNIX books on CD ROM, including UNIX Power Tools,
Sed and Awk, and Learning the Korn Shell. A complete set of all the
UNIX references and tutorials you would ever need at about $130. Buy
this one, even if it means going into debt and not paying the rent.
Update: Seems to have somehow fallen out of print. Ah, well. You can
still buy the dead-tree editions of these books.
The O'Reilly books on Perl. (Actually, any O'Reilly books.)
Other Resources
Fioretti, Marco, "Scripting for X Productivity," Linux Journal, Issue
113, September, 2003, pp. 86-9.
Ben Okopnik's well-written introductory Bash scripting articles in
issues 53, 54, 55, 57, and 59 of the Linux Gazette, and his
explanation of "The Deep, Dark Secrets of Bash" in issue 56.
Chet Ramey's Bash - The GNU Shell, a two-part series published in
issues 3 and 4 of the Linux Journal, July-August 1994.
Mike G's Bash-Programming-Intro HOWTO.
Richard's Unix Scripting Universe.
Chet Ramey's Bash FAQ.
[http://wooledge.org:8000/BashFAQ] Greg's WIKI: Bash FAQ.
Example shell scripts at Lucc's Shell Scripts .
Example shell scripts at [http://www.shelldorado.com] SHELLdorado .
Example shell scripts at Noah Friedman's script site.
[http://bashcookbook.com/bashinfo/] Examples from the The Bash
Scripting Cookbook, by Albing, Vossen, and Newham.
Example shell scripts at [http://www.zazzybob.com] zazzybob.
Steve Parker's Shell Programming Stuff. In fact, all of his shell
scripting books are highly recommended. See also Steve's Arcade Games
written in a shell script.
An excellent collection of Bash scripting tips, tricks, and resources
at the Bash Hackers Wiki.
Giles Orr's Bash-Prompt HOWTO.
The Pixelbeat command-line reference.
Very nice sed, awk, and regular expression tutorials at The UNIX
Grymoire.
The GNU [http://www.gnu.org/software/sed/manual/] sed and
[http://www.gnu.org/software/gawk/manual/] gawk manuals. As you
recall, gawk is the enhanced GNU version of awk.
Many interesting sed scripts at the
[http://sed.sourceforge.net/grabbag/] seder's grab bag.
Tips and tricks at [http://linuxreviews.org] Linux Reviews.
Trent Fisher's groff tutorial.
David Wheeler's Filenames in Shell essay.
"Shelltris" and "shellitaire" at Shell Script Games.
YongYe's wonderfully complex Tetris game script.
Mark Komarinski's Printing-Usage HOWTO.
The Linux USB subsystem (helpful in writing scripts affecting USB
peripherals).
There is some nice material on I/O redirection in
[http://sunsite.ualberta.ca/Documentation/Gnu/textutils-2.0/html_chap
ter/textutils_10.html] chapter 10 of the textutils documentation at
the University of Alberta site.
Rick Hohensee has written the osimpa i386 assembler entirely as Bash
scripts.
dgatwood has a very nice [http://www.shellscriptgames.com/] shell
script games site, featuring a Tetris® clone and solitaire.
Aurelio Marinho Jargas has written a Regular expression wizard. He
has also written an informative [http://guia-er.sf.net] book on
Regular Expressions, in Portuguese.
Ben Tomkins has created the [http://bashnavigator.sourceforge.net]
Bash Navigator directory management tool.
[mailto:opengeometry@yahoo.ca] William Park has been working on a
project to incorporate certain Awk and Python features into Bash.
Among these is a gdbm interface. He has released bashdiff on
[http://freshmeat.net] Freshmeat.net. He has an
[http://linuxgazette.net/108/park.html] article in the November, 2004
issue of the Linux Gazette on adding string functions to Bash, with a
followup article in the December issue, and
[http://linuxgazette.net/110/park.htm] yet another in the January,
2005 issue.
Peter Knowles has written an elaborate Bash script that generates a
book list on the Sony Librie e-book reader. This useful tool
facilitates loading non-DRM user content on the Librie (and the newer
PRS-xxx-series devices).
Tim Waugh's [http://cyberelk.net/tim/xmlto/] xmlto is an elaborate
Bash script for converting Docbook XML documents to other formats.
Philip Patterson's [http://www.gossiplabs.org] logforbash
logging/debugging script.
[http://auctiongallery.sourceforge.net] AuctionGallery, an
application for eBay "power sellers" coded in Bash.
Of historical interest are Colin Needham's original International
Movie Database (IMDB) reader polling scripts, which nicely illustrate
the use of awk for string parsing. Unfortunately, the URL link is
broken.
Fritz Mehner has written a bash-support plugin for the vim text
editor. He has also also come up with his own stylesheet for Bash.
Compare it with the ABS Guide Unofficial Stylesheet.
Penguin Pete has quite a number of shell scripting tips and hints on
his superb site. Highly recommended.
The excellent Bash Reference Manual, by Chet Ramey and Brian Fox,
distributed as part of the bash-2-doc package (available as an rpm).
See especially the instructive example scripts in this package.
John Lion's classic, A Commentary on the Sixth Edition UNIX Operating
System.
The [news:comp.unix.shell] comp.os.unix.shell newsgroup.
The dd thread on [http://www.linuxquestions.org] Linux Questions.
The comp.os.unix.shell FAQ.
Assorted comp.os.unix
[http://www.faqs.org/faqs/by-newsgroup/comp/comp.unix.shell.html]
FAQs.
The Wikipedia article covering dc.
The manpages for bash and bash2, date, expect, expr, find, grep,
gzip, ln, patch, tar, tr, bc, xargs. The texinfo documentation on
bash, dd, m4, gawk, and sed.
________________________________________________________________
Appendix A. Contributed Scripts
These scripts, while not fitting into the text of this document, do
illustrate some interesting shell programming techniques. Some are
useful, too. Have fun analyzing and running them.
Example A-1. mailformat: Formatting an e-mail message
!/bin/bash
mail-format.sh (ver. 1.1): Format e-mail messages.
Gets rid of carets, tabs, and also folds excessively long lines.
=================================================================
Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne $ARGS ] # Correct number of arguments passed to script?
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
if [ -f "$1" ] # Check if file exists.
then
file_name=$1
else
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
MAXWIDTH=70 # Width to fold excessively long lines to.
=================================
A variable can hold a sed script.
It's a useful technique.
sedscript='s/^>//
s/^ *>//
s/^ *//
s/ *//'
=================================
Delete carets and tabs at beginning of lines,
- then fold lines to $MAXWIDTH characters.
sed "$sedscript" $1 | fold -s --width=$MAXWIDTH
# -s option to "fold"
#+ breaks lines at whitespace, if possible.
This script was inspired by an article in a well-known trade journal
- extolling a 164K MS Windows utility with similar functionality.
An nice set of text processing utilities and an efficient
- scripting language provide an alternative to the bloated executables
- of a clunky operating system.
exit $?
Example A-2. rn: A simple-minded file renaming utility
This script is a modification of Example 16-22.
! /bin/bash
rn.sh
Very simpleminded filename "rename" utility (based on "lowercase.sh").
The "ren" utility, by Vladimir Lanin (lanin@csd2.nyu.edu),
- does a much better job of this.
ARGS=2
E_BADARGS=85
ONE=1 # For getting singular/plural right (see below).
if [ $# -ne "$ARGS" ]
then
echo "Usage: basename $0 old-pattern new-pattern"
As in "rn gif jpg", which renames all gif files in working directory to jp
g.
exit $E_BADARGS
fi
number=0 # Keeps track of how many files actually renamed.
for filename in $1 #Traverse all matching files in directory.
do
if [ -f "$filename" ] # If finds match...
then
fname=`basename $filename` # Strip off path.
n=`echo $fname | sed -e "s/$1/$2/"` # Substitute new for old in filenam
e.
mv $fname $n # Rename.
let "number += 1"
fi
done
if [ "$number" -eq "$ONE" ] # For correct grammar.
then
echo "$number file renamed."
else
echo "$number files renamed."
fi
exit $?
Exercises:
What types of files will this not work on?
How can this be fixed?
Example A-3. blank-rename: Renames filenames containing blanks
This is an even simpler-minded version of previous script.
! /bin/bash
blank-rename.sh
Substitutes underscores for blanks in all the filenames in a directory.
ONE=1 # For getting singular/plural right (see below).
number=0 # Keeps track of how many files actually renamed.
FOUND=0 # Successful return value.
for filename in * #Traverse all files in directory.
do
echo "$filename" | grep -q " " # Check whether filename
if [ $? -eq $FOUND ] #+ contains space(s).
then
fname=$filename # Yes, this filename needs work.
n=`echo $fname | sed -e "s/ /_/g"` # Substitute underscore for blank.
mv "$fname" "$n" # Do the actual renaming.
let "number += 1"
fi
done
if [ "$number" -eq "$ONE" ] # For correct grammar.
then
echo "$number file renamed."
else
echo "$number files renamed."
fi
exit 0
Example A-4. encryptedpw: Uploading to an ftp site, using a locally
encrypted password
!/bin/bash
Example "ex72.sh" modified to use encrypted password.
Note that this is still rather insecure,
- since the decrypted password is sent in the clear.
Use something like "ssh" if this is a concern.
E_BADARGS=85
if [ -z "$1" ]
then
echo "Usage: basename $0 filename"
exit $E_BADARGS
fi
Username=bozo # Change to suit.
pword=/home/bozo/secret/password_encrypted.file
File containing encrypted password.
Filename=basename $1 # Strips pathname out of file name.
Server="XXX"
Directory="YYY" # Change above to actual server name & directory.
Password=cruft <$pword # Decrypt password.
Uses the author's own "cruft" file encryption package,
- based on the classic "onetime pad" algorithm,
- Primary-site: ftp://ibiblio.org/pub/Linux/utils/file
ftp -n $Server <<End-Of-Session
user $Username $Password
binary
bell
cd $Directory
put $Filename
bye
End-Of-Session
-n option to "ftp" disables auto-logon.
Note that "bell" rings 'bell' after each file transfer.
exit 0
Example A-5. copy-cd: Copying a data CD
!/bin/bash
copy-cd.sh: copying a data CD
CDROM=/dev/cdrom # CD ROM device
OF=/home/bozo/projects/cdimage.iso # output file
/xxxx/xxxxxxxx/ Change to suit your system.
BLOCKSIZE=2048
SPEED=10 # If unspecified, uses max spd.
DEVICE=/dev/cdrom older version.
DEVICE="1,0,0"
echo; echo "Insert source CD, but do not mount it."
echo "Press ENTER when ready. "
read ready # Wait for input, $ready not used.
echo; echo "Copying the source CD to $OF."
echo "This may take a while. Please be patient."
dd if=$CDROM of=$OF bs=$BLOCKSIZE # Raw device copy.
echo; echo "Remove data CD."
echo "Insert blank CDR."
echo "Press ENTER when ready. "
read ready # Wait for input, $ready not used.
echo "Copying $OF to CDR."
cdrecord -v -isosize speed=$SPEED dev=$DEVICE $OF # Old version.
wodim -v -isosize dev=$DEVICE $OF
Uses Joerg Schilling's "cdrecord" package (see its docs).
http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html
Newer Linux distros may use "wodim" rather than "cdrecord" ...
echo; echo "Done copying $OF to CDR on device $CDROM."
echo "Do you want to erase the image file (y/n)? " # Probably a huge file.
read answer
case "$answer" in
[yY]) rm -f $OF
echo "$OF erased."
;;
- ) echo "$OF not erased.";;
esac
echo
Exercise:
Change the above "case" statement to also accept "yes" and "Yes" as input.
exit 0
Example A-6. Collatz series
!/bin/bash
collatz.sh
The notorious "hailstone" or Collatz series.
- Get the integer "seed" from the command-line.
- NUMBER <-- seed
- Print NUMBER.
- If NUMBER is even, divide by 2, or
5)+ if odd, multiply by 3 and add 1.
- NUMBER <-- result
- Loop back to step 3 (for specified number of iterations).
The theory is that every such sequence,
- no matter how large the initial value,
- eventually settles down to repeating "4,2,1..." cycles,
- even after fluctuating through a wide range of values.
This is an instance of an "iterate,"
- an operation that feeds its output back into its input.
Sometimes the result is a "chaotic" series.
MAX_ITERATIONS=200
For large seed numbers (>32000), try increasing MAX_ITERATIONS.
h=${1:-$$} # Seed.
# Use $PID as seed,
#+ if not specified as command-line arg.
echo
echo "C($h) -*- $MAX_ITERATIONS Iterations"
echo
for ((i=1; i<=MAX_ITERATIONS; i++))
do
echo -n "$h "
^^^
tab
printf does it better ...
COLWIDTH=%7d
printf $COLWIDTH $h
let "remainder = h % 2"
if [ "$remainder" -eq 0 ] # Even?
then
let "h /= 2" # Divide by 2.
else
let "h = h*3 + 1" # Multiply by 3 and add 1.
fi
COLUMNS=10 # Output 10 values per line.
let "line_break = i % $COLUMNS"
if [ "$line_break" -eq 0 ]
then
echo
fi
done
echo
For more information on this strange mathematical function,
- see Computers, Pattern, Chaos, and Beauty, by Pickover, p. 185 ff.,
- as listed in the bibliography.
exit 0
Example A-7. days-between: Days between two dates
!/bin/bash
days-between.sh: Number of days between two dates.
Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY
Note: Script modified to account for changes in Bash, v. 2.05b +,
ARGS=2 # Two command-line parameters expected.
E_PARAM_ERR=85 # Param error.
REFYR=1600 # Reference year.
CENTURY=100
DIY=365
ADJ_DIY=367 # Adjusted for leap year + fraction.
MIY=12
DIM=31
LEAPCYCLE=4
MAXRETVAL=255 # Largest permissible
#+ positive return value from a function.
diff= # Declare global variable for date difference.
value= # Declare global variable for absolute value.
day= # Declare globals for day, month, year.
month=
year=
Param_Error () # Command-line parameters wrong.
{
echo "Usage: basename $0 [M]M/[D]D/YYYY [M]M/[D]D/YYYY"
echo " (date must be after 1/3/1600)"
exit $E_PARAM_ERR
}
Parse_Date () # Parse date from command-line params.
{
month=${1%%/**}
dm=${1%/**} # Day and month.
day=${dm#*/}
let "year = basename $1" # Not a filename, but works just the same.
}
check_date () # Checks for invalid date(s) passed.
{
[ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||
[ "$year" -lt "$REFYR" ] && Param_Error
Exit script on bad value(s).
Uses or-list / and-list.
Exercise: Implement more rigorous date checking.
}
strip_leading_zero () # Better to strip possible leading zero(s)
{ #+ from day and/or month
return ${1#0} #+ since otherwise Bash will interpret them
} #+ as octal values (POSIX.2, sect 2.9.2.1).
day_index () # Gauss' Formula:
{ # Days from March 1, 1600 to date passed as param.
# ^^^^^^^^^^^^^
day=$1
month=$2
year=$3
let "month = $month - 2"
if [ "$month" -le 0 ]
then
let "month += 12"
let "year -= 1"
fi
let "year -= $REFYR"
let "indexyr = $year / $CENTURY"
let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \
+ $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"
For an in-depth explanation of this algorithm, see
#+ http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx
echo $Days
}
calculate_difference () # Difference between two day indices.
{
let "diff = $1 - $2" # Global variable.
}
abs () # Absolute value
{ # Uses global "value" variable.
if [ "$1" -lt 0 ] # If negative
then #+ then
let "value = 0 - $1" #+ change sign,
else #+ else
let "value = $1" #+ leave it alone.
fi
}
if [ $# -ne "$ARGS" ] # Require two command-line params.
then
Param_Error
fi
Parse_Date $1
check_date $day $month $year # See if valid date.
strip_leading_zero $day # Remove any leading zeroes
day=$? #+ on day and/or month.
strip_leading_zero $month
month=$?
let "date1 = day_index $day $month $year"
Parse_Date $2
check_date $day $month $year
strip_leading_zero $day
day=$?
strip_leading_zero $month
month=$?
date2=$(day_index $day $month $year) # Command substitution.
calculate_difference $date1 $date2
abs $diff # Make sure it's positive.
diff=$value
echo $diff
exit 0
Exercise:
If given only one command-line parameter, have the script
- use today's date as the second.
Compare this script with
- the implementation of Gauss' Formula in a C program at
- http://buschencrew.hypermart.net/software/datedif
Example A-8. Making a dictionary
!/bin/bash
makedict.sh [make dictionary]
Modification of /usr/sbin/mkdict (/usr/sbin/cracklib-forman) script.
Original script copyright 1993, by Alec Muffett.
This modified script included in this document in a manner
- consistent with the "LICENSE" document of the "Crack" package
- that the original script is a part of.
This script processes text files to produce a sorted list
- of words found in the files.
This may be useful for compiling dictionaries
- and for other lexicographic purposes.
E_BADARGS=85
if [ ! -r "$1" ] # Need at least one
then #+ valid file argument.
echo "Usage: $0 files-to-process"
exit $E_BADARGS
fi
SORT="sort" # No longer necessary to define
#+ options to sort. Changed from
#+ original script.
cat $* | # Dump specified files to stdout.
tr A-Z a-z | # Convert to lowercase.
tr ' ' '\012' | # New: change spaces to newlines.
tr -cd '\012[a-z][0-9]' | # Get rid of everything
#+ non-alphanumeric (in orig. script).
tr -c '\012a-z' '\012' | # Rather than deleting non-alpha
#+ chars, change them to newlines.
sort | # $SORT options unnecessary now.
uniq | # Remove duplicates.
grep -v '^#' | # Delete lines starting with #.
grep -v '^$' # Delete blank lines.
exit $?
Example A-9. Soundex conversion
!/bin/bash
soundex.sh: Calculate "soundex" code for names
=======================================================
Soundex script
by
Mendel Cooper
thegrendel.abs@gmail.com
reldate: 23 January, 2002
Placed in the Public Domain.
A slightly different version of this script appeared in
- Ed Schaefer's July, 2002 "Shell Corner" column
- in "Unix Review" on-line,
- http://www.unixreview.com/documents/uni1026336632258/
=======================================================
ARGCOUNT=1 # Need name as argument.
E_WRONGARGS=90
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: basename $0 name"
exit $E_WRONGARGS
fi
assign_value () # Assigns numerical value
{ #+ to letters of name.
val1=bfpv # 'b,f,p,v' = 1
val2=cgjkqsxz # 'c,g,j,k,q,s,x,z' = 2
val3=dt # etc.
val4=l
val5=mn
val6=r
Exceptionally clever use of 'tr' follows.
Try to figure out what is going on here.
value=$( echo "$1" \
| tr -d wh \
| tr $val1 1 | tr $val2 2 | tr $val3 3 \
| tr $val4 4 | tr $val5 5 | tr $val6 6 \
| tr -s 123456 \
| tr -d aeiouy )
Assign letter values.
Remove duplicate numbers, except when separated by vowels.
Ignore vowels, except as separators, so delete them last.
Ignore 'w' and 'h', even as separators, so delete them first.
The above command substitution lays more pipe than a plumber .
}
input_name="$1"
echo
echo "Name = $input_name"
Change all characters of name input to lowercase.
name=$( echo $input_name | tr A-Z a-z )
Just in case argument to script is mixed case.
Prefix of soundex code: first letter of name.
char_pos=0 # Initialize character position.
prefix0=${name:$char_pos:1}
prefix=echo $prefix0 | tr a-z A-Z
# Uppercase 1st letter of soundex.
let "char_pos += 1" # Bump character position to 2nd letter of name
.
name1=${name:$char_pos}
++++++++++++++++++++++++++ Exception Patch ++++++++++++++++++++++++++++++
Now, we run both the input name and the name shifted one char
- to the right through the value-assigning function.
If we get the same value out, that means that the first two characters
- of the name have the same value assigned, and that one should cancel.
However, we also need to test whether the first letter of the name
- is a vowel or 'w' or 'h', because otherwise this would bollix things up.
char1=echo $prefix | tr A-Z a-z # First letter of name, lowercased.
assign_value $name
s1=$value
assign_value $name1
s2=$value
assign_value $char1
s3=$value
s3=9$s3 # If first letter of name is a vowel
#+ or 'w' or 'h',
#+ then its "value" will be null (unset).
#+ Therefore, set it to 9, an otherwise
#+ unused value, which can be tested for.
if [[ "$s1" -ne "$s2" || "$s3" -eq 9 ]]
then
suffix=$s2
else
suffix=${s2:$char_pos}
fi
++++++++++++++++++++++ end Exception Patch ++++++++++++++++++++++++++++++
padding=000 # Use at most 3 zeroes to pad.
soun=$prefix$suffix$padding # Pad with zeroes.
MAXLEN=4 # Truncate to maximum of 4 chars.
soundex=${soun:0:$MAXLEN}
echo "Soundex = $soundex"
echo
The soundex code is a method of indexing and classifying names
- by grouping together the ones that sound alike.
The soundex code for a given name is the first letter of the name,
- followed by a calculated three-number code.
Similar sounding names should have almost the same soundex codes.
Examples:
Smith and Smythe both have a "S-530" soundex.
Harrison = H-625
Hargison = H-622
Harriman = H-655
This works out fairly well in practice, but there are numerous anomalies.
The U.S. Census and certain other governmental agencies use soundex,
as do genealogical researchers.
For more information,
- see the "National Archives and Records Administration home page",
- http://www.nara.gov/genealogy/soundex/soundex.html
Exercise:
Simplify the "Exception Patch" section of this script.
exit 0
Example A-10. Game of Life
!/bin/bash
life.sh: "Life in the Slow Lane"
Author: Mendel Cooper
License: GPL3
Version 0.2: Patched by Daniel Albers
Version 0.2.1: Added 2-second delay between generations.
##################################################################### #
This is the Bash script version of John Conway's "Game of Life". #
"Life" is a simple implementation of cellular automata. #
--------------------------------------------------------------------- #
On a rectangular grid, let each "cell" be either "living" or "dead." #
Designate a living cell with a dot, and a dead one with a blank space.#
Begin with an arbitrarily drawn dot-and-blank grid, #
Determine each successive generation by the following rules: #
- Each cell has 8 neighbors, the adjoining cells #
-
left, right, top, bottom, and the 4 diagonals. #
123 #
4*5 The * is the cell under consideration. #
678 #
- A living cell with either 2 or 3 living neighbors remains alive. #
SURVIVE=2 #
- A dead cell with 3 living neighbors comes alive, a "birth." #
BIRTH=3 #
- All other cases result in a dead cell for the next generation. #
##################################################################### #
startfile=gen0 # Read the starting generation from the file "gen0" ...
# Default, if no other file specified when invoking script.
#
if [ -n "$1" ] # Specify another "generation 0" file.
then
startfile="$1"
fi
Abort script if "startfile" not specified
- default file "gen0" not present.
E_NOSTARTFILE=86
if [ ! -e "$startfile" ]
then
echo "Startfile ""$startfile"" missing!"
exit $E_NOSTARTFILE
fi
ALIVE1=.
DEAD1=_
# Represent living and dead cells in the start-up file.
-----------------------------------------------------#
This script uses a 10 x 10 grid (may be increased,
- but a large grid will slow down execution).
ROWS=10
COLS=10
Change above two variables to match desired grid size.
-----------------------------------------------------#
GENERATIONS=10 # How many generations to cycle through.
# Adjust this upwards
#+ if you have time on your hands.
NONE_ALIVE=85 # Exit status on premature bailout,
#+ if no cells left alive.
DELAY=2 # Pause between generations.
TRUE=0
FALSE=1
ALIVE=0
DEAD=1
avar= # Global; holds current generation.
generation=0 # Initialize generation count.
=================================================================
let "cells = $ROWS * $COLS" # How many cells.
Arrays containing "cells."
declare -a initial
declare -a current
display ()
{
alive=0 # How many cells alive at any given time.
# Initially zero.
declare -a arr
arr=( echo "$1" ) # Convert passed arg to array.
element_count=${#arr[*]}
local i
local rowcheck
for ((i=0; i<$element_count; i++))
do
Insert newline at end of each row.
let "rowcheck = $i % COLS"
if [ "$rowcheck" -eq 0 ]
then
echo # Newline.
echo -n " " # Indent.
fi
cell=${arr[i]}
if [ "$cell" = . ]
then
let "alive += 1"
fi
echo -n "$cell" | sed -e 's/_/ /g'
Print out array, changing underscores to spaces.
done
return
}
IsValid () # Test if cell coordinate valid.
{
if [ -z "$1" -o -z "$2" ] # Mandatory arguments missing?
then
return $FALSE
fi
local row
local lower_limit=0 # Disallow negative coordinate.
local upper_limit
local left
local right
let "upper_limit = $ROWS * $COLS - 1" # Total number of cells.
if [ "$1" -lt "$lower_limit" -o "$1" -gt "$upper_limit" ]
then
return $FALSE # Out of array bounds.
fi
row=$2
let "left = $row * $COLS" # Left limit.
let "right = $left + $COLS - 1" # Right limit.
if [ "$1" -lt "$left" -o "$1" -gt "$right" ]
then
return $FALSE # Beyond row boundary.
fi
return $TRUE # Valid coordinate.
}
IsAlive () # Test whether cell is alive.
# Takes array, cell number, and
{ #+ state of cell as arguments.
GetCount "$1" $2 # Get alive cell count in neighborhood.
local nhbd=$?
if [ "$nhbd" -eq "$BIRTH" ] # Alive in any case.
then
return $ALIVE
fi
if [ "$3" = "." -a "$nhbd" -eq "$SURVIVE" ]
then # Alive only if previously alive.
return $ALIVE
fi
return $DEAD # Defaults to dead.
}
GetCount () # Count live cells in passed cell's neighborhood.
# Two arguments needed:
# $1) variable holding array
# $2) cell number
{
local cell_number=$2
local array
local top
local center
local bottom
local r
local row
local i
local t_top
local t_cen
local t_bot
local count=0
local ROW_NHBD=3
array=( echo "$1" )
let "top = $cell_number - $COLS - 1" # Set up cell neighborhood.
let "center = $cell_number - 1"
let "bottom = $cell_number + $COLS - 1"
let "r = $cell_number / $COLS"
for ((i=0; i<$ROW_NHBD; i++)) # Traverse from left to right.
do
let "t_top = $top + $i"
let "t_cen = $center + $i"
let "t_bot = $bottom + $i"
let "row = $r" # Count center row.
IsValid $t_cen $row # Valid cell position?
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_cen]} = "$ALIVE1" ] # Is it alive?
then # If yes, then ...
let "count += 1" # Increment count.
fi
fi
let "row = $r - 1" # Count top row.
IsValid $t_top $row
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_top]} = "$ALIVE1" ] # Redundancy here.
then # Can it be optimized?
let "count += 1"
fi
fi
let "row = $r + 1" # Count bottom row.
IsValid $t_bot $row
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_bot]} = "$ALIVE1" ]
then
let "count += 1"
fi
fi
done
if [ ${array[$cell_number]} = "$ALIVE1" ]
then
let "count -= 1" # Make sure value of tested cell itself
fi #+ is not counted.
return $count
}
next_gen () # Update generation array.
{
local array
local i=0
array=( echo "$1" ) # Convert passed arg to array.
while [ "$i" -lt "$cells" ]
do
IsAlive "$1" $i ${array[$i]} # Is the cell alive?
if [ $? -eq "$ALIVE" ]
then # If alive, then
array[$i]=. #+ represent the cell as a period.
else
array[$i]="_" # Otherwise underscore
fi #+ (will later be converted to space).
let "i += 1"
done
let "generation += 1" # Increment generation count.
Why was the above line commented out?
Set variable to pass as parameter to "display" function.
avar=echo ${array[@]} # Convert array back to string variable.
display "$avar" # Display it.
echo; echo
echo "Generation $generation - $alive alive"
if [ "$alive" -eq 0 ]
then
echo
echo "Premature exit: no more cells alive!"
exit $NONE_ALIVE # No point in continuing
fi #+ if no live cells.
}
=========================================================
main ()
{
Load initial array with contents of startup file.
initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\
Delete lines containing '#' comment character.
sed -e 's/\./\. /g' -e 's/_/_ /g'` )
Remove linefeeds and insert space between elements.
clear # Clear screen.
echo # Title
setterm -reverse on
echo "======================="
setterm -reverse off
echo " $GENERATIONS generations"
echo " of"
echo ""Life in the Slow Lane""
setterm -reverse on
echo "======================="
setterm -reverse off
sleep $DELAY # Display "splash screen" for 2 seconds.
-------- Display first generation. --------
Gen0=echo ${initial[@]}
display "$Gen0" # Display only.
echo; echo
echo "Generation $generation - $alive alive"
sleep $DELAY
let "generation += 1" # Bump generation count.
echo
------- Display second generation. -------
Cur=echo ${initial[@]}
next_gen "$Cur" # Update & display.
sleep $DELAY
let "generation += 1" # Increment generation count.
------ Main loop for displaying subsequent generations ------
while [ "$generation" -le "$GENERATIONS" ]
do
Cur="$avar"
next_gen "$Cur"
let "generation += 1"
sleep $DELAY
done
==============================================================
echo
}
exit 0 # CEOF:EOF
The grid in this script has a "boundary problem."
The the top, bottom, and sides border on a void of dead cells.
Exercise: Change the script to have the grid wrap around,
Exercise: Create a new "gen0" file to seed this script.
Use a 12 x 16 grid, instead of the original 10 x 10 one.
Make the necessary changes to the script,
Exercise: Modify this script so that it can determine the grid size
This would make unnecessary any changes to variables
Exercise: Optimize this script.
It has redundant code.
Example A-11. Data file for Game of Life
gen0
This is an example "generation 0" start-up file for "life.sh".
The "gen0" file is a 10 x 10 grid using a period (.) for live cells,
- and an underscore (_) for dead ones. We cannot simply use spaces
- for dead cells in this file because of a peculiarity in Bash arrays.
[Exercise for the reader: explain this.]
Lines beginning with a '#' are comments, and the script ignores them.
...___
..._
_...
.______.
._
....._
._
..._
....
....
+++
The following script is by Mark Moraes of the University of Toronto.
See the file Moraes-COPYRIGHT for permissions and restrictions. This
file is included in the combined HTML/source tarball of the ABS
Guide.
Example A-12. behead: Removing mail and news message headers
! /bin/sh
Strips off the header from a mail/News message i.e. till the first
Author: Mark Moraes, University of Toronto
==> These comments added by author of this document.
if [ $# -eq 0 ]; then
==> If no command-line args present, then works on file redirected to stdin.
sed -e '1,/^$/d' -e '/^[ ]*$/d'
# --> Delete empty lines and all lines until
# --> first one beginning with white space.
else
==> If command-line args present, then work on files named.
for i do
sed -e '1,/^$/d' -e '/^[ ]*$/d' $i
# --> Ditto, as above.
done
fi
exit
==> Exercise: Add error checking and other options.
==>
==> Note that the small sed script repeats, except for the arg passed.
==> Does it make sense to embed it in a function? Why or why not?
/*
- Copyright University of Toronto 1988, 1989.
- Permission is granted to anyone to use this software for any purpose on
- any computer system, and to alter it and redistribute it freely, subject
- to the following restrictions:
-
- The author and the University of Toronto are not responsible
- for the consequences of use of this software, no matter how awful,
- even if they arise from flaws in it.
-
- The origin of this software must not be misrepresented, either by
- explicit claim or by omission. Since few users ever read sources,
- credits must appear in the documentation.
-
- Altered versions must be plainly marked as such, and must not be
- misrepresented as being the original software. Since few users
- ever read sources, credits must appear in the documentation.
-
- This notice may not be removed or altered.
*/
Antek Sawicki contributed the following script, which makes very
clever use of the parameter substitution operators discussed in
Section 10.2.
Example A-13. password: Generating random 8-character passwords
!/bin/bash
Random password generator for Bash 2.x +
- who generously gave usage permission to the ABS Guide author.
==> Comments added by document author ==>
MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
==> Password will consist of alphanumeric characters.
LENGTH="8"
==> May change 'LENGTH' for longer password.
while [ "${n:=1}" -le "$LENGTH" ]
==> Recall that := is "default substitution" operator.
==> So, if 'n' has not been initialized, set it to 1.
do
PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
# ==> Very clever, but tricky.
# ==> Starting from the innermost nesting...
# ==> ${#MATRIX} returns length of array MATRIX.
# ==> $RANDOM%${#MATRIX} returns random number between 1
# ==> and [length of MATRIX] - 1.
# ==> ${MATRIX:$(($RANDOM%${#MATRIX})):1}
# ==> returns expansion of MATRIX at random position, by length 1.
# ==> See {var:pos:len} parameter substitution in Chapter 9.
# ==> and the associated examples.
# ==> PASS=... simply pastes this result onto previous PASS (concatena
tion).
# ==> To visualize this more clearly, uncomment the following line
# echo "$PASS"
# ==> to see PASS being built up,
# ==> one character at a time, each iteration of the loop.
let n+=1
# ==> Increment 'n' for next pass.
done
echo "$PASS" # ==> Or, redirect to a file, as desired.
exit 0
James R. Van Zandt contributed this script which uses named pipes
and, in his words, "really exercises quoting and escaping."
Example A-14. fifo: Making daily backups, using named pipes
!/bin/bash
==> Script by James R. Van Zandt, and used here with his permission.
==> Comments added by author of this document.
HERE=uname -n # ==> hostname
THERE=bilbo
echo "starting remote backup to $THERE at date +%r"
==> date +%r returns time in 12-hour format, i.e. "08:08:34 PM".
make sure /pipe really is a pipe and not a plain file
rm -rf /pipe
mkfifo /pipe # ==> Create a "named pipe", named "/pipe" ...
==> 'su xyz' runs commands as user "xyz".
==> 'ssh' invokes secure shell (remote login client).
su xyz -c "ssh $THERE "cat > /home/xyz/backup/${HERE}-daily.tar.gz" < /pip
e"&
cd /
tar -czf - bin boot dev etc home info lib man root sbin share usr var > /pip
e
==> Uses named pipe, /pipe, to communicate between processes:
==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe.
==> The end result is this backs up the main directories, from / on down.
==> What are the advantages of a "named pipe" in this situation,
==>+ as opposed to an "anonymous pipe", with |?
==> Will an anonymous pipe even work here?
==> Is it necessary to delete the pipe before exiting the script?
==> How could that be done?
exit 0
Stéphane Chazelas used the following script to demonstrate generating
prime numbers without arrays.
Example A-15. Generating prime numbers using the modulo operator
!/bin/bash
primes.sh: Generate prime numbers, without using arrays.
Script contributed by Stephane Chazelas.
This does not use the classic "Sieve of Eratosthenes" algorithm,
- but instead the more intuitive method of testing each candidate number
- for factors (divisors), using the "%" modulo operator.
LIMIT=1000 # Primes, 2 ... 1000.
Primes()
{
(( n = $1 + 1 )) # Bump to next integer.
shift # Next parameter in list.
echo "n=$n i=$i"
if (( n == LIMIT ))
then echo $*
return
fi
for i; do # "i" set to "@", previous values of $n.
echo "-n=$n i=$i-"
(( i * i > n )) && break # Optimization.
(( n % i )) && continue # Sift out non-primes using modulo operator.
Primes $n $@ # Recursion inside loop.
return
done
Primes $n $@ $n # Recursion outside loop.
# Successively accumulate
#+ positional parameters.
# "$@" is the accumulating list of primes.
}
Primes 1
exit $?
Pipe output of the script to 'fmt' for prettier printing.
Uncomment lines 16 and 24 to help figure out what is going on.
Compare the speed of this algorithm for generating primes
- with the Sieve of Eratosthenes (ex68.sh).
Exercise: Rewrite this script without recursion.
Rick Boivie's revision of Jordi Sanfeliu's tree script.
Example A-16. tree: Displaying a directory tree
!/bin/bash
tree.sh
Written by Rick Boivie.
Used with permission.
This is a revised and simplified version of a script
- by Jordi Sanfeliu (the original author), and patched by Ian Kjos.
This script replaces the earlier version used in
- previous releases of the Advanced Bash Scripting Guide.
Copyright (c) 2002, by Jordi Sanfeliu, Rick Boivie, and Ian Kjos.
==> Comments added by the author of this document.
search () {
for dir in echo *
==> echo * lists all the files in current working directory,
==> Similar effect to for dir in *
==> but "dir in echo *" will not handle filenames with blanks.
do
if [ -d "$dir" ] ; then # ==> If it is a directory (-d)...
zz=0 # ==> Temp variable, keeping track of
# directory level.
while [ $zz != $1 ] # Keep track of inner nested loop.
do
echo -n "| " # ==> Display vertical connector symbol,
# ==> with 2 spaces & no line feed
# in order to indent.
zz=`expr $zz + 1` # ==> Increment zz.
done
if [ -L "$dir" ] ; then # ==> If directory is a symbolic link...
echo "+---$dir" `ls -l $dir | sed 's/^.*'$dir' //'`
# ==> Display horiz. connector and list directory name, but...
# ==> delete date/time part of long listing.
else
echo "+---$dir" # ==> Display horizontal connector symbol...
# ==> and print directory name.
numdirs=`expr $numdirs + 1` # ==> Increment directory count.
if cd "$dir" ; then # ==> If can move to subdirectory...
search `expr $1 + 1` # with recursion ;-)
# ==> Function calls itself.
cd ..
fi
fi
fi
done
}
if [ $# != 0 ] ; then
cd $1 # Move to indicated directory.
#else # stay in current directory
fi
echo "Initial directory = pwd"
numdirs=0
search 0
echo "Total directories = $numdirs"
exit 0
Patsie's version of a directory tree script.
Example A-17. tree2: Alternate directory tree script
!/bin/bash
tree2.sh
Lightly modified/reformatted by ABS Guide author.
Included in ABS Guide with permission of script author (thanks!).
Recursive file/dirsize checking script, by Patsie
This script builds a list of files/directories and their size (du -akx)
and processes this list to a human readable tree shape
The 'du -akx' is only as good as the permissions the owner has.
So preferably run as root* to get the best results, or use only on
directories for which you have read permissions. Anything you can't
read is not in the list.
- ABS Guide author advises caution when running scripts as root!
THIS IS CONFIGURABLE ##########
TOP=5 # Top 5 biggest (sub)directories.
MAXRECURS=5 # Max 5 subdirectories/recursions deep.
E_BL=80 # Blank line already returned.
E_DIR=81 # Directory not specified.
DON'T CHANGE ANYTHING BELOW THIS LINE ##########
PID=$$ # Our own process ID.
SELF=basename $0 # Our own program name.
TMP="/tmp/${SELF}.${PID}.tmp" # Temporary 'du' result.
Convert number to dotted thousand.
function dot { echo " $*" |
sed -e :a -e 's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta' |
tail -c 12; }
Usage: tree
function tree {
recurs="$1" # How deep nested are we?
prefix="$2" # What do we display before file/dirname?
minsize="$3" # What is the minumum file/dirsize?
dirname="$4" # Which directory are we checking?
Get ($TOP) biggest subdirs/subfiles from TMP file.
LIST=`egrep "[[:space:]]${dirname}/[^/]*$" "$TMP" |
awk '{if($1>'$minsize') print;}' | sort -nr | head -$TOP`
[ -z "$LIST" ] && return # Empty list, then go back.
cnt=0
num=echo "$LIST" | wc -l # How many entries in the list.
Main loop
echo "$LIST" | while read size name; do
((cnt+=1)) # Count entry number.
bname=`basename "$name"` # We only need a basename of the entry.
[ -d "$name" ] && bname="$bname/"
# If it's a directory, append a slash.
echo "`dot $size`$prefix +-$bname"
# Display the result.
# Call ourself recursively if it's a directory
#+ and we're not nested too deep ($MAXRECURS).
# The recursion goes up: $((recurs+1))
# The prefix gets a space if it's the last entry,
#+ or a pipe if there are more entries.
# The minimum file/dirsize becomes
#+ a tenth of his parent: $((size/10)).
# Last argument is the full directory name to check.
if [ -d "$name" -a $recurs -lt $MAXRECURS ]; then
[ $cnt -lt $num ] \
|| (tree $((recurs+1)) "$prefix " $((size/10)) "$name") \
&& (tree $((recurs+1)) "$prefix |" $((size/10)) "$name")
fi
done
[ $? -eq 0 ] && echo " $prefix"
Every time we jump back add a 'blank' line.
return $E_BL
We return 80 to tell we added a blank line already.
}
main program ###
rootdir="$@"
[ -d "$rootdir" ] ||
{ echo "$SELF: Usage: $SELF " >&2; exit $E_DIR; }
We should be called with a directory name.
echo "Building inventory list, please wait ..."
# Show "please wait" message.
du -akx "$rootdir" 1>"$TMP" 2>/dev/null
# Build a temporary list of all files/dirs and their size.
size=tail -1 "$TMP" | awk '{print $1}'
# What is our rootdirectory's size?
echo "dot $size $rootdir"
# Display rootdirectory's entry.
tree 0 "" 0 "$rootdir"
# Display the tree below our rootdirectory.
rm "$TMP" 2>/dev/null
# Clean up TMP file.
exit $?
Noah Friedman permitted use of his string function script. It
essentially reproduces some of the C-library string manipulation
functions.
Example A-18. string functions: C-style string functions
!/bin/bash
string.bash --- bash emulation of string(3) library routines
==> Used with his kind permission in this document.
Created: 1992-07-01
Last modified: 1993-09-29
Public domain
Conversion to bash v2 syntax done by Chet Ramey
Commentary:
Code:
:docstring strcat:
Usage: strcat s1 s2
Strcat appends the value of variable s2 to variable s1.
Example:
a="foo"
b="bar"
strcat a b
echo $a
=> foobar
:end docstring:
;;;autoload ==> Autoloading of function commented out.
function strcat ()
{
local s1_val s2_val
s1_val=${!1} # indirect variable expansion
s2_val=${!2}
eval "$1"=\'"${s1_val}${s2_val}"\'
# ==> eval $1='${s1_val}${s2_val}' avoids problems,
# ==> if one of the variables contains a single quote.
}
:docstring strncat:
Usage: strncat s1 s2 $n
Line strcat, but strncat appends a maximum of n characters from the value
of variable s2. It copies fewer if the value of variabl s2 is shorter
than n characters. Echoes result on stdout.
Example:
a=foo
b=barbaz
strncat a b 3
echo $a
=> foobar
:end docstring:
;;;autoload
function strncat ()
{
local s1="$1"
local s2="$2"
local -i n="$3"
local s1_val s2_val
s1_val=${!s1} # ==> indirect variable expansion
s2_val=${!s2}
if [ ${#s2_val} -gt ${n} ]; then
s2_val=${s2_val:0:$n} # ==> substring extraction
fi
eval "$s1"=\'"${s1_val}${s2_val}"\'
# ==> eval $1='${s1_val}${s2_val}' avoids problems,
# ==> if one of the variables contains a single quote.
}
:docstring strcmp:
Usage: strcmp $s1 $s2
Strcmp compares its arguments and returns an integer less than, equal to,
or greater than zero, depending on whether string s1 is lexicographically
less than, equal to, or greater than string s2.
:end docstring:
;;;autoload
function strcmp ()
{
[ "$1" = "$2" ] && return 0
[ "${1}" '<' "${2}" ] > /dev/null && return -1
return 1
}
:docstring strncmp:
Usage: strncmp $s1 $s2 $n
Like strcmp, but makes the comparison by examining a maximum of n
characters (n less than or equal to zero yields equality).
:end docstring:
;;;autoload
function strncmp ()
{
if [ -z "${3}" -o "${3}" -le "0" ]; then
return 0
fi
if [ ${3} -ge ${#1} -a ${3} -ge ${#2} ]; then
strcmp "$1" "$2"
return $?
else
s1=${1:0:$3}
s2=${2:0:$3}
strcmp $s1 $s2
return $?
fi
}
:docstring strlen:
Usage: strlen s
Strlen returns the number of characters in string literal s.
:end docstring:
;;;autoload
function strlen ()
{
eval echo "\${#${1}}"
# ==> Returns the length of the value of the variable
# ==> whose name is passed as an argument.
}
:docstring strspn:
Usage: strspn $s1 $s2
Strspn returns the length of the maximum initial segment of string s1,
which consists entirely of characters from string s2.
:end docstring:
;;;autoload
function strspn ()
{
# Unsetting IFS allows whitespace to be handled as normal chars.
local IFS=
local result="${1%%[!${2}]*}"
echo ${#result}
}
:docstring strcspn:
Usage: strcspn $s1 $s2
Strcspn returns the length of the maximum initial segment of string s1,
which consists entirely of characters not from string s2.
:end docstring:
;;;autoload
function strcspn ()
{
# Unsetting IFS allows whitspace to be handled as normal chars.
local IFS=
local result="${1%%[${2}]*}"
echo ${#result}
}
:docstring strstr:
Usage: strstr s1 s2
Strstr echoes a substring starting at the first occurrence of string s2 in
string s1, or nothing if s2 does not occur in the string. If s2 points to
a string of zero length, strstr echoes s1.
:end docstring:
;;;autoload
function strstr ()
{
# if s2 points to a string of zero length, strstr echoes s1
[ ${#2} -eq 0 ] && { echo "$1" ; return 0; }
# strstr echoes nothing if s2 does not occur in s1
case "$1" in
*$2*) ;;
*) return 1;;
esac
# use the pattern matching code to strip off the match and everything
# following it
first=${1/$2*/}
# then strip off the first unmatched portion of the string
echo "${1##$first}"
}
:docstring strtok:
Usage: strtok s1 s2
Strtok considers the string s1 to consist of a sequence of zero or more
text tokens separated by spans of one or more characters from the
separator string s2. The first call (with a non-empty string s1
specified) echoes a string consisting of the first token on stdout. The
function keeps track of its position in the string s1 between separate
calls, so that subsequent calls made with the first argument an empty
string will work through the string immediately following that token. In
this way subsequent calls will work through the string s1 until no tokens
remain. The separator string s2 may be different from call to call.
When no token remains in s1, an empty value is echoed on stdout.
:end docstring:
;;;autoload
function strtok ()
{
:
}
:docstring strtrunc:
Usage: strtrunc $n $s1 {$s2} {$...}
Used by many functions like strncmp to truncate arguments for comparison.
Echoes the first n characters of each string s1 s2 ... on stdout.
:end docstring:
;;;autoload
function strtrunc ()
{
n=$1 ; shift
for z; do
echo "${z:0:$n}"
done
}
provide string
string.bash ends here
========================================================================== #
==> Everything below here added by the document author.
==> Suggested use of this script is to delete everything below here,
==> and "source" this file into your own scripts.
strcat
string0=one
string1=two
echo
echo "Testing "strcat" function:"
echo "Original "string0" = $string0"
echo ""string1" = $string1"
strcat string0 string1
echo "New "string0" = $string0"
echo
strlen
echo
echo "Testing "strlen" function:"
str=123456789
echo ""str" = $str"
echo -n "Length of "str" = "
strlen str
echo
Exercise:
Add code to test all the other string functions above.
exit 0
Michael Zick's complex array example uses the md5sum check sum
command to encode directory information.
Example A-19. Directory information
! /bin/bash
directory-info.sh
Parses and lists directory information.
NOTE: Change lines 273 and 353 per "README" file.
Michael Zick is the author of this script.
Used here with his permission.
Controls
If overridden by command arguments, they must be in the order:
Arg1: "Descriptor Directory"
Arg2: "Exclude Paths"
Arg3: "Exclude Directories"
Environment Settings override Defaults.
Command arguments override Environment Settings.
Default location for content addressed file descriptors.
MD5UCFS=${1:-${MD5UCFS:-'/tmpfs/ucfs'}}
Directory paths never to list or enter
declare -a \
EXCLUDE_PATHS=${2:-${EXCLUDE_PATHS:-'(/proc /dev /devfs /tmpfs)'}}
Directories never to list or enter
declare -a \
EXCLUDE_DIRS=${3:-${EXCLUDE_DIRS:-'(ucfs lost+found tmp wtmp)'}}
Files never to list or enter
declare -a \
EXCLUDE_FILES=${3:-${EXCLUDE_FILES:-'(core "Name with Spaces")'}}
Here document used as a comment block.
: <<LSfieldsDoc
# # # List Filesystem Directory Information # # # #
ListDirectory "FileGlob" "Field-Array-Name"
or
ListDirectory -of "FileGlob" "Field-Array-Filename"
'-of' meaning 'output to filename'
# #
String format description based on: ls (GNU fileutils) version 4.0.36
Produces a line (or more) formatted:
inode permissions hard-links owner group ...
32736 -rw------- 1 mszick mszick
size day month date hh:mm:ss year path
2756608 Sun Apr 20 08:53:06 2003 /home/mszick/core
Unless it is formatted:
inode permissions hard-links owner group ...
266705 crw-rw---- 1 root uucp
major minor day month date hh:mm:ss year path
4, 68 Sun Apr 20 09:27:33 2003 /dev/ttyS4
NOTE: that pesky comma after the major number
NOTE: the 'path' may be multiple fields:
/home/mszick/core
/proc/982/fd/0 -> /dev/null
/proc/982/fd/1 -> /home/mszick/.xsession-errors
/proc/982/fd/13 -> /tmp/tmpfZVVOCs (deleted)
/proc/982/fd/7 -> /tmp/kde-mszick/ksycoca
/proc/982/fd/8 -> socket:[11586]
/proc/982/fd/9 -> pipe:[11588]
If that isn't enough to keep your parser guessing,
either or both of the path components may be relative:
../Built-Shared -> Built-Static
../linux-2.4.20.tar.bz2 -> ../../../SRCS/linux-2.4.20.tar.bz2
The first character of the 11 (10?) character permissions field:
's' Socket
'd' Directory
'b' Block device
'c' Character device
'l' Symbolic link
NOTE: Hard links not marked - test for identical inode numbers
on identical filesystems.
All information about hard linked files are shared, except
for the names and the name's location in the directory system.
NOTE: A "Hard link" is known as a "File Alias" on some systems.
'-' An undistingushed file
Followed by three groups of letters for: User, Group, Others
Character 1: '-' Not readable; 'r' Readable
Character 2: '-' Not writable; 'w' Writable
Character 3, User and Group: Combined execute and special
'-' Not Executable, Not Special
'x' Executable, Not Special
's' Executable, Special
'S' Not Executable, Special
Character 3, Others: Combined execute and sticky (tacky?)
'-' Not Executable, Not Tacky
'x' Executable, Not Tacky
't' Executable, Tacky
'T' Not Executable, Tacky
Followed by an access indicator
Haven't tested this one, it may be the eleventh character
or it may generate another field
' ' No alternate access
'+' Alternate access
LSfieldsDoc
ListDirectory()
{
local -a T
local -i of=0 # Default return in variable
OLD_IFS=$IFS # Using BASH default ' \t\n'
case "$#" in
3) case "$1" in
-of) of=1 ; shift ;;
* ) return 1 ;;
esac ;;
2) : ;; # Poor man's "continue"
*) return 1 ;;
esac
# NOTE: the (ls) command is NOT quoted (")
T=( $(ls --inode --ignore-backups --almost-all --directory \
--full-time --color=none --time=status --sort=none \
--format=long $1) )
case $of in
# Assign T back to the array whose name was passed as $2
0) eval $2=\( \"\$\{T\[@\]\}\" \) ;;
# Write T into filename passed as $2
1) echo "${T[@]}" > "$2" ;;
esac
return 0
}
# # # Is that string a legal number? # # # #
IsNumber "Var"
# # # There has to be a better way, sigh...
IsNumber()
{
local -i int
if [ $# -eq 0 ]
then
return 1
else
(let int=$1) 2>/dev/null
return $? # Exit status of the let thread
fi
}
# # # Index Filesystem Directory Information # # # #
IndexList "Field-Array-Name" "Index-Array-Name"
or
IndexList -if Field-Array-Filename Index-Array-Name
IndexList -of Field-Array-Name Index-Array-Filename
IndexList -if -of Field-Array-Filename Index-Array-Filename
# #
: <<IndexListDoc
Walk an array of directory fields produced by ListDirectory
Having suppressed the line breaks in an otherwise line oriented
report, build an index to the array element which starts each line.
Each line gets two index entries, the first element of each line
(inode) and the element that holds the pathname of the file.
The first index entry pair (Line-Number==0) are informational:
Index-Array-Name[0] : Number of "Lines" indexed
Index-Array-Name[1] : "Current Line" pointer into Index-Array-Name
The following index pairs (if any) hold element indexes into
the Field-Array-Name per:
Index-Array-Name[Line-Number * 2] : The "inode" field element.
NOTE: This distance may be either +11 or +12 elements.
Index-Array-Name[(Line-Number * 2) + 1] : The "pathname" element.
NOTE: This distance may be a variable number of elements.
Next line index pair for Line-Number+1.
IndexListDoc
IndexList()
{
local -a LIST # Local of listname passed
local -a -i INDEX=( 0 0 ) # Local of index to return
local -i Lidx Lcnt
local -i if=0 of=0 # Default to variable names
case "$#" in # Simplistic option testing
0) return 1 ;;
1) return 1 ;;
2) : ;; # Poor man's continue
3) case "$1" in
-if) if=1 ;;
-of) of=1 ;;
* ) return 1 ;;
esac ; shift ;;
4) if=1 ; of=1 ; shift ; shift ;;
*) return 1
esac
# Make local copy of list
case "$if" in
0) eval LIST=\( \"\$\{$1\[@\]\}\" \) ;;
1) LIST=( $(cat $1) ) ;;
esac
# Grok (grope?) the array
Lcnt=${#LIST[@]}
Lidx=0
until (( Lidx >= Lcnt ))
do
if IsNumber ${LIST[$Lidx]}
then
local -i inode name
local ft
inode=Lidx
local m=${LIST[$Lidx+2]} # Hard Links field
ft=${LIST[$Lidx+1]:0:1} # Fast-Stat
case $ft in
b) ((Lidx+=12)) ;; # Block device
c) ((Lidx+=12)) ;; # Character device
*) ((Lidx+=11)) ;; # Anything else
esac
name=Lidx
case $ft in
-) ((Lidx+=1)) ;; # The easy one
b) ((Lidx+=1)) ;; # Block device
c) ((Lidx+=1)) ;; # Character device
d) ((Lidx+=1)) ;; # The other easy one
l) ((Lidx+=3)) ;; # At LEAST two more fields
A little more elegance here would handle pipes,
- sockets, deleted files - later.
*) until IsNumber ${LIST[$Lidx]} || ((Lidx >= Lcnt))
do
((Lidx+=1))
done
;; # Not required
esac
INDEX[${#INDEX[*]}]=$inode
INDEX[${#INDEX[*]}]=$name
INDEX[0]=${INDEX[0]}+1 # One more "line" found
echo "Line: ${INDEX[0]} Type: $ft Links: $m Inode: \
${LIST[$inode]} Name: ${LIST[$name]}"
else
((Lidx+=1))
fi
done
case "$of" in
0) eval $2=\( \"\$\{INDEX\[@\]\}\" \) ;;
1) echo "${INDEX[@]}" > "$2" ;;
esac
return 0 # What could go wrong?
}
# # # Content Identify File # # # #
DigestFile Input-Array-Name Digest-Array-Name
or
DigestFile -if Input-FileName Digest-Array-Name
# #
Here document used as a comment block.
: <<DigestFilesDoc
The key (no pun intended) to a Unified Content File System (UCFS)
is to distinguish the files in the system based on their content.
Distinguishing files by their name is just so 20th Century.
The content is distinguished by computing a checksum of that content.
This version uses the md5sum program to generate a 128 bit checksum
representative of the file's contents.
There is a chance that two files having different content might
generate the same checksum using md5sum (or any checksum). Should
that become a problem, then the use of md5sum can be replace by a
cyrptographic signature. But until then...
The md5sum program is documented as outputting three fields (and it
does), but when read it appears as two fields (array elements). This
is caused by the lack of whitespace between the second and third field.
So this function gropes the md5sum output and returns:
[0] 32 character checksum in hexidecimal (UCFS filename)
[1] Single character: ' ' text file, '*' binary file
[2] Filesystem (20th Century Style) name
Note: That name may be the character '-' indicating STDIN read.
DigestFilesDoc
DigestFile()
{
local if=0 # Default, variable name
local -a T1 T2
case "$#" in
3) case "$1" in
-if) if=1 ; shift ;;
* ) return 1 ;;
esac ;;
2) : ;; # Poor man's "continue"
*) return 1 ;;
esac
case $if in
0) eval T1=\( \"\$\{$1\[@\]\}\" \)
T2=( $(echo ${T1[@]} | md5sum -) )
;;
1) T2=( $(md5sum $1) )
;;
esac
case ${#T2[@]} in
0) return 1 ;;
1) return 1 ;;
2) case ${T2[1]:0:1} in # SanScrit-2.0.5
\*) T2[${#T2[@]}]=${T2[1]:1}
T2[1]=\*
;;
*) T2[${#T2[@]}]=${T2[1]}
T2[1]=" "
;;
esac
;;
3) : ;; # Assume it worked
*) return 1 ;;
esac
local -i len=${#T2[0]}
if [ $len -ne 32 ] ; then return 1 ; fi
eval $2=\( \"\$\{T2\[@\]\}\" \)
}
# # # Locate File # # # #
LocateFile [-l] FileName Location-Array-Name
or
LocateFile [-l] -of FileName Location-Array-FileName
# #
A file location is Filesystem-id and inode-number
Here document used as a comment block.
: <<StatFieldsDoc
Based on stat, version 2.2
stat -t and stat -lt fields
[0] name
[1] Total size
File - number of bytes
Symbolic link - string length of pathname
[2] Number of (512 byte) blocks allocated
[3] File type and Access rights (hex)
[4] User ID of owner
[5] Group ID of owner
[6] Device number
[7] Inode number
[8] Number of hard links
[9] Device type (if inode device) Major
[10] Device type (if inode device) Minor
[11] Time of last access
May be disabled in 'mount' with noatime
atime of files changed by exec, read, pipe, utime, mknod (mmap
?)
atime of directories changed by addition/deletion of files
[12] Time of last modification
mtime of files changed by write, truncate, utime, mknod
mtime of directories changed by addtition/deletion of files
[13] Time of last change
ctime reflects time of changed inode information (owner, group
permissions, link count
--- Per:
Return code: 0
Size of array: 14
Contents of array
Element 0: /home/mszick
Element 1: 4096
Element 2: 8
Element 3: 41e8
Element 4: 500
Element 5: 500
Element 6: 303
Element 7: 32385
Element 8: 22
Element 9: 0
Element 10: 0
Element 11: 1051221030
Element 12: 1051214068
Element 13: 1051214068
For a link in the form of linkname -> realname
stat -t linkname returns the linkname (link) information
stat -lt linkname returns the realname information
stat -tf and stat -ltf fields
[0] name
[1] ID-0? # Maybe someday, but Linux stat structure
[2] ID-0? # does not have either LABEL nor UUID
# fields, currently information must come
# from file-system specific utilities
These will be munged into:
[1] UUID if possible
[2] Volume Label if possible
Note: 'mount -l' does return the label and could return the UUID
[3] Maximum length of filenames
[4] Filesystem type
[5] Total blocks in the filesystem
[6] Free blocks
[7] Free blocks for non-root user(s)
[8] Block size of the filesystem
[9] Total inodes
[10] Free inodes
--- Per:
Return code: 0
Size of array: 11
Contents of array
Element 0: /home/mszick
Element 1: 0
Element 2: 0
Element 3: 255
Element 4: ef53
Element 5: 2581445
Element 6: 2277180
Element 7: 2146050
Element 8: 4096
Element 9: 1311552
Element 10: 1276425
StatFieldsDoc
LocateFile [-l] FileName Location-Array-Name
LocateFile [-l] -of FileName Location-Array-FileName
LocateFile()
{
local -a LOC LOC1 LOC2
local lk="" of=0
case "$#" in
0) return 1 ;;
1) return 1 ;;
2) : ;;
*) while (( "$#" > 2 ))
do
case "$1" in
-l) lk=-1 ;;
-of) of=1 ;;
*) return 1 ;;
esac
shift
done ;;
esac
More Sanscrit-2.0.5
# LOC1=( $(stat -t $lk $1) )
# LOC2=( $(stat -tf $lk $1) )
# Uncomment above two lines if system has "stat" command installed.
LOC=( ${LOC1[@]:0:1} ${LOC1[@]:3:11}
${LOC2[@]:1:2} ${LOC2[@]:4:1} )
case "$of" in
0) eval $2=\( \"\$\{LOC\[@\]\}\" \) ;;
1) echo "${LOC[@]}" > "$2" ;;
esac
return 0
Which yields (if you are lucky, and have "stat" installed)
--- Location Discriptor ---
Return code: 0
Size of array: 15
Contents of array
Element 0: /home/mszick 20th Century name
Element 1: 41e8 Type and Permissions
Element 2: 500 User
Element 3: 500 Group
Element 4: 303 Device
Element 5: 32385 inode
Element 6: 22 Link count
Element 7: 0 Device Major
Element 8: 0 Device Minor
Element 9: 1051224608 Last Access
Element 10: 1051214068 Last Modify
Element 11: 1051214068 Last Status
Element 12: 0 UUID (to be)
Element 13: 0 Volume Label (to be)
Element 14: ef53 Filesystem type
}
And then there was some test code
ListArray() # ListArray Name
{
local -a Ta
eval Ta=\( \"\$\{$1\[@\]\}\" \)
echo
echo "-*-*- List of Array -*-*-"
echo "Size of array $1: ${#Ta[*]}"
echo "Contents of array $1:"
for (( i=0 ; i<${#Ta[*]} ; i++ ))
do
echo -e "\tElement $i: ${Ta[$i]}"
done
return 0
}
declare -a CUR_DIR
For small arrays
ListDirectory "${PWD}" CUR_DIR
ListArray CUR_DIR
declare -a DIR_DIG
DigestFile CUR_DIR DIR_DIG
echo "The new "name" (checksum) for ${CUR_DIR[9]} is ${DIR_DIG[0]}"
declare -a DIR_ENT
BIG_DIR # For really big arrays - use a temporary file in ramdisk
BIG-DIR # ListDirectory -of "${CUR_DIR[11]}/*" "/tmpfs/junk2"
ListDirectory "${CUR_DIR[11]}/*" DIR_ENT
declare -a DIR_IDX
BIG-DIR # IndexList -if "/tmpfs/junk2" DIR_IDX
IndexList DIR_ENT DIR_IDX
declare -a IDX_DIG
BIG-DIR # DIR_ENT=( $(cat /tmpfs/junk2) )
BIG-DIR # DigestFile -if /tmpfs/junk2 IDX_DIG
DigestFile DIR_ENT IDX_DIG
Small (should) be able to parallize IndexList & DigestFile
Large (should) be able to parallize IndexList & DigestFile & the assignment
echo "The "name" (checksum) for the contents of ${PWD} is ${IDX_DIG[0]}"
declare -a FILE_LOC
LocateFile ${PWD} FILE_LOC
ListArray FILE_LOC
exit 0
Stéphane Chazelas demonstrates object-oriented programming in a Bash
script.
Mariusz Gniazdowski contributed a hash library for use in scripts.
Example A-20. Library of hash functions
Hash:
Hash function library
Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com>
Date: 2005-04-07
Functions making emulating hashes in Bash a little less painful.
Limitations:
- Only global variables are supported.
- Each hash instance generates one global variable per value.
- Variable names collisions are possible
- if you define variable like __hash__hashname_key
- Keys must use chars that can be part of a Bash variable name
- (no dashes, periods, etc.).
- The hash is created as a variable:
... hashname_keyname
So if somone will create hashes like:
myhash_ + mykey = myhash__mykey
myhash + _mykey = myhash__mykey
Then there will be a collision.
(This should not pose a major problem.)
Hash_config_varname_prefix=hash
Emulates: hash[key]=value
Params:
1 - hash
2 - key
3 - value
function hash_set {
eval "${Hash_config_varname_prefix}${1}_${2}=\"${3}\""
}
Emulates: value=hash[key]
Params:
1 - hash
2 - key
3 - value (name of global variable to set)
function hash_get_into {
eval "$3=\"\$${Hash_config_varname_prefix}${1}_${2}\""
}
Emulates: echo hash[key]
Params:
1 - hash
2 - key
3 - echo params (like -n, for example)
function hash_echo {
eval "echo $3 \"\$${Hash_config_varname_prefix}${1}_${2}\""
}
Emulates: hash1[key1]=hash2[key2]
Params:
1 - hash1
2 - key1
3 - hash2
4 - key2
function hash_copy {
eval "${Hash_config_varname_prefix}${1}_${2}\
=> "$${Hash_config_varname_prefix}${3}_${4}""
}
Emulates: hash[keyN-1]=hash[key2]=...hash[key1]
Copies first key to rest of keys.
Params:
1 - hash1
2 - key1
3 - key2
. . .
N - keyN
function hash_dup {
local hashName="$1" keyName="$2"
shift 2
until [ ${#} -le 0 ]; do
eval "${Hash_config_varname_prefix}${hashName}_${1}\
=> "$${Hash_config_varname_prefix}${hashName}_${keyName}""
shift;
done;
}
Emulates: unset hash[key]
Params:
1 - hash
2 - key
function hash_unset {
eval "unset ${Hash_config_varname_prefix}${1}_${2}"
}
Emulates something similar to: ref=&hash[key]
The reference is name of the variable in which value is held.
Params:
1 - hash
2 - key
3 - ref - Name of global variable to set.
function hash_get_ref_into {
eval "$3=\"${Hash_config_varname_prefix}${1}_${2}\""
}
Emulates something similar to: echo &hash[key]
That reference is name of variable in which value is held.
Params:
1 - hash
2 - key
3 - echo params (like -n for example)
function hash_echo_ref {
eval "echo $3 \"${Hash_config_varname_prefix}${1}_${2}\""
}
Emulates something similar to: $$hash[key](param1, param2, ...)
Params:
1 - hash
2 - key
3,4, ... - Function parameters
function hash_call {
local hash key
hash=$1
key=$2
shift 2
eval "eval "$${Hash_config_varname_prefix}${hash}_${key} \"\$@\"""
}
Emulates something similar to: isset(hash[key]) or hash[key]==NULL
Params:
1 - hash
2 - key
Returns:
0 - there is such key
1 - there is no such key
function hash_is_set {
eval "if [[ "${${Hash_config_varname_prefix}${1}_${2}-a}" = "a" &&
"${${Hash_config_varname_prefix}${1}_${2}-b}" = "b" ]]
then return 1; else return 0; fi"
}
Emulates something similar to:
foreach($hash as $key => $value) { fun($key,$value); }
It is possible to write different variations of this function.
Here we use a function call to make it as "generic" as possible.
Params:
1 - hash
2 - function name
function hash_foreach {
local keyname oldIFS="$IFS"
IFS=' '
for i in $(eval "echo ${!${Hash_config_varname_prefix}${1}_*}"); do
keyname=$(eval "echo \${i##${Hash_config_varname_prefix}${1}_}")
eval "$2 $keyname \"\$$i\""
done
IFS="$oldIFS"
}
NOTE: In lines 103 and 116, ampersand changed.
But, it doesn't matter, because these are comment lines anyhow.
Here is an example script using the foregoing hash library.
Example A-21. Colorizing text using hash functions
!/bin/bash
hash-example.sh: Colorizing text.
Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com>
. Hash.lib # Load the library of functions.
hash_set colors red "\033[0;31m"
hash_set colors blue "\033[0;34m"
hash_set colors light_blue "\033[1;34m"
hash_set colors light_red "\033[1;31m"
hash_set colors cyan "\033[0;36m"
hash_set colors light_green "\033[1;32m"
hash_set colors light_gray "\033[0;37m"
hash_set colors green "\033[0;32m"
hash_set colors yellow "\033[1;33m"
hash_set colors light_purple "\033[1;35m"
hash_set colors purple "\033[0;35m"
hash_set colors reset_color "\033[0;00m"
$1 - keyname
$2 - value
try_colors() {
echo -en "$2"
echo "This line is $1."
}
hash_foreach colors try_colors
hash_echo colors reset_color -en
echo -e '\nLet us overwrite some colors with yellow.\n'
It's hard to read yellow text on some terminals.
hash_dup colors yellow red light_green blue green light_gray cyan
hash_foreach colors try_colors
hash_echo colors reset_color -en
echo -e '\nLet us delete them and try colors once more . . .\n'
for i in red light_green blue green light_gray cyan; do
hash_unset colors $i
done
hash_foreach colors try_colors
hash_echo colors reset_color -en
hash_set other txt "Other examples . . ."
hash_echo other txt
hash_get_into other txt text
echo $text
hash_set other my_fun try_colors
hash_call other my_fun purple "hash_echo colors purple"
hash_echo colors reset_color -en
echo; echo "Back to normal?"; echo
exit $?
On some terminals, the "light" colors print in bold,
and end up looking darker than the normal ones.
Why is this?
An example illustrating the mechanics of hashing, but from a
different point of view.
Example A-22. More on hash functions
!/bin/bash
$Id: ha.sh,v 1.2 2005/04/21 23:24:26 oliver Exp $
Copyright 2005 Oliver Beckstein
Released under the GNU Public License
Author of script granted permission for inclusion in ABS Guide.
(Thank you!)
pseudo hash based on indirect parameter expansion
API: access through functions:
create the hash:
newhash Lovers
add entries (note single quotes for spaces)
addhash Lovers Tristan Isolde
addhash Lovers 'Romeo Montague' 'Juliet Capulet'
access value by key
gethash Lovers Tristan ----> Isolde
show all keys
keyshash Lovers ----> 'Tristan' 'Romeo Montague'
Convention: instead of perls' foo{bar} = boing' syntax,
use
'_foo_bar=boing' (two underscores, no spaces)
- store key in _NAME_keys[]
- store value in _NAME_values[] using the same integer index
The integer index for the last entry is _NAME_ptr
NOTE: No error or sanity checks, just bare bones.
function _inihash () {
# private function
# call at the beginning of each procedure
# defines: _keys _values _ptr
#
# Usage: _inihash NAME
local name=$1
_keys=_${name}_keys
_values=_${name}_values
_ptr=_${name}_ptr
}
function newhash () {
# Usage: newhash NAME
# NAME should not contain spaces or dots.
# Actually: it must be a legal name for a Bash variable.
# We rely on Bash automatically recognising arrays.
local name=$1
local _keys _values _ptr
_inihash ${name}
eval ${_ptr}=0
}
function addhash () {
# Usage: addhash NAME KEY 'VALUE with spaces'
# arguments with spaces need to be quoted with single quotes ''
local name=$1 k="$2" v="$3"
local _keys _values _ptr
_inihash ${name}
#echo "DEBUG(addhash): ${_ptr}=${!_ptr}"
eval let ${_ptr}=${_ptr}+1
eval "$_keys[${!_ptr}]=\"${k}\""
eval "$_values[${!_ptr}]=\"${v}\""
}
function gethash () {
# Usage: gethash NAME KEY
# Returns boing
# ERR=0 if entry found, 1 otherwise
# That's not a proper hash --
#+ we simply linearly search through the keys.
local name=$1 key="$2"
local _keys _values _ptr
local k v i found h
_inihash ${name}
# _ptr holds the highest index in the hash
found=0
for i in $(seq 1 ${!_ptr}); do
h="\${${_keys}[${i}]}" # Safer to do it in two steps,
eval k=${h} #+ especially when quoting for spaces.
if [ "${k}" = "${key}" ]; then found=1; break; fi
done;
[ ${found} = 0 ] && return 1;
# else: i is the index that matches the key
h="\${${_values}[${i}]}"
eval echo "${h}"
return 0;
}
function keyshash () {
# Usage: keyshash NAME
# Returns list of all keys defined for hash name.
local name=$1 key="$2"
local _keys _values _ptr
local k i h
_inihash ${name}
# _ptr holds the highest index in the hash
for i in $(seq 1 ${!_ptr}); do
h="\${${_keys}[${i}]}" # Safer to do it in two steps,
eval k=${h} #+ especially when quoting for spaces.
echo -n "'${k}' "
done;
}
Now, let's test it.
(Per comments at the beginning of the script.)
newhash Lovers
addhash Lovers Tristan Isolde
addhash Lovers 'Romeo Montague' 'Juliet Capulet'
Output results.
echo
gethash Lovers Tristan # Isolde
echo
keyshash Lovers # 'Tristan' 'Romeo Montague'
echo; echo
exit 0
Exercise:
Add error checks to the functions.
Now for a script that installs and mounts those cute USB keychain
solid-state "hard drives."
Example A-23. Mounting USB keychain storage devices
!/bin/bash
==> usb.sh
==> Script for mounting and installing pen/keychain USB storage devices.
==> Runs as root at system startup (see below).
==>
==> Newer Linux distros (2004 or later) autodetect
==> and install USB pen drives, and therefore don't need this script.
==> But, it's still instructive.
This code is free software covered by GNU GPL license version 2 or above.
Please refer to http://www.gnu.org/ for the full license text.
Some code lifted from usb-mount by Michael Hamilton's usb-mount (LGPL)
- see http://users.actrix.co.nz/michael/usbmount.html
INSTALL
Put this in /etc/hotplug/usb/diskonkey.
Then look in /etc/hotplug/usb.distmap, and copy all usb-storage entries
- into /etc/hotplug/usb.usermap, substituting "usb-storage" for "diskonkey".
Otherwise this code is only run during the kernel module invocation/removal
- (at least in my tests), which defeats the purpose.
TODO
Handle more than one diskonkey device at one time (e.g. /dev/diskonkey1
- and /mnt/diskonkey1), etc. The biggest problem here is the handling in
- devlabel, which I haven't yet tried.
AUTHOR and SUPPORT
Konstantin Riabitsev, .
Send any problem reports to my email address at the moment.
==> Comments added by ABS Guide author.
SYMLINKDEV=/dev/diskonkey
MOUNTPOINT=/mnt/diskonkey
DEVLABEL=/sbin/devlabel
DEVLABELCONFIG=/etc/sysconfig/devlabel
IAM=$0
Functions lifted near-verbatim from usb-mount code.
function allAttachedScsiUsb {
find /proc/scsi/ -path '/proc/scsi/usb-storage*' -type f |
xargs grep -l 'Attached: Yes'
}
function scsiDevFromScsiUsb {
echo $1 | awk -F"[-/]" '{ n=$(NF-1);
print "/dev/sd" substr("abcdefghijklmnopqrstuvwxyz", n+1, 1) }'
}
if [ "${ACTION}" = "add" ] && [ -f "${DEVICE}" ]; then
##
# lifted from usbcam code.
#
if [ -f /var/run/console.lock ]; then
CONSOLEOWNER=`cat /var/run/console.lock`
elif [ -f /var/lock/console.lock ]; then
CONSOLEOWNER=`cat /var/lock/console.lock`
else
CONSOLEOWNER=
fi
for procEntry in $(allAttachedScsiUsb); do
scsiDev=$(scsiDevFromScsiUsb $procEntry)
# Some bug with usb-storage?
# Partitions are not in /proc/partitions until they are accessed
#+ somehow.
/sbin/fdisk -l $scsiDev >/dev/null
##
# Most devices have partitioning info, so the data would be on
#+ /dev/sd?1. However, some stupider ones don't have any partitioning
#+ and use the entire device for data storage. This tries to
#+ guess semi-intelligently if we have a /dev/sd?1 and if not, then
#+ it uses the entire device and hopes for the better.
#
if grep -q `basename $scsiDev`1 /proc/partitions; then
part="$scsiDev""1"
else
part=$scsiDev
fi
##
# Change ownership of the partition to the console user so they can
#+ mount it.
#
if [ ! -z "$CONSOLEOWNER" ]; then
chown $CONSOLEOWNER:disk $part
fi
##
# This checks if we already have this UUID defined with devlabel.
# If not, it then adds the device to the list.
#
prodid=`$DEVLABEL printid -d $part`
if ! grep -q $prodid $DEVLABELCONFIG; then
# cross our fingers and hope it works
$DEVLABEL add -d $part -s $SYMLINKDEV 2>/dev/null
fi
##
# Check if the mount point exists and create if it doesn't.
#
if [ ! -e $MOUNTPOINT ]; then
mkdir -p $MOUNTPOINT
fi
##
# Take care of /etc/fstab so mounting is easy.
#
if ! grep -q "^$SYMLINKDEV" /etc/fstab; then
# Add an fstab entry
echo -e \
"$SYMLINKDEV\t\t$MOUNTPOINT\t\tauto\tnoauto,owner,kudzu 0 0" \
>> /etc/fstab
fi
done
if [ ! -z "$REMOVER" ]; then
##
# Make sure this script is triggered on device removal.
#
mkdir -p `dirname $REMOVER`
ln -s $IAM $REMOVER
fi
elif [ "${ACTION}" = "remove" ]; then
##
# If the device is mounted, unmount it cleanly.
#
if grep -q "$MOUNTPOINT" /etc/mtab; then
# unmount cleanly
umount -l $MOUNTPOINT
fi
##
# Remove it from /etc/fstab if it's there.
#
if grep -q "^$SYMLINKDEV" /etc/fstab; then
grep -v "^$SYMLINKDEV" /etc/fstab > /etc/.fstab.new
mv -f /etc/.fstab.new /etc/fstab
fi
fi
exit 0
Converting a text file to HTML format.
Example A-24. Converting to HTML
!/bin/bash
tohtml.sh [v. 0.2.01, reldate: 04/13/12, a teeny bit less buggy]
Convert a text file to HTML format.
Author: Mendel Cooper
License: GPL3
Usage: sh tohtml.sh < textfile > htmlfile
Script can easily be modified to accept source and target filenames.
Assumptions:
- Paragraphs in (target) text file are separated by a blank line.
- Jpeg images (*.jpg) are located in "images" subdirectory.
In the target file, the image names are enclosed in square brackets,
for example, [image01.jpg].
- Emphasized (italic) phrases begin with a space+underscore
- or the first character on the line is an underscore,
- and end with an underscore+space or underscore+end-of-line.
Settings
FNTSIZE=2 # Small-medium font size
IMGDIR="images" # Image directory
Headers
HDR01=''
HDR02=''
HDR03=''
HDR10=''
HDR11=''
HDR11a=''
HDR12a=''
HDR12b=''
HDR121=''
HDR13='' # Change background color to suit.
HDR14a='<font size='
HDR14b='>'
Footers
FTR10=''
FTR11=''
Tags
BOLD=""
CENTER=""
END_CENTER=""
LF=""
write_headers ()
{
echo "$HDR01"
echo
echo "$HDR02"
echo "$HDR03"
echo
echo
echo "$HDR10"
echo "$HDR11"
echo "$HDR121"
echo "$HDR11a"
echo "$HDR13"
echo
echo -n "$HDR14a"
echo -n "$FNTSIZE"
echo "$HDR14b"
echo
echo "$BOLD" # Everything in bold (more easily readable).
}
process_text ()
{
while read line # Read one line at a time.
do
{
if [ ! "$line" ] # Blank line?
then # Then new paragraph must follow.
echo
echo "$LF" # Insert two <br> tags.
echo "$LF"
echo
continue # Skip the underscore test.
else # Otherwise . . .
if [[ "$line" =~ \[*jpg\] ]] # Is a graphic?
then # Strip away brackets.
temp=$( echo "$line" | sed -e 's/\[//' -e 's/\]//' )
line=""$CENTER" <img src="\"$IMGDIR"/$temp\"> "$END_CENTER" "
# Add image tag.
# And, center it.
fi
fi
echo "$line" | grep -q _
if [ "$?" -eq 0 ] # If line contains underscore ...
then
# ===================================================
# Convert underscored phrase to italics.
temp=$( echo "$line" |
sed -e 's/ _/ <i>/' -e 's/_/<\/i> /' |
sed -e 's/^_/<i>/' -e 's/_/<\/i>/' )
# Process only underscores prefixed by space,
#+ or at beginning or end of line.
# Do not convert underscores embedded within a word!
line="$temp"
# Slows script execution. Can be optimized?
# ===================================================
fi
echo
echo "$line"
echo
Don't want extra blank lines in generated text!
} # End while
done
} # End process_text ()
write_footers () # Termination tags.
{
echo "$FTR10"
echo "$FTR11"
}
main () {
=========
write_headers
process_text
write_footers
=========
}
exit $?
Exercises:
- Fixup: Check for closing underscore before a comma or period.
- Add a test for the presence of a closing underscore
- in phrases to be italicized.
Here is something to warm the hearts of webmasters and mistresses: a
script that saves weblogs.
Example A-25. Preserving weblogs
!/bin/bash
archiveweblogs.sh v1.0
Slightly modified by document author.
Used with permission.
This script will preserve the normally rotated and
- thrown away weblogs from a default RedHat/Apache installation.
It will save the files with a date/time stamp in the filename,
- bzipped, to a given directory.
Run this from crontab nightly at an off hour,
- as bzip2 can suck up some serious CPU on huge logs:
0 2 * * * /opt/sbin/archiveweblogs.sh
PROBLEM=66
Set this to your backup dir.
BKP_DIR=/opt/backups/weblogs
Default Apache/RedHat stuff
LOG_DAYS="4 3 2 1"
LOG_DIR=/var/log/httpd
LOG_FILES="access_log error_log"
Default RedHat program locations
LS=/bin/ls
MV=/bin/mv
ID=/usr/bin/id
CUT=/bin/cut
COL=/usr/bin/column
BZ2=/usr/bin/bzip2
Are we root?
USER=$ID -u
if [ "X$USER" != "X0" ]; then
echo "PANIC: Only root can run this script!"
exit $PROBLEM
fi
Backup dir exists/writable?
if [ ! -x $BKP_DIR ]; then
echo "PANIC: $BKP_DIR doesn't exist or isn't writable!"
exit $PROBLEM
fi
Move, rename and bzip2 the logs
for logday in $LOG_DAYS; do
for logfile in $LOG_FILES; do
MYFILE="$LOG_DIR/$logfile.$logday"
if [ -w $MYFILE ]; then
DTS=`$LS -lgo --time-style=+%Y%m%d $MYFILE | $COL -t | $CUT -d ' ' -f7`
$MV $MYFILE $BKP_DIR/$logfile.$DTS
$BZ2 $BKP_DIR/$logfile.$DTS
else
# Only spew an error if the file exits (ergo non-writable).
if [ -f $MYFILE ]; then
echo "ERROR: $MYFILE not writable. Skipping."
fi
fi
done
done
exit 0
How to keep the shell from expanding and reinterpreting text strings.
Example A-26. Protecting literal strings
! /bin/bash
protect_literal.sh
set -vx
:<<-'_Protect_Literal_String_Doc'
Copyright (c) Michael S. Zick, 2003; All Rights Reserved
License: Unrestricted reuse in any form, for any purpose.
Warranty: None
Revision: $ID$
Documentation redirected to the Bash no-operation.
Bash will '/dev/null' this block when the script is first read.
(Uncomment the above set command to see this action.)
Remove the first (Sha-Bang) line when sourcing this as a library
procedure. Also comment out the example use code in the two
places where shown.
Usage:
_protect_literal_str 'Whatever string meets your ${fancy}'
Just echos the argument to standard out, hard quotes
restored.
$(_protect_literal_str 'Whatever string meets your ${fancy}')
as the right-hand-side of an assignment statement.
Does:
As the right-hand-side of an assignment, preserves the
hard quotes protecting the contents of the literal during
assignment.
Notes:
The strange names (_*) are used to avoid trampling on
the user's chosen names when this is sourced as a
library.
_Protect_Literal_String_Doc
The 'for illustration' function form
_protect_literal_str() {
Pick an un-used, non-printing character as local IFS.
Not required, but shows that we are ignoring it.
local IFS=$'\x1B' # \ESC character
Enclose the All-Elements-Of in hard quotes during assignment.
local tmp=$'\x27'$@$'\x27'
local tmp=$'''$@$''' # Even uglier.
local len=${#tmp} # Info only.
echo $tmp is $len long. # Output AND information.
}
This is the short-named version.
_pls() {
local IFS=$'x1B' # \ESC character (not required)
echo $'\x27'$@$'\x27' # Hard quoted parameter glob
}
:<<-'_Protect_Literal_String_Test'
# Remove the above "# " to disable this code. # #
See how that looks when printed.
echo
echo "- - Test One - -"
_protect_literal_str 'Hello $user'
_protect_literal_str 'Hello "${username}"'
echo
Which yields:
'Hello $user' is 13 long.
'Hello "${username}"' is 21 long.
Looks as expected, but why all of the trouble?
The difference is hidden inside the Bash internal order
Which shows when you use it on the RHS of an assignment.
Declare an array for test values.
declare -a arrayZ
Assign elements with various types of quotes and escapes.
arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "'Pass: ${pw}'" )
Now list that array and see what is there.
echo "- - Test Two - -"
for (( i=0 ; i<${#arrayZ[*]} ; i++ ))
do
echo Element $i: ${arrayZ[$i]} is: ${#arrayZ[$i]} long.
done
echo
Which yields:
Element 0: zero is: 4 long. # Our marker element
Element 1: 'Hello ${Me}' is: 13 long. # Our "$(_pls '...' )"
Element 2: Hello ${You} is: 12 long. # Quotes are missing
Element 3: 'Pass: ' is: 10 long. # ${pw} expanded to nothing
Now make an assignment with that result.
declare -a array2=( ${arrayZ[@]} )
And print what happened.
echo "- - Test Three - -"
for (( i=0 ; i<${#array2[*]} ; i++ ))
do
echo Element $i: ${array2[$i]} is: ${#array2[$i]} long.
done
echo
Which yields:
Element 0: zero is: 4 long. # Our marker element.
Element 1: Hello ${Me} is: 11 long. # Intended result.
Element 2: Hello is: 5 long. # ${You} expanded to nothing.
Element 3: 'Pass: is: 6 long. # Split on the whitespace.
Element 4: ' is: 1 long. # The end quote is here now.
Our Element 1 has had its leading and trailing hard quotes stripped.
Although not shown, leading and trailing whitespace is also stripped.
Now that the string contents are set, Bash will always, internally,
- hard quote the contents as required during its operations.
Why?
Considering our "$(_pls 'Hello ${Me}')" construction:
" ... " -> Expansion required, strip the quotes.
$( ... ) -> Replace with the result of..., strip this.
_pls ' ... ' -> called with literal arguments, strip the quotes.
The result returned includes hard quotes; BUT the above processing
- has already been done, so they become part of the value assigned.
Similarly, during further usage of the string variable, the ${Me}
- is part of the contents (result) and survives any operations
(Until explicitly told to evaluate the string).
Hint: See what happens when the hard quotes ($'\x27') are replaced
- with soft quotes ($'\x22') in the above procedures.
Interesting also is to remove the addition of any quoting.
_Protect_Literal_String_Test
# Remove the above "# " to disable this code. # #
exit 0
But, what if you want the shell to expand and reinterpret strings?
Example A-27. Unprotecting literal strings
! /bin/bash
unprotect_literal.sh
set -vx
:<<-'_UnProtect_Literal_String_Doc'
Copyright (c) Michael S. Zick, 2003; All Rights Reserved
License: Unrestricted reuse in any form, for any purpose.
Warranty: None
Revision: $ID$
Documentation redirected to the Bash no-operation. Bash will
'/dev/null' this block when the script is first read.
(Uncomment the above set command to see this action.)
Remove the first (Sha-Bang) line when sourcing this as a library
procedure. Also comment out the example use code in the two
places where shown.
Usage:
Complement of the "$(_pls 'Literal String')" function.
(See the protect_literal.sh example.)
StringVar=$(_upls ProtectedSringVariable)
Does:
When used on the right-hand-side of an assignment statement;
makes the substitions embedded in the protected string.
Notes:
The strange names (_*) are used to avoid trampling on
the user's chosen names when this is sourced as a
library.
_UnProtect_Literal_String_Doc
_upls() {
local IFS=$'x1B' # \ESC character (not required)
eval echo $@ # Substitution on the glob.
}
:<<-'_UnProtect_Literal_String_Test'
# Remove the above "# " to disable this code. # #
_pls() {
local IFS=$'x1B' # \ESC character (not required)
echo $'\x27'$@$'\x27' # Hard quoted parameter glob
}
Declare an array for test values.
declare -a arrayZ
Assign elements with various types of quotes and escapes.
arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "'Pass: ${pw}'" )
Now make an assignment with that result.
declare -a array2=( ${arrayZ[@]} )
Which yielded:
Element 0: zero is: 4 long # Our marker element.
Element 1: Hello ${Me} is: 11 long # Intended result.
Element 2: Hello is: 5 long # ${You} expanded to nothing.
Element 3: 'Pass: is: 6 long # Split on the whitespace.
Element 4: ' is: 1 long # The end quote is here now.
set -vx
Initialize 'Me' to something for the embedded ${Me} substitution.
This needs to be done ONLY just prior to evaluating the
(This is why it was protected to begin with.)
Me="to the array guy."
Set a string variable destination to the result.
newVar=$(_upls ${array2[1]})
Show what the contents are.
echo $newVar
Do we really need a function to do this?
newerVar=$(eval echo ${array2[1]})
echo $newerVar
I guess not, but the _upls function gives us a place to hang
This helps when we forget what a # construction like:
What if Me isn't set when the protected string is evaluated?
unset Me
newestVar=$(_upls ${array2[1]})
echo $newestVar
Just gone, no hints, no runs, no errors.
Why in the world?
Setting the contents of a string variable containing character
- sequences that have a meaning in Bash is a general problem in
This problem is now solved in eight lines of code
- (and four pages of description).
Where is all this going?
Dynamic content Web pages as an array of Bash strings.
Content set per request by a Bash 'eval' command
- on the stored page template.
Not intended to replace PHP, just an interesting thing to do.
Don't have a webserver application?
No problem, check the example directory of the Bash source;
- there is a Bash script for that also.
_UnProtect_Literal_String_Test
# Remove the above "# " to disable this code. # #
exit 0
This interesting script helps hunt down spammers.
Example A-28. Spammer Identification
!/bin/bash
$Id: is_spammer.bash,v 1.12.2.11 2004/10/01 21:42:33 mszick Exp $
Above line is RCS info.
The latest version of this script is available from http://www.morethan.org.
Spammer-identification
by Michael S. Zick
Used in the ABS Guide with permission.
Documentation
See also "Quickstart" at end of script.
:<<-'_is_spammer_Doc'
Copyright (c) Michael S. Zick, 2004
License: Unrestricted reuse in any form, for any purpose.
Warranty: None -{Its a script; the user is on their own.}-
Impatient?
Application code: goto "# # # Hunt the Spammer' program code # # #"
Example output: ":<<-'_is_spammer_outputs_'"
How to use: Enter script name without arguments.
Or goto "Quickstart" at end of script.
Provides
Given a domain name or IP(v4) address as input:
Does an exhaustive set of queries to find the associated
network resources (short of recursing into TLDs).
Checks the IP(v4) addresses found against Blacklist
nameservers.
If found to be a blacklisted IP(v4) address,
reports the blacklist text records.
(Usually hyper-links to the specific report.)
Requires
A working Internet connection.
(Exercise: Add check and/or abort if not on-line when running script.)
Bash with arrays (2.05b+).
The external program 'dig' --
a utility program provided with the 'bind' set of programs.
Specifically, the version which is part of Bind series 9.x
See: http://www.isc.org
All usages of 'dig' are limited to wrapper functions,
which may be rewritten as required.
See: dig_wrappers.bash for details.
("Additional documentation" -- below)
Usage
Script requires a single argument, which may be:
1) A domain name;
2) An IP(v4) address;
3) A filename, with one name or address per line.
Script accepts an optional second argument, which may be:
1) A Blacklist server name;
2) A filename, with one Blacklist server name per line.
If the second argument is not provided, the script uses
a built-in set of (free) Blacklist servers.
See also, the Quickstart at the end of this script (after 'exit').
Return Codes
0 - All OK
1 - Script failure
2 - Something is Blacklisted
Optional environment variables
SPAMMER_TRACE
If set to a writable file,
script will log an execution flow trace.
SPAMMER_DATA
If set to a writable file, script will dump its
discovered data in the form of GraphViz file.
See: http://www.research.att.com/sw/tools/graphviz
SPAMMER_LIMIT
Limits the depth of resource tracing.
Default is 2 levels.
A setting of 0 (zero) means 'unlimited' . . .
Caution: script might recurse the whole Internet!
A limit of 1 or 2 is most useful when processing
a file of domain names and addresses.
A higher limit can be useful when hunting spam gangs.
Additional documentation
Download the archived set of scripts
explaining and illustrating the function contained within this script.
http://bash.deta.in/mszick_clf.tar.bz2
Study notes
This script uses a large number of functions.
Nearly all general functions have their own example script.
Each of the example scripts have tutorial level comments.
Scripting project
Add support for IP(v6) addresses.
IP(v6) addresses are recognized but not processed.
Advanced project
Add the reverse lookup detail to the discovered information.
Report the delegation chain and abuse contacts.
Modify the GraphViz file output to include the
newly discovered information.
_is_spammer_Doc
Special IFS settings used for string parsing. ####
Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
No Whitespace == Line Feed:Carriage Return
NO_WSP=$'\x0A'$'\x0D'
Field separator for dotted decimal IP addresses
ADR_IFS=${NO_WSP}'.'
Array to dotted string conversions
DOT_IFS='.'${WSP_IFS}
# Pending operations stack machine # #
This set of functions described in func_stack.bash.
(See "Additional documentation" above.)
Global stack of pending operations.
declare -f -a pending
Global sentinel for stack runners
declare -i p_ctrl
Global holder for currently executing function
declare -f pend_current
# Debug version only - remove for regular use # #
The function stored in pend_hook is called
immediately before each pending function is
evaluated. Stack clean, pend_current set.
This thingy demonstrated in pend_hook.bash.
declare -f pend_hook
The do nothing function
pend_dummy() { : ; }
Clear and initialize the function stack.
pend_init() {
unset _pending_[@]
pend_func pend_stop_mark
_pend_hook_='pend_dummy' # Debug only.
}
Discard the top function on the stack.
pend_pop() {
if [ ${#_pending_[@]} -gt 0 ]
then
local -i _top_
_top_=${#_pending_[@]}-1
unset _pending_[$_top_]
fi
}
pend_func function_name [$(printf '%q\n' arguments)]
pend_func() {
local IFS=${NO_WSP}
set -f
_pending_[${#_pending_[@]}]=$@
set +f
}
The function which stops the release:
pend_stop_mark() {
_p_ctrl_=0
}
pend_mark() {
pend_func pend_stop_mark
}
Execute functions until 'pend_stop_mark' . . .
pend_release() {
local -i _top_ # Declare _top_ as integer.
_p_ctrl_=${#_pending_[@]}
while [ ${_p_ctrl_} -gt 0 ]
do
_top_=${#_pending_[@]}-1
_pend_current_=${_pending_[$_top_]}
unset _pending_[$_top_]
$_pend_hook_ # Debug only.
eval $_pend_current_
done
}
Drop functions until 'pend_stop_mark' . . .
pend_drop() {
local -i _top_
local _pd_ctrl_=${#_pending_[@]}
while [ ${_pd_ctrl_} -gt 0 ]
do
_top_=$_pd_ctrl_-1
if [ "${_pending_[$_top_]}" == 'pend_stop_mark' ]
then
unset _pending_[$_top_]
break
else
unset _pending_[$_top_]
_pd_ctrl_=$_top_
fi
done
if [ ${#_pending_[@]} -eq 0 ]
then
pend_func pend_stop_mark
fi
}
Array editors ####
This function described in edit_exact.bash.
(See "Additional documentation," above.)
edit_exact <excludes_array_name> <target_array_name>
edit_exact() {
[ $# -eq 2 ] ||
[ $# -eq 3 ] || return 1
local -a _ee_Excludes
local -a _ee_Target
local _ee_x
local _ee_t
local IFS=${NO_WSP}
set -f
eval _ee_Excludes=\( \$\{$1\[@\]\} \)
eval _ee_Target=\( \$\{$2\[@\]\} \)
local _ee_len=${#_ee_Target[@]} # Original length.
local _ee_cnt=${#_ee_Excludes[@]} # Exclude list length.
[ ${_ee_len} -ne 0 ] || return 0 # Can't edit zero length.
[ ${_ee_cnt} -ne 0 ] || return 0 # Can't edit zero length.
for (( x = 0; x < ${_ee_cnt} ; x++ ))
do
_ee_x=${_ee_Excludes[$x]}
for (( n = 0 ; n < ${_ee_len} ; n++ ))
do
_ee_t=${_ee_Target[$n]}
if [ x"${_ee_t}" == x"${_ee_x}" ]
then
unset _ee_Target[$n] # Discard match.
[ $# -eq 2 ] && break # If 2 arguments, then done.
fi
done
done
eval $2=\( \$\{_ee_Target\[@\]\} \)
set +f
return 0
}
This function described in edit_by_glob.bash.
edit_by_glob <excludes_array_name> <target_array_name>
edit_by_glob() {
[ $# -eq 2 ] ||
[ $# -eq 3 ] || return 1
local -a _ebg_Excludes
local -a _ebg_Target
local _ebg_x
local _ebg_t
local IFS=${NO_WSP}
set -f
eval _ebg_Excludes=\( \$\{$1\[@\]\} \)
eval _ebg_Target=\( \$\{$2\[@\]\} \)
local _ebg_len=${#_ebg_Target[@]}
local _ebg_cnt=${#_ebg_Excludes[@]}
[ ${_ebg_len} -ne 0 ] || return 0
[ ${_ebg_cnt} -ne 0 ] || return 0
for (( x = 0; x < ${_ebg_cnt} ; x++ ))
do
_ebg_x=${_ebg_Excludes[$x]}
for (( n = 0 ; n < ${_ebg_len} ; n++ ))
do
[ $# -eq 3 ] && _ebg_x=${_ebg_x}'*' # Do prefix edit
if [ ${_ebg_Target[$n]:=} ] #+ if defined & set.
then
_ebg_t=${_ebg_Target[$n]/#${_ebg_x}/}
[ ${#_ebg_t} -eq 0 ] && unset _ebg_Target[$n]
fi
done
done
eval $2=\( \$\{_ebg_Target\[@\]\} \)
set +f
return 0
}
This function described in unique_lines.bash.
unique_lines <in_name> <out_name>
unique_lines() {
[ $# -eq 2 ] || return 1
local -a _ul_in
local -a _ul_out
local -i _ul_cnt
local -i _ul_pos
local _ul_tmp
local IFS=${NO_WSP}
set -f
eval _ul_in=\( \$\{$1\[@\]\} \)
_ul_cnt=${#_ul_in[@]}
for (( _ul_pos = 0 ; _ul_pos < ${_ul_cnt} ; _ul_pos++ ))
do
if [ ${_ul_in[${_ul_pos}]:=} ] # If defined & not empty
then
_ul_tmp=${_ul_in[${_ul_pos}]}
_ul_out[${#_ul_out[@]}]=${_ul_tmp}
for (( zap = _ul_pos ; zap < ${_ul_cnt} ; zap++ ))
do
[ ${_ul_in[${zap}]:=} ] &&
[ 'x'${_ul_in[${zap}]} == 'x'${_ul_tmp} ] &&
unset _ul_in[${zap}]
done
fi
done
eval $2=\( \$\{_ul_out\[@\]\} \)
set +f
return 0
}
This function described in char_convert.bash.
to_lower
to_lower() {
[ $# -eq 1 ] || return 1
local _tl_out
_tl_out=${1//A/a}
_tl_out=${_tl_out//B/b}
_tl_out=${_tl_out//C/c}
_tl_out=${_tl_out//D/d}
_tl_out=${_tl_out//E/e}
_tl_out=${_tl_out//F/f}
_tl_out=${_tl_out//G/g}
_tl_out=${_tl_out//H/h}
_tl_out=${_tl_out//I/i}
_tl_out=${_tl_out//J/j}
_tl_out=${_tl_out//K/k}
_tl_out=${_tl_out//L/l}
_tl_out=${_tl_out//M/m}
_tl_out=${_tl_out//N/n}
_tl_out=${_tl_out//O/o}
_tl_out=${_tl_out//P/p}
_tl_out=${_tl_out//Q/q}
_tl_out=${_tl_out//R/r}
_tl_out=${_tl_out//S/s}
_tl_out=${_tl_out//T/t}
_tl_out=${_tl_out//U/u}
_tl_out=${_tl_out//V/v}
_tl_out=${_tl_out//W/w}
_tl_out=${_tl_out//X/x}
_tl_out=${_tl_out//Y/y}
_tl_out=${_tl_out//Z/z}
echo ${_tl_out}
return 0
}
Application helper functions ####
Not everybody uses dots as separators (APNIC, for example).
This function described in to_dot.bash
to_dot
to_dot() {
[ $# -eq 1 ] || return 1
echo ${1//[#|@|%]/.}
return 0
}
This function described in is_number.bash.
is_number
is_number() {
[ "$#" -eq 1 ] || return 1 # is blank?
[ x"$1" == 'x0' ] && return 0 # is zero?
local -i tst
let tst=$1 2>/dev/null # else is numeric!
return $?
}
This function described in is_address.bash.
is_address
is_address() {
[ $# -eq 1 ] || return 1 # Blank ==> false
local -a _ia_input
local IFS=${ADR_IFS}
_ia_input=( $1 )
if [ ${#_ia_input[@]} -eq 4 ] &&
is_number ${_ia_input[0]} &&
is_number ${_ia_input[1]} &&
is_number ${_ia_input[2]} &&
is_number ${_ia_input[3]} &&
[ ${_ia_input[0]} -lt 256 ] &&
[ ${_ia_input[1]} -lt 256 ] &&
[ ${_ia_input[2]} -lt 256 ] &&
[ ${_ia_input[3]} -lt 256 ]
then
return 0
else
return 1
fi
}
This function described in split_ip.bash.
split_ip <IP_address>
- <array_name_norm> [<array_name_rev>]
split_ip() {
[ $# -eq 3 ] || # Either three
[ $# -eq 2 ] || return 1 #+ or two arguments
local -a _si_input
local IFS=${ADR_IFS}
_si_input=( $1 )
IFS=${WSP_IFS}
eval $2=\(\ \$\{_si_input\[@\]\}\ \)
if [ $# -eq 3 ]
then
# Build query order array.
local -a _dns_ip
_dns_ip[0]=${_si_input[3]}
_dns_ip[1]=${_si_input[2]}
_dns_ip[2]=${_si_input[1]}
_dns_ip[3]=${_si_input[0]}
eval $3=\(\ \$\{_dns_ip\[@\]\}\ \)
fi
return 0
}
This function described in dot_array.bash.
dot_array <array_name>
dot_array() {
[ $# -eq 1 ] || return 1 # Single argument required.
local -a _da_input
eval _da_input=\(\ \$\{$1\[@\]\}\ \)
local IFS=${DOT_IFS}
local _da_output=${_da_input[@]}
IFS=${WSP_IFS}
echo ${_da_output}
return 0
}
This function described in file_to_array.bash
file_to_array <file_name> <line_array_name>
file_to_array() {
[ $# -eq 2 ] || return 1 # Two arguments required.
local IFS=${NO_WSP}
local -a _fta_tmp_
_fta_tmp_=( $(cat $1) )
eval $2=\( \$\{_fta_tmp_\[@\]\} \)
return 0
}
Columnized print of an array of multi-field strings.
col_print <array_name> <min_space> <
col_print() {
[ $# -gt 2 ] || return 0
local -a _cp_inp
local -a _cp_spc
local -a _cp_line
local _cp_min
local _cp_mcnt
local _cp_pos
local _cp_cnt
local _cp_tab
local -i _cp
local -i _cpf
local _cp_fld
# WARNING: FOLLOWING LINE NOT BLANK -- IT IS QUOTED SPACES.
local _cp_max='
'
set -f
local IFS=${NO_WSP}
eval _cp_inp=\(\ \$\{$1\[@\]\}\ \)
[ ${#_cp_inp[@]} -gt 0 ] || return 0 # Empty is easy.
_cp_mcnt=$2
_cp_min=${_cp_max:1:${_cp_mcnt}}
shift
shift
_cp_cnt=$#
for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))
do
_cp_spc[${#_cp_spc[@]}]="${_cp_max:2:$1}" #"
shift
done
_cp_cnt=${#_cp_inp[@]}
for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))
do
_cp_pos=1
IFS=${NO_WSP}$'\x20'
_cp_line=( ${_cp_inp[${_cp}]} )
IFS=${NO_WSP}
for (( _cpf = 0 ; _cpf < ${#_cp_line[@]} ; _cpf++ ))
do
_cp_tab=${_cp_spc[${_cpf}]:${_cp_pos}}
if [ ${#_cp_tab} -lt ${_cp_mcnt} ]
then
_cp_tab="${_cp_min}"
fi
echo -n "${_cp_tab}"
(( _cp_pos = ${_cp_pos} + ${#_cp_tab} ))
_cp_fld="${_cp_line[${_cpf}]}"
echo -n ${_cp_fld}
(( _cp_pos = ${_cp_pos} + ${#_cp_fld} ))
done
echo
done
set +f
return 0
}
# # 'Hunt the Spammer' data flow # # #
Application return code
declare -i _hs_RC
Original input, from which IP addresses are removed
After which, domain names to check
declare -a uc_name
Original input IP addresses are moved here
After which, IP addresses to check
declare -a uc_address
Names against which address expansion run
Ready for name detail lookup
declare -a chk_name
Addresses against which name expansion run
Ready for address detail lookup
declare -a chk_address
Recursion is depth-first-by-name.
The expand_input_address maintains this list
- to prohibit looking up addresses twice during
declare -a been_there_addr
been_there_addr=( '127.0.0.1' ) # Whitelist localhost
Names which we have checked (or given up on)
declare -a known_name
Addresses which we have checked (or given up on)
declare -a known_address
List of zero or more Blacklist servers to check.
Each 'known_address' will be checked against each server,
- with negative replies and failures suppressed.
declare -a list_server
Indirection limit - set to zero == no limit
indirect=${SPAMMER_LIMIT:=2}
# # 'Hunt the Spammer' information output data # # #
Any domain name may have multiple IP addresses.
Any IP address may have multiple domain names.
Therefore, track unique address-name pairs.
declare -a known_pair
declare -a reverse_pair
In addition to the data flow variables; known_address
- known_name and list_server, the following are output to the
- external graphics interface file.
Authority chain, parent -> SOA fields.
declare -a auth_chain
Reference chain, parent name -> child name
declare -a ref_chain
DNS chain - domain name -> address
declare -a name_address
Name and service pairs - domain name -> service
declare -a name_srvc
Name and resource pairs - domain name -> Resource Record
declare -a name_resource
Parent and Child pairs - parent name -> child name
This MAY NOT be the same as the ref_chain followed!
declare -a parent_child
Address and Blacklist hit pairs - address->server
declare -a address_hits
Dump interface file data
declare -f _dot_dump
_dot_dump=pend_dummy # Initially a no-op
Data dump is enabled by setting the environment variable SPAMMER_DATA
- to the name of a writable file.
declare _dot_file
Helper function for the dump-to-dot-file function
dump_to_dot <array_name>
dump_to_dot() {
local -a _dda_tmp
local -i _dda_cnt
local _dda_form=' '${2}'%04u %s\n'
local IFS=${NO_WSP}
eval _dda_tmp=\(\ \$\{$1\[@\]\}\ \)
_dda_cnt=${#_dda_tmp[@]}
if [ ${_dda_cnt} -gt 0 ]
then
for (( _dda = 0 ; _dda < _dda_cnt ; _dda++ ))
do
printf "${_dda_form}" \
"${_dda}" "${_dda_tmp[${_dda}]}" >>${_dot_file}
done
fi
}
Which will also set _dot_dump to this function . . .
dump_dot() {
local -i _dd_cnt
echo '# Data vintage: '$(date -R) >${_dot_file}
echo '# ABS Guide: is_spammer.bash; v2, 2004-msz' >>${_dot_file}
echo >>${_dot_file}
echo 'digraph G {' >>${_dot_file}
if [ ${#known_name[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known domain name nodes' >>${_dot_file}
_dd_cnt=${#known_name[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf ' N%04u [label="%s"] ;\n' \
"${_dd}" "${known_name[${_dd}]}" >>${_dot_file}
done
fi
if [ ${#known_address[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known address nodes' >>${_dot_file}
_dd_cnt=${#known_address[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf ' A%04u [label="%s"] ;\n' \
"${_dd}" "${known_address[${_dd}]}" >>${_dot_file}
done
fi
echo >>${_dot_file}
echo '/*' >>${_dot_file}
echo ' * Known relationships :: User conversion to' >>${_dot_file}
echo ' * graphic form by hand or program required.' >>${_dot_file}
echo ' *' >>${_dot_file}
if [ ${#auth_chain[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Authority ref. edges followed & field source.' >>${_dot_file}
dump_to_dot auth_chain AC
fi
if [ ${#ref_chain[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Name ref. edges followed and field source.' >>${_dot_file}
dump_to_dot ref_chain RC
fi
if [ ${#name_address[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->address edges' >>${_dot_file}
dump_to_dot name_address NA
fi
if [ ${#name_srvc[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->service edges' >>${_dot_file}
dump_to_dot name_srvc NS
fi
if [ ${#name_resource[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->resource edges' >>${_dot_file}
dump_to_dot name_resource NR
fi
if [ ${#parent_child[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known parent->child edges' >>${_dot_file}
dump_to_dot parent_child PC
fi
if [ ${#list_server[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known Blacklist nodes' >>${_dot_file}
_dd_cnt=${#list_server[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf ' LS%04u [label="%s"] ;\n' \
"${_dd}" "${list_server[${_dd}]}" >>${_dot_file}
done
fi
unique_lines address_hits address_hits
if [ ${#address_hits[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known address->Blacklist_hit edges' >>${_dot_file}
echo '# CAUTION: dig warnings can trigger false hits.' >>${_dot_file}
dump_to_dot address_hits AH
fi
echo >>${_dot_file}
echo ' *' >>${_dot_file}
echo ' * That is a lot of relationships. Happy graphing.' >>${_dot_file}
echo ' */' >>${_dot_file}
echo '}' >>${_dot_file}
return 0
}
# # 'Hunt the Spammer' execution flow # # #
Execution trace is enabled by setting the
- environment variable SPAMMER_TRACE to the name of a writable file.
declare -a _trace_log
declare _log_file
Function to fill the trace log
trace_logger() {
_trace_log[${#_trace_log[@]}]=${_pend_current_}
}
Dump trace log to file function variable.
declare -f _log_dump
_log_dump=pend_dummy # Initially a no-op.
Dump the trace log to a file.
dump_log() {
local -i _dl_cnt
_dl_cnt=${#_trace_log[@]}
for (( _dl = 0 ; _dl < _dl_cnt ; _dl++ ))
do
echo ${_trace_log[${_dl}]} >> ${_log_file}
done
_dl_cnt=${#_pending_[@]}
if [ ${_dl_cnt} -gt 0 ]
then
_dl_cnt=${_dl_cnt}-1
echo '# # # Operations stack not empty # # #' >> ${_log_file}
for (( _dl = ${_dl_cnt} ; _dl >= 0 ; _dl-- ))
do
echo ${_pending_[${_dl}]} >> ${_log_file}
done
fi
}
# Utility program 'dig' wrappers # #
These wrappers are derived from the
- examples shown in dig_wrappers.bash.
The major difference is these return
- their results as a list in an array.
See dig_wrappers.bash for details and
- use that script to develop any changes.
Short form answer: 'dig' parses answer.
Forward lookup :: Name -> Address
short_fwd <domain_name> <array_name>
short_fwd() {
local -a _sf_reply
local -i _sf_rc
local -i _sf_cnt
IFS=${NO_WSP}
echo -n '.'
echo 'sfwd: '${1}
_sf_reply=( $(dig +short ${1} -c in -t a 2>/dev/null) )
_sf_rc=$?
if [ ${_sf_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='## Lookup error '${_sf_rc}' on '${1}' ##'
[ ${_sf_rc} -ne 9 ] && pend_drop
return ${_sf_rc}
else
# Some versions of 'dig' return warnings on stdout.
_sf_cnt=${#_sf_reply[@]}
for (( _sf = 0 ; _sf < ${_sf_cnt} ; _sf++ ))
do
[ 'x'${_sf_reply[${_sf}]:0:2} == 'x;;' ] &&
unset _sf_reply[${_sf}]
done
eval $2=\( \$\{_sf_reply\[@\]\} \)
fi
return 0
}
Reverse lookup :: Address -> Name
short_rev <ip_address> <array_name>
short_rev() {
local -a _sr_reply
local -i _sr_rc
local -i _sr_cnt
IFS=${NO_WSP}
echo -n '.'
echo 'srev: '${1}
_sr_reply=( $(dig +short -x ${1} 2>/dev/null) )
_sr_rc=$?
if [ ${_sr_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='## Lookup error '${_sr_rc}' on '${1}' ##'
[ ${_sr_rc} -ne 9 ] && pend_drop
return ${_sr_rc}
else
# Some versions of 'dig' return warnings on stdout.
_sr_cnt=${#_sr_reply[@]}
for (( _sr = 0 ; _sr < ${_sr_cnt} ; _sr++ ))
do
[ 'x'${_sr_reply[${_sr}]:0:2} == 'x;;' ] &&
unset _sr_reply[${_sr}]
done
eval $2=\( \$\{_sr_reply\[@\]\} \)
fi
return 0
}
Special format lookup used to query blacklist servers.
short_text <ip_address> <array_name>
short_text() {
local -a _st_reply
local -i _st_rc
local -i _st_cnt
IFS=${NO_WSP}
echo 'stxt: '${1}
_st_reply=( $(dig +short ${1} -c in -t txt 2>/dev/null) )
_st_rc=$?
if [ ${_st_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='##Text lookup error '${_st_rc}' on '${1}'##
'
[ ${_st_rc} -ne 9 ] && pend_drop
return ${_st_rc}
else
# Some versions of 'dig' return warnings on stdout.
_st_cnt=${#_st_reply[@]}
for (( _st = 0 ; _st < ${#_st_cnt} ; _st++ ))
do
[ 'x'${_st_reply[${_st}]:0:2} == 'x;;' ] &&
unset _st_reply[${_st}]
done
eval $2=\( \$\{_st_reply\[@\]\} \)
fi
return 0
}
The long forms, a.k.a., the parse it yourself versions
RFC 2782 Service lookups
dig +noall +nofail +answer _ldap._tcp.openldap.org -t srv
..<domain_name>
_ldap._tcp.openldap.org. 3600 IN SRV 0 0 389 ldap.openldap.org.
domain TTL Class SRV Priority Weight Port Target
Forward lookup :: Name -> poor man's zone transfer
long_fwd <domain_name> <array_name>
long_fwd() {
local -a _lf_reply
local -i _lf_rc
local -i _lf_cnt
IFS=${NO_WSP}
echo -n ':'
echo 'lfwd: '${1}
_lf_reply=( $(
dig +noall +nofail +answer +authority +additional \
${1} -t soa ${1} -t mx ${1} -t any 2>/dev/null) )
_lf_rc=$?
if [ ${_lf_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='# Zone lookup err '${_lf_rc}' on '${1}' #'
[ ${_lf_rc} -ne 9 ] && pend_drop
return ${_lf_rc}
else
# Some versions of 'dig' return warnings on stdout.
_lf_cnt=${#_lf_reply[@]}
for (( _lf = 0 ; _lf < ${_lf_cnt} ; _lf++ ))
do
[ 'x'${_lf_reply[${_lf}]:0:2} == 'x;;' ] &&
unset _lf_reply[${_lf}]
done
eval $2=\( \$\{_lf_reply\[@\]\} \)
fi
return 0
}
The reverse lookup domain name corresponding to the IPv6 address:
4321:0:1:2:3:4:567:89ab
would be (nibble, I.E: Hexdigit) reversed:
b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA.
Reverse lookup :: Address -> poor man's delegation chain
long_rev <rev_ip_address> <array_name>
long_rev() {
local -a _lr_reply
local -i _lr_rc
local -i _lr_cnt
local _lr_dns
_lr_dns=${1}'.in-addr.arpa.'
IFS=${NO_WSP}
echo -n ':'
echo 'lrev: '${1}
_lr_reply=( $(
dig +noall +nofail +answer +authority +additional \
${_lr_dns} -t soa ${_lr_dns} -t any 2>/dev/null) )
_lr_rc=$?
if [ ${_lr_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='# Deleg lkp error '${_lr_rc}' on '${1}' #'
[ ${_lr_rc} -ne 9 ] && pend_drop
return ${_lr_rc}
else
# Some versions of 'dig' return warnings on stdout.
_lr_cnt=${#_lr_reply[@]}
for (( _lr = 0 ; _lr < ${_lr_cnt} ; _lr++ ))
do
[ 'x'${_lr_reply[${_lr}]:0:2} == 'x;;' ] &&
unset _lr_reply[${_lr}]
done
eval $2=\( \$\{_lr_reply\[@\]\} \)
fi
return 0
}
# Application specific functions # #
Mung a possible name; suppresses root and TLDs.
name_fixup
name_fixup(){
local -a _nf_tmp
local -i _nf_end
local _nf_str
local IFS
_nf_str=$(to_lower ${1})
_nf_str=$(to_dot ${_nf_str})
_nf_end=${#_nf_str}-1
[ ${_nf_str:${_nf_end}} != '.' ] &&
_nf_str=${_nf_str}'.'
IFS=${ADR_IFS}
_nf_tmp=( ${_nf_str} )
IFS=${WSP_IFS}
_nf_end=${#_nf_tmp[@]}
case ${_nf_end} in
0) # No dots, only dots.
echo
return 1
;;
1) # Only a TLD.
echo
return 1
;;
2) # Maybe okay.
echo ${_nf_str}
return 0
# Needs a lookup table?
if [ ${#_nf_tmp[1]} -eq 2 ]
then # Country coded TLD.
echo
return 1
else
echo ${_nf_str}
return 0
fi
;;
esac
echo ${_nf_str}
return 0
}
Grope and mung original input(s).
split_input() {
[ ${#uc_name[@]} -gt 0 ] || return 0
local -i _si_cnt
local -i _si_len
local _si_str
unique_lines uc_name uc_name
_si_cnt=${#uc_name[@]}
for (( _si = 0 ; _si < _si_cnt ; _si++ ))
do
_si_str=${uc_name[$_si]}
if is_address ${_si_str}
then
uc_address[${#uc_address[@]}]=${_si_str}
unset uc_name[$_si]
else
if ! uc_name[$_si]=$(name_fixup ${_si_str})
then
unset ucname[$_si]
fi
fi
done
uc_name=( ${uc_name[@]} )
_si_cnt=${#uc_name[@]}
_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd name input(s).#'
_si_cnt=${#uc_address[@]}
_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd addr input(s).#'
return 0
}
# Discovery functions -- recursively interlocked by external data # #
# The leading 'if list is empty; return 0' in each is required. # #
Recursion limiter
limit_chk() <next_level>
limit_chk() {
local -i _lc_lmt
# Check indirection limit.
if [ ${indirect} -eq 0 ] || [ $# -eq 0 ]
then
# The 'do-forever' choice
echo 1 # Any value will do.
return 0 # OK to continue.
else
# Limiting is in effect.
if [ ${indirect} -lt ${1} ]
then
echo ${1} # Whatever.
return 1 # Stop here.
else
_lc_lmt=${1}+1 # Bump the given limit.
echo ${_lc_lmt} # Echo it.
return 0 # OK to continue.
fi
fi
}
For each name in uc_name:
Move name to chk_name.
Add addresses to uc_address.
Pend expand_input_address.
Repeat until nothing new found.
expand_input_name <indirection_limit>
expand_input_name() {
[ ${#uc_name[@]} -gt 0 ] || return 0
local -a _ein_addr
local -a _ein_new
local -i _ucn_cnt
local -i _ein_cnt
local _ein_tst
_ucn_cnt=${#uc_name[@]}
if ! _ein_cnt=$(limit_chk ${1})
then
return 0
fi
for (( _ein = 0 ; _ein < _ucn_cnt ; _ein++ ))
do
if short_fwd ${uc_name[${_ein}]} _ein_new
then
for (( _ein_cnt = 0 ; _ein_cnt < ${#_ein_new[@]}; _ein_cnt++ ))
do
_ein_tst=${_ein_new[${_ein_cnt}]}
if is_address ${_ein_tst}
then
_ein_addr[${#_ein_addr[@]}]=${_ein_tst}
fi
done
fi
done
unique_lines _ein_addr _ein_addr # Scrub duplicates.
edit_exact chk_address _ein_addr # Scrub pending detail.
edit_exact known_address _ein_addr # Scrub already detailed.
if [ ${#_ein_addr[@]} -gt 0 ] # Anything new?
then
uc_address=( ${uc_address[@]} ${_ein_addr[@]} )
pend_func expand_input_address ${1}
_trace_log[${#_trace_log[@]}]='#Add '${#_ein_addr[@]}' unchkd addr inp.#'
fi
edit_exact chk_name uc_name # Scrub pending detail.
edit_exact known_name uc_name # Scrub already detailed.
if [ ${#uc_name[@]} -gt 0 ]
then
chk_name=( ${chk_name[@]} ${uc_name[@]} )
pend_func detail_each_name ${1}
fi
unset uc_name[@]
return 0
}
For each address in uc_address:
Move address to chk_address.
Add names to uc_name.
Pend expand_input_name.
Repeat until nothing new found.
expand_input_address <indirection_limit>
expand_input_address() {
[ ${#uc_address[@]} -gt 0 ] || return 0
local -a _eia_addr
local -a _eia_name
local -a _eia_new
local -i _uca_cnt
local -i _eia_cnt
local _eia_tst
unique_lines uc_address _eia_addr
unset uc_address[@]
edit_exact been_there_addr _eia_addr
_uca_cnt=${#_eia_addr[@]}
[ ${_uca_cnt} -gt 0 ] &&
been_there_addr=( ${been_there_addr[@]} ${_eia_addr[@]} )
for (( _eia = 0 ; _eia < _uca_cnt ; _eia++ ))
do
if short_rev ${_eia_addr[${_eia}]} _eia_new
then
for (( _eia_cnt = 0 ; _eia_cnt < ${#_eia_new[@]} ; _eia_cnt++ ))
do
_eia_tst=${_eia_new[${_eia_cnt}]}
if _eia_tst=$(name_fixup ${_eia_tst})
then
_eia_name[${#_eia_name[@]}]=${_eia_tst}
fi
done
fi
done
unique_lines _eia_name _eia_name # Scrub duplicates.
edit_exact chk_name _eia_name # Scrub pending detail.
edit_exact known_name _eia_name # Scrub already detailed.
if [ ${#_eia_name[@]} -gt 0 ] # Anything new?
then
uc_name=( ${uc_name[@]} ${_eia_name[@]} )
pend_func expand_input_name ${1}
_trace_log[${#_trace_log[@]}]='#Add '${#_eia_name[@]}' unchkd name inp.#'
fi
edit_exact chk_address _eia_addr # Scrub pending detail.
edit_exact known_address _eia_addr # Scrub already detailed.
if [ ${#_eia_addr[@]} -gt 0 ] # Anything new?
then
chk_address=( ${chk_address[@]} ${_eia_addr[@]} )
pend_func detail_each_address ${1}
fi
return 0
}
The parse-it-yourself zone reply.
The input is the chk_name list.
detail_each_name <indirection_limit>
detail_each_name() {
[ ${#chk_name[@]} -gt 0 ] || return 0
local -a _den_chk # Names to check
local -a _den_name # Names found here
local -a _den_address # Addresses found here
local -a _den_pair # Pairs found here
local -a _den_rev # Reverse pairs found here
local -a _den_tmp # Line being parsed
local -a _den_auth # SOA contact being parsed
local -a _den_new # The zone reply
local -a _den_pc # Parent-Child gets big fast
local -a _den_ref # So does reference chain
local -a _den_nr # Name-Resource can be big
local -a _den_na # Name-Address
local -a _den_ns # Name-Service
local -a _den_achn # Chain of Authority
local -i _den_cnt # Count of names to detail
local -i _den_lmt # Indirection limit
local _den_who # Named being processed
local _den_rec # Record type being processed
local _den_cont # Contact domain
local _den_str # Fixed up name string
local _den_str2 # Fixed up reverse
local IFS=${WSP_IFS}
# Local, unique copy of names to check
unique_lines chk_name _den_chk
unset chk_name[@] # Done with globals.
# Less any names already known
edit_exact known_name _den_chk
_den_cnt=${#_den_chk[@]}
# If anything left, add to known_name.
[ ${_den_cnt} -gt 0 ] &&
known_name=( ${known_name[@]} ${_den_chk[@]} )
# for the list of (previously) unknown names . . .
for (( _den = 0 ; _den < _den_cnt ; _den++ ))
do
_den_who=${_den_chk[${_den}]}
if long_fwd ${_den_who} _den_new
then
unique_lines _den_new _den_new
if [ ${#_den_new[@]} -eq 0 ]
then
_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}
fi
# Parse each line in the reply.
for (( _line = 0 ; _line < ${#_den_new[@]} ; _line++ ))
do
IFS=${NO_WSP}$'\x09'$'\x20'
_den_tmp=( ${_den_new[${_line}]} )
IFS=${WSP_IFS}
# If usable record and not a warning message . . .
if [ ${#_den_tmp[@]} -gt 4 ] && [ 'x'${_den_tmp[0]} != 'x;;' ]
then
_den_rec=${_den_tmp[3]}
_den_nr[${#_den_nr[@]}]=${_den_who}' '${_den_rec}
# Begin at RFC1033 (+++)
case ${_den_rec} in
[] [] SOA
SOA) # Start Of Authority
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str}' SOA'
# SOA origin -- domain name of master zone record
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str2}
_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str2}' SOA.O'
fi
# Responsible party e-mail address (possibly bogus).
# Possibility of first.last@domain.name ignored.
set -f
if _den_str2=$(name_fixup ${_den_tmp[5]})
then
IFS=${ADR_IFS}
_den_auth=( ${_den_str2} )
IFS=${WSP_IFS}
if [ ${#_den_auth[@]} -gt 2 ]
then
_den_cont=${_den_auth[1]}
for (( _auth = 2 ; _auth < ${#_den_auth[@]} ; _auth++ ))
do
_den_cont=${_den_cont}'.'${_den_auth[${_auth}]}
done
_den_name[${#_den_name[@]}]=${_den_cont}'.'
_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_cont}'. SOA.C'
fi
fi
set +f
fi
;;
A) # IP(v4) Address Record
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}
_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' A'
else
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'
_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain A'
fi
_den_address[${#_den_address[@]}]=${_den_tmp[4]}
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}
;;
NS) # Name Server Record
# Domain name being serviced (may be other than current)
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' NS'
# Domain name of service provider
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str2}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' NSH'
_den_ns[${#_den_ns[@]}]=${_den_str2}' NS'
_den_pc[${#_den_pc[@]}]=${_den_str}' '${_den_str2}
fi
fi
;;
MX) # Mail Server Record
# Domain name being serviced (wildcards not handled here)
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MX'
fi
# Domain name of service provider
if _den_str=$(name_fixup ${_den_tmp[5]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MXH'
_den_ns[${#_den_ns[@]}]=${_den_str}' MX'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
;;
PTR) # Reverse address record
# Special name
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' PTR'
# Host name (not a CNAME)
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_rev[${#_den_rev[@]}]=${_den_str}' '${_den_str2}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' PTRH'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
fi
;;
AAAA) # IP(v6) Address Record
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}
_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' AAAA'
else
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'
_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain'
fi
# No processing for IPv6 addresses
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}
;;
CNAME) # Alias name record
# Nickname
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CNAME'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
# Hostname
if _den_str=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CHOST'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
;;
TXT)
;;
esac
fi
done
else # Lookup error == 'A' record 'unknown address'
_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}
fi
done
# Control dot array growth.
unique_lines _den_achn _den_achn # Works best, all the same.
edit_exact auth_chain _den_achn # Works best, unique items.
if [ ${#_den_achn[@]} -gt 0 ]
then
IFS=${NO_WSP}
auth_chain=( ${auth_chain[@]} ${_den_achn[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_ref _den_ref # Works best, all the same.
edit_exact ref_chain _den_ref # Works best, unique items.
if [ ${#_den_ref[@]} -gt 0 ]
then
IFS=${NO_WSP}
ref_chain=( ${ref_chain[@]} ${_den_ref[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_na _den_na
edit_exact name_address _den_na
if [ ${#_den_na[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_address=( ${name_address[@]} ${_den_na[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_ns _den_ns
edit_exact name_srvc _den_ns
if [ ${#_den_ns[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_srvc=( ${name_srvc[@]} ${_den_ns[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_nr _den_nr
edit_exact name_resource _den_nr
if [ ${#_den_nr[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_resource=( ${name_resource[@]} ${_den_nr[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_pc _den_pc
edit_exact parent_child _den_pc
if [ ${#_den_pc[@]} -gt 0 ]
then
IFS=${NO_WSP}
parent_child=( ${parent_child[@]} ${_den_pc[@]} )
IFS=${WSP_IFS}
fi
# Update list known_pair (Address and Name).
unique_lines _den_pair _den_pair
edit_exact known_pair _den_pair
if [ ${#_den_pair[@]} -gt 0 ] # Anything new?
then
IFS=${NO_WSP}
known_pair=( ${known_pair[@]} ${_den_pair[@]} )
IFS=${WSP_IFS}
fi
# Update list of reverse pairs.
unique_lines _den_rev _den_rev
edit_exact reverse_pair _den_rev
if [ ${#_den_rev[@]} -gt 0 ] # Anything new?
then
IFS=${NO_WSP}
reverse_pair=( ${reverse_pair[@]} ${_den_rev[@]} )
IFS=${WSP_IFS}
fi
# Check indirection limit -- give up if reached.
if ! _den_lmt=$(limit_chk ${1})
then
return 0
fi
Execution engine is LIFO. Order of pend operations is important.
Did we define any new addresses?
unique_lines _den_address _den_address # Scrub duplicates.
edit_exact known_address _den_address # Scrub already processed.
edit_exact un_address _den_address # Scrub already waiting.
if [ ${#_den_address[@]} -gt 0 ] # Anything new?
then
uc_address=( ${uc_address[@]} ${_den_address[@]} )
pend_func expand_input_address ${_den_lmt}
_trace_log[${#_trace_log[@]}]='# Add '${#_den_address[@]}' unchkd addr. #'
fi
Did we find any new names?
unique_lines _den_name _den_name # Scrub duplicates.
edit_exact known_name _den_name # Scrub already processed.
edit_exact uc_name _den_name # Scrub already waiting.
if [ ${#_den_name[@]} -gt 0 ] # Anything new?
then
uc_name=( ${uc_name[@]} ${_den_name[@]} )
pend_func expand_input_name ${_den_lmt}
_trace_log[${#_trace_log[@]}]='#Added '${#_den_name[@]}' unchkd name#'
fi
return 0
}
The parse-it-yourself delegation reply
Input is the chk_address list.
detail_each_address <indirection_limit>
detail_each_address() {
[ ${#chk_address[@]} -gt 0 ] || return 0
unique_lines chk_address chk_address
edit_exact known_address chk_address
if [ ${#chk_address[@]} -gt 0 ]
then
known_address=( ${known_address[@]} ${chk_address[@]} )
unset chk_address[@]
fi
return 0
}
# Application specific output functions # #
Pretty print the known pairs.
report_pairs() {
echo
echo 'Known network pairs.'
col_print known_pair 2 5 30
if [ ${#auth_chain[@]} -gt 0 ]
then
echo
echo 'Known chain of authority.'
col_print auth_chain 2 5 30 55
fi
if [ ${#reverse_pair[@]} -gt 0 ]
then
echo
echo 'Known reverse pairs.'
col_print reverse_pair 2 5 55
fi
return 0
}
Check an address against the list of blacklist servers.
A good place to capture for GraphViz: address->status(server(reports))
check_lists <ip_address>
check_lists() {
[ $# -eq 1 ] || return 1
local -a _cl_fwd_addr
local -a _cl_rev_addr
local -a _cl_reply
local -i _cl_rc
local -i _ls_cnt
local _cl_dns_addr
local _cl_lkup
split_ip ${1} _cl_fwd_addr _cl_rev_addr
_cl_dns_addr=$(dot_array _cl_rev_addr)'.'
_ls_cnt=${#list_server[@]}
echo ' Checking address '${1}
for (( _cl = 0 ; _cl < _ls_cnt ; _cl++ ))
do
_cl_lkup=${_cl_dns_addr}${list_server[${_cl}]}
if short_text ${_cl_lkup} _cl_reply
then
if [ ${#_cl_reply[@]} -gt 0 ]
then
echo ' Records from '${list_server[${_cl}]}
address_hits[${#address_hits[@]}]=${1}' '${list_server[${_cl}]}
_hs_RC=2
for (( _clr = 0 ; _clr < ${#_cl_reply[@]} ; _clr++ ))
do
echo ' '${_cl_reply[${_clr}]}
done
fi
fi
done
return 0
}
# The usual application glue # #
Who did it?
credits() {
echo
echo 'Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz'
}
How to use it?
(See also, "Quickstart" at end of script.)
usage() {
cat <<-'_usage_statement_'
The script is_spammer.bash requires either one or two arguments.
arg 1) May be one of:
a) A domain name
b) An IPv4 address
c) The name of a file with any mix of names
and addresses, one per line.
arg 2) May be one of:
a) A Blacklist server domain name
b) The name of a file with Blacklist server
domain names, one per line.
c) If not present, a default list of (free)
Blacklist servers is used.
d) If a filename of an empty, readable, file
is given,
Blacklist server lookup is disabled.
All script output is written to stdout.
Return codes: 0 -> All OK, 1 -> Script failure,
2 -> Something is Blacklisted.
Requires the external program 'dig' from the 'bind-9'
set of DNS programs. See: http://www.isc.org
The domain name lookup depth limit defaults to 2 levels.
Set the environment variable SPAMMER_LIMIT to change.
SPAMMER_LIMIT=0 means 'unlimited'
Limit may also be set on the command-line.
If arg#1 is an integer, the limit is set to that value
and then the above argument rules are applied.
Setting the environment variable 'SPAMMER_DATA' to a filename
will cause the script to write a GraphViz graphic file.
For the development version;
Setting the environment variable 'SPAMMER_TRACE' to a filename
will cause the execution engine to log a function call trace.
usage_statement
}
The default list of Blacklist servers:
Many choices, see: http://www.spews.org/lists.html
declare -a default_servers
See: http://www.spamhaus.org (Conservative, well maintained)
default_servers[0]='sbl-xbl.spamhaus.org'
See: http://ordb.org (Open mail relays)
default_servers[1]='relays.ordb.org'
See: http://www.spamcop.net/ (You can report spammers here)
default_servers[2]='bl.spamcop.net'
See: http://www.spews.org (An 'early detect' system)
default_servers[3]='l2.spews.dnsbl.sorbs.net'
See: http://www.dnsbl.us.sorbs.net/using.shtml
default_servers[4]='dnsbl.sorbs.net'
See: http://dsbl.org/usage (Various mail relay lists)
default_servers[5]='list.dsbl.org'
default_servers[6]='multihop.dsbl.org'
default_servers[7]='unconfirmed.dsbl.org'
User input argument #1
setup_input() {
if [ -e ${1} ] && [ -r ${1} ] # Name of readable file
then
file_to_array ${1} uc_name
echo 'Using filename >'${1}'< as input.'
else
if is_address ${1} # IP address?
then
uc_address=( ${1} )
echo 'Starting with address >'${1}'<'
else # Must be a name.
uc_name=( ${1} )
echo 'Starting with domain name >'${1}'<'
fi
fi
return 0
}
User input argument #2
setup_servers() {
if [ -e ${1} ] && [ -r ${1} ] # Name of a readable file
then
file_to_array ${1} list_server
echo 'Using filename >'${1}'< as blacklist server list.'
else
list_server=( ${1} )
echo 'Using blacklist server >'${1}'<'
fi
return 0
}
User environment variable SPAMMER_TRACE
live_log_die() {
if [ ${SPAMMER_TRACE:=} ] # Wants trace log?
then
if [ ! -e ${SPAMMER_TRACE} ]
then
if ! touch ${SPAMMER_TRACE} 2>/dev/null
then
pend_func echo $(printf '%q\n' \
'Unable to create log file >'${SPAMMER_TRACE}'<')
pend_release
exit 1
fi
_log_file=${SPAMMER_TRACE}
_pend_hook_=trace_logger
_log_dump=dump_log
else
if [ ! -w ${SPAMMER_TRACE} ]
then
pend_func echo $(printf '%q\n' \
'Unable to write log file >'${SPAMMER_TRACE}'<')
pend_release
exit 1
fi
_log_file=${SPAMMER_TRACE}
echo '' > ${_log_file}
_pend_hook_=trace_logger
_log_dump=dump_log
fi
fi
return 0
}
User environment variable SPAMMER_DATA
data_capture() {
if [ ${SPAMMER_DATA:=} ] # Wants a data dump?
then
if [ ! -e ${SPAMMER_DATA} ]
then
if ! touch ${SPAMMER_DATA} 2>/dev/null
then
pend_func echo $(printf '%q]n' \
'Unable to create data output file >'${SPAMMER_DATA}'<')
pend_release
exit 1
fi
_dot_file=${SPAMMER_DATA}
_dot_dump=dump_dot
else
if [ ! -w ${SPAMMER_DATA} ]
then
pend_func echo $(printf '%q\n' \
'Unable to write data output file >'${SPAMMER_DATA}'<')
pend_release
exit 1
fi
_dot_file=${SPAMMER_DATA}
_dot_dump=dump_dot
fi
fi
return 0
}
Grope user specified arguments.
do_user_args() {
if [ $# -gt 0 ] && is_number $1
then
indirect=$1
shift
fi
case $# in # Did user treat us well?
1)
if ! setup_input $1 # Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
list_server=( ${default_servers[@]} )
_list_cnt=${#list_server[@]}
echo 'Using default blacklist server list.'
echo 'Search depth limit: '${indirect}
;;
2)
if ! setup_input $1 # Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
if ! setup_servers $2 # Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
echo 'Search depth limit: '${indirect}
;;
*)
pend_func usage
pend_release
$_log_dump
exit 1
;;
esac
return 0
}
A general purpose debug tool.
list_array <array_name>
list_array() {
[ $# -eq 1 ] || return 1 # One argument required.
local -a _la_lines
set -f
local IFS=${NO_WSP}
eval _la_lines=\(\ \$\{$1\[@\]\}\ \)
echo
echo "Element count "${#_la_lines[@]}" array "${1}
local _ln_cnt=${#_la_lines[@]}
for (( _i = 0; _i < ${_ln_cnt}; _i++ ))
do
echo 'Element '$_i' >'${_la_lines[$_i]}'<'
done
set +f
return 0
}
# 'Hunt the Spammer' program code # #
pend_init # Ready stack engine.
pend_func credits # Last thing to print.
# Deal with user # #
live_log_die # Setup debug trace log.
data_capture # Setup data capture file.
echo
do_user_args $@
# Haven't exited yet - There is some hope # #
Discovery group - Execution engine is LIFO - pend
in reverse order of execution.
_hs_RC=0 # Hunt the Spammer return code
pend_mark
pend_func report_pairs # Report name-address pairs.
# The two detail_* are mutually recursive functions.
# They also pend expand_* functions as required.
# These two (the last of ???) exit the recursion.
pend_func detail_each_address # Get all resources of addresses.
pend_func detail_each_name # Get all resources of names.
# The two expand_* are mutually recursive functions,
#+ which pend additional detail_* functions as required.
pend_func expand_input_address 1 # Expand input names by address.
pend_func expand_input_name 1 # #xpand input addresses by name.
# Start with a unique set of names and addresses.
pend_func unique_lines uc_address uc_address
pend_func unique_lines uc_name uc_name
# Separate mixed input of names and addresses.
pend_func split_input
pend_release
# Pairs reported -- Unique list of IP addresses found
echo
_ip_cnt=${#known_address[@]}
if [ ${#list_server[@]} -eq 0 ]
then
echo 'Blacklist server list empty, none checked.'
else
if [ ${_ip_cnt} -eq 0 ]
then
echo 'Known address list empty, none checked.'
else
_ip_cnt=${_ip_cnt}-1 # Start at top.
echo 'Checking Blacklist servers.'
for (( _ip = _ip_cnt ; _ip >= 0 ; _ip-- ))
do
pend_func check_lists $( printf '%q\n' ${known_address[$_ip]} )
done
fi
fi
pend_release
$_dot_dump # Graphics file dump
$_log_dump # Execution trace
echo
Example output from script #
:<<-'is_spammer_outputs'
./is_spammer.bash 0 web4.alojamentos7.com
Starting with domain name >web4.alojamentos7.com<
Using default blacklist server list.
Search depth limit: 0
.:....::::...:::...:::.......::..::...:::.......::
Known network pairs.
66.98.208.97 web4.alojamentos7.com.
66.98.208.97 ns1.alojamentos7.com.
69.56.202.147 ns2.alojamentos.ws.
66.98.208.97 alojamentos7.com.
66.98.208.97 web.alojamentos7.com.
69.56.202.146 ns1.alojamentos.ws.
69.56.202.146 alojamentos.ws.
66.235.180.113 ns1.alojamentos.org.
66.235.181.192 ns2.alojamentos.org.
66.235.180.113 alojamentos.org.
66.235.180.113 web6.alojamentos.org.
216.234.234.30 ns1.theplanet.com.
12.96.160.115 ns2.theplanet.com.
216.185.111.52 mail1.theplanet.com.
69.56.141.4 spooling.theplanet.com.
216.185.111.40 theplanet.com.
216.185.111.40 www.theplanet.com.
216.185.111.52 mail.theplanet.com.
Checking Blacklist servers.
Checking address 66.98.208.97
Records from dnsbl.sorbs.net
"Spam Received See: http://www.dnsbl.sorbs.net/lookup.shtml?66.98.208.97"
Checking address 69.56.202.147
Checking address 69.56.202.146
Checking address 66.235.180.113
Checking address 66.235.181.192
Checking address 216.185.111.40
Checking address 216.234.234.30
Checking address 12.96.160.115
Checking address 216.185.111.52
Checking address 69.56.141.4
Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz
is_spammer_outputs
exit ${_hs_RC}
The script ignores everything from here on down #
- because of the 'exit' command, just above. #
Quickstart
=> ========
Prerequisites
Bash version 2.05b or 3.00 (bash --version)
A version of Bash which supports arrays. Array
support is included by default Bash configurations.
'dig,' version 9.x.x (dig $HOSTNAME, see first line of output)
A version of dig which supports the +short options.
See: dig_wrappers.bash for details.
Optional Prerequisites
'named,' a local DNS caching program. Any flavor will do.
Do twice: dig $HOSTNAME
Check near bottom of output for: SERVER: 127.0.0.1#53
That means you have one running.
Optional Graphics Support
'date,' a standard *nix thing. (date -R)
dot Program to convert graphic description file to a
diagram. (dot -V)
A part of the Graph-Viz set of programs.
See: [http://www.research.att.com/sw/tools/graphviz||GraphViz]
'dotty,' a visual editor for graphic description files.
Also a part of the Graph-Viz set of programs.
Quick Start
In the same directory as the is_spammer.bash script;
Do: ./is_spammer.bash
Usage Details
- Blacklist server choices.
(a) To use default, built-in list: Do nothing.
(b) To use your own list:
i. Create a file with a single Blacklist server
domain name per line.
ii. Provide that filename as the last argument to
the script.
(c) To use a single Blacklist server: Last argument
to the script.
(d) To disable Blacklist lookups:
i. Create an empty file (touch spammer.nul)
Your choice of filename.
ii. Provide the filename of that empty file as the
last argument to the script.
- Search depth limit.
(a) To use the default value of 2: Do nothing.
(b) To set a different limit:
A limit of 0 means: no limit.
i. export SPAMMER_LIMIT=1
or whatever limit you want.
ii. OR provide the desired limit as the first
argument to the script.
- Optional execution trace log.
(a) To use the default setting of no log output: Do nothing.
(b) To write an execution trace log:
export SPAMMER_TRACE=spammer.log
or whatever filename you want.
- Optional graphic description file.
(a) To use the default setting of no graphic file: Do nothing.
(b) To write a Graph-Viz graphic description file:
export SPAMMER_DATA=spammer.dot
or whatever filename you want.
- Where to start the search.
(a) Starting with a single domain name:
i. Without a command-line search limit: First
argument to script.
ii. With a command-line search limit: Second
argument to script.
(b) Starting with a single IP address:
i. Without a command-line search limit: First
argument to script.
ii. With a command-line search limit: Second
argument to script.
(c) Starting with (mixed) multiple name(s) and/or address(es):
Create a file with one name or address per line.
Your choice of filename.
i. Without a command-line search limit: Filename as
first argument to script.
ii. With a command-line search limit: Filename as
second argument to script.
- What to do with the display output.
(a) To view display output on screen: Do nothing.
(b) To save display output to a file: Redirect stdout to a filename.
(c) To discard display output: Redirect stdout to /dev/null.
- Temporary end of decision making.
press RETURN
wait (optionally, watch the dots and colons).
- Optionally check the return code.
(a) Return code 0: All OK
(b) Return code 1: Script setup failure
(c) Return code 2: Something was blacklisted.
- Where is my graph (diagram)?
The script does not directly produce a graph (diagram).
It only produces a graphic description file. You can
process the graphic descriptor file that was output
with the 'dot' program.
Until you edit that descriptor file, to describe the
relationships you want shown, all that you will get is
a bunch of labeled name and address nodes.
All of the script's discovered relationships are within
a comment block in the graphic descriptor file, each
with a descriptive heading.
The editing required to draw a line between a pair of
nodes from the information in the descriptor file may
be done with a text editor.
Given these lines somewhere in the descriptor file:
Known domain name nodes
N0000 [label="guardproof.info."] ;
N0002 [label="third.guardproof.info."] ;
Known address nodes
A0000 [label="61.141.32.197"] ;
/*
Known name->address edges
NA0000 third.guardproof.info. 61.141.32.197
Known parent->child edges
PC0000 guardproof.info. third.guardproof.info.
*/
Turn that into the following lines by substituting node
identifiers into the relationships:
Known domain name nodes
N0000 [label="guardproof.info."] ;
N0002 [label="third.guardproof.info."] ;
Known address nodes
A0000 [label="61.141.32.197"] ;
PC0000 guardproof.info. third.guardproof.info.
N0000->N0002 ;
NA0000 third.guardproof.info. 61.141.32.197
N0002->A0000 ;
/*
Known name->address edges
NA0000 third.guardproof.info. 61.141.32.197
Known parent->child edges
PC0000 guardproof.info. third.guardproof.info.
*/
Process that with the 'dot' program, and you have your
first network diagram.
In addition to the conventional graphic edges, the
descriptor file includes similar format pair-data that
describes services, zone records (sub-graphs?),
blacklisted addresses, and other things which might be
interesting to include in your graph. This additional
information could be displayed as different node
shapes, colors, line sizes, etc.
The descriptor file can also be read and edited by a
Bash script (of course). You should be able to find
most of the functions required within the
"is_spammer.bash" script.
End Quickstart.
Additional Note
=> ====
Michael Zick points out that there is a "makeviz.bash" interactive
Web site at rediris.es. Can't give the full URL, since this is not
a publically accessible site.
Another anti-spam script.
Example A-29. Spammer Hunt
!/bin/bash
whx.sh: "whois" spammer lookup
Author: Walter Dnes
Slight revisions (first section) by ABS Guide author.
Used in ABS Guide with permission.
Needs version 3.x or greater of Bash to run (because of =~ operator).
Commented by script author and ABS Guide author.
E_BADARGS=85 # Missing command-line arg.
E_NOHOST=86 # Host not found.
E_TIMEOUT=87 # Host lookup timed out.
E_UNDEF=88 # Some other (undefined) error.
HOSTWAIT=10 # Specify up to 10 seconds for host query reply.
# The actual wait may be a bit longer.
OUTFILE=whois.txt # Output file.
PORT=4321
if [ -z "$1" ] # Check for (required) command-line arg.
then
echo "Usage: $0 domain name or IP address"
exit $E_BADARGS
fi
if [[ "$1" =~ [a-zA-Z][a-zA-Z]$ ]] # Ends in two alpha chars?
then # It's a domain name &&
#+ must do host lookup.
IPADDR=$(host -W $HOSTWAIT $1 | awk '{print $4}')
# Doing host lookup
#+ to get IP address.
# Extract final field.
else
IPADDR="$1" # Command-line arg was IP address.
fi
echo; echo "IP Address is: "$IPADDR""; echo
if [ -e "$OUTFILE" ]
then
rm -f "$OUTFILE"
echo "Stale output file "$OUTFILE" removed."; echo
fi
Sanity checks.
(This section needs more work.)
===============================
if [ -z "$IPADDR" ]
No response.
then
echo "Host not found!"
exit $E_NOHOST # Bail out.
fi
if [[ "$IPADDR" =~ ^[;;] ]]
;; Connection timed out; no servers could be reached.
then
echo "Host lookup timed out!"
exit $E_TIMEOUT # Bail out.
fi
if [[ "$IPADDR" =~ [(NXDOMAIN)]$ ]]
Host xxxxxxxxx.xxx not found: 3(NXDOMAIN)
then
echo "Host not found!"
exit $E_NOHOST # Bail out.
fi
if [[ "$IPADDR" =~ [(SERVFAIL)]$ ]]
Host xxxxxxxxx.xxx not found: 2(SERVFAIL)
then
echo "Host not found!"
exit $E_NOHOST # Bail out.
fi
======================== Main body of script ========================
AFRINICquery() {
Define the function that queries AFRINIC. Echo a notification to the
- screen, and then run the actual query, redirecting output to $OUTFILE.
echo "Searching for $IPADDR in whois.afrinic.net"
whois -h whois.afrinic.net "$IPADDR" > $OUTFILE
Check for presence of reference to an rwhois.
Warn about non-functional rwhois.infosat.net server
- and attempt rwhois query.
if grep -e "^remarks: .*rwhois.[^ ]+" "$OUTFILE"
then
echo " " >> $OUTFILE
echo "***" >> $OUTFILE
echo "***" >> $OUTFILE
echo "Warning: rwhois.infosat.net was not working \
as of 2005/02/02" >> $OUTFILE
echo " when this script was written." >> $OUTFILE
echo "***" >> $OUTFILE
echo "***" >> $OUTFILE
echo " " >> $OUTFILE
RWHOIS=`grep "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\
sed "s/\(^.*\)\(rwhois\..*\)\(:4.*\)/\2/"`
whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE
fi
}
APNICquery() {
echo "Searching for $IPADDR in whois.apnic.net"
whois -h whois.apnic.net "$IPADDR" > $OUTFILE
Just about every country has its own internet registrar.
I don't normally bother consulting them, because the regional registry
- usually supplies sufficient information.
There are a few exceptions, where the regional registry simply
- refers to the national registry for direct data.
These are Japan and South Korea in APNIC, and Brasil in LACNIC.
The following if statement checks $OUTFILE (whois.txt) for the presence
- of "KR" (South Korea) or "JP" (Japan) in the country field.
If either is found, the query is re-run against the appropriate
if grep -E "^country:[ ]+KR$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.krnic.net"
whois -h whois.krnic.net "$IPADDR" >> $OUTFILE
elif grep -E "^country:[ ]+JP$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.nic.ad.jp"
whois -h whois.nic.ad.jp "$IPADDR"/e >> $OUTFILE
fi
}
ARINquery() {
echo "Searching for $IPADDR in whois.arin.net"
whois -h whois.arin.net "$IPADDR" > $OUTFILE
Several large internet providers listed by ARIN have their own
- internal whois service, referred to as "rwhois".
A large block of IP addresses is listed with the provider
To get the IP addresses of 2nd-level ISPs or other large customers,
- one has to refer to the rwhois server on port 4321.
I originally started with a bunch of "if" statements checking for
This approach is unwieldy, and there's always another rwhois server
- that I didn't know about.
A more elegant approach is to check $OUTFILE for a reference
- to a whois server, parse that server name out of the comment section,
- and re-run the query against the appropriate rwhois server.
The parsing looks a bit ugly, with a long continued line inside
But it only has to be done once, and will work as new servers are added.
@ ABS Guide author comment: it isn't all that ugly, and is, in fact,
@+ an instructive use of Regular Expressions.
if grep -E "^Comment: .*rwhois.[^ ]+" "$OUTFILE"
then
RWHOIS=`grep -e "^Comment:.*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\
sed "s/^\(.*\)\(rwhois\.[^ ]\+\)\(.*$\)/\2/"`
echo "Searching for $IPADDR in ${RWHOIS}"
whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE
fi
}
LACNICquery() {
echo "Searching for $IPADDR in whois.lacnic.net"
whois -h whois.lacnic.net "$IPADDR" > $OUTFILE
The following if statement checks $OUTFILE (whois.txt) for
- the presence of "BR" (Brasil) in the country field.
If it is found, the query is re-run against whois.registro.br.
if grep -E "^country:[ ]+BR$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.registro.br"
whois -h whois.registro.br "$IPADDR" >> $OUTFILE
fi
}
RIPEquery() {
echo "Searching for $IPADDR in whois.ripe.net"
whois -h whois.ripe.net "$IPADDR" > $OUTFILE
}
Initialize a few variables.
- slash8 is the most significant octet
- slash16 consists of the two most significant octets
- octet2 is the second most significant octet
slash8=echo $IPADDR | cut -d. -f 1
if [ -z "$slash8" ] # Yet another sanity check.
then
echo "Undefined error!"
exit $E_UNDEF
fi
slash16=echo $IPADDR | cut -d. -f 1-2
^ Period specified as 'cut" delimiter.
if [ -z "$slash16" ]
then
echo "Undefined error!"
exit $E_UNDEF
fi
octet2=echo $slash16 | cut -d. -f 2
if [ -z "$octet2" ]
then
echo "Undefined error!"
exit $E_UNDEF
fi
Check for various odds and ends of reserved space.
There is no point in querying for those addresses.
if [ $slash8 == 0 ]; then
echo $IPADDR is '"This Network"' space; Not querying
elif [ $slash8 == 10 ]; then
echo $IPADDR is RFC1918 space; Not querying
elif [ $slash8 == 14 ]; then
echo $IPADDR is '"Public Data Network"' space; Not querying
elif [ $slash8 == 127 ]; then
echo $IPADDR is loopback space; Not querying
elif [ $slash16 == 169.254 ]; then
echo $IPADDR is link-local space; Not querying
elif [ $slash8 == 172 ] && [ $octet2 -ge 16 ] && [ $octet2 -le 31 ];then
echo $IPADDR is RFC1918 space; Not querying
elif [ $slash16 == 192.168 ]; then
echo $IPADDR is RFC1918 space; Not querying
elif [ $slash8 -ge 224 ]; then
echo $IPADDR is either Multicast or reserved space; Not querying
elif [ $slash8 -ge 200 ] && [ $slash8 -le 201 ]; then LACNICquery "$IPADDR"
elif [ $slash8 -ge 202 ] && [ $slash8 -le 203 ]; then APNICquery "$IPADDR"
elif [ $slash8 -ge 210 ] && [ $slash8 -le 211 ]; then APNICquery "$IPADDR"
elif [ $slash8 -ge 218 ] && [ $slash8 -le 223 ]; then APNICquery "$IPADDR"
If we got this far without making a decision, query ARIN.
If a reference is found in $OUTFILE to APNIC, AFRINIC, LACNIC, or RIPE,
- query the appropriate whois server.
else
ARINquery "$IPADDR"
if grep "whois.afrinic.net" "$OUTFILE"; then
AFRINICquery "$IPADDR"
elif grep -E "^OrgID:[ ]+RIPE$" "$OUTFILE"; then
RIPEquery "$IPADDR"
elif grep -E "^OrgID:[ ]+APNIC$" "$OUTFILE"; then
APNICquery "$IPADDR"
elif grep -E "^OrgID:[ ]+LACNIC$" "$OUTFILE"; then
LACNICquery "$IPADDR"
fi
fi
@ ---------------------------------------------------------------
Try also:
wget http://logi.cc/nw/whois.php3?ACTION=doQuery&DOMAIN=$IPADDR
@ ---------------------------------------------------------------
We've now finished the querying.
Echo a copy of the final result to the screen.
cat $OUTFILE
Or "less $OUTFILE" . . .
exit 0
@ ABS Guide author comments:
@ Nothing fancy here, but still a very useful tool for hunting spammers.
@ Sure, the script can be cleaned up some, and it's still a bit buggy,
@+ (exercise for reader), but all the same, it's a nice piece of coding
@+ by Walter Dnes.
@ Thank you!
"Little Monster's" front end to wget.
Example A-30. Making wget easier to use
!/bin/bash
wgetter2.bash
Author: Little Monster [monster@monstruum.co.uk]
==> Used in ABS Guide with permission of script author.
==> This script still needs debugging and fixups (exercise for reader).
==> It could also use some additional editing in the comments.
This is wgetter2 --
- a Bash script to make wget a bit more friendly, and save typing.
Carefully crafted by Little Monster.
More or less complete on 02/02/2005.
If you think this script can be improved,
- email me at: monster@monstruum.co.uk
==> and cc: to the author of the ABS Guide, please.
This script is licenced under the GPL.
You are free to copy, alter and re-use it,
- but please don't try to claim you wrote it.
Log your changes here instead.
=======================================================================
changelog:
07/02/2005. Fixups by Little Monster.
02/02/2005. Minor additions by Little Monster.
(See after # +++++++++++ )
29/01/2005. Minor stylistic edits and cleanups by author of ABS Guide.
Added exit error codes.
22/11/2004. Finished initial version of second version of wgetter:
wgetter2 is born.
01/12/2004. Changed 'runn' function so it can be run 2 ways --
either ask for a file name or have one input on the CL.
01/12/2004. Made sensible handling of no URL's given.
01/12/2004. Made loop of main options, so you don't
have to keep calling wgetter 2 all the time.
Runs as a session instead.
01/12/2004. Added looping to 'runn' function.
Simplified and improved.
01/12/2004. Added state to recursion setting.
Enables re-use of previous value.
05/12/2004. Modified the file detection routine in the 'runn' function
so it's not fooled by empty values, and is cleaner.
01/02/2004. Added cookie finding routine from later version (which
isn't ready yet), so as not to have hard-coded paths.
=======================================================================
Error codes for abnormal exit.
E_USAGE=67 # Usage message, then quit.
E_NO_OPTS=68 # No command-line args entered.
E_NO_URLS=69 # No URLs passed to script.
E_NO_SAVEFILE=70 # No save filename passed to script.
E_USER_EXIT=71 # User decides to quit.
Basic default wget command we want to use.
This is the place to change it, if required.
NB: if using a proxy, set http_proxy = yourproxy in .wgetrc.
Otherwise delete --proxy=on, below.
====================================================================
CommandA="wget -nc -c -t 5 --progress=bar --random-wait --proxy=on -r"
====================================================================
Set some other variables and explain them.
pattern=" -A .jpg,.JPG,.jpeg,.JPEG,.gif,.GIF,.htm,.html,.shtml,.php"
# wget's option to only get certain types of file.
# comment out if not using
today=date +%F # Used for a filename.
home=$HOME # Set HOME to an internal variable.
# In case some other path is used, change it here.
depthDefault=3 # Set a sensible default recursion.
Depth=$depthDefault # Otherwise user feedback doesn't tie in properly.
RefA="" # Set blank referring page.
Flag="" # Default to not saving anything,
#+ or whatever else might be wanted in future.
lister="" # Used for passing a list of urls directly to wget.
Woptions="" # Used for passing wget some options for itself.
inFile="" # Used for the run function.
newFile="" # Used for the run function.
savePath="$home/w-save"
Config="$home/.wgetter2rc"
# This is where some variables can be stored,
#+ if permanently changed from within the script.
Cookie_List="$home/.cookielist"
# So we know where the cookies are kept . . .
cFlag="" # Part of the cookie file selection routine.
Define the options available. Easy to change letters here if needed.
These are the optional options; you don't just wait to be asked.
save=s # Save command instead of executing it.
cook=c # Change cookie file for this session.
help=h # Usage guide.
list=l # Pass wget the -i option and URL list.
runn=r # Run saved commands as an argument to the option.
inpu=i # Run saved commands interactively.
wopt=w # Allow to enter options to pass directly to wget.
if [ -z "$1" ]; then # Make sure we get something for wget to eat.
echo "You must at least enter a URL or option!"
echo "-$help for usage."
exit $E_NO_OPTS
fi
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
added added added added added added added added added added added added
if [ ! -e "$Config" ]; then # See if configuration file exists.
echo "Creating configuration file, $Config"
echo "# This is the configuration file for wgetter2" > "$Config"
echo "# Your customised settings will be saved in this file" >> "$Config"
else
source $Config # Import variables we set outside the script.
fi
if [ ! -e "$Cookie_List" ]; then
Set up a list of cookie files, if there isn't one.
echo "Hunting for cookies . . ."
find -name cookies.txt >> $Cookie_List # Create the list of cookie files.
fi # Isolate this in its own 'if' statement,
#+ in case we got interrupted while searching.
if [ -z "$cFlag" ]; then # If we haven't already done this . . .
echo # Make a nice space after the command prompt.
echo "Looks like you haven't set up your source of cookies yet."
n=0 # Make sure the counter
#+ doesn't contain random values.
while read; do
Cookies[$n]=$REPLY # Put the cookie files we found into an array.
echo "$n) ${Cookies[$n]}" # Create a menu.
n=$(( n + 1 )) # Increment the counter.
done < $Cookie_List # Feed the read statement.
echo "Enter the number of the cookie file you want to use."
echo "If you won't be using cookies, just press RETURN."
echo
echo "I won't be asking this again. Edit $Config"
echo "If you decide to change at a later date"
echo "or use the -${cook} option for per session changes."
read
if [ ! -z $REPLY ]; then # User didn't just press return.
Cookie=" --load-cookies ${Cookies[$REPLY]}"
# Set the variable here as well as in the config file.
echo "Cookie=\" --load-cookies ${Cookies[$REPLY]}\"" >> $Config
fi
echo "cFlag=1" >> $Config # So we know not to ask again.
fi
end added section end added section end added section end added section
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Another variable.
This one may or may not be subject to variation.
A bit like the small print.
CookiesON=$Cookie
echo "cookie file is $CookiesON" # For debugging.
echo "home is ${home}" # For debugging.
# Got caught with this one!
wopts()
{
echo "Enter options to pass to wget."
echo "It is assumed you know what you're doing."
echo
echo "You can pass their arguments here too."
That is to say, everything passed here is passed to wget.
read Wopts
Read in the options to be passed to wget.
Woptions=" $Wopts"
^ Why the leading space?
Assign to another variable.
Just for fun, or something . . .
echo "passing options ${Wopts} to wget"
Mainly for debugging.
Is cute.
return
}
save_func()
{
echo "Settings will be saved."
if [ ! -d $savePath ]; then # See if directory exists.
mkdir $savePath # Create the directory to save things in
#+ if it isn't already there.
fi
Flag=S
Tell the final bit of code what to do.
Set a flag since stuff is done in main.
return
}
usage() # Tell them how it works.
{
echo "Welcome to wgetter. This is a front end to wget."
echo "It will always run wget with these options:"
echo "$CommandA"
echo "and the pattern to match: $pattern \
(which you can change at the top of this script)."
echo "It will also ask you for recursion depth, \
and if you want to use a referring page."
echo "Wgetter accepts the following options:"
echo ""
echo "-$help : Display this help."
echo "-$save : Save the command to a file $savePath/wget-($today) \
instead of running it."
echo "-$runn : Run saved wget commands instead of starting a new one -"
echo "Enter filename as argument to this option."
echo "-$inpu : Run saved wget commands interactively --"
echo "The script will ask you for the filename."
echo "-$cook : Change the cookies file for this session."
echo "-$list : Tell wget to use URL's from a list instead of \
from the command-line."
echo "-$wopt : Pass any other options direct to wget."
echo ""
echo "See the wget man page for additional options \
you can pass to wget."
echo ""
exit $E_USAGE # End here. Don't process anything else.
}
list_func() # Gives the user the option to use the -i option to wget,
#+ and a list of URLs.
{
while [ 1 ]; do
echo "Enter the name of the file containing URL's (press q to change
your mind)."
read urlfile
if [ ! -e "$urlfile" ] && [ "$urlfile" != q ]; then
# Look for a file, or the quit option.
echo "That file does not exist!"
elif [ "$urlfile" = q ]; then # Check quit option.
echo "Not using a url list."
return
else
echo "using $urlfile."
echo "If you gave url's on the command-line, I'll use those first."
# Report wget standard behaviour to the user.
lister=" -i $urlfile" # This is what we want to pass to wget.
return
fi
done
}
cookie_func() # Give the user the option to use a different cookie file.
{
while [ 1 ]; do
echo "Change the cookies file. Press return if you don't want to change
it."
read Cookies
NB: this is not the same as Cookie, earlier.
There is an 's' on the end.
Bit like chocolate chips.
if [ -z "$Cookies" ]; then # Escape clause for wusses.
return
elif [ ! -e "$Cookies" ]; then
echo "File does not exist. Try again." # Keep em going . . .
else
CookiesON=" --load-cookies $Cookies" # File is good -- use it!
return
fi
done
}
run_func()
{
if [ -z "$OPTARG" ]; then
Test to see if we used the in-line option or the query one.
if [ ! -d "$savePath" ]; then # If directory doesn't exist . . .
echo "$savePath does not appear to exist."
echo "Please supply path and filename of saved wget commands:"
read newFile
until [ -f "$newFile" ]; do # Keep going till we get something.
echo "Sorry, that file does not exist. Please try again."
# Try really hard to get something.
read newFile
done
if [ -z ( grep wget ${newfile} ) ]; then
# Assume they haven't got the right file and bail out.
echo "Sorry, that file does not contain wget commands. Aborting."
exit
fi
This is bogus code.
It doesn't actually work.
If anyone wants to fix it, feel free!
filePath="${newFile}"
else
echo "Save path is $savePath"
echo "Please enter name of the file which you want to use."
echo "You have a choice of:"
ls $savePath # Give them a choice.
read inFile
until [ -f "$savePath/$inFile" ]; do # Keep going till
#+ we get something.
if [ ! -f "${savePath}/${inFile}" ]; then # If file doesn't exist.
echo "Sorry, that file does not exist. Please choose from:"
ls $savePath # If a mistake is made.
read inFile
fi
done
filePath="${savePath}/${inFile}" # Make one variable . . .
fi
else filePath="${savePath}/${OPTARG}" # Which can be many things . . .
fi
if [ ! -f "$filePath" ]; then # If a bogus file got through.
echo "You did not specify a suitable file."
echo "Run this script with the -${save} option first."
echo "Aborting."
exit $E_NO_SAVEFILE
fi
echo "Using: $filePath"
while read; do
eval $REPLY
echo "Completed: $REPLY"
done < $filePath # Feed the actual file we are using into a 'while' loop.
exit
}
Fish out any options we are using for the script.
This is based on the demo in "Learning The Bash Shell" (O'Reilly).
while getopts ":$save$cook$help$list$runn:$inpu$wopt" opt
do
case $opt in
$save) save_func;; # Save some wgetter sessions for later.
$cook) cookie_func;; # Change cookie file.
$help) usage;; # Get help.
$list) list_func;; # Allow wget to use a list of URLs.
$runn) run_func;; # Useful if you are calling wgetter from,
#+ for example, a cron script.
$inpu) run_func;; # When you don't know what your files are named.
$wopt) wopts;; # Pass options directly to wget.
\?) echo "Not a valid option."
echo "Use -${wopt} to pass options directly to wget,"
echo "or -${help} for help";; # Catch anything else.
esac
done
shift $((OPTIND - 1)) # Do funky magic stuff with $#.
if [ -z "$1" ] && [ -z "$lister" ]; then
# We should be left with at least one URL
#+ on the command-line, unless a list is
#+ being used -- catch empty CL's.
echo "No URL's given! You must enter them on the same line as wgetter2."
echo "E.g., wgetter2 http://somesite http://anothersite."
echo "Use $help option for more information."
exit $E_NO_URLS # Bail out, with appropriate error code.
fi
URLS=" $@"
Use this so that URL list can be changed if we stay in the option loop.
while [ 1 ]; do
This is where we ask for the most used options.
(Mostly unchanged from version 1 of wgetter)
if [ -z $curDepth ]; then
Current=""
else Current=" Current value is $curDepth"
fi
echo "How deep should I go? \
(integer: Default is $depthDefault.$Current)"
read Depth # Recursion -- how far should we go?
inputB="" # Reset this to blank on each pass of the loop.
echo "Enter the name of the referring page (default is none)."
read inputB # Need this for some sites.
echo "Do you want to have the output logged to the terminal"
echo "(y/n, default is yes)?"
read noHide # Otherwise wget will just log it to a file.
case $noHide in # Now you see me, now you don't.
y|Y ) hide="";;
n|N ) hide=" -b";;
* ) hide="";;
esac
if [ -z ${Depth} ]; then
# User accepted either default or current depth,
#+ in which case Depth is now empty.
if [ -z ${curDepth} ]; then
# See if a depth was set on a previous iteration.
Depth="$depthDefault"
# Set the default recursion depth if nothing
#+ else to use.
else Depth="$curDepth" # Otherwise, set the one we used before.
fi
fi
Recurse=" -l $Depth" # Set how deep we want to go.
curDepth=$Depth # Remember setting for next time.
if [ ! -z $inputB ]; then
RefA=" --referer=$inputB" # Option to use referring page.
fi
WGETTER="${CommandA}${pattern}${hide}${RefA}${Recurse}\
${CookiesON}${lister}${Woptions}${URLS}"
Just string the whole lot together . . .
NB: no embedded spaces.
They are in the individual elements so that if any are empty,
#+ we don't get an extra space.
if [ -z "${CookiesON}" ] && [ "$cFlag" = "1" ] ; then
echo "Warning -- can't find cookie file"
# This should be changed,
#+ in case the user has opted to not use cookies.
fi
if [ "$Flag" = "S" ]; then
echo "$WGETTER" >> $savePath/wget-${today}
# Create a unique filename for today, or append to it if it exists.
echo "$inputB" >> $savePath/site-list-${today}
# Make a list, so it's easy to refer back to,
#+ since the whole command is a bit confusing to look at.
echo "Command saved to the file $savePath/wget-${today}"
# Tell the user.
echo "Referring page URL saved to the file$ \
savePath/site-list-${today}"
# Tell the user.
Saver=" with save option"
# Stick this somewhere, so it appears in the loop if set.
else
echo "*****************"
echo "*****Getting*****"
echo "*****************"
echo ""
echo "$WGETTER"
echo ""
echo "*****************"
eval "$WGETTER"
fi
echo ""
echo "Starting over$Saver."
echo "If you want to stop, press q."
echo "Otherwise, enter some URL's:"
# Let them go again. Tell about save option being set.
read
case $REPLY in
# Need to change this to a 'trap' clause.
q|Q ) exit $E_USER_EXIT;; # Exercise for the reader?
* ) URLS=" $REPLY";;
esac
echo ""
done
exit 0
Example A-31. A podcasting script
!/bin/bash
bashpodder.sh:
By Linc 10/1/2004
Find the latest script at
- http://linc.homeunix.org:8080/scripts/bashpodder
Last revision 12/14/2004 - Many Contributors!
If you use this and have made improvements or have comments
- drop me an email at linc dot fessenden at gmail dot com
I'd appreciate it!
==> ABS Guide extra comments.
==> Author of this script has kindly granted permission
==>+ for inclusion in ABS Guide.
==> ################################################################
==> What is "podcasting"?
==> It's broadcasting "radio shows" over the Internet.
==> These shows can be played on iPods and other music file players.
==> This script makes it possible.
==> See documentation at the script author's site, above.
==> ################################################################
Make script crontab friendly:
cd $(dirname $0)
==> Change to directory where this script lives.
datadir is the directory you want podcasts saved to:
datadir=$(date +%Y-%m-%d)
==> Will create a date-labeled directory, named: YYYY-MM-DD
Check for and create datadir if necessary:
if test ! -d $datadir
then
mkdir $datadir
fi
Delete any temp file:
rm -f temp.log
Read the bp.conf file and wget any url not already
while read podcast
do # ==> Main action follows.
file=$(wget -q $podcast -O - | tr '\r' '\n' | tr ' " | \
sed -n 's/.url="([^"])".*/\1/p')
for url in $file
do
echo $url >> temp.log
if ! grep "$url" podcast.log > /dev/null
then
wget -q -P $datadir "$url"
fi
done
done < bp.conf
Move dynamically created log file to permanent log file:
cat podcast.log >> temp.log
sort temp.log | uniq > podcast.log
rm temp.log
Create an m3u playlist:
ls $datadir | grep -v m3u > $datadir/podcast.m3u
exit 0
For a different scripting approach to Podcasting,
see Phil Salkie's article,
"Internet Radio to Podcast with Shell Tools"
in the September, 2005 issue of LINUX JOURNAL,
http://www.linuxjournal.com/article/8171
Example A-32. Nightly backup to a firewire HD
!/bin/bash
nightly-backup.sh
http://www.richardneill.org/source.php#nightly-backup-rsync
This is Free Software licensed under the GNU GPL.
==> Included in ABS Guide with script author's kind permission.
==> (Thanks!)
This does a backup from the host computer to a locally connected
- firewire HDD using rsync and ssh.
(Script should work with USB-connected device (see lines 40-43).
It then rotates the backups.
Run it via cron every night at 5am.
This only backs up the home directory.
If ownerships (other than the user's) should be preserved,
- then run the rsync process as root (and re-instate the -o).
We save every day for 7 days, then every week for 4 weeks,
- then every month for 3 months.
See: http://www.mikerubel.org/computers/rsync_snapshots/
- for more explanation of the theory.
Save as: $HOME/bin/nightly-backup_firewire-hdd.sh
Known bugs:
i) Ideally, we want to exclude ~/.tmp and the browser caches.
ii) If the user is sitting at the computer at 5am,
To some extent, this is a
BEGIN CONFIGURATION SECTION ############################################
LOCAL_USER=rjn # User whose home directory should be backed up.
MOUNT_POINT=/backup # Mountpoint of backup drive.
# NO trailing slash!
# This must be unique (eg using a udev symlink)
MOUNT_POINT=/media/disk # For USB-connected device.
SOURCE_DIR=/home/$LOCAL_USER # NO trailing slash - it DOES matter to rsync.
BACKUP_DEST_DIR=$MOUNT_POINT/backup/hostname -s.${LOCAL_USER}.nightly_backup
DRY_RUN=false #If true, invoke rsync with -n, to do a dry run.
# Comment out or set to false for normal use.
VERBOSE=false # If true, make rsync verbose.
# Comment out or set to false otherwise.
COMPRESS=false # If true, compress.
# Good for internet, bad on LAN.
# Comment out or set to false otherwise.
Exit Codes ###
E_VARS_NOT_SET=64
E_COMMANDLINE=65
E_MOUNT_FAIL=70
E_NOSOURCEDIR=71
E_UNMOUNTED=72
E_BACKUP=73
END CONFIGURATION SECTION ##############################################
Check that all the important variables have been set:
if [ -z "$LOCAL_USER" ] ||
[ -z "$SOURCE_DIR" ] ||
[ -z "$MOUNT_POINT" ] ||
[ -z "$BACKUP_DEST_DIR" ]
then
echo 'One of the variables is not set! Edit the file: $0. BACKUP FAILED.'
exit $E_VARS_NOT_SET
fi
if [ "$#" != 0 ] # If command-line param(s) . . .
then # Here document(ation).
cat <<-ENDOFTEXT
Automatic Nightly backup run from cron.
Read the source for more details: $0
The backup directory is $BACKUP_DEST_DIR .
It will be created if necessary; initialisation is no longer required.
WARNING: Contents of $BACKUP_DEST_DIR are rotated.
Directories named 'backup.\$i' will eventually be DELETED.
We keep backups from every day for 7 days (1-8),
then every week for 4 weeks (9-12),
then every month for 3 months (13-15).
You may wish to add this to your crontab using 'crontab -e'
# Back up files: $SOURCE_DIR to $BACKUP_DEST_DIR
#+ every night at 3:15 am
15 03 * * * /home/$LOCAL_USER/bin/nightly-backup_firewire-hdd.sh
Don't forget to verify the backups are working,
especially if you don't read cron's mail!"
ENDOFTEXT
exit $E_COMMANDLINE
fi
Parse the options.
==================
if [ "$DRY_RUN" == "true" ]; then
DRY_RUN="-n"
echo "WARNING:"
echo "THIS IS A 'DRY RUN'!"
echo "No data will actually be transferred!"
else
DRY_RUN=""
fi
if [ "$VERBOSE" == "true" ]; then
VERBOSE="-v"
else
VERBOSE=""
fi
if [ "$COMPRESS" == "true" ]; then
COMPRESS="-z"
else
COMPRESS=""
fi
Every week (actually of 8 days) and every month,
- extra backups are preserved.
DAY_OF_MONTH=date +%d # Day of month (01..31).
if [ $DAY_OF_MONTH = 01 ]; then # First of month.
MONTHSTART=true
elif [ $DAY_OF_MONTH = 08 \
-o $DAY_OF_MONTH = 16 \
-o $DAY_OF_MONTH = 24 ]; then
# Day 8,16,24 (use 8, not 7 to better handle 31-day months)
WEEKSTART=true
fi
Check that the HDD is mounted.
At least, check that something is mounted here!
We can use something unique to the device, rather than just guessing
- the scsi-id by having an appropriate udev rule in
- /etc/udev/rules.d/10-rules.local
- and by putting a relevant entry in /etc/fstab.
Eg: this udev rule:
BUS="scsi", KERNEL="sd*", SYSFS{vendor}="WDC WD16",
SYSFS{model}="00JB-00GVA0 ", NAME="%k", SYMLINK="lacie_1394d%n"
if mount | grep $MOUNT_POINT >/dev/null; then
echo "Mount point $MOUNT_POINT is indeed mounted. OK"
else
echo -n "Attempting to mount $MOUNT_POINT..."
# If it isn't mounted, try to mount it.
sudo mount $MOUNT_POINT 2>/dev/null
if mount | grep $MOUNT_POINT >/dev/null; then
UNMOUNT_LATER=TRUE
echo "OK"
# Note: Ensure that this is also unmounted
#+ if we exit prematurely with failure.
else
echo "FAILED"
echo -e "Nothing is mounted at $MOUNT_POINT. BACKUP FAILED!"
exit $E_MOUNT_FAIL
fi
fi
Check that source dir exists and is readable.
if [ ! -r $SOURCE_DIR ] ; then
echo "$SOURCE_DIR does not exist, or cannot be read. BACKUP FAILED."
exit $E_NOSOURCEDIR
fi
Check that the backup directory structure is as it should be.
If not, create it.
Create the subdirectories.
Note that backup.0 will be created as needed by rsync.
for ((i=1;i<=15;i++)); do
if [ ! -d $BACKUP_DEST_DIR/backup.$i ]; then
if /bin/mkdir -p $BACKUP_DEST_DIR/backup.$i ; then
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ No [ ] test brackets. Why?
echo "Warning: directory $BACKUP_DEST_DIR/backup.$i is missing,"
echo "or was not initialised. (Re-)creating it."
else
echo "ERROR: directory $BACKUP_DEST_DIR/backup.$i"
echo "is missing and could not be created."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd
sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
fi
done
Set the permission to 700 for security
- on an otherwise permissive multi-user system.
if ! /bin/chmod 700 $BACKUP_DEST_DIR ; then
echo "ERROR: Could not set permissions on $BACKUP_DEST_DIR to 700."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
Before we exit, unmount the mount point if necessary.
cd ; sudo umount $MOUNT_POINT \
&& echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
Create the symlink: current -> backup.1 if required.
A failure here is not critical.
cd $BACKUP_DEST_DIR
if [ ! -h current ] ; then
if ! /bin/ln -s backup.1 current ; then
echo "WARNING: could not create symlink current -> backup.1"
fi
fi
Now, do the rsync.
echo "Now doing backup with rsync..."
echo "Source dir: $SOURCE_DIR"
echo -e "Backup destination dir: $BACKUP_DEST_DIR\n"
/usr/bin/rsync $DRY_RUN $VERBOSE -a -S --delete --modify-window=60 \
--link-dest=../backup.1 $SOURCE_DIR $BACKUP_DEST_DIR/backup.0/
Only warn, rather than exit if the rsync failed,
- since it may only be a minor problem.
E.g., if one file is not readable, rsync will fail.
This shouldn't prevent the rotation.
Not using, e.g., date +%a since these directories
- are just full of links and don't consume that much space.
if [ $? != 0 ]; then
BACKUP_JUSTINCASE=backup.date +%F_%T.justincase
echo "WARNING: the rsync process did not entirely succeed."
echo "Something might be wrong."
echo "Saving an extra copy at: $BACKUP_JUSTINCASE"
echo "WARNING: if this occurs regularly, a LOT of space will be consumed,"
echo "even though these are just hard-links!"
fi
Save a readme in the backup parent directory.
Save another one in the recent subdirectory.
echo "Backup of $SOURCE_DIR on hostname was last run on \
echo "This backup of $SOURCE_DIR on `hostname` was created on \
If we are not in a dry run, rotate the backups.
[ -z "$DRY_RUN" ] &&
Check how full the backup disk is.
Warn if 90%. if 98% or more, we'll probably fail, so give up.
(Note: df can output to more than one line.)
We test this here, rather than before
#+ so that rsync may possibly have a chance.
DISK_FULL_PERCENT=`/bin/df $BACKUP_DEST_DIR |
tr "\n" ' ' | awk '{print $12}' | grep -oE [0-9]+ `
echo "Disk space check on backup partition \
$MOUNT_POINT $DISK_FULL_PERCENT% full."
if [ $DISK_FULL_PERCENT -gt 90 ]; then
echo "Warning: Disk is greater than 90% full."
fi
if [ $DISK_FULL_PERCENT -gt 98 ]; then
echo "Error: Disk is full! Giving up."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd; sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
Create an extra backup.
If this copy fails, give up.
if [ -n "$BACKUP_JUSTINCASE" ]; then
if ! /bin/cp -al $BACKUP_DEST_DIR/backup.0 \
$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE
then
echo "ERROR: Failed to create extra copy \
$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE"
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd ;sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
fi
At start of month, rotate the oldest 8.
if [ "$MONTHSTART" == "true" ]; then
echo -e "\nStart of month. \
Removing oldest backup: $BACKUP_DEST_DIR/backup.15" &&
/bin/rm -rf $BACKUP_DEST_DIR/backup.15 &&
echo "Rotating monthly,weekly backups: \
$BACKUP_DEST_DIR/backup.[8-14] -> $BACKUP_DEST_DIR/backup.[9-15]" &&
/bin/mv $BACKUP_DEST_DIR/backup.14 $BACKUP_DEST_DIR/backup.15 &&
/bin/mv $BACKUP_DEST_DIR/backup.13 $BACKUP_DEST_DIR/backup.14 &&
/bin/mv $BACKUP_DEST_DIR/backup.12 $BACKUP_DEST_DIR/backup.13 &&
/bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&
/bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&
/bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&
/bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9
At start of week, rotate the second-oldest 4.
elif [ "$WEEKSTART" == "true" ]; then
echo -e "\nStart of week. \
Removing oldest weekly backup: $BACKUP_DEST_DIR/backup.12" &&
/bin/rm -rf $BACKUP_DEST_DIR/backup.12 &&
echo "Rotating weekly backups: \
$BACKUP_DEST_DIR/backup.[8-11] -> $BACKUP_DEST_DIR/backup.[9-12]" &&
/bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&
/bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&
/bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&
/bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9
else
echo -e "\nRemoving oldest daily backup: $BACKUP_DEST_DIR/backup.8" &&
/bin/rm -rf $BACKUP_DEST_DIR/backup.8
fi &&
Every day, rotate the newest 8.
echo "Rotating daily backups: \
$BACKUP_DEST_DIR/backup.[1-7] -> $BACKUP_DEST_DIR/backup.[2-8]" &&
/bin/mv $BACKUP_DEST_DIR/backup.7 $BACKUP_DEST_DIR/backup.8 &&
/bin/mv $BACKUP_DEST_DIR/backup.6 $BACKUP_DEST_DIR/backup.7 &&
/bin/mv $BACKUP_DEST_DIR/backup.5 $BACKUP_DEST_DIR/backup.6 &&
/bin/mv $BACKUP_DEST_DIR/backup.4 $BACKUP_DEST_DIR/backup.5 &&
/bin/mv $BACKUP_DEST_DIR/backup.3 $BACKUP_DEST_DIR/backup.4 &&
/bin/mv $BACKUP_DEST_DIR/backup.2 $BACKUP_DEST_DIR/backup.3 &&
/bin/mv $BACKUP_DEST_DIR/backup.1 $BACKUP_DEST_DIR/backup.2 &&
/bin/mv $BACKUP_DEST_DIR/backup.0 $BACKUP_DEST_DIR/backup.1 &&
SUCCESS=true
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
Unmount the mount point if it wasn't mounted to begin with.
cd ; sudo umount $MOUNT_POINT && echo "Unmounted $MOUNT_POINT again."
fi
if [ "$SUCCESS" == "true" ]; then
echo 'SUCCESS!'
exit 0
fi
Should have already exited if backup worked.
echo 'BACKUP FAILED! Is this just a dry run? Is the disk full?) '
exit $E_BACKUP
Example A-33. An expanded cd command
cdll
by Phil Braham
############################################
Latest version of this script available from
http://freshmeat.net/projects/cd/
############################################
.cd_new
An enhancement of the Unix cd command
There are unlimited stack entries and special entries. The stack
entries keep the last cd_maxhistory
directories that have been used. The special entries can be
assigned to commonly used directories.
The special entries may be pre-assigned by setting the environment
variables CDSn or by using the -u or -U command.
The following is a suggestion for the .profile file:
. cdll # Set up the cd command
alias cd='cd_new' # Replace the cd command
cd -U # Upload pre-assigned entries for
#+ the stack and special entries
cd -D # Set non-default mode
alias @="cd_new @" # Allow @ to be used to get history
For help type:
cd -h or
cd -H
Version 1.2.1
Written by Phil Braham - Realtime Software Pty Ltd
(realtime@mpx.com.au)
Please send any suggestions or enhancements to the author (also at
phil@braham.net)
cd_hm ()
{
${PRINTF} "%s" "cd [dir] [0-9] [@[s|h] [-g [<dir>]] [-d] \
[-D] [-r] [dir|0-9] [-R] [|0-9]
[-s] [-S] [-u] [-U] [-f] [-F] [-h] [-H] [-v]
<dir> Go to directory
0-n Go to previous directory (0 is previous, 1 is last but 1 etc)
n is up to max history (default is 50)
@ List history and special entries
@h List history entries
@s List special entries
-g [<dir>] Go to literal name (bypass special names)
This is to allow access to dirs called '0','1','-h' etc
-d Change default action - verbose. (See note)
-D Change default action - silent. (See note)
-s<n> Go to the special entry <n>*
-S<n> Go to the special entry <n>
and replace it with the current dir*
-r<n> [<dir>] Go to directory <dir>
and then put it on special entry <n>*
-R<n> [<dir>] Go to directory <dir>
and put current dir on special entry <n>*
-a<n> Alternative suggested directory. See note below.
-f [<file>] File entries to <file>.
-u [<file>] Update entries from <file>.
If no filename supplied then default file
(${CDPath}${2:-"$CDFile"}) is used
-F and -U are silent versions
-v Print version number
-h Help
-H Detailed help
*The special entries (0 - 9) are held until log off, replaced by another
entry or updated with the -u command
Alternative suggested directories:
If a directory is not found then CD will suggest any
possibilities. These are directories starting with the same letters
and if any are found they are listed prefixed with -a<n>
where <n> is a number.
It's possible to go to the directory by entering cd -a<n>
on the command line.
The directory for -r<n> or -R<n> may be a number.
For example:
$ cd -r3 4 Go to history entry 4 and put it on special entry 3
$ cd -R3 4 Put current dir on the special entry 3
and go to history entry 4
$ cd -s3 Go to special entry 3
Note that commands R,r,S and s may be used without a number
and refer to 0:
$ cd -s Go to special entry 0
$ cd -S Go to special entry 0 and make special
entry 0 current dir
$ cd -r 1 Go to history entry 1 and put it on special entry 0
$ cd -r Go to history entry 0 and put it on special entry 0
"
if ${TEST} "$CD_MODE" = "PREV"
then
${PRINTF} "$cd_mnset"
else
${PRINTF} "$cd_mset"
fi
}
cd_Hm ()
{
cd_hm
${PRINTF} "%s" "
The previous directories (0-$cd_maxhistory) are stored in the
environment variables CD[0] - CD[$cd_maxhistory]
Similarly the special directories S0 - $cd_maxspecial are in
the environment variable CDS[0] - CDS[$cd_maxspecial]
and may be accessed from the command line
The default pathname for the -f and -u commands is $CDPath
The default filename for the -f and -u commands is $CDFile
Set the following environment variables:
CDL_PROMPTLEN - Set to the length of prompt you require.
Prompt string is set to the right characters of the
current directory.
If not set then prompt is left unchanged
CDL_PROMPT_PRE - Set to the string to prefix the prompt.
Default is:
non-root: \"\\[\\e[01;34m\\]\" (sets colour to blue).
root: \"\\[\\e[01;31m\\]\" (sets colour to red).
CDL_PROMPT_POST - Set to the string to suffix the prompt.
Default is:
non-root: \"\\[\\e[00m\\]$\"
(resets colour and displays $).
root: \"\\[\\e[00m\\]#\"
(resets colour and displays #).
CDPath - Set the default path for the -f & -u options.
Default is home directory
CDFile - Set the default filename for the -f & -u options.
Default is cdfile
"
cd_version
}
cd_version ()
{
printf "Version: ${VERSION_MAJOR}.${VERSION_MINOR} Date: ${VERSION_DATE}\n"
}
Truncate right.
params:
p1 - string
p2 - length to truncate to
returns string in tcd
cd_right_trunc ()
{
local tlen=${2}
local plen=${#1}
local str="${1}"
local diff
local filler="<--"
if ${TEST} ${plen} -le ${tlen}
then
tcd="${str}"
else
let diff=${plen}-${tlen}
elen=3
if ${TEST} ${diff} -le 2
then
let elen=${diff}
fi
tlen=-${tlen}
let tlen=${tlen}+${elen}
tcd=${filler:0:elen}${str:tlen}
fi
}
Three versions of do history:
cd_dohistory - packs history and specials side by side
cd_dohistoryH - Shows only hstory
cd_dohistoryS - Shows only specials
cd_dohistory ()
{
cd_getrc
${PRINTF} "History:\n"
local -i count=${cd_histcount}
while ${TEST} ${count} -ge 0
do
cd_right_trunc "${CD[count]}" ${cd_lchar}
${PRINTF} "%2d %-${cd_lchar}.${cd_lchar}s " ${count} "${tcd}"
cd_right_trunc "${CDS[count]}" ${cd_rchar}
${PRINTF} "S%d %-${cd_rchar}.${cd_rchar}s\n" ${count} "${tcd}"
count=${count}-1
done
}
cd_dohistoryH ()
{
cd_getrc
${PRINTF} "History:\n"
local -i count=${cd_maxhistory}
while ${TEST} ${count} -ge 0
do
${PRINTF} "${count} %-${cd_flchar}.${cd_flchar}s\n" ${CD[$count]}
count=${count}-1
done
}
cd_dohistoryS ()
{
cd_getrc
${PRINTF} "Specials:\n"
local -i count=${cd_maxspecial}
while ${TEST} ${count} -ge 0
do
${PRINTF} "S${count} %-${cd_flchar}.${cd_flchar}s\n" ${CDS[$count]}
count=${count}-1
done
}
cd_getrc ()
{
cd_flchar=$(stty -a | awk -F \;
'/rows/ { print $2 $3 }' | awk -F \ '{ print $4 }')
if ${TEST} ${cd_flchar} -ne 0
then
cd_lchar=${cd_flchar}/2-5
cd_rchar=${cd_flchar}/2-5
cd_flchar=${cd_flchar}-5
else
cd_flchar=${FLCHAR:=75}
# cd_flchar is used for for the @s & @h history
cd_lchar=${LCHAR:=35}
cd_rchar=${RCHAR:=35}
fi
}
cd_doselection ()
{
local -i nm=0
cd_doflag="TRUE"
if ${TEST} "${CD_MODE}" = "PREV"
then
if ${TEST} -z "$cd_npwd"
then
cd_npwd=0
fi
fi
tm=$(echo "${cd_npwd}" | cut -b 1)
if ${TEST} "${tm}" = "-"
then
pm=$(echo "${cd_npwd}" | cut -b 2)
nm=$(echo "${cd_npwd}" | cut -d $pm -f2)
case "${pm}" in
a) cd_npwd=${cd_sugg[$nm]} ;;
s) cd_npwd="${CDS[$nm]}" ;;
S) cd_npwd="${CDS[$nm]}" ; CDS[$nm]=`pwd` ;;
r) cd_npwd="$2" ; cd_specDir=$nm ; cd_doselection "$1" "$2";;
R) cd_npwd="$2" ; CDS[$nm]=`pwd` ; cd_doselection "$1" "$2";;
esac
fi
if ${TEST} "${cd_npwd}" != "." -a "${cd_npwd}" \
!= ".." -a "${cd_npwd}" -le ${cd_maxhistory} >>/dev/null 2>&1
then
cd_npwd=${CD[$cd_npwd]}
else
case "$cd_npwd" in
@) cd_dohistory ; cd_doflag="FALSE" ;;
@h) cd_dohistoryH ; cd_doflag="FALSE" ;;
@s) cd_dohistoryS ; cd_doflag="FALSE" ;;
-h) cd_hm ; cd_doflag="FALSE" ;;
-H) cd_Hm ; cd_doflag="FALSE" ;;
-f) cd_fsave "SHOW" $2 ; cd_doflag="FALSE" ;;
-u) cd_upload "SHOW" $2 ; cd_doflag="FALSE" ;;
-F) cd_fsave "NOSHOW" $2 ; cd_doflag="FALSE" ;;
-U) cd_upload "NOSHOW" $2 ; cd_doflag="FALSE" ;;
-g) cd_npwd="$2" ;;
-d) cd_chdefm 1; cd_doflag="FALSE" ;;
-D) cd_chdefm 0; cd_doflag="FALSE" ;;
-r) cd_npwd="$2" ; cd_specDir=0 ; cd_doselection "$1" "$2";;
-R) cd_npwd="$2" ; CDS[0]=`pwd` ; cd_doselection "$1" "$2";;
-s) cd_npwd="${CDS[0]}" ;;
-S) cd_npwd="${CDS[0]}" ; CDS[0]=`pwd` ;;
-v) cd_version ; cd_doflag="FALSE";;
esac
fi
}
cd_chdefm ()
{
if ${TEST} "${CD_MODE}" = "PREV"
then
CD_MODE=""
if ${TEST} $1 -eq 1
then
${PRINTF} "${cd_mset}"
fi
else
CD_MODE="PREV"
if ${TEST} $1 -eq 1
then
${PRINTF} "${cd_mnset}"
fi
fi
}
cd_fsave ()
{
local sfile=${CDPath}${2:-"$CDFile"}
if ${TEST} "$1" = "SHOW"
then
${PRINTF} "Saved to %s\n" $sfile
fi
${RM} -f ${sfile}
local -i count=0
while ${TEST} ${count} -le ${cd_maxhistory}
do
echo "CD[$count]=\"${CD[$count]}\"" >> ${sfile}
count=${count}+1
done
count=0
while ${TEST} ${count} -le ${cd_maxspecial}
do
echo "CDS[$count]=\"${CDS[$count]}\"" >> ${sfile}
count=${count}+1
done
}
cd_upload ()
{
local sfile=${CDPath}${2:-"$CDFile"}
if ${TEST} "${1}" = "SHOW"
then
${PRINTF} "Loading from %s\n" ${sfile}
fi
. ${sfile}
}
cd_new ()
{
local -i count
local -i choose=0
cd_npwd="${1}"
cd_specDir=-1
cd_doselection "${1}" "${2}"
if ${TEST} ${cd_doflag} = "TRUE"
then
if ${TEST} "${CD[0]}" != "`pwd`"
then
count=$cd_maxhistory
while ${TEST} $count -gt 0
do
CD[$count]=${CD[$count-1]}
count=${count}-1
done
CD[0]=`pwd`
fi
command cd "${cd_npwd}" 2>/dev/null
if ${TEST} $? -eq 1
then
${PRINTF} "Unknown dir: %s\n" "${cd_npwd}"
local -i ftflag=0
for i in "${cd_npwd}"*
do
if ${TEST} -d "${i}"
then
if ${TEST} ${ftflag} -eq 0
then
${PRINTF} "Suggest:\n"
ftflag=1
fi
${PRINTF} "\t-a${choose} %s\n" "$i"
cd_sugg[$choose]="${i}"
choose=${choose}+1
fi
done
fi
fi
if ${TEST} ${cd_specDir} -ne -1
then
CDS[${cd_specDir}]=`pwd`
fi
if ${TEST} ! -z "${CDL_PROMPTLEN}"
then
cd_right_trunc "${PWD}" ${CDL_PROMPTLEN}
cd_rp=${CDL_PROMPT_PRE}${tcd}${CDL_PROMPT_POST}
export PS1="$(echo -ne ${cd_rp})"
fi
}
Initialisation here #
VERSION_MAJOR="1"
VERSION_MINOR="2.1"
VERSION_DATE="24-MAY-2003"
alias cd=cd_new
Set up commands
RM=/bin/rm
TEST=test
PRINTF=printf # Use builtin printf
Change this to modify the default pre- and post prompt strings. #
These only come into effect if CDL_PROMPTLEN is set. #
if ${TEST} ${EUID} -eq 0
then
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="$HOSTNAME@"}
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;31m\\]"} # Root is in red
CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]#"}
else
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;34m\\]"} # Users in blue
CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]$"}
fi
cd_maxhistory defines the max number of history entries allowed.
typeset -i cd_maxhistory=50
cd_maxspecial defines the number of special entries.
typeset -i cd_maxspecial=9
cd_histcount defines the number of entries displayed in
typeset -i cd_histcount=9
export CDPath=${HOME}/
Change these to use a different #
- default path and filename #
export CDFile=${CDFILE:=cdfile} # for the -u and -f commands #
#
typeset -i cd_lchar cd_rchar cd_flchar
# This is the number of chars to allow for the #
cd_flchar=${FLCHAR:=75} #+ cd_flchar is used for for the @s & @h history#
typeset -ax CD CDS
cd_mset="\n\tDefault mode is now set - entering cd with no parameters \
has the default action\n\tUse cd -d or -D for cd to go to \
previous directory with no parameters\n"
cd_mnset="\n\tNon-default mode is now set - entering cd with no \
parameters is the same as entering cd 0\n\tUse cd -d or \
-D to change default cd action\n"
==================================================================== #
: <<DOCUMENTATION
Written by Phil Braham. Realtime Software Pty Ltd.
Released under GNU license. Free to use. Please pass any modifications
or comments to the author Phil Braham:
realtime@mpx.com.au
=> =====================================================================
cdll is a replacement for cd and incorporates similar functionality to
the bash pushd and popd commands but is independent of them.
This version of cdll has been tested on Linux using Bash. It will work
on most Linux versions but will probably not work on other shells without
modification.
Introduction
=> ==========
cdll allows easy moving about between directories. When changing to a new
directory the current one is automatically put onto a stack. By default
50 entries are kept, but this is configurable. Special directories can be
kept for easy access - by default up to 10, but this is configurable. The
most recent stack entries and the special entries can be easily viewed.
The directory stack and special entries can be saved to, and loaded from,
a file. This allows them to be set up on login, saved before logging out
or changed when moving project to project.
In addition, cdll provides a flexible command prompt facility that allows,
for example, a directory name in colour that is truncated from the left
if it gets too long.
Setting up cdll
=> =============
Copy cdll to either your local home directory or a central directory
such as /usr/bin (this will require root access).
Copy the file cdfile to your home directory. It will require read and
write access. This a default file that contains a directory stack and
special entries.
To replace the cd command you must add commands to your login script.
The login script is one or more of:
/etc/profile
~/.bash_profile
~/.bash_login
~/.profile
~/.bashrc
/etc/bash.bashrc.local
To setup your login, ~/.bashrc is recommended, for global (and root) setup
add the commands to /etc/bash.bashrc.local
To set up on login, add the command:
. <dir>/cdll
For example if cdll is in your local home directory:
. ~/cdll
If in /usr/bin then:
. /usr/bin/cdll
If you want to use this instead of the buitin cd command then add:
alias cd='cd_new'
We would also recommend the following commands:
alias @='cd_new @'
cd -U
cd -D
If you want to use cdll's prompt facilty then add the following:
CDL_PROMPTLEN=nn
Where nn is a number described below. Initially 99 would be suitable
number.
Thus the script looks something like this:
######################################################################
# CD Setup
######################################################################
CDL_PROMPTLEN=21 # Allow a prompt length of up to 21 characters
. /usr/bin/cdll # Initialise cdll
alias cd='cd_new' # Replace the built in cd command
alias @='cd_new @' # Allow @ at the prompt to display history
cd -U # Upload directories
cd -D # Set default action to non-posix
######################################################################
The full meaning of these commands will become clear later.
There are a couple of caveats. If another program changes the directory
without calling cdll, then the directory won't be put on the stack and
also if the prompt facility is used then this will not be updated. Two
programs that can do this are pushd and popd. To update the prompt and
stack simply enter:
cd .
Note that if the previous entry on the stack is the current directory
then the stack is not updated.
Usage
=> ===
cd [dir] [0-9] [@[s|h] [-g ] [-d] [-D] [-r]
[dir|0-9] [-R] [|0-9] [-s] [-S]
[-u] [-U] [-f] [-F] [-h] [-H] [-v]
<dir> Go to directory
0-n Goto previous directory (0 is previous,
1 is last but 1, etc.)
n is up to max history (default is 50)
@ List history and special entries (Usually available as $ @)
@h List history entries
@s List special entries
-g [<dir>] Go to literal name (bypass special names)
This is to allow access to dirs called '0','1','-h' etc
-d Change default action - verbose. (See note)
-D Change default action - silent. (See note)
-s<n> Go to the special entry <n>
-S<n> Go to the special entry <n>
and replace it with the current dir
-r<n> [<dir>] Go to directory <dir>
and then put it on special entry <n>
-R<n> [<dir>] Go to directory <dir>
and put current dir on special entry <n>
-a<n> Alternative suggested directory. See note below.
-f [<file>] File entries to <file>.
-u [<file>] Update entries from <file>.
If no filename supplied then default file (~/cdfile) is used
-F and -U are silent versions
-v Print version number
-h Help
-H Detailed help
Examples
=> ======
These examples assume non-default mode is set (that is, cd with no
parameters will go to the most recent stack directory), that aliases
have been set up for cd and @ as described above and that cd's prompt
facility is active and the prompt length is 21 characters.
/home/phil$ @
# List the entries with the @
History:
# Output of the @ command
.....
# Skipped these entries for brevity
1 /home/phil/ummdev S1 /home/phil/perl
# Most recent two history entries
0 /home/phil/perl/eg S0 /home/phil/umm/ummdev
# and two special entries are shown
/home/phil$ cd /home/phil/utils/Cdll
# Now change directories
/home/phil/utils/Cdll$ @
# Prompt reflects the directory.
History:
# New history
.....
1 /home/phil/perl/eg S1 /home/phil/perl
# History entry 0 has moved to 1
0 /home/phil S0 /home/phil/umm/ummdev
# and the most recent has entered
To go to a history entry:
/home/phil/utils/Cdll$ cd 1
# Go to history entry 1.
/home/phil/perl/eg$
# Current directory is now what was 1
To go to a special entry:
/home/phil/perl/eg$ cd -s1
# Go to special entry 1
/home/phil/umm/ummdev$
# Current directory is S1
To go to a directory called, for example, 1:
/home/phil$ cd -g 1
# -g ignores the special meaning of 1
/home/phil/1$
To put current directory on the special list as S1:
cd -r1 . # OR
cd -R1 . # These have the same effect if the directory is
#+ . (the current directory)
To go to a directory and add it as a special
The directory for -r or -R may be a number.
For example:
$ cd -r3 4 Go to history entry 4 and put it on special entry 3
$ cd -R3 4 Put current dir on the special entry 3 and go to
history entry 4
$ cd -s3 Go to special entry 3
Note that commands R,r,S and s may be used without a number and
refer to 0:
$ cd -s Go to special entry 0
$ cd -S Go to special entry 0 and make special entry 0
current dir
$ cd -r 1 Go to history entry 1 and put it on special entry 0
$ cd -r Go to history entry 0 and put it on special entry 0
Alternative suggested directories:
If a directory is not found, then CD will suggest any
possibilities. These are directories starting with the same letters
and if any are found they are listed prefixed with -a<n>
where <n> is a number. It's possible to go to the directory
by entering cd -a<n> on the command line.
Use cd -d or -D to change default cd action. cd -H will show
current action.
The history entries (0-n) are stored in the environment variables
CD[0] - CD[n]
Similarly the special directories S0 - 9 are in the environment
variable CDS[0] - CDS[9]
and may be accessed from the command line, for example:
ls -l ${CDS[3]}
cat ${CD[8]}/file.txt
The default pathname for the -f and -u commands is ~
The default filename for the -f and -u commands is cdfile
Configuration
=> ===========
The following environment variables can be set:
CDL_PROMPTLEN - Set to the length of prompt you require.
Prompt string is set to the right characters of the current
directory. If not set, then prompt is left unchanged. Note
that this is the number of characters that the directory is
shortened to, not the total characters in the prompt.
CDL_PROMPT_PRE - Set to the string to prefix the prompt.
Default is:
non-root: "\\[\\e[01;34m\\]" (sets colour to blue).
root: "\\[\\e[01;31m\\]" (sets colour to red).
CDL_PROMPT_POST - Set to the string to suffix the prompt.
Default is:
non-root: "\\[\\e[00m\\]$"
(resets colour and displays $).
root: "\\[\\e[00m\\]#"
(resets colour and displays #).
Note:
CDL_PROMPT_PRE & _POST only t
CDPath - Set the default path for the -f & -u options.
Default is home directory
CDFile - Set the default filename for the -f & -u options.
Default is cdfile
There are three variables defined in the file cdll which control the
number of entries stored or displayed. They are in the section labeled
'Initialisation here' towards the end of the file.
cd_maxhistory - The number of history entries stored.
Default is 50.
cd_maxspecial - The number of special entries allowed.
Default is 9.
cd_histcount - The number of history and special entries
displayed. Default is 9.
Note that cd_maxspecial should be >= cd_histcount to avoid displaying
special entries that can't be set.
Version: 1.2.1 Date: 24-MAY-2003
DOCUMENTATION
Example A-34. A soundcard setup script
!/bin/bash
soundcard-on.sh
Script author: Mkarcher
http://www.thinkwiki.org/wiki ...
/Script_for_configuring_the_CS4239_sound_chip_in_PnP_mode
ABS Guide author made minor changes and added comments.
Couldn't contact script author to ask for permission to use, but ...
- the script was released under the FDL,
- so its use here should be both legal and ethical.
Sound-via-pnp-script for Thinkpad 600E
- and possibly other computers with onboard CS4239/CS4610
- that do not work with the PCI driver
- and are not recognized by the PnP code of snd-cs4236.
Also for some 770-series Thinkpads, such as the 770x.
Run as root user, of course.
These are old and very obsolete laptop computers,
- but this particular script is very instructive,
- as it shows how to set up and hack device files.
Search for sound card pnp device:
for dev in /sys/bus/pnp/devices/*
do
grep CSC0100 $dev/id > /dev/null && WSSDEV=$dev
grep CSC0110 $dev/id > /dev/null && CTLDEV=$dev
done
On 770x:
WSSDEV = /sys/bus/pnp/devices/00:07
CTLDEV = /sys/bus/pnp/devices/00:06
These are symbolic links to /sys/devices/pnp0/ ...
Activate devices:
Thinkpad boots with devices disabled unless "fast boot" is turned off
echo activate > $WSSDEV/resources
echo activate > $CTLDEV/resources
Parse resource settings.
{ read # Discard "state = active" (see below).
read bla port1
read bla port2
read bla port3
read bla irq
read bla dma1
read bla dma2
The "bla's" are labels in the first field: "io," "state," etc.
These are discarded.
Hack: with PnPBIOS: ports are: port1: WSS, port2:
#+ OPL, port3: sb (unneeded)
with ACPI-PnP:ports are: port1: OPL, port2: sb, port3: WSS
(ACPI bios seems to be wrong here, the PnP-card-code in snd-cs4236.c
#+ uses the PnPBIOS port order)
Detect port order using the fixed OPL port as reference.
if [ ${port2%%-*} = 0x388 ]
^^^^ Strip out everything following hyphen in port address.
So, if port1 is 0x530-0x537
#+ we're left with 0x530 -- the start address of the port.
then
PnPBIOS: usual order
port=${port1%%-*}
oplport=${port2%%-*}
else
ACPI: mixed-up order
port=${port3%%-*}
oplport=${port1%%-*}
fi
} < $WSSDEV/resources
To see what's going on here:
cat /sys/devices/pnp0/00:07/resources
state = active
io 0x530-0x537
io 0x388-0x38b
io 0x220-0x233
irq 5
dma 1
dma 0
^^^ "bla" labels in first field (discarded).
{ read # Discard first line, as above.
read bla port1
cport=${port1%%-*}
^^^^
Just want start address of port.
} < $CTLDEV/resources
Load the module:
modprobe --ignore-install snd-cs4236 port=$port cport=$cport\
fm_port=$oplport irq=$irq dma1=$dma1 dma2=$dma2 isapnp=0 index=0
See the modprobe manpage.
exit $?
Example A-35. Locating split paragraphs in a text file
!/bin/bash
find-splitpara.sh
Finds split paragraphs in a text file,
- and tags the line numbers.
ARGCOUNT=1 # Expect one arg.
OFF=0 # Flag states.
ON=1
E_WRONGARGS=85
file="$1" # Target filename.
lineno=1 # Line number. Start at 1.
Flag=$OFF # Blank line flag.
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: basename $0 FILENAME"
exit $E_WRONGARGS
fi
file_read () # Scan file for pattern, then print line.
{
while read line
do
if [[ "$line" =~ ^[a-z] && $Flag -eq $ON ]]
then # Line begins with lowercase character, following blank line.
echo -n "$lineno:: "
echo "$line"
fi
if [[ "$line" =~ ^$ ]]
then # If blank line,
Flag=$ON #+ set flag.
else
Flag=$OFF
fi
((lineno++))
done
} < $file # Redirect file into function's stdin.
file_read
exit $?
This is line one of an example paragraph, bla, bla, bla.
This is line two, and line three should follow on next line, but
there is a blank line separating the two parts of the paragraph.
Running this script on a file containing the above paragraph
yields:
4:: there is a blank line separating the two parts of the paragraph.
There will be additional output for all the other split paragraphs
in the target file.
Example A-36. Insertion sort
!/bin/bash
insertion-sort.bash: Insertion sort implementation in Bash
Heavy use of Bash array features:
URL: http://www.lugmen.org.ar/~jjo/jjotip/insertion-sort.bash.d
Lightly reformatted by ABS Guide author.
License: GPLv2
Used in ABS Guide with author's permission (thanks!).
Test with: ./insertion-sort.bash -t
Or: bash insertion-sort.bash -t
The following doesn't work:
sh insertion-sort.bash -t
Why not? Hint: which Bash-specific features are disabled
- when running a script by 'sh script.sh'?
: ${DEBUG:=0} # Debug, override with: DEBUG=1 ./scriptname . . .
Parameter substitution -- set DEBUG to 0 if not previously set.
Global array: "list"
typeset -a list
Load whitespace-separated numbers from stdin.
if [ "$1" = "-t" ]; then
DEBUG=1
read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) # Random list.
^ ^ process substition
else
read -a list
fi
numelem=${#list[*]}
Shows the list, marking the element whose index is $1
- by surrounding it with the two chars passed as $2.
Whole line prefixed with $3.
showlist()
{
echo "$3"${list[@]:0:$1} ${2:0:1}${list[$1]}${2:1:1} ${list[@]:$1+1};
}
Loop pivot -- from second element to end of list.
for(( i=1; i<numelem; i++ )) do
((DEBUG))&&showlist i "[]" " "
# From current _pivot_, back to first element.
for(( j=i; j; j-- )) do
# Search for the 1st elem. less than current "pivot" . . .
[[ "${list[j-1]}" -le "${list[i]}" ]] && break
done
(( i==j )) && continue ## No insertion was needed for this element.
# . . . Move list[i] (pivot) to the left of list[j]:
list=(${list[@]:0:j} ${list[i]} ${list[j]}\
# {0,j-1} {i} {j}
${list[@]:j+1:i-(j+1)} ${list[@]:i+1})
# {j+1,i-1} {i+1,last}
((DEBUG))&&showlist j "<>" "*"
done
echo
echo "------"
echo $'Result:\n'${list[@]}
exit $?
Example A-37. Standard Deviation
!/bin/bash
sd.sh: Standard Deviation
The Standard Deviation indicates how consistent a set of data is.
It shows to what extent the individual data points deviate from the
- arithmetic mean, i.e., how much they "bounce around" (or cluster).
It is essentially the average deviation-distance of the
- data points from the mean.
=========================================================== #
To calculate the Standard Deviation:
1 Find the arithmetic mean (average) of all the data points.
2 Subtract each data point from the arithmetic mean,
and square that difference.
3 Add all of the individual difference-squares in # 2.
4 Divide the sum in # 3 by the number of data points.
This is known as the "variance."
5 The square root of # 4 gives the Standard Deviation.
=========================================================== #
count=0 # Number of data points; global.
SC=9 # Scale to be used by bc. Nine decimal places.
E_DATAFILE=90 # Data file error.
----------------- Set data file ---------------------
if [ ! -z "$1" ] # Specify filename as cmd-line arg?
then
datafile="$1" # ASCII text file,
else #+ one (numerical) data point per line!
datafile=sample.dat
fi # See example data file, below.
if [ ! -e "$datafile" ]
then
echo """$datafile"" does not exist!"
exit $E_DATAFILE
fi
arith_mean ()
{
local rt=0 # Running total.
local am=0 # Arithmetic mean.
local ct=0 # Number of data points.
while read value # Read one data point at a time.
do
rt=$(echo "scale=$SC; $rt + $value" | bc)
(( ct++ ))
done
am=$(echo "scale=$SC; $rt / $ct" | bc)
echo $am; return $ct # This function "returns" TWO values!
Caution: This little trick will not work if $ct > 255!
To handle a larger number of data points,
#+ simply comment out the "return $ct" above.
} <"$datafile" # Feed in data file.
sd ()
{
mean1=$1 # Arithmetic mean (passed to function).
n=$2 # How many data points.
sum2=0 # Sum of squared differences ("variance").
avg2=0 # Average of $sum2.
sdev=0 # Standard Deviation.
while read value # Read one line at a time.
do
diff=$(echo "scale=$SC; $mean1 - $value" | bc)
# Difference between arith. mean and data point.
dif2=$(echo "scale=$SC; $diff * $diff" | bc) # Squared.
sum2=$(echo "scale=$SC; $sum2 + $dif2" | bc) # Sum of squares.
done
avg2=$(echo "scale=$SC; $sum2 / $n" | bc) # Avg. of sum of squares.
sdev=$(echo "scale=$SC; sqrt($avg2)" | bc) # Square root =
echo $sdev # Standard Deviation.
} <"$datafile" # Rewinds data file.
======================================================= #
mean=$(arith_mean); count=$? # Two returns from function!
std_dev=$(sd $mean $count)
echo
echo "Number of data points in ""$datafile"" = $count"
echo "Arithmetic mean (average) = $mean"
echo "Standard Deviation = $std_dev"
echo
======================================================= #
exit
This script could stand some drastic streamlining,
- but not at the cost of reduced legibility, please.
++++++++++++++++++++++++++++++++++++++++ #
A sample data file (sample1.dat):
18.35
19.0
18.88
18.91
18.64
$ sh sd.sh sample1.dat
Number of data points in "sample1.dat" = 5
Arithmetic mean (average) = 18.756000000
Standard Deviation = .235338054
++++++++++++++++++++++++++++++++++++++++ #
Example A-38. A pad file generator for shareware authors
!/bin/bash
pad.sh
PAD (xml) file creator
- Released to the Public Domain.
Generates a "PAD" descriptor file for shareware
- packages, according to the specifications
http://www.asp-shareware.org/pad
Accepts (optional) save filename as a command-line argument.
if [ -n "$1" ]
then
savefile=$1
else
savefile=save_file.xml # Default save_file name.
fi
===== PAD file headers =====
HDR1=""
HDR2="<XML_DIZ_INFO>"
HDR3="<MASTER_PAD_VERSION_INFO>"
HDR4="\t<MASTER_PAD_VERSION>1.15</MASTER_PAD_VERSION>"
HDR5="\t<MASTER_PAD_INFO>Portable Application Description, or PAD
for short, is a data set that is used by shareware authors to
disseminate information to anyone interested in their software products.
To find out more go to http://www.asp-shareware.org/pad</MASTER_PAD_INFO>"
HDR6="</MASTER_PAD_VERSION_INFO>"
============================
fill_in ()
{
if [ -z "$2" ]
then
echo -n "$1? " # Get user input.
else
echo -n "$1 $2? " # Additional query?
fi
read var # May paste to fill in field.
# This shows how flexible "read" can be.
if [ -z "$var" ]
then
echo -e "\t\t<$1 />" >>$savefile # Indent with 2 tabs.
return
else
echo -e "\t\t<$1>$var</$1>" >>$savefile
return ${#var} # Return length of input string.
fi
}
check_field_length () # Check length of program description fields.
{
$1 = maximum field length
$2 = actual field length
if [ "$2" -gt "$1" ]
then
echo "Warning: Maximum field length of $1 characters exceeded!"
fi
}
clear # Clear screen.
echo "PAD File Creator"
echo "--- ---- -------"
echo
Write File Headers to file.
echo $HDR1 >$savefile
echo $HDR2 >>$savefile
echo $HDR3 >>$savefile
echo -e $HDR4 >>$savefile
echo -e $HDR5 >>$savefile
echo $HDR6 >>$savefile
Company_Info
echo "COMPANY INFO"
CO_HDR="Company_Info"
echo "<$CO_HDR>" >>$savefile
fill_in Company_Name
fill_in Address_1
fill_in Address_2
fill_in City_Town
fill_in State_Province
fill_in Zip_Postal_Code
fill_in Country
If applicable:
fill_in ASP_Member "[Y/N]"
fill_in ASP_Member_Number
fill_in ESC_Member "[Y/N]"
fill_in Company_WebSite_URL
clear # Clear screen between sections.
Contact_Info
echo "CONTACT INFO"
CONTACT_HDR="Contact_Info"
echo "<$CONTACT_HDR>" >>$savefile
fill_in Author_First_Name
fill_in Author_Last_Name
fill_in Author_Email
fill_in Contact_First_Name
fill_in Contact_Last_Name
fill_in Contact_Email
echo -e "\t</$CONTACT_HDR>" >>$savefile
END Contact_Info
clear
Support_Info
echo "SUPPORT INFO"
SUPPORT_HDR="Support_Info"
echo "<$SUPPORT_HDR>" >>$savefile
fill_in Sales_Email
fill_in Support_Email
fill_in General_Email
fill_in Sales_Phone
fill_in Support_Phone
fill_in General_Phone
fill_in Fax_Phone
echo -e "\t</$SUPPORT_HDR>" >>$savefile
END Support_Info
echo "</$CO_HDR>" >>$savefile
END Company_Info
clear
Program_Info
echo "PROGRAM INFO"
PROGRAM_HDR="Program_Info"
echo "<$PROGRAM_HDR>" >>$savefile
fill_in Program_Name
fill_in Program_Version
fill_in Program_Release_Month
fill_in Program_Release_Day
fill_in Program_Release_Year
fill_in Program_Cost_Dollars
fill_in Program_Cost_Other
fill_in Program_Type "[Shareware/Freeware/GPL]"
fill_in Program_Release_Status "[Beta, Major Upgrade, etc.]"
fill_in Program_Install_Support
fill_in Program_OS_Support "[Win9x/Win2k/Linux/etc.]"
fill_in Program_Language "[English/Spanish/etc.]"
echo; echo
File_Info
echo "FILE INFO"
FILEINFO_HDR="File_Info"
echo "<$FILEINFO_HDR>" >>$savefile
fill_in Filename_Versioned
fill_in Filename_Previous
fill_in Filename_Generic
fill_in Filename_Long
fill_in File_Size_Bytes
fill_in File_Size_K
fill_in File_Size_MB
echo -e "\t</$FILEINFO_HDR>" >>$savefile
END File_Info
clear
Expire_Info
echo "EXPIRE INFO"
EXPIRE_HDR="Expire_Info"
echo "<$EXPIRE_HDR>" >>$savefile
fill_in Has_Expire_Info "Y/N"
fill_in Expire_Count
fill_in Expire_Based_On
fill_in Expire_Other_Info
fill_in Expire_Month
fill_in Expire_Day
fill_in Expire_Year
echo -e "\t</$EXPIRE_HDR>" >>$savefile
END Expire_Info
clear
More Program_Info
echo "ADDITIONAL PROGRAM INFO"
fill_in Program_Change_Info
fill_in Program_Specific_Category
fill_in Program_Categories
fill_in Includes_JAVA_VM "[Y/N]"
fill_in Includes_VB_Runtime "[Y/N]"
fill_in Includes_DirectX "[Y/N]"
END More Program_Info
echo "</$PROGRAM_HDR>" >>$savefile
END Program_Info
clear
Program Description
echo "PROGRAM DESCRIPTIONS"
PROGDESC_HDR="Program_Descriptions"
echo "<$PROGDESC_HDR>" >>$savefile
LANG="English"
echo "<$LANG>" >>$savefile
fill_in Keywords "[comma + space separated]"
echo
echo "45, 80, 250, 450, 2000 word program descriptions"
echo "(may cut and paste into field)"
It would be highly appropriate to compose the following
- "Char_Desc" fields with a text editor,
- then cut-and-paste the text into the answer fields.
echo
echo " |---------------45 characters---------------|"
fill_in Char_Desc_45
check_field_length 45 "$?"
echo
fill_in Char_Desc_80
check_field_length 80 "$?"
fill_in Char_Desc_250
check_field_length 250 "$?"
fill_in Char_Desc_450
fill_in Char_Desc_2000
echo "</$LANG>" >>$savefile
echo "</$PROGDESC_HDR>" >>$savefile
END Program Description
clear
echo "Done."; echo; echo
echo "Save file is: ""$savefile"""
exit 0
Example A-39. A man page editor
!/bin/bash
maned.sh
A rudimentary man page editor
Version: 0.1 (Alpha, probably buggy)
Reldate: 16 June 2008
License: GPL3
savefile= # Global, used in multiple functions.
E_NOINPUT=90 # User input missing (error). May or may not be critical.
=========== Markup Tags ============ #
TopHeader=".TH"
NameHeader=".SH NAME"
SyntaxHeader=".SH SYNTAX"
SynopsisHeader=".SH SYNOPSIS"
InstallationHeader=".SH INSTALLATION"
DescHeader=".SH DESCRIPTION"
OptHeader=".SH OPTIONS"
FilesHeader=".SH FILES"
EnvHeader=".SH ENVIRONMENT"
AuthHeader=".SH AUTHOR"
BugsHeader=".SH BUGS"
SeeAlsoHeader=".SH SEE ALSO"
BOLD=".B"
Add more tags, as needed.
See groff docs for markup meanings.
==================================== #
start ()
{
clear # Clear screen.
echo "ManEd"
echo "-----"
echo
echo "Simple man page creator"
echo "Author: Mendel Cooper"
echo "License: GPL3"
echo; echo; echo
}
progname ()
{
echo -n "Program name? "
read name
echo -n "Manpage section? [Hit RETURN for default ("1") ] "
read section
if [ -z "$section" ]
then
section=1 # Most man pages are in section 1.
fi
if [ -n "$name" ]
then
savefile=""$name"."$section"" # Filename suffix = section.
echo -n "$1 " >>$savefile
name1=$(echo "$name" | tr a-z A-Z) # Change to uppercase,
#+ per man page convention.
echo -n "$name1" >>$savefile
else
echo "Error! No input." # Mandatory input.
exit $E_NOINPUT # Critical!
# Exercise: The script-abort if no filename input is a bit clumsy.
# Rewrite this section so a default filename is used
#+ if no input.
fi
echo -n " "$section"">>$savefile # Append, always append.
echo -n "Version? "
read ver
echo -n " "Version $ver "">>$savefile
echo >>$savefile
echo -n "Short description [0 - 5 words]? "
read sdesc
echo "$NameHeader">>$savefile
echo ""$BOLD" "$name"">>$savefile
echo "- "$sdesc"">>$savefile
}
fill_in ()
{ # This function more or less copied from "pad.sh" script.
echo -n "$2? " # Get user input.
read var # May paste (a single line only!) to fill in field.
if [ -n "$var" ]
then
echo "$1 " >>$savefile
echo -n "$var" >>$savefile
else # Don't append empty field to file.
return $E_NOINPUT # Not critical here.
fi
echo >>$savefile
}
end ()
{
clear
echo -n "Would you like to view the saved man page (y/n)? "
read ans
if [ "$ans" = "n" -o "$ans" = "N" ]; then exit; fi
exec less "$savefile" # Exit script and hand off control to "less" ...
#+ ... which formats for viewing man page source.
}
---------------------------------------- #
start
progname "$TopHeader"
fill_in "$SynopsisHeader" "Synopsis"
fill_in "$DescHeader" "Long description"
May paste in single line of text.
fill_in "$OptHeader" "Options"
fill_in "$FilesHeader" "Files"
fill_in "$AuthHeader" "Author"
fill_in "$BugsHeader" "Bugs"
fill_in "$SeeAlsoHeader" "See also"
fill_in "$OtherHeader" ... as necessary.
end # ... exit not needed.
---------------------------------------- #
Note that the generated man page will usually
- require manual fine-tuning with a text editor.
However, it's a distinct improvement upon
- writing man source from scratch
- or even editing a blank man page template.
The main deficiency of the script is that it permits
- pasting only a single text line into the input fields.
This may be a long, cobbled-together line, which groff
will automatically wrap and hyphenate.
However, if you want multiple (newline-separated) paragraphs,
- these must be inserted by manual text editing on the
- script-generated man page.
Exercise (difficult): Fix this!
This script is not nearly as elaborate as the
- full-featured "manedit" package
- http://freshmeat.net/projects/manedit/
- but it's much easier to use.
Example A-40. Petals Around the Rose
!/bin/bash -i
petals.sh
Petals Around the Rose #
Version 0.1 Created by Serghey Rodin #
Version 0.2 Modded by ABS Guide Author #
License: GPL3 #
Used in ABS Guide with permission. #
##################################################################### #
hits=0 # Correct guesses.
WIN=6 # Mastered the game.
ALMOST=5 # One short of mastery.
EXIT=exit # Give up early?
RANDOM=$$ # Seeds the random number generator from PID of script.
Bones (ASCII graphics for dice)
bone1[1]="| |"
bone1[2]="| o |"
bone1[3]="| o |"
bone1[4]="| o o |"
bone1[5]="| o o |"
bone1[6]="| o o |"
bone2[1]="| o |"
bone2[2]="| |"
bone2[3]="| o |"
bone2[4]="| |"
bone2[5]="| o |"
bone2[6]="| o o |"
bone3[1]="| |"
bone3[2]="| o |"
bone3[3]="| o |"
bone3[4]="| o o |"
bone3[5]="| o o |"
bone3[6]="| o o |"
bone="+---------+"
Functions
instructions () {
clear
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[34;47m' # Blue type.
"cat document"
cat <<INSTRUCTIONSZZZ
The name of the game is Petals Around the Rose,
and that name is significant.
Five dice will roll and you must guess the "answer" for each roll.
It will be zero or an even number.
After your guess, you will be told the answer for the roll, but . . .
that's ALL the information you will get.
Six consecutive correct guesses admits you to the
Fellowship of the Rose.
INSTRUCTIONSZZZ
echo -e "\033[0m" # Turn off blue.
else clear
fi
}
fortune ()
{
RANGE=7
FLOOR=0
number=0
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE" # 1 - 6.
done
return $number
}
throw () { # Calculate each individual die.
fortune; B1=$?
fortune; B2=$?
fortune; B3=$?
fortune; B4=$?
fortune; B5=$?
calc () { # Function embedded within a function!
case "$1" in
3 ) rose=2;;
5 ) rose=4;;
* ) rose=0;;
esac # Simplified algorithm.
# Doesn't really get to the heart of the matter.
return $rose
}
answer=0
calc "$B1"; answer=$(expr $answer + $(echo $?))
calc "$B2"; answer=$(expr $answer + $(echo $?))
calc "$B3"; answer=$(expr $answer + $(echo $?))
calc "$B4"; answer=$(expr $answer + $(echo $?))
calc "$B5"; answer=$(expr $answer + $(echo $?))
}
game ()
{ # Generate graphic display of dice throw.
throw
echo -e "\033[1m" # Bold.
echo -e "\n"
echo -e "$bone\t$bone\t$bone\t$bone\t$bone"
echo -e \
"${bone1[$B1]}\t${bone1[$B2]}\t${bone1[$B3]}\t${bone1[$B4]}\t${bone1[$B5]}"
echo -e \
"${bone2[$B1]}\t${bone2[$B2]}\t${bone2[$B3]}\t${bone2[$B4]}\t${bone2[$B5]}"
echo -e \
"${bone3[$B1]}\t${bone3[$B2]}\t${bone3[$B3]}\t${bone3[$B4]}\t${bone3[$B5]}"
echo -e "$bone\t$bone\t$bone\t$bone\t$bone"
echo -e "\n\n\t\t"
echo -e "\033[0m" # Turn off bold.
echo -n "There are how many petals around the rose? "
}
============================================================== #
instructions
while [ "$petal" != "$EXIT" ] # Main loop.
do
game
read petal
echo "$petal" | grep [0-9] >/dev/null # Filter response for digit.
# Otherwise just roll dice again.
if [ "$?" -eq 0 ] # If-loop #1.
then
if [ "$petal" == "$answer" ]; then # If-loop #2.
echo -e "\nCorrect. There are $petal petals around the rose.\n"
(( hits++ ))
if [ "$hits" -eq "$WIN" ]; then # If-loop #3.
echo -e '\E[31;47m' # Red type.
echo -e "\033[1m" # Bold.
echo "You have unraveled the mystery of the Rose Petals!"
echo "Welcome to the Fellowship of the Rose!!!"
echo "(You are herewith sworn to secrecy.)"; echo
echo -e "\033[0m" # Turn off red & bold.
break # Exit!
else echo "You have $hits correct so far."; echo
if [ "$hits" -eq "$ALMOST" ]; then
echo "Just one more gets you to the heart of the mystery!"; echo
fi
fi # Close if-loop #3.
else
echo -e "\nWrong. There are $answer petals around the rose.\n"
hits=0 # Reset number of correct guesses.
fi # Close if-loop #2.
echo -n "Hit ENTER for the next roll, or type \"exit\" to end. "
read
if [ "$REPLY" = "$EXIT" ]; then exit
fi
fi # Close if-loop #1.
clear
done # End of main (while) loop.
exit $?
Resources:
- http://en.wikipedia.org/wiki/Petals_Around_the_Rose
(Wikipedia entry.)
- http://www.borrett.id.au/computing/petals-bg.htm
(How Bill Gates coped with the Petals Around the Rose challenge.)
Example A-41. Quacky: a Perquackey-type word game
!/bin/bash
qky.sh
QUACKEY: a somewhat simplified version of Perquackey [TM]. #
version 0.1.02 03 May, 2008 #
License: GPL3 #
WLIST=/usr/share/dict/word.lst
^^^^^^^^ Word list file found here.
ASCII word list, one word per line, UNIX format.
A suggested list is the script author's "yawl" word list package.
http://bash.deta.in/yawl-0.3.2.tar.gz
or
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
NONCONS=0 # Word not constructable from letter set.
CONS=1 # Constructable.
SUCCESS=0
NG=1
FAILURE=''
NULL=0 # Zero out value of letter (if found).
MINWLEN=3 # Minimum word length.
MAXCAT=5 # Maximum number of words in a given category.
PENALTY=200 # General-purpose penalty for unacceptable words.
total=
E_DUP=70 # Duplicate word error.
TIMEOUT=10 # Time for word input.
NVLET=10 # 10 letters for non-vulnerable.
VULET=13 # 13 letters for vulnerable (not yet implemented!).
declare -a Words
declare -a Status
declare -a Score=( 0 0 0 0 0 0 0 0 0 0 0 )
letters=( a n s r t m l k p r b c i d s i d z e w u e t f
e y e r e f e g t g h h i t r s c i t i d i j a t a o l a
m n a n o v n w o s e l n o s p a q e e r a b r s a o d s
t g t i t l u e u v n e o x y m r k )
Letter distribution table shamelessly borrowed from "Wordy" game,
- ca. 1992, written by a certain fine fellow named Mendel Cooper.
declare -a LS
numelements=${#letters[@]}
randseed="$1"
instructions ()
{
clear
echo "Welcome to QUACKEY, the anagramming word construction game."; echo
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[31;47m' # Red foreground. '\E[34;47m' for blue.
cat <<INSTRUCTION1
QUACKEY is a variant of Perquackey [TM].
The rules are the same, but the scoring is simplified
and plurals of previously played words are allowed.
"Vulnerable" play is not yet implemented,
but it is otherwise feature-complete.
As the game begins, the player gets 10 letters.
The object is to construct valid dictionary words
of at least 3-letter length from the letterset.
Each word-length category
-- 3-letter, 4-letter, 5-letter, ... --
fills up with the fifth word entered,
and no further words in that category are accepted.
The penalty for too-short (two-letter), duplicate, unconstructable,
and invalid (not in dictionary) words is -200. The same penalty applies
to attempts to enter a word in a filled-up category.
INSTRUCTION1
echo -n "Hit ENTER for next page of instructions. "; read az1
cat <<INSTRUCTION2
The scoring mostly corresponds to classic Perquackey:
The first 3-letter word scores 60, plus 10 for each additional one.
The first 4-letter word scores 120, plus 20 for each additional one.
The first 5-letter word scores 200, plus 50 for each additional one.
The first 6-letter word scores 300, plus 100 for each additional one.
The first 7-letter word scores 500, plus 150 for each additional one.
The first 8-letter word scores 750, plus 250 for each additional one.
The first 9-letter word scores 1000, plus 500 for each additional one.
The first 10-letter word scores 2000, plus 2000 for each additional one.
Category completion bonuses are:
3-letter words 100
4-letter words 200
5-letter words 400
6-letter words 800
7-letter words 2000
8-letter words 10000
This is a simplification of the absurdly baroque Perquackey bonus
scoring system.
INSTRUCTION2
echo -n "Hit ENTER for final page of instructions. "; read az1
cat <<INSTRUCTION3
Hitting just ENTER for a word entry ends the game.
Individual word entry is timed to a maximum of 10 seconds.
- ** Timing out on an entry ends the game. ***
Aside from that, the game is untimed.
Game statistics are automatically saved to a file.
For competitive ("duplicate") play, a previous letterset
may be duplicated by repeating the script's random seed,
command-line parameter $1.
For example, "qky 7633" specifies the letterset
c a d i f r h u s k ...
INSTRUCTION3
echo; echo -n "Hit ENTER to begin game. "; read az1
echo -e "\033[0m" # Turn off red.
else clear
fi
clear
}
seed_random ()
{ # Seed random number generator.
if [ -n "$randseed" ] # Can specify random seed.
then #+ for play in competitive mode.
RANDOM="$randseed"
echo "RANDOM seed set to "$randseed""
else
randseed="$$" # Or get random seed from process ID.
echo "RANDOM seed not specified, set to Process ID of script ($$)."
fi
RANDOM="$randseed"
echo
}
get_letset ()
{
element=0
echo -n "Letterset:"
for lset in $(seq $NVLET)
do # Pick random letters to fill out letterset.
LS[element]="${letters[$((RANDOM%numelements))]}"
((element++))
done
echo
echo "${LS[@]}"
}
add_word ()
{
wrd="$1"
local idx=0
Status[0]=""
Status[3]=""
Status[4]=""
while [ "${Words[idx]}" != '' ]
do
if [ "${Words[idx]}" = "$wrd" ]
then
Status[3]="Duplicate-word-PENALTY"
let "Score[0]= 0 - $PENALTY"
let "Score[1]-=$PENALTY"
return $E_DUP
fi
((idx++))
done
Words[idx]="$wrd"
get_score
}
get_score()
{
local wlen=0
local score=0
local bonus=0
local first_word=0
local add_word=0
local numwords=0
wlen=${#wrd}
numwords=${Score[wlen]}
Score[2]=0
Status[4]="" # Initialize "bonus" to 0.
case "$wlen" in
3) first_word=60
add_word=10;;
4) first_word=120
add_word=20;;
5) first_word=200
add_word=50;;
6) first_word=300
add_word=100;;
7) first_word=500
add_word=150;;
8) first_word=750
add_word=250;;
9) first_word=1000
add_word=500;;
- first_word=2000
add_word=2000;; # This category modified from original rules!
esac
((Score[wlen]++))
if [ ${Score[wlen]} -eq $MAXCAT ]
then # Category completion bonus scoring simplified!
case $wlen in
3 ) bonus=100;;
4 ) bonus=200;;
5 ) bonus=400;;
6 ) bonus=800;;
7 ) bonus=2000;;
8 ) bonus=10000;;
esac # Needn't worry about 9's and 10's.
Status[4]="Category-$wlen-completion***BONUS***"
Score[2]=$bonus
else
Status[4]="" # Erase it.
fi
let "score = $first_word + $add_word * $numwords"
if [ "$numwords" -eq 0 ]
then
Score[0]=$score
else
Score[0]=$add_word
fi # All this to distinguish last-word score
#+ from total running score.
let "Score[1] += ${Score[0]}"
let "Score[1] += ${Score[2]}"
}
get_word ()
{
local wrd=''
read -t $TIMEOUT wrd # Timed read.
echo $wrd
}
is_constructable ()
{ # This is the most complex and difficult-to-write function.
local -a local_LS=( "${LS[@]}" ) # Local copy of letter set.
local is_found=0
local idx=0
local pos
local strlen
local local_word=( "$1" )
strlen=${#local_word}
while [ "$idx" -lt "$strlen" ]
do
is_found=$(expr index "${local_LS[*]}" "${local_word:idx:1}")
if [ "$is_found" -eq "$NONCONS" ] # Not constructable!
then
echo "$FAILURE"; return
else
((pos = ($is_found - 1) / 2)) # Compensate for spaces betw. letters!
local_LS[pos]=$NULL # Zero out used letters.
((idx++)) # Bump index.
fi
done
echo "$SUCCESS"
return
}
is_valid ()
{ # Surprisingly easy to check if word in dictionary ...
fgrep -qw "$1" "$WLIST" # ... courtesy of 'grep' ...
echo $?
}
check_word ()
{
if [ -z "$1" ]
then
return
fi
Status[1]=""
Status[2]=""
Status[3]=""
Status[4]=""
iscons=$(is_constructable "$1")
if [ "$iscons" ]
then
Status[1]="constructable"
v=$(is_valid "$1")
if [ "$v" -eq "$SUCCESS" ]
then
Status[2]="valid"
strlen=${#1}
if [ ${Score[strlen]} -eq "$MAXCAT" ] # Category full!
then
Status[3]="Category-$strlen-overflow-PENALTY"
return $NG
fi
case "$strlen" in
1 | 2 )
Status[3]="Two-letter-word-PENALTY"
return $NG;;
* )
Status[3]=""
return $SUCCESS;;
esac
else
Status[3]="Not-valid-PENALTY"
return $NG
fi
else
Status[3]="Not-constructable-PENALTY"
return $NG
fi
FIXME: Streamline the above code block.
}
display_words ()
{
local idx=0
local wlen0
clear
echo "Letterset: ${LS[@]}"
echo "Threes: Fours: Fives: Sixes: Sevens: Eights:"
echo "------------------------------------------------------------"
while [ "${Words[idx]}" != '' ]
do
wlen0=${#Words[idx]}
case "$wlen0" in
3) ;;
4) echo -n " " ;;
5) echo -n " " ;;
6) echo -n " " ;;
7) echo -n " " ;;
8) echo -n " " ;;
esac
echo "${Words[idx]}"
((idx++))
done
FIXME: The word display is pretty crude.
}
play ()
{
word="Start game" # Dummy word, to start ...
while [ "$word" ] # If player just hits return (null word),
do #+ then game ends.
echo "$word: "${Status[@]}""
echo -n "Last score: [${Score[0]}] TOTAL score: [${Score[1]}]: Next
word: "
total=${Score[1]}
word=$(get_word)
check_word "$word"
if [ "$?" -eq "$SUCCESS" ]
then
add_word "$word"
else
let "Score[0]= 0 - $PENALTY"
let "Score[1]-=$PENALTY"
fi
display_words
done # Exit game.
FIXME: The play () function calls too many other functions.
This verges on "spaghetti code" !!!
}
end_of_game ()
{ # Save and display stats.
#######################Autosave##########################
savefile=qky.save.$$
^^ PID of script
echo date >> $savefile
echo "Letterset # $randseed (random seed) ">> $savefile
echo -n "Letterset: " >> $savefile
echo "${LS[@]}" >> $savefile
echo "---------" >> $savefile
echo "Words constructed:" >> $savefile
echo "${Words[@]}" >> $savefile
echo >> $savefile
echo "Score: $total" >> $savefile
echo "Statistics for this round saved in ""$savefile"""
#########################################################
echo "Score for this round: $total"
echo "Words: ${Words[@]}"
}
---------#
instructions
seed_random
get_letset
play
end_of_game
---------#
exit $?
TODO:
- Clean up code!
- Prettify the display_words () function (maybe with widgets?).
- Improve the time-out ... maybe change to untimed entry,
- but with a time limit for the overall round.
- An on-screen countdown timer would be nice.
- Implement "vulnerable" mode of play for compatibility with classic
- Improve save-to-file capability (and maybe make it optional).
- Fix bugs!!!
For more info, reference:
http://bash.deta.in/qky.README.html
Example A-42. Nim
!/bin/bash
nim.sh: Game of Nim
Author: Mendel Cooper
Reldate: 15 July 2008
License: GPL3
ROWS=5 # Five rows of pegs (or matchsticks).
WON=91 # Exit codes to keep track of wins/losses.
LOST=92 # Possibly useful if running in batch mode.
QUIT=99
peg_msg= # Peg/Pegs?
Rows=( 0 5 4 3 2 1 ) # Array holding play info.
${Rows[0]} holds total number of pegs, updated after each turn.
Other array elements hold number of pegs in corresponding row.
instructions ()
{
clear
tput bold
echo "Welcome to the game of Nim."; echo
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[33;41m' # Yellow fg., over red bg.; bold.
cat <<INSTRUCTIONS
Nim is a game with roots in the distant past.
This particular variant starts with five rows of pegs.
1: | | | | |
2: | | | |
3: | | |
4: | |
5: |
The number at the left identifies the row.
The human player moves first, and alternates turns with the bot.
A turn consists of removing at least one peg from a single row.
It is permissable to remove ALL the pegs from a row.
For example, in row 2, above, the player can remove 1, 2, 3, or 4 pegs.
The player who removes the last peg loses.
The strategy consists of trying to be the one who removes
the next-to-last peg(s), leaving the loser with the final peg.
To exit the game early, hit ENTER during your turn.
INSTRUCTIONS
echo; echo -n "Hit ENTER to begin game. "; read azx
echo -e "\033[0m" # Restore display.
else tput sgr0; clear
fi
clear
}
tally_up ()
{
let "Rows[0] = ${Rows[1]} + ${Rows[2]} + ${Rows[3]} + ${Rows[4]} + \
${Rows[5]}" # Add up how many pegs remaining.
}
display ()
{
index=1 # Start with top row.
echo
while [ "$index" -le "$ROWS" ]
do
p=${Rows[index]}
echo -n "$index: " # Show row number.
------------------------------------------------
Two concurrent inner loops.
indent=$index
while [ "$indent" -gt 0 ]
do
echo -n " " # Staggered rows.
((indent--)) # Spacing between pegs.
done
while [ "$p" -gt 0 ]
do
echo -n "| "
((p--))
done
-----------------------------------------------
echo
((index++))
done
tally_up
rp=${Rows[0]}
if [ "$rp" -eq 1 ]
then
peg_msg=peg
final_msg="Game over."
else # Game not yet over . . .
peg_msg=pegs
final_msg="" # . . . So "final message" is blank.
fi
echo " $rp $peg_msg remaining."
echo " "$final_msg""
echo
}
player_move ()
{
echo "Your move:"
echo -n "Which row? "
while read idx
do # Validity check, etc.
if [ -z "$idx" ] # Hitting return quits.
then
echo "Premature exit."; echo
tput sgr0 # Restore display.
exit $QUIT
fi
if [ "$idx" -gt "$ROWS" -o "$idx" -lt 1 ] # Bounds check.
then
echo "Invalid row number!"
echo -n "Which row? "
else
break
fi
# TODO:
# Add check for non-numeric input.
# Also, script crashes on input outside of range of long double.
# Fix this.
done
echo -n "Remove how many? "
while read num
do # Validity check.
if [ -z "$num" ]
then
echo "Premature exit."; echo
tput sgr0 # Restore display.
exit $QUIT
fi
if [ "$num" -gt ${Rows[idx]} -o "$num" -lt 1 ]
then
echo "Cannot remove $num!"
echo -n "Remove how many? "
else
break
fi
done
TODO:
Add check for non-numeric input.
Also, script crashes on input outside of range of long double.
Fix this.
let "Rows[idx] -= $num"
display
tally_up
if [ ${Rows[0]} -eq 1 ]
then
echo " Human wins!"
echo " Congratulations!"
tput sgr0 # Restore display.
echo
exit $WON
fi
if [ ${Rows[0]} -eq 0 ]
then # Snatching defeat from the jaws of victory . . .
echo " Fool!"
echo " You just removed the last peg!"
echo " Bot wins!"
tput sgr0 # Restore display.
echo
exit $LOST
fi
}
bot_move ()
{
row_b=0
while [[ $row_b -eq 0 || ${Rows[row_b]} -eq 0 ]]
do
row_b=$RANDOM # Choose random row.
let "row_b %= $ROWS"
done
num_b=0
r0=${Rows[row_b]}
if [ "$r0" -eq 1 ]
then
num_b=1
else
let "num_b = $r0 - 1"
# Leave only a single peg in the row.
fi # Not a very strong strategy,
#+ but probably a bit better than totally random.
let "Rows[row_b] -= $num_b"
echo -n "Bot: "
echo "Removing from row $row_b ... "
if [ "$num_b" -eq 1 ]
then
peg_msg=peg
else
peg_msg=pegs
fi
echo " $num_b $peg_msg."
display
tally_up
if [ ${Rows[0]} -eq 1 ]
then
echo " Bot wins!"
tput sgr0 # Restore display.
exit $WON
fi
}
================================================== #
instructions # If human player needs them . . .
tput bold # Bold characters for easier viewing.
display # Show game board.
while [ true ] # Main loop.
do # Alternate human and bot turns.
player_move
bot_move
done
================================================== #
Exercise:
Improve the bot's strategy.
There is, in fact, a Nim strategy that can force a win.
See the Wikipedia article on Nim: http://en.wikipedia.org/wiki/Nim
Recode the bot to use this strategy (rather difficult).
Curiosities:
Nim played a prominent role in Alain Resnais' 1961 New Wave film,
In 1978, Leo Christopherson wrote an animated version of Nim,
- Android Nim, for the TRS-80 Model I.
Example A-43. A command-line stopwatch
!/bin/sh
sw.sh
A command-line Stopwatch
Author: Pádraig Brady
http://www.pixelbeat.org/scripts/sw
(Minor reformatting by ABS Guide author.)
Used in ABS Guide with script author's permission.
Notes:
This script starts a few processes per lap, in addition to
the shell loop processing, so the assumption is made that
this takes an insignificant amount of time compared to
the response time of humans (~.1s) (or the keyboard
interrupt rate (~.05s)).
'?' for splits must be entered twice if characters
(erroneously) entered before it (on the same line).
'?' since not generating a signal may be slightly delayed
on heavily loaded systems.
Lap timings on ubuntu may be slightly delayed due to:
https://bugs.launchpad.net/bugs/62511
Changes:
V1.0, 23 Aug 2005, Initial release
V1.1, 26 Jul 2007, Allow both splits and laps from single invocation.
Only start timer after a key is pressed.
Indicate lap number
Cache programs at startup so there is less error
due to startup delays.
V1.2, 01 Aug 2007, Work around date commands that don't have
nanoseconds.
Use stty to change interrupt keys to space for
laps etc.
Ignore other input as it causes problems.
V1.3, 01 Aug 2007, Testing release.
V1.4, 02 Aug 2007, Various tweaks to get working under ubuntu
and Mac OS X.
V1.5, 27 Jun 2008, set LANG=C as got vague bug report about it.
export LANG=C
ulimit -c 0 # No coredumps from SIGQUIT.
trap '' TSTP # Ignore Ctrl-Z just in case.
save_tty=stty -g && trap "stty $save_tty" EXIT # Restore tty on exit.
stty quit ' ' # Space for laps rather than Ctrl-.
stty eof '?' # ? for splits rather than Ctrl-D.
stty -echo # Don't echo input.
cache_progs() {
stty > /dev/null
date > /dev/null
grep . < /dev/null
(echo "import time" | python) 2> /dev/null
bc < /dev/null
sed '' < /dev/null
printf '1' > /dev/null
/usr/bin/time false 2> /dev/null
cat < /dev/null
}
cache_progs # To minimise startup delay.
date +%s.%N | grep -qF 'N' && use_python=1 # If date lacks nanoseconds.
now() {
if [ "$use_python" ]; then
echo "import time; print time.time()" 2>/dev/null | python
else
printf "%.2f" `date +%s.%N`
fi
}
fmt_seconds() {
seconds=$1
mins=`echo $seconds/60 | bc`
if [ "$mins" != "0" ]; then
seconds=`echo "$seconds - ($mins*60)" | bc`
echo "$mins:$seconds"
else
echo "$seconds"
fi
}
total() {
end=`now`
total=`echo "$end - $start" | bc`
fmt_seconds $total
}
stop() {
[ "$lapped" ] && lap "$laptime" "display"
total
exit
}
lap() {
laptime=`echo "$1" | sed -n 's/.*real[^0-9.]*\(.*\)/\1/p'`
[ ! "$laptime" -o "$laptime" = "0.00" ] && return
# Signals too frequent.
laptotal=`echo $laptime+0$laptotal | bc`
if [ "$2" = "display" ]; then
lapcount=`echo 0$lapcount+1 | bc`
laptime=`fmt_seconds $laptotal`
echo $laptime "($lapcount)"
lapped="true"
laptotal="0"
fi
}
echo -n "Space for lap | ? for split | Ctrl-C to stop | Space to start...">&2
while true; do
trap true INT QUIT # Set signal handlers.
laptime=`/usr/bin/time -p 2>&1 cat >/dev/null`
ret=$?
trap '' INT QUIT # Ignore signals within this script.
if [ $ret -eq 1 -o $ret -eq 2 -o $ret -eq 130 ]; then # SIGINT = stop
[ ! "$start" ] && { echo >&2; exit; }
stop
elif [ $ret -eq 3 -o $ret -eq 131 ]; then # SIGQUIT = lap
if [ ! "$start" ]; then
start=`now` || exit 1
echo >&2
continue
fi
lap "$laptime" "display"
else # eof = split
[ ! "$start" ] && continue
total
lap "$laptime" # Update laptotal.
fi
done
exit $?
Example A-44. An all-purpose shell scripting homework assignment
solution
!/bin/bash
homework.sh: All-purpose homework assignment solution.
Author: M. Leo Cooper
If you substitute your own name as author, then it is plagiarism,
- possibly a lesser sin than cheating on your homework!
License: Public Domain
This script may be turned in to your instructor
- in fulfillment of ALL shell scripting homework assignments.
It's sparsely commented, but you, the student, can easily remedy that.
The script author repudiates all responsibility!
DLA=1
P1=2
P2=4
P3=7
PP1=0
PP2=8
MAXL=9
E_LZY=99
declare -a L
L[0]="3 4 0 17 29 8 13 18 19 17 20 2 19 14 17 28"
L[1]="8 29 12 14 18 19 29 4 12 15 7 0 19 8 2 0 11 11 24 29 17 4 6 17 4 19"
L[2]="29 19 7 0 19 29 8 29 7 0 21 4 29 13 4 6 11 4 2 19 4 3"
L[3]="19 14 29 2 14 12 15 11 4 19 4 29 19 7 8 18 29"
L[4]="18 2 7 14 14 11 22 14 17 10 29 0 18 18 8 6 13 12 4 13 19 26"
L[5]="15 11 4 0 18 4 29 0 2 2 4 15 19 29 12 24 29 7 20 12 1 11 4 29"
L[6]="4 23 2 20 18 4 29 14 5 29 4 6 17 4 6 8 14 20 18 29"
L[7]="11 0 25 8 13 4 18 18 27"
L[8]="0 13 3 29 6 17 0 3 4 29 12 4 29 0 2 2 14 17 3 8 13 6 11 24 26"
L[9]="19 7 0 13 10 29 24 14 20 26"
declare -a \
alph=( A B C D E F G H I J K L M N O P Q R S T U V W X Y Z . , : ' ' )
pt_lt ()
{
echo -n "${alph[$1]}"
echo -n -e "\a"
sleep $DLA
}
b_r ()
{
echo -e '\E[31;48m\033[1m'
}
cr ()
{
echo -e "\a"
sleep $DLA
}
restore ()
{
echo -e '\033[0m' # Bold off.
tput sgr0 # Normal.
}
p_l ()
{
for ltr in $1
do
pt_lt "$ltr"
done
}
b_r
for i in $(seq 0 $MAXL)
do
p_l "${L[i]}"
if [[ "$i" -eq "$P1" || "$i" -eq "$P2" || "$i" -eq "$P3" ]]
then
cr
elif [[ "$i" -eq "$PP1" || "$i" -eq "$PP2" ]]
then
cr; cr
fi
done
restore
echo
exit $E_LZY
A typical example of an obfuscated script that is difficult
- to understand, and frustrating to maintain.
In your career as a sysadmin, you'll run into these critters
Example A-45. The Knight's Tour
!/bin/bash
ktour.sh
author: mendel cooper
reldate: 12 Jan 2009
license: public domain
(Not much sense GPLing something that's pretty much in the common
The Knight's Tour, a classic problem. #
===================================== #
The knight must move onto every square of the chess board, #
but cannot revisit any square he has already visited. #
And just why is Sir Knight unwelcome for a return visit? #
Could it be that he has a habit of partying into the wee hours #
Possibly he leaves pizza crusts in the bed, empty beer bottles #
- all over the floor, and clogs the plumbing. . . . #
------------------------------------------------------------- #
Usage: ktour.sh [start-square] [stupid] #
Note that start-square can be a square number #
- in the range 0 - 63 ... or #
a square designator in conventional chess notation, #
such as a1, f5, h3, etc. #
If start-square-number not supplied, #
- then starts on a random square somewhere on the board. #
"stupid" as second parameter sets the stupid strategy. #
Examples: #
ktour.sh 23 starts on square #23 (h3) #
ktour.sh g6 stupid starts on square #46, #
using "stupid" (non-Warnsdorff) strategy. #
DEBUG= # Set this to echo debugging info to stdout.
SUCCESS=0
FAIL=99
BADMOVE=-999
FAILURE=1
LINELEN=21 # How many moves to display per line.
---------------------------------------- #
Board array params
ROWS=8 # 8 x 8 board.
COLS=8
let "SQUARES = $ROWS * $COLS"
let "MAX = $SQUARES - 1"
MIN=0
64 squares on board, indexed from 0 to 63.
VISITED=1
UNVISITED=-1
UNVSYM="##"
---------------------------------------- #
Global variables.
startpos= # Starting position (square #, 0 - 63).
currpos= # Current position.
movenum= # Move number.
CRITPOS=37 # Have to patch for f5 starting position!
declare -i board
Use a one-dimensional array to simulate a two-dimensional one.
This can make life difficult and result in ugly kludges; see below.
declare -i moves # Offsets from current knight position.
initialize_board ()
{
local idx
for idx in {0..63}
do
board[$idx]=$UNVISITED
done
}
print_board ()
{
local idx
echo " _____________________________________"
for row in {7..0} # Reverse order of rows ...
do #+ so it prints in chessboard order.
let "rownum = $row + 1" # Start numbering rows at 1.
echo -n "$rownum |" # Mark board edge with border and
for column in {0..7} #+ "algebraic notation."
do
let "idx = $ROWS*$row + $column"
if [ ${board[idx]} -eq $UNVISITED ]
then
echo -n "$UNVSYM " ##
else # Mark square with move number.
printf "%02d " "${board[idx]}"; echo -n " "
fi
done
echo -e -n "\b\b\b|" # \b is a backspace.
echo # -e enables echoing escaped chars.
done
echo " -------------------------------------"
echo " a b c d e f g h"
}
failure()
{ # Whine, then bail out.
echo
print_board
echo
echo " Waah!!! Ran out of squares to move to!"
echo -n " Knight's Tour attempt ended"
echo " on $(to_algebraic $currpos) [square #$currpos]"
echo " after just $movenum moves!"
echo
exit $FAIL
}
xlat_coords () # Translate x/y coordinates to board position
{ #+ (board-array element #).
For user input of starting board position as x/y coords.
This function not used in initial release of ktour.sh.
May be used in an updated version, for compatibility with
#+ standard implementation of the Knight's Tour in C, Python, etc.
if [ -z "$1" -o -z "$2" ]
then
return $FAIL
fi
local xc=$1
local yc=$2
let "board_index = $xc * $ROWS + yc"
if [ $board_index -lt $MIN -o $board_index -gt $MAX ]
then
return $FAIL # Strayed off the board!
else
return $board_index
fi
}
to_algebraic () # Translate board position (board-array element #)
{ #+ to standard algebraic notation used by chess players.
if [ -z "$1" ]
then
return $FAIL
fi
local element_no=$1 # Numerical board position.
local col_arr=( a b c d e f g h )
local row_arr=( 1 2 3 4 5 6 7 8 )
let "row_no = $element_no / $ROWS"
let "col_no = $element_no % $ROWS"
t1=${col_arr[col_no]}; t2=${row_arr[row_no]}
local apos=$t1$t2 # Concatenate.
echo $apos
}
from_algebraic () # Translate standard algebraic chess notation
{ #+ to numerical board position (board-array element #).
# Or recognize numerical input & return it unchanged.
if [ -z "$1" ]
then
return $FAIL
fi # If no command-line arg, then will default to random start pos.
local ix
local ix_count=0
local b_index # Board index [0-63]
local alpos="$1"
arow=${alpos:0:1} # position = 0, length = 1
acol=${alpos:1:1}
if [[ $arow =~ [[:digit:]] ]] # Numerical input?
then # POSIX char class
if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!
then return $FAIL
else if [ $alpos -gt $MAX ] # Off board?
then return $FAIL
else return $alpos # Return digit(s) unchanged . . .
fi #+ if within range.
fi
fi
if [[ $acol -eq $MIN || $acol -gt $ROWS ]]
then # Outside of range 1 - 8?
return $FAIL
fi
for ix in a b c d e f g h
do # Convert column letter to column number.
if [ "$arow" = "$ix" ]
then
break
fi
((ix_count++)) # Find index count.
done
((acol--)) # Decrementing converts to zero-based array.
let "b_index = $ix_count + $acol * $ROWS"
if [ $b_index -gt $MAX ] # Off board?
then
return $FAIL
fi
return $b_index
}
generate_moves () # Calculate all valid knight moves,
{ #+ relative to current position ($1),
#+ and store in ${moves} array.
local kt_hop=1 # One square :: short leg of knight move.
local kt_skip=2 # Two squares :: long leg of knight move.
local valmov=0 # Valid moves.
local row_pos; let "row_pos = $1 % $COLS"
let "move1 = -$kt_skip + $ROWS" # 2 sideways to-the-left, 1 up
if [[ `expr $row_pos - $kt_skip` -lt $MIN ]] # An ugly, ugly kludge!
then # Can't move off board.
move1=$BADMOVE # Not even temporarily.
else
((valmov++))
fi
let "move2 = -$kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-left, 2 up
if [[ `expr $row_pos - $kt_hop` -lt $MIN ]] # Kludge continued ...
then
move2=$BADMOVE
else
((valmov++))
fi
let "move3 = $kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-right, 2 up
if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]
then
move3=$BADMOVE
else
((valmov++))
fi
let "move4 = $kt_skip + $ROWS" # 2 sideways to-the-right, 1 up
if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]
then
move4=$BADMOVE
else
((valmov++))
fi
let "move5 = $kt_skip - $ROWS" # 2 sideways to-the-right, 1 dn
if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]
then
move5=$BADMOVE
else
((valmov++))
fi
let "move6 = $kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-right, 2 dn
if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]
then
move6=$BADMOVE
else
((valmov++))
fi
let "move7 = -$kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-left, 2 dn
if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]
then
move7=$BADMOVE
else
((valmov++))
fi
let "move8 = -$kt_skip - $ROWS" # 2 sideways to-the-left, 1 dn
if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]
then
move8=$BADMOVE
else
((valmov++))
fi # There must be a better way to do this.
local m=( $valmov $move1 $move2 $move3 $move4 $move5 $move6 $move7 $move8 )
${moves[0]} = number of valid moves.
${moves[1]} ... ${moves[8]} = possible moves.
echo "${m[*]}" # Elements of array to stdout for capture in a var.
}
is_on_board () # Is position actually on the board?
{
if [[ "$1" -lt "$MIN" || "$1" -gt "$MAX" ]]
then
return $FAILURE
else
return $SUCCESS
fi
}
do_move () # Move the knight!
{
local valid_moves=0
local aapos
currposl="$1"
lmin=$ROWS
iex=0
squarel=
mpm=
mov=
declare -a p_moves
########################## DECIDE-MOVE #############################
if [ $startpos -ne $CRITPOS ]
then # CRITPOS = square #37
decide_move
else # Needs a special patch for startpos=37 !!!
decide_move_patched # Why this particular move and no other ???
fi
####################################################################
(( ++movenum )) # Increment move count.
let "square = $currposl + ${moves[iex]}"
################## DEBUG ###############
if [ "$DEBUG" ]
then debug # Echo debugging information.
fi
##############################################
if [[ "$square" -gt $MAX || "$square" -lt $MIN ||
${board[square]} -ne $UNVISITED ]]
then
(( --movenum )) # Decrement move count,
echo "RAN OUT OF SQUARES!!!" #+ since previous one was invalid.
return $FAIL
fi
board[square]=$movenum
currpos=$square # Update current position.
((valid_moves++)); # moves[0]=$valid_moves
aapos=$(to_algebraic $square)
echo -n "$aapos "
test $(( $Moves % $LINELEN )) -eq 0 && echo
Print LINELEN=21 moves per line. A valid tour shows 3 complete lines.
return $valid_moves # Found a square to move to!
}
do_move_stupid() # Dingbat algorithm,
{ #+ courtesy of script author, not Warnsdorff.
local valid_moves=0
local movloc
local squareloc
local aapos
local cposloc="$1"
for movloc in {1..8}
do # Move to first-found unvisited square.
let "squareloc = $cposloc + ${moves[movloc]}"
is_on_board $squareloc
if [ $? -eq $SUCCESS ] && [ ${board[squareloc]} -eq $UNVISITED ]
then # Add conditions to above if-test to improve algorithm.
(( ++movenum ))
board[squareloc]=$movenum
currpos=$squareloc # Update current position.
((valid_moves++)); # moves[0]=$valid_moves
aapos=$(to_algebraic $squareloc)
echo -n "$aapos "
test $(( $Moves % $LINELEN )) -eq 0 && echo # Print 21 moves/line.
return $valid_moves # Found a square to move to!
fi
done
return $FAIL
If no square found in all 8 loop iterations,
#+ then Knight's Tour attempt ends in failure.
Dingbat algorithm will typically fail after about 30 - 40 moves,
#+ but executes much faster than Warnsdorff's in do_move() function.
}
decide_move () # Which move will we make?
{ # But, fails on startpos=37 !!!
for mov in {1..8}
do
let "squarel = $currposl + ${moves[mov]}"
is_on_board $squarel
if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]
then # Find accessible square with least possible future moves.
# This is Warnsdorff's algorithm.
# What happens is that the knight wanders toward the outer edge
#+ of the board, then pretty much spirals inward.
# Given two or more possible moves with same value of
#+ least-possible-future-moves, this implementation chooses
#+ the _first_ of those moves.
# This means that there is not necessarily a unique solution
#+ for any given starting position.
possible_moves $squarel
mpm=$?
p_moves[mov]=$mpm
if [ $mpm -lt $lmin ] # If less than previous minimum ...
then # ^^
lmin=$mpm # Update minimum.
iex=$mov # Save index.
fi
fi
done
}
decide_move_patched () # Decide which move to make,
{ # ^^^^^^^ #+ but only if startpos=37 !!!
for mov in {1..8}
do
let "squarel = $currposl + ${moves[mov]}"
is_on_board $squarel
if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]
then
possible_moves $squarel
mpm=$?
p_moves[mov]=$mpm
if [ $mpm -le $lmin ] # If less-than-or equal to prev. minimum!
then # ^^
lmin=$mpm
iex=$mov
fi
fi
done # There has to be a better way to do this.
}
possible_moves () # Calculate number of possible moves,
{ #+ given the current position.
if [ -z "$1" ]
then
return $FAIL
fi
local curr_pos=$1
local valid_movl=0
local icx=0
local movl
local sq
declare -a movesloc
movesloc=( $(generate_moves $curr_pos) )
for movl in {1..8}
do
let "sq = $curr_pos + ${movesloc[movl]}"
is_on_board $sq
if [ $? -eq $SUCCESS ] && [ ${board[sq]} -eq $UNVISITED ]
then
((valid_movl++));
fi
done
return $valid_movl # Found a square to move to!
}
strategy ()
{
echo
if [ -n "$STUPID" ]
then
for Moves in {1..63}
do
cposl=$1
moves=( $(generate_moves $currpos) )
do_move_stupid "$currpos"
if [ $? -eq $FAIL ]
then
failure
fi
done
fi
Don't need an "else" clause here,
#+ because Stupid Strategy will always fail and exit!
for Moves in {1..63}
do
cposl=$1
moves=( $(generate_moves $currpos) )
do_move "$currpos"
if [ $? -eq $FAIL ]
then
failure
fi
done
# Could have condensed above two do-loops into a single one,
echo #+ but this would have slowed execution.
print_board
echo
echo "Knight's Tour ends on $(to_algebraic $currpos) [square #$currpos]."
return $SUCCESS
}
debug ()
{ # Enable this by setting DEBUG=1 near beginning of script.
local n
echo "================================="
echo " At move number $movenum:"
echo " *** possible moves = $mpm ***"
echo "### square = $square ###"
echo "lmin = $lmin"
echo "${moves[@]}"
for n in {1..8}
do
echo -n "($n):${p_moves[n]} "
done
echo
echo "iex = $iex :: moves[iex] = ${moves[iex]}"
echo "square = $square"
echo "================================="
echo
} # Gives pretty complete status after ea. move.
=============================================================== #
int main () {
from_algebraic "$1"
startpos=$?
if [ "$startpos" -eq "$FAIL" ] # Okay even if no $1.
then # ^^^^^^^^^^^ Okay even if input -lt 0.
echo "No starting square specified (or illegal input)."
let "startpos = $RANDOM % $SQUARES" # 0 - 63 permissable range.
fi
if [ "$2" = "stupid" ]
then
STUPID=1
echo -n " ### Stupid Strategy ###"
else
STUPID=''
echo -n " *** Warnsdorff's Algorithm ***"
fi
initialize_board
movenum=0
board[startpos]=$movenum # Mark each board square with move number.
currpos=$startpos
algpos=$(to_algebraic $startpos)
echo; echo "Starting from $algpos [square #$startpos] ..."; echo
echo -n "Moves:"
strategy "$currpos"
echo
exit 0 # return 0;
} # End of main() pseudo-function.
=============================================================== #
Exercises:
- Extend this example to a 10 x 10 board or larger.
- Improve the "stupid strategy" by modifying the
do_move_stupid function.
Hint: Prevent straying into corner squares in early moves
(the exact opposite of Warnsdorff's algorithm!).
- This script could stand considerable improvement and
streamlining, especially in the poorly-written
generate_moves() function
and in the DECIDE-MOVE patch in the do_move() function.
Must figure out why standard algorithm fails for startpos=37 ...
- but not on any other, including symmetrical startpos=26.
Possibly, when calculating possible moves, counts the move back
- to the originating square. If so, it might be a relatively easy fix.
Example A-46. Magic Squares
!/bin/bash
msquare.sh
Magic Square generator (odd-order squares only!)
Author: mendel cooper
reldate: 19 Jan. 2009
License: Public Domain
A C-program by the very talented Kwon Young Shin inspired this script.
http://user.chollian.net/~brainstm/MagicSquare.htm
Definition: A "magic square" is a two-dimensional array
of integers in which all the rows, columns,
and long diagonals add up to the same number.
Being "square," the array has the same number
of rows and columns. That number is the "order."
An example of a magic square of order 3 is:
8 1 6
3 5 7
4 9 2
All the rows, columns, and the two long diagonals add up to 15.
Globals
EVEN=2
MAXSIZE=31 # 31 rows x 31 cols.
E_usage=90 # Invocation error.
dimension=
declare -i square
usage_message ()
{
echo "Usage: $0 order"
echo " ... where "order" (square size) is an ODD integer"
echo " in the range 3 - 31."
Actually works for squares up to order 159,
#+ but large squares will not display pretty-printed in a term window.
Try increasing MAXSIZE, above.
exit $E_usage
}
calculate () # Here's where the actual work gets done.
{
local row col index dimadj j k cell_val=1
dimension=$1
let "dimadj = $dimension * 3"; let "dimadj /= 2" # x 1.5, then truncate.
for ((j=0; j < dimension; j++))
do
for ((k=0; k < dimension; k++))
do # Calculate indices, then convert to 1-dim. array index.
# Bash doesn't support multidimensional arrays. Pity.
let "col = $k - $j + $dimadj"; let "col %= $dimension"
let "row = $j * 2 - $k + $dimension"; let "row %= $dimension"
let "index = $row*($dimension) + $col"
square[$index]=cell_val; ((cell_val++))
done
done
} # Plain math, visualization not required.
print_square () # Output square, one row at a time.
{
local row col idx d1
let "d1 = $dimension - 1" # Adjust for zero-indexed array.
for row in $(seq 0 $d1)
do
for col in $(seq 0 $d1)
do
let "idx = $row * $dimension + $col"
printf "%3d " "${square[idx]}"; echo -n " "
done # Displays up to 13th order neatly in 80-column term window.
echo # Newline after each row.
done
}
if [[ -z "$1" ]] || [[ "$1" -gt $MAXSIZE ]]
then
usage_message
fi
let "test_even = $1 % $EVEN"
if [ $test_even -eq 0 ]
then # Can't handle even-order squares.
usage_message
fi
calculate $1
print_square # echo "${square[@]}" # DEBUG
exit $?
Exercises:
- Add a function to calculate the sum of each row, column,
and long diagonal. The sums must match.
This is the "magic constant" of that particular order square.
- Have the print_square function auto-calculate how much space
to allot between square elements for optimized display.
This might require parameterizing the "printf" line.
- Add appropriate functions for generating magic squares
with an even number of rows/columns.
This is non-trivial(!).
See the URL for Kwon Young Shin, above, for help.
Example A-47. Fifteen Puzzle
!/bin/bash
fifteen.sh
Classic "Fifteen Puzzle"
Author: Antonio Macchi
Lightly edited and commented by ABS Guide author.
Used in ABS Guide with permission. (Thanks!)
The invention of the Fifteen Puzzle is attributed to either
- Sam Loyd or Noyes Palmer Chapman.
The puzzle was wildly popular in the late 19th-century.
Object: Rearrange the numbers so they read in order,
- from 1 - 15: ________________
| 1 2 3 4 |
| 5 6 7 8 |
| 9 10 11 12 |
| 13 14 15 |
Constants #
SQUARES=16 #
FAIL=70 #
E_PREMATURE_EXIT=80 #
Data #
Puzzle=( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 " " )
Functions #
function swap
{
local tmp
tmp=${Puzzle[$1]}
Puzzle[$1]=${Puzzle[$2]}
Puzzle[$2]=$tmp
}
function Jumble
{ # Scramble the pieces at beginning of round.
local i pos1 pos2
for i in {1..100}
do
pos1=$(( $RANDOM % $SQUARES))
pos2=$(( $RANDOM % $SQUARES ))
swap $pos1 $pos2
done
}
function PrintPuzzle
{
local i1 i2 puzpos
puzpos=0
clear
echo "Enter quit to exit."; echo # Better that than Ctl-C.
echo ",----.----.----.----." # Top border.
for i1 in {1..4}
do
for i2 in {1..4}
do
printf "| %2s " "${Puzzle[$puzpos]}"
(( puzpos++ ))
done
echo "|" # Right-side border.
test $i1 = 4 || echo "+----+----+----+----+"
done
echo "'----'----'----'----'" # Bottom border.
}
function GetNum
{ # Test for valid input.
local puznum garbage
while true
do
echo "Moves: $moves" # Also counts invalid moves.
read -p "Number to move: " puznum garbage
if [ "$puznum" = "quit" ]; then echo; exit $E_PREMATURE_EXIT; fi
test -z "$puznum" -o -n "${puznum//[0-9]/}" && continue
test $puznum -gt 0 -a $puznum -lt $SQUARES && break
done
return $puznum
}
function GetPosFromNum
{ # $1 = puzzle-number
local puzpos
for puzpos in {0..15}
do
test "${Puzzle[$puzpos]}" = "$1" && break
done
return $puzpos
}
function Move
{ # $1=Puzzle-pos
test $1 -gt 3 && test "${Puzzle[$(( $1 - 4 ))]}" = " "\
&& swap $1 $(( $1 - 4 )) && return 0
test $(( $1%4 )) -ne 3 && test "${Puzzle[$(( $1 + 1 ))]}" = " "\
&& swap $1 $(( $1 + 1 )) && return 0
test $1 -lt 12 && test "${Puzzle[$(( $1 + 4 ))]}" = " "\
&& swap $1 $(( $1 + 4 )) && return 0
test $(( $1%4 )) -ne 0 && test "${Puzzle[$(( $1 - 1 ))]}" = " " &&\
swap $1 $(( $1 - 1 )) && return 0
return 1
}
function Solved
{
local pos
for pos in {0..14}
do
test "${Puzzle[$pos]}" = $(( $pos + 1 )) || return $FAIL
# Check whether number in each square = square number.
done
return 0 # Successful solution.
}
MAIN () #######################{
moves=0
Jumble
while true # Loop continuously until puzzle solved.
do
echo; echo
PrintPuzzle
echo
while true
do
GetNum
puznum=$?
GetPosFromNum $puznum
puzpos=$?
((moves++))
Move $puzpos && break
done
Solved && break
done
echo;echo
PrintPuzzle
echo; echo "BRAVO!"; echo
exit 0
}
Exercise:
Rewrite the script to display the letters A - O,
- rather than the numbers 1 - 15.
Example A-48. The Towers of Hanoi, graphic version
! /bin/bash
The Towers Of Hanoi
Original script (hanoi.bash) copyright (C) 2000 Amit Singh.
All Rights Reserved.
http://hanoi.kernelthread.com
hanoi2.bash
Version 2.00: modded for ASCII-graphic display.
Version 2.01: fixed no command-line param bug.
Uses code contributed by Antonio Macchi,
- with heavy editing by ABS Guide author.
This variant falls under the original copyright, see above.
Used in ABS Guide with Amit Singh's permission (thanks!).
Variables && sanity check ###
E_NOPARAM=86
E_BADPARAM=87 # Illegal no. of disks passed to script.
E_NOEXIT=88
DISKS=${1:-$E_NOPARAM} # Must specify how many disks.
Moves=0
MWIDTH=7
MARGIN=2
Arbitrary "magic" constants; work okay for relatively small # of disks.
BASEWIDTH=51 # Original code.
let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods.
Above "algorithm" could likely stand improvement.
Display variables ###
let "disks1 = $DISKS - 1"
let "spaces1 = $DISKS"
let "spaces2 = 2 * $DISKS"
let "lastmove_t = $DISKS - 1" # Final move?
declare -a Rod1 Rod2 Rod3
######################### ###
function repeat { # $1=char $2=number of repetitions
local n # Repeat-print a character.
for (( n=0; n<$2; n++ )); do
echo -n "$1"
done
}
function FromRod {
local rod summit weight sequence
while true; do
rod=$1
test ${rod/[^123]/} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for summit in $sequence; do
eval weight=\${Rod${rod}[$summit]}
test $weight -ne 0 &&
{ echo "$rod $summit $weight"; return; }
done
done
}
function ToRod { # $1=previous (FromRod) weight
local rod firstfree weight sequence
while true; do
rod=$2
test ${rod/[^123]} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for firstfree in $sequence; do
eval weight=\${Rod${rod}[$firstfree]}
test $weight -gt 0 && { (( firstfree++ )); break; }
done
test $weight -gt $1 -o $firstfree = 0 &&
{ echo "$rod $firstfree"; return; }
done
}
function PrintRods {
local disk rod empty fill sp sequence
repeat " " $spaces1
echo -n "|"
repeat " " $spaces2
echo -n "|"
repeat " " $spaces2
echo "|"
sequence=$(echo $(seq 0 $disks1 | tac))
for disk in $sequence; do
for rod in {1..3}; do
eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))
eval fill=\${Rod${rod}[$disk]}
repeat " " $empty
test $fill -gt 0 && repeat "*" $fill || echo -n "|"
repeat " " $empty
done
echo
done
repeat "=" $basewidth # Print "base" beneath rods.
echo
}
display ()
{
echo
PrintRods
Get rod-number, summit and weight
first=( FromRod $1 )
eval Rod${first[0]}[${first[1]}]=0
Get rod-number and first-free position
second=( ToRod ${first[2]} $2 )
eval Rod${second[0]}[${second[1]}]=${first[2]}
echo; echo; echo
if [ "${Rod3[lastmove_t]}" = 1 ]
then # Last move? If yes, then display final position.
echo "+ Final Position: $Moves moves"; echo
PrintRods
fi
}
From here down, almost the same as original (hanoi.bash) script.
dohanoi() { # Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
if [ "$Moves" -ne 0 ]
then
echo "+ Position after move $Moves"
fi
((Moves++))
echo -n " Next move will be: "
echo $2 "-->" $3
display $2 $3
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
setup_arrays ()
{
local dim n elem
let "dim1 = $1 - 1"
elem=$dim1
for n in $(seq 0 $dim1)
do
let "Rod1[$elem] = 2 * $n + 1"
Rod2[$n]=0
Rod3[$n]=0
((elem--))
done
}
Main ###
setup_arrays $DISKS
echo; echo "+ Start Position"
case $# in
1) case $(($1>0)) in # Must have at least one disk.
1)
disks=$1
dohanoi $1 1 3 2
Total moves = 2^n - 1, where n = number of disks.
echo
exit 0;
;;
*)
echo "$0: Illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
clear
echo "usage: $0 N"
echo " Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
exit $E_NOEXIT # Shouldn't exit here.
Note:
Redirect script output to a file, otherwise it scrolls off display.
Example A-49. The Towers of Hanoi, alternate graphic version
! /bin/bash
The Towers Of Hanoi
Original script (hanoi.bash) copyright (C) 2000 Amit Singh.
All Rights Reserved.
http://hanoi.kernelthread.com
hanoi2.bash
Version 2: modded for ASCII-graphic display.
Uses code contributed by Antonio Macchi,
- with heavy editing by ABS Guide author.
This variant also falls under the original copyright, see above.
Used in ABS Guide with Amit Singh's permission (thanks!).
Variables #
E_NOPARAM=86
E_BADPARAM=87 # Illegal no. of disks passed to script.
E_NOEXIT=88
DELAY=2 # Interval, in seconds, between moves. Change, if desired.
DISKS=$1
Moves=0
MWIDTH=7
MARGIN=2
Arbitrary "magic" constants, work okay for relatively small # of disks.
BASEWIDTH=51 # Original code.
let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods.
Above "algorithm" could likely stand improvement.
Display variables.
let "disks1 = $DISKS - 1"
let "spaces1 = $DISKS"
let "spaces2 = 2 * $DISKS"
let "lastmove_t = $DISKS - 1" # Final move?
declare -a Rod1 Rod2 Rod3
function repeat { # $1=char $2=number of repetitions
local n # Repeat-print a character.
for (( n=0; n<$2; n++ )); do
echo -n "$1"
done
}
function FromRod {
local rod summit weight sequence
while true; do
rod=$1
test ${rod/[^123]/} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for summit in $sequence; do
eval weight=\${Rod${rod}[$summit]}
test $weight -ne 0 &&
{ echo "$rod $summit $weight"; return; }
done
done
}
function ToRod { # $1=previous (FromRod) weight
local rod firstfree weight sequence
while true; do
rod=$2
test ${rod/[^123]} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for firstfree in $sequence; do
eval weight=\${Rod${rod}[$firstfree]}
test $weight -gt 0 && { (( firstfree++ )); break; }
done
test $weight -gt $1 -o $firstfree = 0 &&
{ echo "$rod $firstfree"; return; }
done
}
function PrintRods {
local disk rod empty fill sp sequence
tput cup 5 0
repeat " " $spaces1
echo -n "|"
repeat " " $spaces2
echo -n "|"
repeat " " $spaces2
echo "|"
sequence=$(echo $(seq 0 $disks1 | tac))
for disk in $sequence; do
for rod in {1..3}; do
eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))
eval fill=\${Rod${rod}[$disk]}
repeat " " $empty
test $fill -gt 0 && repeat "*" $fill || echo -n "|"
repeat " " $empty
done
echo
done
repeat "=" $basewidth # Print "base" beneath rods.
echo
}
display ()
{
echo
PrintRods
Get rod-number, summit and weight
first=( FromRod $1 )
eval Rod${first[0]}[${first[1]}]=0
Get rod-number and first-free position
second=( ToRod ${first[2]} $2 )
eval Rod${second[0]}[${second[1]}]=${first[2]}
if [ "${Rod3[lastmove_t]}" = 1 ]
then # Last move? If yes, then display final position.
tput cup 0 0
echo; echo "+ Final Position: $Moves moves"
PrintRods
fi
sleep $DELAY
}
From here down, almost the same as original (hanoi.bash) script.
dohanoi() { # Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
if [ "$Moves" -ne 0 ]
then
tput cup 0 0
echo; echo "+ Position after move $Moves"
fi
((Moves++))
echo -n " Next move will be: "
echo $2 "-->" $3
display $2 $3
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
setup_arrays ()
{
local dim n elem
let "dim1 = $1 - 1"
elem=$dim1
for n in $(seq 0 $dim1)
do
let "Rod1[$elem] = 2 * $n + 1"
Rod2[$n]=0
Rod3[$n]=0
((elem--))
done
}
Main ###
trap "tput cnorm" 0
tput civis
clear
setup_arrays $DISKS
tput cup 0 0
echo; echo "+ Start Position"
case $# in
1) case $(($1>0)) in # Must have at least one disk.
1)
disks=$1
dohanoi $1 1 3 2
Total moves = 2^n - 1, where n = # of disks.
echo
exit 0;
;;
*)
echo "$0: Illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo " Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
exit $E_NOEXIT # Shouldn't exit here.
Exercise:
There is a minor bug in the script that causes the display of
- the next-to-last move to be skipped.
Example A-50. An alternate version of the getopt-simple.sh script
!/bin/bash
UseGetOpt.sh
UseGetOpt () {
declare inputOptions
declare -r E_OPTERR=85
declare -r ScriptName=${0##*/}
declare -r ShortOpts="adf:hlt"
declare -r LongOpts="aoption,debug,file:,help,log,test"
DoSomething () {
echo "The function name is '${FUNCNAME}'"
# Recall that $FUNCNAME is an internal variable
#+ holding the name of the function it is in.
}
inputOptions=$(getopt -o "${ShortOpts}" --long \
"${LongOpts}" --name "${ScriptName}" -- "${@}")
if [[ ($? -ne 0) || ($# -eq 0) ]]; then
echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"
exit $E_OPTERR
fi
eval set -- "${inputOptions}"
Only for educational purposes. Can be removed.
#-----------------------------------------------
echo "++ Test: Number of arguments: [$#]"
echo '++ Test: Looping through "$@"'
for a in "$@"; do
echo " ++ [$a]"
done
#-----------------------------------------------
while true; do
case "${1}" in
--aoption | -a) # Argument found.
echo "Option [$1]"
;;
--debug | -d) # Enable informational messages.
echo "Option [$1] Debugging enabled"
;;
--file | -f) # Check for optional argument.
case "$2" in #+ Double colon is optional argument.
"") # Not there.
echo "Option [$1] Use default"
shift
;;
*) # Got it
echo "Option [$1] Using input [$2]"
shift
;;
esac
DoSomething
;;
--log | -l) # Enable Logging.
echo "Option [$1] Logging enabled"
;;
--test | -t) # Enable testing.
echo "Option [$1] Testing enabled"
;;
--help | -h)
echo "Option [$1] Display help"
break
;;
--) # Done! $# is argument number for "--", $@ is "--"
echo "Option [$1] Dash Dash"
break
;;
*)
echo "Major internal error!"
exit 8
;;
esac
echo "Number of arguments: [$#]"
shift
done
shift
Only for educational purposes. Can be removed.
#----------------------------------------------------------------------
echo "++ Test: Number of arguments after "--" is [$#] They are: [$@]"
echo '++ Test: Looping through "$@"'
for a in "$@"; do
echo " ++ [$a]"
done
#----------------------------------------------------------------------
}
M A I N ########################
If you remove "function UseGetOpt () {" and corresponding "}",
- you can uncomment the "exit 0" line below, and invoke this script
- with the various options from the command-line.
exit 0
echo "Test 1"
UseGetOpt -f myfile one "two three" four
echo;echo "Test 2"
UseGetOpt -h
echo;echo "Test 3 - Short Options"
UseGetOpt -adltf myfile anotherfile
echo;echo "Test 4 - Long Options"
UseGetOpt --aoption --debug --log --test --file myfile anotherfile
exit
Example A-51. The version of the UseGetOpt.sh example used in the Tab
Expansion appendix
!/bin/bash
UseGetOpt-2.sh
Modified version of the script for illustrating tab-expansion
See the "Introduction to Tab Expansion" appendix.
Possible options: -a -d -f -l -t -h
UseGetOpt () {
declare inputOptions
declare -r E_OPTERR=85
declare -r ScriptName=${0##*/}
declare -r ShortOpts="adf:hlt"
declare -r LongOpts="aoption,debug,file:,help,log,test"
DoSomething () {
echo "The function name is '${FUNCNAME}'"
}
inputOptions=$(getopt -o "${ShortOpts}" --long \
"${LongOpts}" --name "${ScriptName}" -- "${@}")
if [[ ($? -ne 0) || ($# -eq 0) ]]; then
echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"
exit $E_OPTERR
fi
eval set -- "${inputOptions}"
while true; do
case "${1}" in
--aoption | -a) # Argument found.
echo "Option [$1]"
;;
--debug | -d) # Enable informational messages.
echo "Option [$1] Debugging enabled"
;;
--file | -f) # Check for optional argument.
case "$2" in #+ Double colon is optional argument.
"") # Not there.
echo "Option [$1] Use default"
shift
;;
*) # Got it
echo "Option [$1] Using input [$2]"
shift
;;
esac
DoSomething
;;
--log | -l) # Enable Logging.
echo "Option [$1] Logging enabled"
;;
--test | -t) # Enable testing.
echo "Option [$1] Testing enabled"
;;
--help | -h)
echo "Option [$1] Display help"
break
;;
--) # Done! $# is argument number for "--", $@ is "--"
echo "Option [$1] Dash Dash"
break
;;
*)
echo "Major internal error!"
exit 8
;;
esac
echo "Number of arguments: [$#]"
shift
done
shift
}
exit
Example A-52. Cycling through all the possible color backgrounds
!/bin/bash
show-all-colors.sh
Displays all 256 possible background colors, using ANSI escape sequences.
Author: Chetankumar Phulpagare
Used in ABS Guide with permission.
T1=8
T2=6
T3=36
offset=0
for num1 in {0..7}
do {
for num2 in {0,1}
do {
shownum=`echo "$offset + $T1 * ${num2} + $num1" | bc`
echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"
}
done
echo
}
done
offset=16
for num1 in {0..5}
do {
for num2 in {0..5}
do {
for num3 in {0..5}
do {
shownum=`echo "$offset + $T2 * ${num3} \
+ $num2 + $T3 * ${num1}" | bc`
echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"
}
done
echo
}
done
}
done
offset=232
for num1 in {0..23}
do {
shownum=expr $offset + $num1
echo -en "\E[0;48;5;${shownum}m ${shownum}\E[0m"
}
done
echo
Example A-53. Morse Code Practice
!/bin/bash
sam.sh, v. .01a
Still Another Morse (code training script)
With profuse apologies to Sam (F.B.) Morse.
Author: Mendel Cooper
License: GPL3
Reldate: 05/25/11
Morse code training script.
Converts arguments to audible dots and dashes.
Note: lowercase input only at this time.
Get the wav files from the source tarball:
http://bash.deta.in/abs-guide-latest.tar.bz2
DOT='soundfiles/dot.wav'
DASH='soundfiles/dash.wav'
Maybe move soundfiles to /usr/local/sounds?
LETTERSPACE=300000 # Microseconds.
WORDSPACE=980000
Nice and slow, for beginners. Maybe 5 wpm?
EXIT_MSG="May the Morse be with you!"
E_NOARGS=75 # No command-line args?
declare -A morse # Associative array!
======================================= #
morse[a]="dot; dash"
morse[b]="dash; dot; dot; dot"
morse[c]="dash; dot; dash; dot"
morse[d]="dash; dot; dot"
morse[e]="dot"
morse[f]="dot; dot; dash; dot"
morse[g]="dash; dash; dot"
morse[h]="dot; dot; dot; dot"
morse[i]="dot; dot;"
morse[j]="dot; dash; dash; dash"
morse[k]="dash; dot; dash"
morse[l]="dot; dash; dot; dot"
morse[m]="dash; dash"
morse[n]="dash; dot"
morse[o]="dash; dash; dash"
morse[p]="dot; dash; dash; dot"
morse[q]="dash; dash; dot; dash"
morse[r]="dot; dash; dot"
morse[s]="dot; dot; dot"
morse[t]="dash"
morse[u]="dot; dot; dash"
morse[v]="dot; dot; dot; dash"
morse[w]="dot; dash; dash"
morse[x]="dash; dot; dot; dash"
morse[y]="dash; dot; dash; dash"
morse[z]="dash; dash; dot; dot"
morse[0]="dash; dash; dash; dash; dash"
morse[1]="dot; dash; dash; dash; dash"
morse[2]="dot; dot; dash; dash; dash"
morse[3]="dot; dot; dot; dash; dash"
morse[4]="dot; dot; dot; dot; dash"
morse[5]="dot; dot; dot; dot; dot"
morse[6]="dash; dot; dot; dot; dot"
morse[7]="dash; dash; dot; dot; dot"
morse[8]="dash; dash; dash; dot; dot"
morse[9]="dash; dash; dash; dash; dot"
The following must be escaped or quoted.
morse[?]="dot; dot; dash; dash; dot; dot"
morse[.]="dot; dash; dot; dash; dot; dash"
morse[,]="dash; dash; dot; dot; dash; dash"
morse[/]="dash; dot; dot; dash; dot"
morse[@]="dot; dash; dash; dot; dash; dot"
======================================= #
play_letter ()
{
eval ${morse[$1]} # Play dots, dashes from appropriate sound files.
Why is 'eval' necessary here?
usleep $LETTERSPACE # Pause in between letters.
}
extract_letters ()
{ # Slice string apart, letter by letter.
local pos=0 # Starting at left end of string.
local len=1 # One letter at a time.
strlen=${#1}
while [ $pos -lt $strlen ]
do
letter=${1:pos:len}
# ^^^^^^^^^^^^ See Chapter 10.1.
play_letter $letter
echo -n "*" # Mark letter just played.
((pos++))
done
}
Play the sounds ############
dot() { aplay "$DOT" 2&>/dev/null; }
dash() { aplay "$DASH" 2&>/dev/null; }
no_args ()
{
declare -a usage
usage=( $0 word1 word2 ... )
echo "Usage:"; echo
echo ${usage[*]}
for index in 0 1 2 3
do
extract_letters ${usage[index]}
usleep $WORDSPACE
echo -n " " # Print space between words.
done
echo "Usage: $0 word1 word2 ... "
echo; echo
}
int main()
{
clear # Clear the terminal screen.
echo " SAM"
echo "Still Another Morse code trainer"
echo " Author: Mendel Cooper"
echo; echo;
if [ -z "$1" ]
then
no_args
echo; echo; echo "$EXIT_MSG"; echo
exit $E_NOARGS
fi
echo; echo "$*" # Print text that will be played.
until [ -z "$1" ]
do
extract_letters $1
shift # On to next word.
usleep $WORDSPACE
echo -n " " # Print space between words.
done
echo; echo; echo "$EXIT_MSG"; echo
exit 0
}
Exercises:
- Have the script accept either lowercase or uppercase words
- as arguments. Hint: Use 'tr' . . .
- Have the script optionally accept input from a text file.
Example A-54. Base64 encoding/decoding
!/bin/bash
base64.sh: Bash implementation of Base64 encoding and decoding.
Used in ABSG with permission (thanks!).
Encode or decode original Base64 (and also Base64url)
Usage:
Encode
$ ./base64.sh < binary-file > binary-file.base64
Decode
$ ./base64.sh -d < binary-file.base64 > binary-file
Reference:
[1] RFC4648 - "The Base16, Base32, and Base64 Data Encodings"
http://tools.ietf.org/html/rfc4648#section-5
The base64_charset[] array contains entire base64 charset,
and additionally the character "=" ...
base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Nice illustration of brace expansion.
Uncomment the ### line below to use base64url encoding instead of
base64_charset=( {A..Z} {a..z} {0..9} - _ = )
Output text width when encoding
- (64 characters, just like openssl output).
text_width=64
function display_base64_char {
Convert a 6-bit number (between 0 and 63) into its corresponding values
- in Base64, then display the result with the specified text width.
printf "${base64_charset[$1]}"; (( width++ ))
(( width % text_width == 0 )) && printf "\n"
}
function encode_base64 {
Encode three 8-bit hexadecimal codes into four 6-bit numbers.
We need two local int array variables:
c8[]: to store the codes of the 8-bit characters to encode
c6[]: to store the corresponding encoded values on 6-bit
declare -a -i c8 c6
Convert hexadecimal to decimal.
c8=( $(printf "ibase=16; ${1:0:2}\n${1:2:2}\n${1:4:2}\n" | bc) )
Let's play with bitwise operators
#+ (3x8-bit into 4x6-bits conversion).
(( c6[0] = c8[0] >> 2 ))
(( c6[1] = ((c8[0] & 3) << 4) | (c8[1] >> 4) ))
The following operations depend on the c8 element number.
case ${#c8[*]} in
3) (( c6[2] = ((c8[1] & 15) << 2) | (c8[2] >> 6) ))
(( c6[3] = c8[2] & 63 )) ;;
2) (( c6[2] = (c8[1] & 15) << 2 ))
(( c6[3] = 64 )) ;;
1) (( c6[2] = c6[3] = 64 )) ;;
esac
for char in ${c6[@]}; do
display_base64_char ${char}
done
}
function decode_base64 {
Decode four base64 characters into three hexadecimal ASCII characters.
c8[]: to store the codes of the 8-bit characters
c6[]: to store the corresponding Base64 values on 6-bit
declare -a -i c8 c6
Find decimal value corresponding to the current base64 character.
for current_char in ${1:0:1} ${1:1:1} ${1:2:1} ${1:3:1}; do
[ "${current_char}" = "=" ] && break
position=0
while [ "${current_char}" != "${base64_charset[${position}]}" ]; do
(( position++ ))
done
c6=( ${c6[*]} ${position} )
done
Let's play with bitwise operators
#+ (4x8-bit into 3x6-bits conversion).
(( c8[0] = (c6[0] << 2) | (c6[1] >> 4) ))
The next operations depends on the c6 elements number.
case ${#c6[*]} in
3) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))
(( c8[2] = (c6[2] & 3) << 6 )); unset c8[2] ;;
4) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))
(( c8[2] = ( (c6[2] & 3) << 6) | c6[3] )) ;;
esac
for char in ${c8[*]}; do
printf "\x$(printf "%x" ${char})"
done
}
main ()
if [ "$1" = "-d" ]; then # decode
Reformat STDIN in pseudo 4x6-bit groups.
content=$(cat - | tr -d "\n" | sed -r "s/(.{4})/\1 /g")
for chars in ${content}; do decode_base64 ${chars}; done
else
Make a hexdump of stdin and reformat in 3-byte groups.
content=$(cat - | xxd -ps -u | sed -r "s/(\w{6})/\1 /g" |
tr -d "\n")
for chars in ${content}; do encode_base64 ${chars}; done
echo
fi
Example A-55. Inserting text in a file using sed
!/bin/bash
Prepends a string at a specified line
- in files with names ending in "sample"
- in the current working directory.
000000000000000000000000000000000000
This script overwrites files!
Be careful running it in a directory
- where you have important files!!!
000000000000000000000000000000000000
Create a couple of files to operate on ...
01sample
02sample
... etc.
These files must not be empty, else the prepend will not work.
lineno=1 # Append at line 1 (prepend).
filespec="*sample" # Filename pattern to operate on.
string=$(whoami) # Will set your username as string to insert.
# It could just as easily be any other string.
for file in $filespec # Specify which files to alter.
do # ^^^^^^^^^
sed -i ""$lineno"i "$string"" $file
^^ -i option edits files in-place.
^ Insert (i) command.
echo ""$file" altered!"
done
echo "Warning: files possibly clobbered!"
exit 0
Exercise:
Add error checking to this script.
It needs it badly.
Example A-56. The Gronsfeld Cipher
!/bin/bash
gronsfeld.bash
License: GPL3
Reldate 06/23/11
This is an implementation of the Gronsfeld Cipher.
It's essentially a stripped-down variant of the
- polyalphabetic Vigenère Tableau, but with only 10 alphabets.
The classic Gronsfeld has a numeric sequence as the key word,
- but here we substitute a letter string, for ease of use.
Allegedly, this cipher was invented by the eponymous Count Gronsfeld
- in the 17th Century. It was at one time considered to be unbreakable.
Note that this is ###not### a secure cipher by modern standards.
Global Variables #
Enc_suffix="29379" # Encrypted text output with this 5-digit suffix.
# This functions as a decryption flag,
#+ and when used to generate passwords adds security.
Default_key="gronsfeldk"
# The script uses this if key not entered below
# (at "Keychain").
# Change the above two values frequently
#+ for added security.
GROUPLEN=5 # Output in groups of 5 letters, per tradition.
alpha1=( abcdefghijklmnopqrstuvwxyz )
alpha2=( {A..Z} ) # Output in all caps, per tradition.
# Use alpha2=( {a..z} ) for password generator.
wraplen=26 # Wrap around if past end of alphabet.
dflag= # Decrypt flag (set if $Enc_suffix present).
E_NOARGS=76 # Missing command-line args?
DEBUG=77 # Debugging flag.
declare -a offsets # This array holds the numeric shift values for
#+ encryption/decryption.
Keychain#########
key= ### Put key here!!!
# 10 characters!
Function
: ()
{ # Encrypt or decrypt, depending on whether $dflag is set.
Why ": ()" as a function name? Just to prove that it can be done.
local idx keydx mlen off1 shft
local plaintext="$1"
local mlen=${#plaintext}
for (( idx=0; idx<$mlen; idx++ ))
do
let "keydx = $idx % $keylen"
shft=${offsets[keydx]}
if [ -n "$dflag" ]
then # Decrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) - $shft"
# Shift backward to decrypt.
else # Encrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) + $shft"
# Shift forward to encrypt.
test $(( $idx % $GROUPLEN)) = 0 && echo -n " " # Groups of 5 letters.
# Comment out above line for output as a string without whitespace,
#+ for example, if using the script as a password generator.
fi
((off1--)) # Normalize. Why is this necessary?
if [ $off1 -lt 0 ]
then # Catch negative indices.
let "off1 += $wraplen"
fi
((off1 %= $wraplen)) # Wrap around if past end of alphabet.
echo -n "${alpha2[off1]}"
done
if [ -z "$dflag" ]
then
echo " $Enc_suffix"
echo "$Enc_suffix" # For password generator.
else
echo
fi
} # End encrypt/decrypt function.
int main () {
Check for command-line args.
if [ -z "$1" ]
then
echo "Usage: $0 TEXT TO ENCODE/DECODE"
exit $E_NOARGS
fi
if [ ${!#} == "$Enc_suffix" ]
^^^^^ Final command-line arg.
then
dflag=ON
echo -n "+" # Flag decrypted text with a "+" for easy ID.
fi
if [ -z "$key" ]
then
key="$Default_key" # "gronsfeldk" per above.
fi
keylen=${#key}
for (( idx=0; idx<$keylen; idx++ ))
do # Calculate shift values for encryption/decryption.
offsets[idx]=$(expr index "${alpha1[*]}" ${key:idx:1}) # Normalize.
((offsets[idx]--)) # Necessary because "expr index" starts at 1,
#+ whereas array count starts at 0.
Generate array of numerical offsets corresponding to the key.
There are simpler ways to accomplish this.
done
args=$(echo "$*" | sed -e 's/ //g' | tr A-Z a-z | sed -e 's/[0-9]//g')
Remove whitespace and digits from command-line args.
Can modify to also remove punctuation characters, if desired.
# Debug:
# echo "$args"; exit $DEBUG
: "$args" # Call the function named ":".
: is a null operator, except . . . when it's a function name!
exit $? # } End-of-script
************************************************************** #
This script can function as a password generator,
- with several minor mods, see above.
That would allow an easy-to-remember password, even the word
- "password" itself, which encrypts to vrgfotvo29379
- a fairly secure password not susceptible to a dictionary attack.
Or, you could use your own name (surely that's easy to remember!).
For example, Bozo Bozeman encrypts to hfnbttdppkt29379.
************************************************************** #
Example A-57. Bingo Number Generator
!/bin/bash
bingo.sh
Bingo number generator
Reldate 20Aug12, License: Public Domain
This script generates bingo numbers.
Hitting a key generates a new number.
Hitting 'q' terminates the script.
In a given run of the script, there will be no duplicate numbers.
When the script terminates, it prints a log of the numbers generated.
MIN=1 # Lowest allowable bingo number.
MAX=75 # Highest allowable bingo number.
COLS=15 # Numbers in each column (B I N G O).
SINGLE_DIGIT_MAX=9
declare -a Numbers
Prefix=(B I N G O)
initialize_Numbers ()
{ # Zero them out to start.
They'll be incremented if chosen.
local index=0
until [ "$index" -gt $MAX ]
do
Numbers[index]=0
((index++))
done
Numbers[0]=1 # Flag zero, so it won't be selected.
}
generate_number ()
{
local number
while [ 1 ]
do
let "number = $(expr $RANDOM % $MAX)"
if [ ${Numbers[number]} -eq 0 ] # Number not yet called.
then
let "Numbers[number]+=1" # Flag it in the array.
break # And terminate loop.
fi # Else if already called, loop and generate another number.
done
Exercise: Rewrite this more elegantly as an until-loop.
return $number
}
print_numbers_called ()
{ # Print out the called number log in neat columns.
# echo ${Numbers[@]}
local pre2=0 # Prefix a zero, so columns will align
#+ on single-digit numbers.
echo "Number Stats"
for (( index=1; index<=MAX; index++))
do
count=${Numbers[index]}
let "t = $index - 1" # Normalize, since array begins with index 0.
let "column = $(expr $t / $COLS)"
pre=${Prefix[column]}
echo -n "${Prefix[column]} "
if [ $(expr $t % $COLS) -eq 0 ]
then
echo # Newline at end of row.
fi
if [ "$index" -gt $SINGLE_DIGIT_MAX ] # Check for single-digit number.
then
echo -n "$pre$index#$count "
else # Prefix a zero.
echo -n "$pre$pre2$index#$count "
fi
done
}
main () {
RANDOM=$$ # Seed random number generator.
initialize_Numbers # Zero out the number tracking array.
clear
echo "Bingo Number Caller"; echo
while [[ "$key" != "q" ]] # Main loop.
do
read -s -n1 -p "Hit a key for the next number [q to exit] " key
Usually 'q' exits, but not always.
Can always hit Ctl-C if q fails.
echo
generate_number; new_number=$?
let "column = $(expr $new_number / $COLS)"
echo -n "${Prefix[column]} " # B-I-N-G-O
echo $new_number
done
echo; echo
Game over ...
print_numbers_called
echo; echo "[#0 = not called . . . #1 = called]"
echo
exit 0
}
Certainly, this script could stand some improvement.
See also the author's Instructable:
www.instructables.com/id/Binguino-An-Arduino-based-Bingo-Number-Generato/
To end this section, a review of the basics . . . and more.
Example A-58. Basics Reviewed
!/bin/bash
basics-reviewed.bash
File extension == *.bash == specific to Bash
Copyright (c) Michael S. Zick, 2003; All rights reserved.
License: Use in any form, for any purpose.
Revision: $ID$
Edited for layout by M.C.
(author of the "Advanced Bash Scripting Guide")
Fixes and updates (04/08) by Cliff Bamford.
This script tested under Bash versions 2.04, 2.05a and 2.05b.
It may not work with earlier versions.
This demonstration script generates one --intentional--
- "command not found" error message. See line 436.
The current Bash maintainer, Chet Ramey, has fixed the items noted
- for later versions of Bash.
###-------------------------------------------###
### Pipe the output of this script to 'more' ###
###+ else it will scroll off the page. ###
### ###
### You may also redirect its output ###
###+ to a file for examination. ###
###-------------------------------------------###
Most of the following points are described at length in
- the text of the foregoing "Advanced Bash Scripting Guide."
This demonstration script is mostly just a reorganized presentation.
-- msz
Variables are not typed unless otherwise specified.
Variables are named. Names must contain a non-digit.
File descriptor names (as in, for example: 2>&1)
Parameters and Bash array elements are numbered.
(Parameters are very similar to Bash arrays.)
A variable name may be undefined (null reference).
unset VarNull
A variable name may be defined but empty (null contents).
VarEmpty='' # Two, adjacent, single quotes.
A variable name may be defined and non-empty.
VarSomething='Literal'
A variable may contain:
- A whole number as a signed 32-bit (or larger) integer
A variable may also be an array.
A string may contain embedded blanks and may be treated
- as if it where a function name with optional arguments.
The names of variables and the names of functions
- are in different namespaces.
A variable may be defined as a Bash array either explicitly or
- implicitly by the syntax of the assignment statement.
Explicit:
declare -a ArrayVar
The echo command is a builtin.
echo $VarSomething
The printf command is a builtin.
Translate %s as: String-Format
printf %s $VarSomething # No linebreak specified, none output.
echo # Default, only linebreak output.
The Bash parser word breaks on whitespace.
Whitespace, or the lack of it is significant.
(This holds true in general; there are, of course, exceptions.)
Translate the DOLLAR_SIGN character as: Content-Of.
Extended-Syntax way of writing Content-Of:
echo ${VarSomething}
The ${ ... } Extended-Syntax allows more than just the variable
In general, $VarSomething can always be written as: ${VarSomething}.
Call this script with arguments to see the following in action.
Outside of double-quotes, the special characters @ and *
- specify identical behavior.
May be pronounced as: All-Elements-Of.
Without specification of a name, they refer to the
- pre-defined parameter Bash-Array.
Glob-Pattern references
echo $* # All parameters to script or function
echo ${*} # Same
Bash disables filename expansion for Glob-Patterns.
Only character matching is active.
All-Elements-Of references
echo $@ # Same as above
echo ${@} # Same as above
Within double-quotes, the behavior of Glob-Pattern references
- depends on the setting of IFS (Input Field Separator).
Within double-quotes, All-Elements-Of references behave the same.
Specifying only the name of a variable holding a string refers
- to all elements (characters) of a string.
To specify an element (character) of a string,
- the Extended-Syntax reference notation (see below) MAY be used.
Specifying only the name of a Bash array references
- the subscript zero element,
- NOT the FIRST DEFINED nor the FIRST WITH CONTENTS element.
Additional qualification is needed to reference other elements,
- which means that the reference MUST be written in Extended-Syntax.
The general form is: ${name[subscript]}.
The string forms may also be used: ${name:subscript}
- for Bash-Arrays when referencing the subscript zero element.
Bash-Arrays are implemented internally as linked lists,
- not as a fixed area of storage as in some programming languages.
Characteristics of Bash arrays (Bash-Arrays):
If not otherwise specified, Bash-Array subscripts begin with
- subscript number zero. Literally: [0]
This is called zero-based indexing.
If not otherwise specified, Bash-Arrays are subscript packed
- (sequential subscripts without subscript gaps).
Negative subscripts are not allowed.
Elements of a Bash-Array need not all be of the same type.
Elements of a Bash-Array may be undefined (null reference).
That is, a Bash-Array may be "subscript sparse."
Elements of a Bash-Array may be defined and empty (null contents).
Elements of a Bash-Array may contain:
- A whole number as a signed 32-bit (or larger) integer
- A string formated so that it appears to be a function name
Defined elements of a Bash-Array may be undefined (unset).
That is, a subscript packed Bash-Array may be changed
- into a subscript sparse Bash-Array.
Elements may be added to a Bash-Array by defining an element
For these reasons, I have been calling them "Bash-Arrays".
I'll return to the generic term "array" from now on.
-- msz
echo "========================================================="
Lines 202 - 334 supplied by Cliff Bamford. (Thanks!)
Demo --- Interaction with Arrays, quoting, IFS, echo, * and @ ---
- all affect how things work
ArrayVar[0]='zero' # 0 normal
ArrayVar[1]=one # 1 unquoted literal
ArrayVar[2]='two' # 2 normal
ArrayVar[3]='three' # 3 normal
ArrayVar[4]='I am four' # 4 normal with spaces
ArrayVar[5]='five' # 5 normal
unset ArrayVar[6] # 6 undefined
ArrayValue[7]='seven' # 7 normal
ArrayValue[8]='' # 8 defined but empty
ArrayValue[9]='nine' # 9 normal
echo '--- Here is the array we are using for this test'
echo
echo "ArrayVar[0]='zero' # 0 normal"
echo "ArrayVar[1]=one # 1 unquoted literal"
echo "ArrayVar[2]='two' # 2 normal"
echo "ArrayVar[3]='three' # 3 normal"
echo "ArrayVar[4]='I am four' # 4 normal with spaces"
echo "ArrayVar[5]='five' # 5 normal"
echo "unset ArrayVar[6] # 6 undefined"
echo "ArrayValue[7]='seven' # 7 normal"
echo "ArrayValue[8]='' # 8 defined but empty"
echo "ArrayValue[9]='nine' # 9 normal"
echo
echo
echo '---Case0: No double-quotes, Default IFS of space,tab,newline ---'
IFS=$'\x20'$'\x09'$'\x0A' # In exactly this order.
echo 'Here is: printf %q {${ArrayVar[*]}'
printf %q ${ArrayVar[*]}
echo
echo 'Here is: printf %q {${ArrayVar[@]}'
printf %q ${ArrayVar[@]}
echo
echo 'Here is: echo ${ArrayVar[*]}'
echo ${ArrayVar[@]}
echo 'Here is: echo {${ArrayVar[@]}'
echo ${ArrayVar[@]}
echo
echo '---Case1: Within double-quotes - Default IFS of space-tab-
newline ---'
IFS=$'\x20'$'\x09'$'\x0A' # These three bytes,
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case2: Within double-quotes - IFS is q'
IFS='q'
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case3: Within double-quotes - IFS is ^'
IFS='^'
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case4: Within double-quotes - IFS is ^ followed by
space,tab,newline'
IFS=$'^'$'\x20'$'\x09'$'\x0A' # ^ + space tab newline
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case6: Within double-quotes - IFS set and empty '
IFS=''
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case7: Within double-quotes - IFS is unset'
unset IFS
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---End of Cases---'
echo "========================================================="; echo
Put IFS back to the default.
Default is exactly these three bytes.
IFS=$'\x20'$'\x09'$'\x0A' # In exactly this order.
Interpretation of the above outputs:
A Glob-Pattern is I/O; the setting of IFS matters.
An All-Elements-Of does not consider IFS settings.
Note the different output using the echo command and the
- quoted format operator of the printf command.
Recall:
Parameters are similar to arrays and have the similar behaviors.
The above examples demonstrate the possible variations.
To retain the shape of a sparse array, additional script
The source code of Bash has a routine to output the
- [subscript]=value array assignment format.
As of version 2.05b, that routine is not used,
- but that might change in future releases.
The length of a string, measured in non-null elements (characters):
echo
echo '- - Non-quoted references - -'
echo 'Non-Null character count: '${#VarSomething}' characters.'
test='Lit'$'\x00''eral' # $'\x00' is a null character.
echo ${#test} # See that?
The length of an array, measured in defined elements,
- including null content elements.
echo
echo 'Defined content count: '${#ArrayVar[@]}' elements.'
That is NOT the maximum subscript (4).
That is NOT the range of the subscripts (1 . . 4 inclusive).
It IS the length of the linked list.
Both the maximum subscript and the range of the subscripts may
- be found with additional script programming.
The length of a string, measured in non-null elements (characters):
echo
echo '- - Quoted, Glob-Pattern references - -'
echo 'Non-Null character count: '"${#VarSomething}"' characters.'
The length of an array, measured in defined elements,
- including null-content elements.
echo
echo 'Defined element count: '"${#ArrayVar[*]}"' elements.'
Interpretation: Substitution does not effect the ${# ... } operation.
Suggestion:
Always use the All-Elements-Of character
- if that is what is intended (independence from IFS).
Define a simple function.
I include an underscore in the name
- to make it distinctive in the examples below.
Bash separates variable names and function names
The Mark-One eyeball isn't that advanced.
_simple() {
echo -n 'SimpleFunc'$@ # Newlines are swallowed in
} #+ result returned in any case.
The ( ... ) notation invokes a command or function.
The $( ... ) notation is pronounced: Result-Of.
Invoke the function _simple
echo
echo '- - Output of function _simple - -'
_simple # Try passing arguments.
echo
or
(_simple) # Try passing arguments.
echo
echo '- Is there a variable of that name? -'
echo $_simple not defined # No variable by that name.
Invoke the result of function _simple (Error msg intended)
$(_simple) # Gives an error message:
line 436: SimpleFunc: command not found
echo
The first word of the result of function _simple
- is neither a valid Bash command nor the name of a defined function.
This demonstrates that the output of _simple is subject to evaluation.
Interpretation:
A function can be used to generate in-line Bash commands.
A simple function where the first word of result IS a bash command:
_print() {
echo -n 'printf %q '$@
}
echo '- - Outputs of function _print - -'
_print parm1 parm2 # An Output NOT A Command.
echo
$(_print parm1 parm2) # Executes: printf %q parm1 parm2
# See above IFS examples for the
#+ various possibilities.
echo
$(_print $VarSomething) # The predictable result.
echo
Function variables
echo
echo '- - Function variables - -'
A variable may represent a signed integer, a string or an array.
A string may be used like a function name with optional arguments.
set -vx # Enable if desired
declare -f funcVar #+ in namespace of functions
funcVar=_print # Contains name of function.
$funcVar parm1 # Same as _print at this point.
echo
funcVar=$(_print ) # Contains result of function.
$funcVar # No input, No output.
$funcVar $VarSomething # The predictable result.
echo
funcVar=$(_print $VarSomething) # $VarSomething replaced HERE.
$funcVar # The expansion is part of the
echo #+ variable contents.
funcVar="$(_print $VarSomething)" # $VarSomething replaced HERE.
$funcVar # The expansion is part of the
echo #+ variable contents.
The difference between the unquoted and the double-quoted versions
- above can be seen in the "protect_literal.sh" example.
The first case above is processed as two, unquoted, Bash-Words.
The second case above is processed as one, quoted, Bash-Word.
Delayed replacement
echo
echo '- - Delayed replacement - -'
funcVar="$(_print '$VarSomething')" # No replacement, single Bash-Word.
eval $funcVar # $VarSomething replaced HERE.
echo
VarSomething='NewThing'
eval $funcVar # $VarSomething replaced HERE.
echo
Restore the original setting trashed above.
VarSomething=Literal
There are a pair of functions demonstrated in the
- "protect_literal.sh" and "unprotect_literal.sh" examples.
These are general purpose functions for delayed replacement literals
REVIEW:
A string can be considered a Classic-Array of elements (characters).
A string operation applies to all elements (characters) of the string
The notation: ${array_name[@]} represents all elements of the
The Extended-Syntax string operations can be applied to all
This may be thought of as a For-Each operation on a vector of strings.
Parameters are similar to an array.
The initialization of a parameter array for a script
- and a parameter array for a function only differ
- in the initialization of ${0}, which never changes its setting.
Subscript zero of the script's parameter array contains
Subscript zero of a function's parameter array DOES NOT contain
- the name of the function.
The name of the current function is accessed by the $FUNCNAME variable.
A quick, review list follows (quick, not short).
echo
echo '- - Test (but not change) - -'
echo '- null reference -'
echo -n ${VarNull-'NotSet'}' ' # NotSet
echo ${VarNull} # NewLine only
echo -n ${VarNull:-'NotSet'}' ' # NotSet
echo ${VarNull} # Newline only
echo '- null contents -'
echo -n ${VarEmpty-'Empty'}' ' # Only the space
echo ${VarEmpty} # Newline only
echo -n ${VarEmpty:-'Empty'}' ' # Empty
echo ${VarEmpty} # Newline only
echo '- contents -'
echo ${VarSomething-'Content'} # Literal
echo ${VarSomething:-'Content'} # Literal
echo '- Sparse Array -'
echo ${ArrayVar[@]-'not set'}
ASCII-Art time
State Y==yes, N==no
Unset Y Y ${# ... } == 0
Empty N Y ${# ... } == 0
Contents N N ${# ... } > 0
Either the first and/or the second part of the tests
- may be a command or a function invocation string.
echo
echo '- - Test 1 for undefined - -'
declare -i t
_decT() {
t=$t-1
}
Null reference, set: t == -1
t=${#VarNull} # Results in zero.
${VarNull- _decT } # Function executes, t now -1.
echo $t
Null contents, set: t == 0
t=${#VarEmpty} # Results in zero.
${VarEmpty- _decT } # _decT function NOT executed.
echo $t
Contents, set: t == number of non-null characters
VarSomething='_simple' # Set to valid function name.
t=${#VarSomething} # non-zero length
${VarSomething- _decT } # Function _simple executed.
echo $t # Note the Append-To action.
Exercise: clean up that example.
unset t
unset _decT
VarSomething=Literal
echo
echo '- - Test and Change - -'
echo '- Assignment if null reference -'
echo -n ${VarNull='NotSet'}' ' # NotSet NotSet
echo ${VarNull}
unset VarNull
echo '- Assignment if null reference -'
echo -n ${VarNull:='NotSet'}' ' # NotSet NotSet
echo ${VarNull}
unset VarNull
echo '- No assignment if null contents -'
echo -n ${VarEmpty='Empty'}' ' # Space only
echo ${VarEmpty}
VarEmpty=''
echo '- Assignment if null contents -'
echo -n ${VarEmpty:='Empty'}' ' # Empty Empty
echo ${VarEmpty}
VarEmpty=''
echo '- No change if already has contents -'
echo ${VarSomething='Content'} # Literal
echo ${VarSomething:='Content'} # Literal
"Subscript sparse" Bash-Arrays
Bash-Arrays are subscript packed, beginning with
- subscript zero unless otherwise specified.
The initialization of ArrayVar was one way
- to "otherwise specify". Here is the other way:
echo
declare -a ArraySparse
ArraySparse=( [1]=one [2]='' [4]='four' )
[0]=null reference, [2]=null content, [3]=null reference
echo '- - Array-Sparse List - -'
Within double-quotes, default IFS, Glob-Pattern
IFS=$'\x20'$'\x09'$'\x0A'
printf %q "${ArraySparse[*]}"
echo
Note that the output does not distinguish between "null content"
Both print as escaped whitespace.
Note also that the output does NOT contain escaped whitespace
- for the "null reference(s)" prior to the first defined element.
This behavior of 2.04, 2.05a and 2.05b has been reported
- and may change in a future version of Bash.
To output a sparse array and maintain the [subscript]=value
- relationship without change requires a bit of programming.
One possible code fragment:
local l=${#ArraySparse[@]} # Count of defined elements
local f=0 # Count of found subscripts
local i=0 # Subscript to test
( # Anonymous in-line function
for (( l=${#ArraySparse[@]}, f = 0, i = 0 ; f < l ; i++ ))
do
# 'if defined then...'
${ArraySparse[$i]+ eval echo '\ ['$i']='${ArraySparse[$i]} ; (( f++ ))
}
done
)
The reader coming upon the above code fragment cold
- might want to review "command lists" and "multiple commands on a line"
- in the text of the foregoing "Advanced Bash Scripting Guide."
Note:
The "read -a array_name" version of the "read" command
- begins filling array_name at subscript zero.
ArraySparse does not define a value at subscript zero.
The user needing to read/write a sparse array to either
- external storage or a communications socket must invent
- a read/write code pair suitable for their purpose.
Exercise: clean it up.
unset ArraySparse
echo
echo '- - Conditional alternate (But not change)- -'
echo '- No alternate if null reference -'
echo -n ${VarNull+'NotSet'}' '
echo ${VarNull}
unset VarNull
echo '- No alternate if null reference -'
echo -n ${VarNull:+'NotSet'}' '
echo ${VarNull}
unset VarNull
echo '- Alternate if null contents -'
echo -n ${VarEmpty+'Empty'}' ' # Empty
echo ${VarEmpty}
VarEmpty=''
echo '- No alternate if null contents -'
echo -n ${VarEmpty:+'Empty'}' ' # Space only
echo ${VarEmpty}
VarEmpty=''
echo '- Alternate if already has contents -'
Alternate literal
echo -n ${VarSomething+'Content'}' ' # Content Literal
echo ${VarSomething}
Invoke function
echo -n ${VarSomething:+ $(_simple) }' ' # SimpleFunc Literal
echo ${VarSomething}
echo
echo '- - Sparse Array - -'
echo ${ArrayVar[@]+'Empty'} # An array of 'Empty'(ies)
echo
echo '- - Test 2 for undefined - -'
declare -i t
_incT() {
t=$t+1
}
Note:
This is the same test used in the sparse array
Null reference, set: t == -1
t=${#VarNull}-1 # Results in minus-one.
${VarNull+ _incT } # Does not execute.
echo $t' Null reference'
Null contents, set: t == 0
t=${#VarEmpty}-1 # Results in minus-one.
${VarEmpty+ _incT } # Executes.
echo $t' Null content'
Contents, set: t == (number of non-null characters)
t=${#VarSomething}-1 # non-null length minus-one
${VarSomething+ _incT } # Executes.
echo $t' Contents'
Exercise: clean up that example.
unset t
unset _incT
${name?err_msg} ${name:?err_msg}
These follow the same rules but always exit afterwards
- if an action is specified following the question mark.
The action following the question mark may be a literal
${name?} ${name:?} are test-only, the return can be tested.
Element operations
echo
echo '- - Trailing sub-element selection - -'
Strings, Arrays and Positional parameters
Call this script with multiple arguments
- to see the parameter selections.
echo '- All -'
echo ${VarSomething:0} # all non-null characters
echo ${ArrayVar[@]:0} # all elements with content
echo ${@:0} # all parameters with content;
# ignoring parameter[0]
echo
echo '- All after -'
echo ${VarSomething:1} # all non-null after character[0]
echo ${ArrayVar[@]:1} # all after element[0] with content
echo ${@:2} # all after param[1] with content
echo
echo '- Range after -'
echo ${VarSomething:4:3} # ral
# Three characters after
# character[3]
echo '- Sparse array gotch -'
echo ${ArrayVar[@]:1:2} # four - The only element with content.
# Two elements after (if that many exist).
# the FIRST WITH CONTENTS
#+ (the FIRST WITH CONTENTS is being
#+ considered as if it
#+ were subscript zero).
Executed as if Bash considers ONLY array elements with CONTENT
printf %q "${ArrayVar[@]:0:3}" # Try this one
In versions 2.04, 2.05a and 2.05b,
- Bash does not handle sparse arrays as expected using this notation.
The current Bash maintainer, Chet Ramey, has corrected this.
echo '- Non-sparse array -'
echo ${@:2:2} # Two parameters following parameter[1]
New victims for string vector examples:
stringZ=abcABC123ABCabc
arrayZ=( abcabc ABCABC 123123 ABCABC abcabc )
sparseZ=( [1]='abcabc' [3]='ABCABC' [4]='' [5]='123123' )
echo
echo ' - - Victim string - -'$stringZ'- - '
echo ' - - Victim array - -'${arrayZ[@]}'- - '
echo ' - - Sparse array - -'${sparseZ[@]}'- - '
echo ' - [0]==null ref, [2]==null ref, [4]==null content - '
echo ' - [1]=abcabc [3]=ABCABC [5]=123123 - '
echo ' - non-null-reference count: '${#sparseZ[@]}' elements'
echo
echo '- - Prefix sub-element removal - -'
echo '- - Glob-Pattern match must include the first character. - -'
echo '- - Glob-Pattern may be a literal or a function result. - -'
echo
Function returning a simple, Literal, Glob-Pattern
_abc() {
echo -n 'abc'
}
echo '- Shortest prefix -'
echo ${stringZ#123} # Unchanged (not a prefix).
echo ${stringZ#$(_abc)} # ABC123ABCabc
echo ${arrayZ[@]#abc} # Applied to each element.
echo ${sparseZ[@]#abc} # Version-2.05b core dumps.
Has since been fixed by Chet Ramey.
The -it would be nice- First-Subscript-Of
echo ${#sparseZ[@]#*} # This is NOT valid Bash.
echo
echo '- Longest prefix -'
echo ${stringZ##1*3} # Unchanged (not a prefix)
echo ${stringZ##a*C} # abc
echo ${arrayZ[@]##a*c} # ABCABC 123123 ABCABC
echo ${sparseZ[@]##a*c} # Version-2.05b core dumps.
Has since been fixed by Chet Ramey.
echo
echo '- - Suffix sub-element removal - -'
echo '- - Glob-Pattern match must include the last character. - -'
echo '- - Glob-Pattern may be a literal or a function result. - -'
echo
echo '- Shortest suffix -'
echo ${stringZ%1*3} # Unchanged (not a suffix).
echo ${stringZ%$(_abc)} # abcABC123ABC
echo ${arrayZ[@]%abc} # Applied to each element.
echo ${sparseZ[@]%abc} # Version-2.05b core dumps.
Has since been fixed by Chet Ramey.
The -it would be nice- Last-Subscript-Of
echo ${#sparseZ[@]%*} # This is NOT valid Bash.
echo
echo '- Longest suffix -'
echo ${stringZ%%1*3} # Unchanged (not a suffix)
echo ${stringZ%%b*c} # a
echo ${arrayZ[@]%%b*c} # a ABCABC 123123 ABCABC a
echo ${sparseZ[@]%%b*c} # Version-2.05b core dumps.
Has since been fixed by Chet Ramey.
echo
echo '- - Sub-element replacement - -'
echo '- - Sub-element at any location in string. - -'
echo '- - First specification is a Glob-Pattern - -'
echo '- - Glob-Pattern may be a literal or Glob-Pattern function result. - -'
echo '- - Second specification may be a literal or function result. - -'
echo '- - Second specification may be unspecified. Pronounce that'
echo ' as: Replace-With-Nothing (Delete) - -'
echo
Function returning a simple, Literal, Glob-Pattern
_123() {
echo -n '123'
}
echo '- Replace first occurrence -'
echo ${stringZ/$(_123)/999} # Changed (123 is a component).
echo ${stringZ/ABC/xyz} # xyzABC123ABCabc
echo ${arrayZ[@]/ABC/xyz} # Applied to each element.
echo ${sparseZ[@]/ABC/xyz} # Works as expected.
echo
echo '- Delete first occurrence -'
echo ${stringZ/$(_123)/}
echo ${stringZ/ABC/}
echo ${arrayZ[@]/ABC/}
echo ${sparseZ[@]/ABC/}
The replacement need not be a literal,
- since the result of a function invocation is allowed.
This is general to all forms of replacement.
echo
echo '- Replace first occurrence with Result-Of -'
echo ${stringZ/$(_123)/$(_simple)} # Works as expected.
echo ${arrayZ[@]/ca/$(_simple)} # Applied to each element.
echo ${sparseZ[@]/ca/$(_simple)} # Works as expected.
echo
echo '- Replace all occurrences -'
echo ${stringZ//[b2]/X} # X-out b's and 2's
echo ${stringZ//abc/xyz} # xyzABC123ABCxyz
echo ${arrayZ[@]//abc/xyz} # Applied to each element.
echo ${sparseZ[@]//abc/xyz} # Works as expected.
echo
echo '- Delete all occurrences -'
echo ${stringZ//[b2]/}
echo ${stringZ//abc/}
echo ${arrayZ[@]//abc/}
echo ${sparseZ[@]//abc/}
echo
echo '- - Prefix sub-element replacement - -'
echo '- - Match must include the first character. - -'
echo
echo '- Replace prefix occurrences -'
echo ${stringZ/#[b2]/X} # Unchanged (neither is a prefix).
echo ${stringZ/#$(_abc)/XYZ} # XYZABC123ABCabc
echo ${arrayZ[@]/#abc/XYZ} # Applied to each element.
echo ${sparseZ[@]/#abc/XYZ} # Works as expected.
echo
echo '- Delete prefix occurrences -'
echo ${stringZ/#[b2]/}
echo ${stringZ/#$(_abc)/}
echo ${arrayZ[@]/#abc/}
echo ${sparseZ[@]/#abc/}
echo
echo '- - Suffix sub-element replacement - -'
echo '- - Match must include the last character. - -'
echo
echo '- Replace suffix occurrences -'
echo ${stringZ/%[b2]/X} # Unchanged (neither is a suffix).
echo ${stringZ/%$(_abc)/XYZ} # abcABC123ABCXYZ
echo ${arrayZ[@]/%abc/XYZ} # Applied to each element.
echo ${sparseZ[@]/%abc/XYZ} # Works as expected.
echo
echo '- Delete suffix occurrences -'
echo ${stringZ/%[b2]/}
echo ${stringZ/%$(_abc)/}
echo ${arrayZ[@]/%abc/}
echo ${sparseZ[@]/%abc/}
echo
echo '- - Special cases of null Glob-Pattern - -'
echo
echo '- Prefix all -'
null substring pattern means 'prefix'
echo ${stringZ/#/NEW} # NEWabcABC123ABCabc
echo ${arrayZ[@]/#/NEW} # Applied to each element.
echo ${sparseZ[@]/#/NEW} # Applied to null-content also.
# That seems reasonable.
echo
echo '- Suffix all -'
null substring pattern means 'suffix'
echo ${stringZ/%/NEW} # abcABC123ABCabcNEW
echo ${arrayZ[@]/%/NEW} # Applied to each element.
echo ${sparseZ[@]/%/NEW} # Applied to null-content also.
# That seems reasonable.
echo
echo '- - Special case For-Each Glob-Pattern - -'
echo '- - - - This is a nice-to-have dream - - - -'
echo
_GenFunc() {
echo -n ${0} # Illustration only.
# Actually, that would be an arbitrary computation.
}
All occurrences, matching the AnyThing pattern.
Currently //*/ does not match null-content nor null-reference.
/#/ and /%/ does match null-content but not null-reference.
echo ${sparseZ[@]//*/$(_GenFunc)}
A possible syntax would be to make
- the parameter notation used within this construct mean:
${1} - The full element
${2} - The prefix, if any, to the matched sub-element
${3} - The matched sub-element
${4} - The suffix, if any, to the matched sub-element
echo ${sparseZ[@]//*/$(_GenFunc ${3})} # Same as ${1} here.
Perhaps it will be implemented in a future version of Bash.
exit 0
Example A-59. Testing execution times of various commands
!/bin/bash
test-execution-time.sh
Example by Erik Brandsberg, for testing execution time
Referenced in the "Optimizations" section of "Miscellany" chapter.
count=50000
echo "Math tests"
echo "Math via $(( ))"
time for (( i=0; i< $count; i++))
do
result=$(( $i%2 ))
done
echo "Math via expr:"
time for (( i=0; i< $count; i++))
do
result=expr "$i%2"
done
echo "Math via let:"
time for (( i=0; i< $count; i++))
do
let result=$i%2
done
echo
echo "Conditional testing tests"
echo "Test via case:"
time for (( i=0; i< $count; i++))
do
case $(( $i%2 )) in
0) : ;;
1) : ;;
esac
done
echo "Test with if [], no quotes:"
time for (( i=0; i< $count; i++))
do
if [ $(( $i%2 )) = 0 ]; then
:
else
:
fi
done
echo "Test with if [], quotes:"
time for (( i=0; i< $count; i++))
do
if [ "$(( $i%2 ))" = "0" ]; then
:
else
:
fi
done
echo "Test with if [], using -eq:"
time for (( i=0; i< $count; i++))
do
if [ $(( $i%2 )) -eq 0 ]; then
:
else
:
fi
done
exit $?
Example A-60. Associative arrays vs. conventional arrays (execution
times)
!/bin/bash
assoc-arr-test.sh
Benchmark test script to compare execution times of
numeric-indexed array vs. associative array.
Thank you, Erik Brandsberg.
count=100000 # May take a while for some of the tests below.
declare simple # Can change to 20000, if desired.
declare -a array1
declare -A array2
declare -a array3
declare -A array4
echo "===Assignment tests==="
echo
echo "Assigning a simple variable:"
References $i twice to equalize lookup times.
time for (( i=0; i< $count; i++)); do
simple=$i$i
done
echo "---"
echo "Assigning a numeric index array entry:"
time for (( i=0; i< $count; i++)); do
array1[$i]=$i
done
echo "---"
echo "Overwriting a numeric index array entry:"
time for (( i=0; i< $count; i++)); do
array1[$i]=$i
done
echo "---"
echo "Linear reading of numeric index array:"
time for (( i=0; i< $count; i++)); do
simple=array1[$i]
done
echo "---"
echo "Assigning an associative array entry:"
time for (( i=0; i< $count; i++)); do
array2[$i]=$i
done
echo "---"
echo "Overwriting an associative array entry:"
time for (( i=0; i< $count; i++)); do
array2[$i]=$i
done
echo "---"
echo "Linear reading an associative array entry:"
time for (( i=0; i< $count; i++)); do
simple=array2[$i]
done
echo "---"
echo "Assigning a random number to a simple variable:"
time for (( i=0; i< $count; i++)); do
simple=$RANDOM
done
echo "---"
echo "Assign a sparse numeric index array entry randomly into 64k cells:"
time for (( i=0; i< $count; i++)); do
array3[$RANDOM]=$i
done
echo "---"
echo "Reading sparse numeric index array entry:"
time for value in "${array3[@]}"i; do
simple=$value
done
echo "---"
echo "Assigning a sparse associative array entry randomly into 64k cells:"
time for (( i=0; i< $count; i++)); do
array4[$RANDOM]=$i
done
echo "---"
echo "Reading sparse associative index array entry:"
time for value in "${array4[@]}"; do
simple=$value
done
exit $?
________________________________________________________________
Appendix B. Reference Cards
The following reference cards provide a useful summary of certain
scripting concepts. The foregoing text treats these matters in more
depth, as well as giving usage examples.
Table B-1. Special Shell Variables
Variable Meaning
$0 Filename of script
$1 Positional parameter #1
$2 - $9 Positional parameters #2 - #9
${10} Positional parameter #10
$# Number of positional parameters
"$*" All the positional parameters (as a single word) *
"$@" All the positional parameters (as separate strings)
${#*} Number of positional parameters
${#@} Number of positional parameters
$? Return value
$$ Process ID (PID) of script
$- Flags passed to script (using set)
$_ Last argument of previous command
$! Process ID (PID) of last job run in background
- Must be quoted, otherwise it defaults to $@.
Table B-2. TEST Operators: Binary Comparison
Operator Meaning ----- Operator Meaning
Arithmetic Comparison String Comparison
-eq Equal to = Equal to
== Equal to
-ne Not equal to != Not equal to
-lt Less than < Less than (ASCII) *
-le Less than or equal to
-gt Greater than > Greater than (ASCII) *
-ge Greater than or equal to
-z String is empty
-n String is not empty
Arithmetic Comparison within double parentheses (( ... ))
Greater than
= Greater than or equal to
< Less than
<= Less than or equal to
- If within a double-bracket [[ ... ]] test construct, then no escape
\ is needed.
Table B-3. TEST Operators: Files
Operator Tests Whether ----- Operator Tests Whether
-e File exists -s File is not zero size
-f File is a regular file
-d File is a directory -r File has read permission
-h File is a symbolic link -w File has write permission
-L File is a symbolic link -x File has execute permission
-b File is a block device
-c File is a character device -g sgid flag set
-p File is a pipe -u suid flag set
-S File is a socket -k "sticky bit" set
-t File is associated with a terminal
-N File modified since it was last read F1 -nt F2 File F1 is newer
than F2 *
-O You own the file F1 -ot F2 File F1 is older than F2 *
-G Group id of file same as yours F1 -ef F2 Files F1 and F2 are
hard links to the same file *
! NOT (inverts sense of above tests)
- Binary operator (requires two operands).
Table B-4. Parameter Substitution and Expansion
Expression Meaning
${var} Value of var (same as $var)
${var-$DEFAULT} If var not set, evaluate expression as $DEFAULT *
${var:-$DEFAULT} If var not set or is empty, evaluate expression as
$DEFAULT *
${var=$DEFAULT} If var not set, evaluate expression as $DEFAULT *
${var:=$DEFAULT} If var not set or is empty, evaluate expression as
$DEFAULT *
${var+$OTHER} If var set, evaluate expression as $OTHER, otherwise as
null string
${var:+$OTHER} If var set, evaluate expression as $OTHER, otherwise
as null string
${var?$ERR_MSG} If var not set, print $ERR_MSG and abort script with
an exit status of 1.*
${var:?$ERR_MSG} If var not set, print $ERR_MSG and abort script with
an exit status of 1.*
${!varprefix*} Matches all previously declared variables beginning
with varprefix
${!varprefix@} Matches all previously declared variables beginning
with varprefix
- If var is set, evaluate the expression as $var with no
side-effects.
Note that some of the above behavior of operators has changed from
earlier versions of Bash.
Table B-5. String Operations
Expression Meaning
${#string} Length of $string
${string:position} Extract substring from $string at $position
${string:position:length} Extract $length characters substring from
$string at $position [zero-indexed, first character is at position 0]
${string#substring} Strip shortest match of $substring from front of
$string
${string##substring} Strip longest match of $substring from front of
$string
${string%substring} Strip shortest match of $substring from back of
$string
${string%%substring} Strip longest match of $substring from back of
$string
${string/substring/replacement} Replace first match of $substring
with $replacement
${string//substring/replacement} Replace all matches of $substring
with $replacement
${string/#substring/replacement} If $substring matches front end of
$string, substitute $replacement for $substring
${string/%substring/replacement} If $substring matches back end of
$string, substitute $replacement for $substring
expr match "$string" '$substring' Length of matching $substring* at
beginning of $string
expr "$string" : '$substring' Length of matching $substring* at
beginning of $string
expr index "$string" $substring Numerical position in $string of
first character in $substring* that matches [0 if no match, first
character counts as position 1]
expr substr $string $position $length Extract $length characters from
$string starting at $position [0 if no match, first character counts
as position 1]
expr match "$string" '($substring)' Extract $substring*, searching
from beginning of $string
expr "$string" : '($substring)' Extract $substring* , searching
from beginning of $string
expr match "$string" '.($substring)' Extract $substring,
searching from end of $string
expr "$string" : '.($substring)' Extract $substring, searching
from end of $string
- Where $substring is a Regular Expression.
Table B-6. Miscellaneous Constructs
Expression Interpretation
Brackets
if [ CONDITION ] Test construct
if [[ CONDITION ]] Extended test construct
Array[1]=element1 Array initialization
[a-z] Range of characters within a Regular Expression
Curly Brackets
${variable} Parameter substitution
${!variable} Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...} Brace expansion
{a..z} Extended brace expansion
{} Text replacement, after find and xargs
Parentheses
( command1; command2 ) Command group executed within a subshell
Array=(element1 element2 element3) Array initialization
result=$(COMMAND) Command substitution, new style
(COMMAND) Process substitution
<(COMMAND) Process substitution
Double Parentheses
(( var = 78 )) Integer arithmetic
var=$(( 20 + 5 )) Integer arithmetic, with variable assignment
(( var++ )) C-style variable increment
(( var-- )) C-style variable decrement
(( var0 = var1<98?9:21 )) C-style ternary operation
Quoting
"$variable" "Weak" quoting
'string' 'Strong' quoting
Back Quotes
result=COMMAND Command substitution, classic style
________________________________________________________________
Appendix C. A Sed and Awk Micro-Primer
This is a very brief introduction to the sed and awk text processing
utilities. We will deal with only a few basic commands here, but that
will suffice for understanding simple sed and awk constructs within
shell scripts.
sed: a non-interactive text file editor
awk: a field-oriented pattern processing language with a C-style
syntax
For all their differences, the two utilities share a similar
invocation syntax, use regular expressions , read input by default
from stdin, and output to stdout. These are well-behaved UNIX tools,
and they work together well. The output from one can be piped to the
other, and their combined capabilities give shell scripts some of the
power of Perl.
Note
One important difference between the utilities is that while shell
scripts can easily pass arguments to sed, it is more cumbersome for
awk (see Example 36-5 and Example 28-2).
________________________________________________________________
C.1. Sed
Sed is a non-interactive [141] stream editor. It receives text input,
whether from stdin or from a file, performs certain operations on
specified lines of the input, one line at a time, then outputs the
result to stdout or to a file. Within a shell script, sed is usually
one of several tool components in a pipe.
Sed determines which lines of its input that it will operate on from
the address range passed to it. [142] Specify this address range
either by line number or by a pattern to match. For example, 3d
signals sed to delete line 3 of the input, and /Windows/d tells sed
that you want every line of the input containing a match to "Windows"
deleted.
Of all the operations in the sed toolkit, we will focus primarily on
the three most commonly used ones. These are printing (to stdout),
deletion, and substitution.
Table C-1. Basic sed operators
Operator Name Effect
[address-range]/p print Print [specified address range]
[address-range]/d delete Delete [specified address range]
s/pattern1/pattern2/ substitute Substitute pattern2 for first
instance of pattern1 in a line
[address-range]/s/pattern1/pattern2/ substitute Substitute pattern2
for first instance of pattern1 in a line, over address-range
[address-range]/y/pattern1/pattern2/ transform replace any character
in pattern1 with the corresponding character in pattern2, over
address-range (equivalent of tr)
[address] i pattern Filename insert Insert pattern at address
indicated in file Filename. Usually used with -i in-place option.
g global Operate on every pattern match within each matched line of
input
Note
Unless the g (global) operator is appended to a substitute command,
the substitution operates only on the first instance of a pattern
match within each line.
From the command-line and in a shell script, a sed operation may
require quoting and certain options.
sed -e '/^$/d' $filename
The -e option causes the next string to be interpreted as an editing instruc
tion.
(If passing only a single instruction to sed, the "-e" is optional.)
The "strong" quotes ('') protect the RE characters in the instruction
- from reinterpretation as special characters by the body of the script.
(This reserves RE expansion of the instruction for sed.)
Operates on the text contained in file $filename.
In certain cases, a sed editing command will not work with single
quotes.
filename=file1.txt
pattern=BEGIN
sed "/^$pattern/d" "$filename" # Works as specified.
sed '/^$pattern/d' "$filename" has unexpected results.
In this instance, with strong quoting (' ... '),
Note
Sed uses the -e option to specify that the following string is an
instruction or set of instructions. If there is only a single
instruction contained in the string, then this may be omitted.
sed -n '/xzy/p' $filename
The -n option tells sed to print only those lines matching the pattern.
Otherwise all input lines would print.
The -e option not necessary here since there is only a single editing instru
ction.
Table C-2. Examples of sed operators
Notation Effect
8d Delete 8th line of input.
/^$/d Delete all blank lines.
1,/^$/d Delete from beginning of input up to, and including first
blank line.
/Jones/p Print only lines containing "Jones" (with -n option).
s/Windows/Linux/ Substitute "Linux" for first instance of "Windows"
found in each input line.
s/BSOD/stability/g Substitute "stability" for every instance of
"BSOD" found in each input line.
s/ *$// Delete all spaces at the end of every line.
s/00*/0/g Compress all consecutive sequences of zeroes into a single
zero.
echo "Working on it." | sed -e '1i How far are you along?' Prints
"How far are you along?" as first line, "Working on it" as second.
5i 'Linux is great.' file.txt Inserts 'Linux is great.' at line 5 of
the file file.txt.
/GUI/d Delete all lines containing "GUI".
s/GUI//g Delete all instances of "GUI", leaving the remainder of each
line intact.
Substituting a zero-length string for another is equivalent to
deleting that string within a line of input. This leaves the
remainder of the line intact. Applying s/GUI// to the line
The most important parts of any application are its GUI and sound effects
results in
The most important parts of any application are its and sound effects
A backslash forces the sed replacement command to continue on to the
next line. This has the effect of using the newline at the end of the
first line as the replacement string.
s/^ */\
/g
This substitution replaces line-beginning spaces with a newline. The
net result is to replace paragraph indents with a blank line between
paragraphs.
An address range followed by one or more operations may require open
and closed curly brackets, with appropriate newlines.
/[0-9A-Za-z]/,/^$/{
/^$/d
}
This deletes only the first of each set of consecutive blank lines.
That might be useful for single-spacing a text file, but retaining
the blank line(s) between paragraphs.
Note
The usual delimiter that sed uses is /. However, sed allows other
delimiters, such as %. This is useful when / is part of a replacement
string, as in a file pathname. See Example 11-10 and Example 16-32.
Tip
A quick way to double-space a text file is sed G filename.
For illustrative examples of sed within shell scripts, see:
1. Example 36-1
2. Example 36-2
3. Example 16-3
4. Example A-2
5. Example 16-17
6. Example 16-27
7. Example A-12
8. Example A-16
9. Example A-17
- Example 16-32
- Example 11-10
- Example 16-48
- Example A-1
- Example 16-14
- Example 16-12
- Example A-10
- Example 19-12
- Example 16-19
- Example A-29
- Example A-31
- Example A-24
- Example A-43
- Example A-55
For a more extensive treatment of sed, refer to the pertinent
references in the Bibliography.
________________________________________________________________
C.2. Awk
Awk [143] is a full-featured text processing language with a syntax
reminiscent of C. While it possesses an extensive set of operators
and capabilities, we will cover only a few of these here - the ones
most useful in shell scripts.
Awk breaks each line of input passed to it into fields. By default, a
field is a string of consecutive characters delimited by whitespace,
though there are options for changing this. Awk parses and operates
on each separate field. This makes it ideal for handling structured
text files -- especially tables -- data organized into consistent
chunks, such as rows and columns.
Strong quoting and curly brackets enclose blocks of awk code within a
shell script.
$1 is field #1, $2 is field #2, etc.
echo one two | awk '{print $1}'
one
echo one two | awk '{print $2}'
two
But what is field #0 ($0)?
echo one two | awk '{print $0}'
one two
All the fields!
awk '{print $3}' $filename
Prints field #3 of file $filename to stdout.
awk '{print $1 $5 $6}' $filename
Prints fields #1, #5, and #6 of file $filename.
awk '{print $0}' $filename
Prints the entire file!
Same effect as: cat $filename . . . or . . . sed '' $filename
We have just seen the awk print command in action. The only other
feature of awk we need to deal with here is variables. Awk handles
variables similarly to shell scripts, though a bit more flexibly.
{ total += ${column_number} }
This adds the value of column_number to the running total of total>.
Finally, to print "total", there is an END command block, executed
after the script has processed all its input.
END { print total }
Corresponding to the END, there is a BEGIN, for a code block to be
performed before awk starts processing its input.
The following example illustrates how awk can add text-parsing tools
to a shell script.
Example C-1. Counting Letter Occurrences
! /bin/sh
letter-count2.sh: Counting letter occurrences in a text file.
Script by nyal [nyal@voila.fr].
Used in ABS Guide with permission.
Recommented and reformatted by ABS Guide author.
Version 1.1: Modified to work with gawk 3.1.3.
(Will still work with earlier versions.)
INIT_TAB_AWK=""
Parameter to initialize awk script.
count_case=0
FILE_PARSE=$1
E_PARAMERR=85
usage()
{
echo "Usage: letter-count.sh file letters" 2>&1
# For example: ./letter-count2.sh filename.txt a b c
exit $E_PARAMERR # Too few arguments passed to script.
}
if [ ! -f "$1" ] ; then
echo "$1: No such file." 2>&1
usage # Print usage message and exit.
fi
if [ -z "$2" ] ; then
echo "$2: No letters specified." 2>&1
usage
fi
shift # Letters specified.
for letter in echo $@ # For each one . . .
do
INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \
"$letter"; final_tab[${count_case}] = 0; "
Pass as parameter to awk script below.
count_case=expr $count_case + 1
done
DEBUG:
echo $INIT_TAB_AWK;
cat $FILE_PARSE |
Pipe the target file to the following awk script.
Earlier version of script:
awk -v tab_search=0 -v final_tab=0 -v tab=0 -v \
nb_letter=0 -v chara=0 -v chara2=0 \
awk \
"BEGIN { $INIT_TAB_AWK } \
{ split($0, tab, ""); \
for (chara in tab) \
{ for (chara2 in tab_search) \
{ if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \
END { for (chara in final_tab) \
{ print tab_search[chara] " => " final_tab[chara] } }"
Nothing all that complicated, just . . .
- for-loops, if-tests, and a couple of specialized functions.
exit $?
Compare this script to letter-count.sh.
For simpler examples of awk within shell scripts, see:
1. Example 15-14
2. Example 20-8
3. Example 16-32
4. Example 36-5
5. Example 28-2
6. Example 15-20
7. Example 29-3
8. Example 29-4
9. Example 11-3
- Example 16-61
- Example 9-16
- Example 16-4
- Example 10-6
- Example 36-19
- Example 11-9
- Example 36-4
- Example 16-53
- Example T-3
That's all the awk we'll cover here, folks, but there's lots more to
learn. See the appropriate references in the Bibliography.
________________________________________________________________
Appendix D. Parsing and Managing Pathnames
Emmanual Rouat contributed the following example of parsing and
transforming filenames and, in particular, pathnames. It draws
heavily on the functionality of sed.
!/usr/bin/env bash
Management of PATH, LD_LIBRARY_PATH, MANPATH variables...
By Emmanuel Rouat
(Inspired by the bash documentation 'pathfuncs' and on
discussions found on stackoverflow:
http://stackoverflow.com/questions/370047/
http://stackoverflow.com/questions/273909/#346860 )
Last modified: Sat Sep 22 12:01:55 CEST 2012
The following functions handle spaces correctly.
These functions belong in .bash_profile rather than in
.bashrc, I guess.
The modular aspect of these functions should make it easy
to expand them to handle path substitutions instead
of path removal etc....
See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/
(item 43) for an explanation of the 'duplicate-entries' removal
(it's a nice trick!)
Show $@ (usually PATH) as list.
function p_show() { local p="$@" && for p; do [[ ${!p} ]] &&
echo -e ${!p//:/\n}; done }
Filter out empty lines, multiple/trailing slashes, and duplicate entries.
function p_filter()
{ awk '/^[ \t]*$/ {next} {sub(//+$/, "");gsub(//+/, "/")}!x[$0]++' ;}
Rebuild list of items into ':' separated word (PATH-like).
function p_build() { paste -sd: ;}
Clean $1 (typically PATH) and rebuild it
function p_clean()
{ local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}
Remove $1 from $2 (found on stackoverflow, with modifications).
function p_rm()
{ local d=$(echo $1 | p_filter) p=${2} &&
eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}
Same as previous, but filters on a pattern (dangerous...
- don't use 'bin' or '/' as pattern!).
function p_rmpat()
{ local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |
p_filter | grep -v "${d}" | p_build)' ;}
Delete $1 from $2 and append it cleanly.
function p_append()
{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
eval ${p}='$(p_show ${p} d | p_build)' ;}
Delete $1 from $2 and prepend it cleanly.
function p_prepend()
{ local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
eval ${p}='$(p_show d ${p} | p_build)' ;}
Some tests:
echo
MYPATH="/bin:/usr/bin/:/bin://bin/"
p_append "/project//my project/bin" MYPATH
echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'"
echo "(result should be: /bin:/usr/bin:/project/my project/bin)"
echo $MYPATH
echo
MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin"
p_prepend "/project//my project/bin" MYOTHERPATH
echo "Prepend '/project//my project/bin' \
to '/bin:/usr/bin/:/bin:/project//my project/bin/'"
echo "(result should be: /project/my project/bin:/bin:/usr/bin)"
echo $MYOTHERPATH
echo
p_prepend "/project//my project/bin" FOOPATH # FOOPATH doesn't exist.
echo "Prepend '/project//my project/bin' to an unset variable"
echo "(result should be: /project/my project/bin)"
echo $FOOPATH
echo
BARPATH="/a:/b/://b c://a:/my local pub"
p_clean BARPATH
echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'"
echo "(result should be: /a:/b:/b c:/my local pub)"
echo $BARPATH
David Wheeler kindly permitted me to use his instructive examples.
Doing it correctly: A quick summary
by David Wheeler
http://www.dwheeler.com/essays/filenames-in-shell.html
So, how can you process filenames correctly in shell? Here's a quick
summary about how to do it correctly, for the impatient who "just want the
answer". In short: Double-quote to use "$variable" instead of $variable,
set IFS to just newline and tab, prefix all globs/filenames so they cannot
begin with "-" when expanded, and use one of a few templates that work
correctly. Here are some of those templates that work correctly:
IFS="$(printf '\n\t')"
Remove SPACE, so filenames with spaces work well.
Correct glob use:
#+ always use "for" loop, prefix glob, check for existence:
for file in ./* ; do # Use "./" ... NEVER bare "" ...
if [ -e "$file" ] ; then # Make sure it isn't an empty match.
COMMAND ... "$file" ...
fi
done
Correct glob use, but requires nonstandard bash extension.
shopt -s nullglob # Bash extension,
#+ so that empty glob matches will work.
for file in ./* ; do # Use "./", NEVER bare ""
COMMAND ... "$file" ...
done
These handle all filenames correctly;
#+ can be unwieldy if COMMAND is large:
find ... -exec COMMAND... {} ;
find ... -exec COMMAND... {} + # If multiple files are okay for COMMAND.
This skips filenames with control characters
#+ (including tab and newline).
IFS="$(printf '\n\t')"
controlchars="$(printf '[\001-\037\177]')"
for file in $(find . ! -name "$controlchars"') ; do
COMMAND "$file" ...
done
Okay if filenames can't contain tabs or newlines --
#+ beware the assumption.
IFS="$(printf '\n\t')"
for file in $(find .) ; do
COMMAND "$file" ...
done
Requires nonstandard but common extensions in find and xargs:
find . -print0 | xargs -0 COMMAND
Requires nonstandard extensions to find and to shell (bash works).
variables might not stay set once the loop ends:
find . -print0 | while IFS="" read -r -d "" file ; do ...
COMMAND "$file" # Use quoted "$file", not $file, everywhere.
done
Requires nonstandard extensions to find and to shell (bash works).
Underlying system must include named pipes (FIFOs)
#+ or the /dev/fd mechanism.
In this version, variables do stay set after the loop ends,
and you can read from stdin.
#+ (Change the 4 to another number if fd 4 is needed.)
while IFS="" read -r -d "" file <&4 ; do
COMMAND "$file" # Use quoted "$file" -- not $file, everywhere.
done 4< <(find . -print0)
Named pipe version.
Requires nonstandard extensions to find and to shell's read (bash ok).
Underlying system must include named pipes (FIFOs).
Again, in this version, variables do stay set after the loop ends,
and you can read from stdin.
(Change the 4 to something else if fd 4 needed).
mkfifo mypipe
find . -print0 > mypipe &
while IFS="" read -r -d "" file <&4 ; do
COMMAND "$file" # Use quoted "$file", not $file, everywhere.
done 4< mypipe
________________________________________________________________
Appendix E. Exit Codes With Special Meanings
Table E-1. Reserved Exit Codes
Exit Code Number Meaning Example Comments
1 Catchall for general errors let "var1 = 1/0" Miscellaneous errors,
such as "divide by zero" and other impermissible operations
2 Misuse of shell builtins (according to Bash documentation)
empty_function() {} Missing keyword or command, or permission problem
(and diff return code on a failed binary file comparison).
126 Command invoked cannot execute /dev/null Permission problem or
command is not an executable
127 "command not found" illegal_command Possible problem with $PATH
or a typo
128 Invalid argument to exit exit 3.14159 exit takes only integer
args in the range 0 - 255 (see first footnote)
128+n Fatal error signal "n" kill -9 $PPID of script $? returns 137
(128 + 9)
130 Script terminated by Control-C Ctl-C Control-C is fatal error
signal 2, (130 = 128 + 2, see above)
255* Exit status out of range exit -1 exit takes only integer args in
the range 0 - 255
According to the above table, exit codes 1 - 2, 126 - 165, and 255
[144] have special meanings, and should therefore be avoided for
user-specified exit parameters. Ending a script with exit 127 would
certainly cause confusion when troubleshooting (is the error code a
"command not found" or a user-defined one?). However, many scripts
use an exit 1 as a general bailout-upon-error. Since exit code 1
signifies so many possible errors, it is not particularly useful in
debugging.
There has been an attempt to systematize exit status numbers (see
/usr/include/sysexits.h), but this is intended for C and C++
programmers. A similar standard for scripting might be appropriate.
The author of this document proposes restricting user-defined exit
codes to the range 64 - 113 (in addition to 0, for success), to
conform with the C/C++ standard. This would allot 50 valid codes, and
make troubleshooting scripts more straightforward. [145] All
user-defined exit codes in the accompanying examples to this document
conform to this standard, except where overriding circumstances
exist, as in Example 9-2.
Note
Issuing a $? from the command-line after a shell script exits gives
results consistent with the table above only from the Bash or sh
prompt. Running the C-shell or tcsh may give different values in some
cases.
________________________________________________________________
Appendix F. A Detailed Introduction to I/O and I/O Redirection
written by Stéphane Chazelas, and revised by the document author
A command expects the first three file descriptors to be available.
The first, fd 0 (standard input, stdin), is for reading. The other
two (fd 1, stdout and fd 2, stderr) are for writing.
There is a stdin, stdout, and a stderr associated with each command.
ls 2>&1 means temporarily connecting the stderr of the ls command to
the same "resource" as the shell's stdout.
By convention, a command reads its input from fd 0 (stdin), prints
normal output to fd 1 (stdout), and error ouput to fd 2 (stderr). If
one of those three fd's is not open, you may encounter problems:
bash$ cat /etc/passwd >&-
cat: standard output: Bad file descriptor
For example, when xterm runs, it first initializes itself. Before
running the user's shell, xterm opens the terminal device
(/dev/pts/ or something similar) three times.
At this point, Bash inherits these three file descriptors, and each
command (child process) run by Bash inherits them in turn, except
when you redirect the command. Redirection means reassigning one of
the file descriptors to another file (or a pipe, or anything
permissible). File descriptors may be reassigned locally (for a
command, a command group, a subshell, a while or if or case or for
loop...), or globally, for the remainder of the shell (using exec).
ls > /dev/null means running ls with its fd 1 connected to /dev/null.
bash$ lsof -a -p $$ -d0,1,2
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
bash 363 bozo 0u CHR 136,1 3 /dev/pts/1
bash 363 bozo 1u CHR 136,1 3 /dev/pts/1
bash 363 bozo 2u CHR 136,1 3 /dev/pts/1
bash$ exec 2> /dev/null
bash$ lsof -a -p $$ -d0,1,2
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
bash 371 bozo 0u CHR 136,1 3 /dev/pts/1
bash 371 bozo 1u CHR 136,1 3 /dev/pts/1
bash 371 bozo 2w CHR 1,3 120 /dev/null
bash$ bash -c 'lsof -a -p $$ -d0,1,2' | cat
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
lsof 379 root 0u CHR 136,1 3 /dev/pts/1
lsof 379 root 1w FIFO 0,0 7118 pipe
lsof 379 root 2u CHR 136,1 3 /dev/pts/1
bash$ echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)"
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
lsof 426 root 0u CHR 136,1 3 /dev/pts/1
lsof 426 root 1w FIFO 0,0 7520 pipe
lsof 426 root 2w FIFO 0,0 7520 pipe
This works for different types of redirection.
Exercise: Analyze the following script.
! /usr/bin/env bash
mkfifo /tmp/fifo1 /tmp/fifo2
while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1
exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)
exec 3>&1
(
(
(
while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \
| tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2
echo 1st, to stdout
sleep 1
echo 2nd, to stderr >&2
sleep 1
echo 3rd, to fd 3 >&3
sleep 1
echo 4th, to fd 4 >&4
sleep 1
echo 5th, to fd 5 >&5
sleep 1
echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5
sleep 1
echo 7th, to fd 6 >&6
sleep 1
echo 8th, to fd 7 >&7
sleep 1
echo 9th, to fd 8 >&8
) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&-
) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&-
) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&-
rm -f /tmp/fifo1 /tmp/fifo2
For each command and subshell, figure out which fd points to what.
Good luck!
exit 0
________________________________________________________________
Appendix G. Command-Line Options
Many executables, whether binaries or script files, accept options to
modify their run-time behavior. For example: from the command-line,
typing command -o would invoke command, with option o.
________________________________________________________________
G.1. Standard Command-Line Options
Over time, there has evolved a loose standard for the meanings of
command-line option flags. The GNU utilities conform more closely to
this "standard" than older UNIX utilities.
Traditionally, UNIX command-line options consist of a dash, followed
by one or more lowercase letters. The GNU utilities added a
double-dash, followed by a complete word or compound word.
The two most widely-accepted options are:
* -h
--help
Help: Give usage message and exit.
* -v
--version
Version: Show program version and exit.
Other common options are:
* -a
--all
All: show all information or operate on all arguments.
* -l
--list
List: list files or arguments without taking other action.
* -o
Output filename
* -q
--quiet
Quiet: suppress stdout.
* -r
-R
--recursive
Recursive: Operate recursively (down directory tree).
* -v
--verbose
Verbose: output additional information to stdout or stderr.
* -z
--compress
Compress: apply compression (usually gzip).
However:
* In tar and gawk:
-f
--file
File: filename follows.
* In cp, mv, rm:
-f
--force
Force: force overwrite of target file(s).
Caution
Many UNIX and Linux utilities deviate from this "standard," so it is
dangerous to assume that a given option will behave in a standard
way. Always check the man page for the command in question when in
doubt.
A complete table of recommended options for the GNU utilities is
available at [http://www.gnu.org/prep/standards/] the GNU standards
page.
________________________________________________________________
G.2. Bash Command-Line Options
Bash itself has a number of command-line options. Here are some of
the more useful ones.
* -c
Read commands from the following string and assign any arguments
to the positional parameters.
bash$ bash -c 'set a b c d; IFS="+-;"; echo "$*"'
a+b+c+d
* -r
--restricted
Runs the shell, or a script, in restricted mode.
* --posix
Forces Bash to conform to POSIX mode.
* --version
Display Bash version information and exit.
* --
End of options. Anything further on the command line is an
argument, not an option.
________________________________________________________________
Appendix H. Important Files
startup files
These files contain the aliases and environmental variables
made available to Bash running as a user shell and to all Bash
scripts invoked after system initialization.
/etc/profile
Systemwide defaults, mostly setting the environment (all
Bourne-type shells, not just Bash [146])
/etc/bashrc
systemwide functions and aliases for Bash
$HOME/.bash_profile
user-specific Bash environmental default settings, found in
each user's home directory (the local counterpart to
/etc/profile)
$HOME/.bashrc
user-specific Bash init file, found in each user's home
directory (the local counterpart to /etc/bashrc). Only
interactive shells and user scripts read this file. See
Appendix M for a sample .bashrc file.
logout file
$HOME/.bash_logout
user-specific instruction file, found in each user's home
directory. Upon exit from a login (Bash) shell, the commands
in this file execute.
data files
/etc/passwd
A listing of all the user accounts on the system, their
identities, their home directories, the groups they belong to,
and their default shell. Note that the user passwords are not
stored in this file, [147] but in /etc/shadow in encrypted
form.
system configuration files
/etc/sysconfig/hwconf
Listing and description of attached hardware devices. This
information is in text form and can be extracted and parsed.
bash$ grep -A 5 AUDIO /etc/sysconfig/hwconf
class: AUDIO
bus: PCI
detached: 0
driver: snd-intel8x0
desc: "Intel Corporation 82801CA/CAM AC'97 Audio Controller"
vendorId: 8086
Note
This file is present on Red Hat and Fedora Core installations, but
may be missing from other distros.
________________________________________________________________
Appendix I. Important System Directories
Sysadmins and anyone else writing administrative scripts should be
intimately familiar with the following system directories.
* /bin
Binaries (executables). Basic system programs and utilities (such
as bash).
* /usr/bin [148]
More system binaries.
* /usr/local/bin
Miscellaneous binaries local to the particular machine.
* /sbin
System binaries. Basic system administrative programs and
utilities (such as fsck).
* /usr/sbin
More system administrative programs and utilities.
* /etc
Et cetera. Systemwide configuration scripts.
Of particular interest are the /etc/fstab (filesystem table),
/etc/mtab (mounted filesystem table), and the /etc/inittab files.
* /etc/rc.d
Boot scripts, on Red Hat and derivative distributions of Linux.
* /usr/share/doc
Documentation for installed packages.
* /usr/man
The systemwide manpages.
* /dev
Device directory. Entries (but not mount points) for physical and
virtual devices. See Chapter 29.
* /proc
Process directory. Contains information and statistics about
running processes and kernel parameters. See Chapter 29.
* /sys
Systemwide device directory. Contains information and statistics
about device and device names. This is newly added to Linux with
the 2.6.X kernels.
* /mnt
Mount. Directory for mounting hard drive partitions, such as
/mnt/dos, and physical devices. In newer Linux distros, the
/media directory has taken over as the preferred mount point for
I/O devices.
* /media
In newer Linux distros, the preferred mount point for I/O
devices, such as CD/DVD drives or USB flash drives.
* /var
Variable (changeable) system files. This is a catchall
"scratchpad" directory for data generated while a Linux/UNIX
machine is running.
* /var/log
Systemwide log files.
* /var/spool/mail
User mail spool.
* /lib
Systemwide library files.
* /usr/lib
More systemwide library files.
* /tmp
System temporary files.
* /boot
System boot directory. The kernel, module links, system map, and
boot manager reside here.
Warning
Altering files in this directory may result in an unbootable system.
________________________________________________________________
Appendix J. An Introduction to Programmable Completion
The programmable completion feature in Bash permits typing a partial
command, then pressing the [Tab] key to auto-complete the command
sequence. [149] If multiple completions are possible, then [Tab]
lists them all. Let's see how it works.
bash$ xtra[Tab]
xtraceroute xtrapin xtrapproto
xtraceroute.real xtrapinfo xtrapreset
xtrapchar xtrapout xtrapstats
bash$ xtrac[Tab]
xtraceroute xtraceroute.real
bash$ xtraceroute.r[Tab]
xtraceroute.real
Tab completion also works for variables and path names.
bash$ echo $BASH[Tab]
$BASH $BASH_COMPLETION $BASH_SUBSHELL
$BASH_ARGC $BASH_COMPLETION_DIR $BASH_VERSINFO
$BASH_ARGV $BASH_LINENO $BASH_VERSION
$BASH_COMMAND $BASH_SOURCE
bash$ echo /usr/local/[Tab]
bin/ etc/ include/ libexec/ sbin/ src/
doc/ games/ lib/ man/ share/
The Bash complete and compgen builtins make it possible for tab
completion to recognize partial parameters and options to commands.
In a very simple case, we can use complete from the command-line to
specify a short list of acceptable parameters.
bash$ touch sample_command
bash$ touch file1.txt file2.txt file2.doc file30.txt file4.zzz
bash$ chmod +x sample_command
bash$ complete -f -X '!*.txt' sample_command
bash$ ./sample[Tab][Tab]
sample_command
file1.txt file2.txt file30.txt
The -f option to complete specifies filenames, and -X the filter
pattern.
For anything more complex, we could write a script that specifies a
list of acceptable command-line parameters. The compgen builtin
expands a list of arguments to generate completion matches.
Let us take a modified version of the UseGetOpt.sh script as an
example command. This script accepts a number of command-line
parameters, preceded by either a single or double dash. And here is
the corresponding completion script, by convention given a filename
corresponding to its associated command.
Example J-1. Completion script for UseGetOpt.sh
file: UseGetOpt-2
UseGetOpt-2.sh parameter-completion
_UseGetOpt-2 () # By convention, the function name
{ #+ starts with an underscore.
local cur
Pointer to current completion word.
By convention, it's named "cur" but this isn't strictly necessary.
COMPREPLY=() # Array variable storing the possible completions.
cur=${COMP_WORDS[COMP_CWORD]}
case "$cur" in
-*)
COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \
--file --log --test --help --' -- $cur ) );;
Generate the completion matches and load them into $COMPREPLY array.
xx) May add more cases here.
yy)
zz)
esac
return 0
}
complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh
^^ ^^^^^^^^^^^^ Invokes the function _UseGetOpt-2.
Now, let's try it.
bash$ source UseGetOpt-2
bash$ ./UseGetOpt-2.sh -[Tab]
-- --aoption --debug --file --help --log --test
-a -d -f -h -l -t
bash$ ./UseGetOpt-2.sh --[Tab]
-- --aoption --debug --file --help --log --test
We begin by sourcing the "completion script." This sets the
command-line parameters. [150]
In the first instance, hitting [Tab] after a single dash, the output
is all the possible parameters preceded by one or more dashes.
Hitting [Tab] after two dashes gives the possible parameters preceded
by two or more dashes.
Now, just what is the point of having to jump through flaming hoops
to enable command-line tab completion? It saves keystrokes. [151]
--
Resources:
Bash [http://freshmeat.net/projects/bashcompletion] programmable
completion project
Mitch Frazier's Linux Journal article, More on Using the Bash
Complete Command
Steve's excellent two-part article, "An Introduction to Bash
Completion": Part 1 and
[http://www.debian-administration.org/article/An_introduction_to_bash
_completion_part_2] Part 2
________________________________________________________________
Appendix K. Localization
Localization is an undocumented Bash feature.
A localized shell script echoes its text output in the language
defined as the system's locale. A Linux user in Berlin, Germany,
would get script output in German, whereas his cousin in Berlin,
Maryland, would get output from the same script in English.
To create a localized script, use the following template to write all
messages to the user (error messages, prompts, etc.).
!/bin/bash
localized.sh
Script by Stéphane Chazelas,
- modified by Bruno Haible, bugfixed by Alfredo Pironti.
. gettext.sh
E_CDERROR=65
error()
{
printf "$@" >&2
exit $E_CDERROR
}
cd $var || error "eval_gettext \"Can\'t cd to \\\$var.\""
The triple backslashes (escapes) in front of $var needed
- "because eval_gettext expects a string
- where the variable values have not yet been substituted."
-- per Bruno Haible
read -p "gettext \"Enter the value: \"" var
...
Alfredo Pironti comments:
This script has been modified to not use the $"..." syntax in
- favor of the "
gettext \"...\"" syntax.
This is ok, but with the new localized.sh program, the commands
- "bash -D filename" and "bash --dump-po-string filename"
- (because those command are only searching for the $"..." strings)!
The ONLY way to extract strings from the new file is to use the
'xgettext' program. However, the xgettext program is buggy.
Note that 'xgettext' has another bug.
The shell fragment:
gettext -s "I like Bash"
will be correctly extracted, but . . .
xgettext -s "I like Bash"
. . . fails!
'xgettext' will extract "-s" because
- the command only extracts the
- very first argument after the 'gettext' word.
Escape characters:
To localize a sentence like
echo -e "Hello\tworld!"
echo -e "gettext \"Hello\\tworld\""
The "double escape character" before the `t' is needed because
- 'gettext' will search for a string like: 'Hello\tworld'
This is because gettext will read one literal `')
- and will output a string like "Bonjour\tmonde",
- so the 'echo' command will display the message correctly.
You may not use
echo "gettext -e \"Hello\tworld\""
- due to the xgettext bug explained above.
Let's localize the following shell fragment:
echo "-h display help and exit"
First, one could do this:
echo "gettext \"-h display help and exit\""
This way 'xgettext' will work ok,
- but the 'gettext' program will read "-h" as an option!
One solution could be
echo "gettext -- \"-h display help and exit\""
This way 'gettext' will work,
- but 'xgettext' will extract "--", as referred to above.
The workaround you may use to get this string localized is
echo -e "gettext \"\\0-h display help and exit\""
We have added a \0 (NULL) at the beginning of the sentence.
This way 'gettext' works correctly, as does 'xgettext.'
Moreover, the NULL character won't change the behavior
bash$ bash -D localized.sh
"Can't cd to %s."
"Enter the value: "
This lists all the localized text. (The -D option lists double-quoted
strings prefixed by a $, without executing the script.)
bash$ bash --dump-po-strings localized.sh
: a:6
msgid "Can't cd to %s."
msgstr ""
#: a:7
msgid "Enter the value: "
msgstr ""
The --dump-po-strings option to Bash resembles the -D option, but
uses gettext "po" format.
Note
Bruno Haible points out:
Starting with gettext-0.12.2, xgettext -o - localized.sh is
recommended instead of bash --dump-po-strings localized.sh, because
xgettext . . .
- understands the gettext and eval_gettext commands (whereas bash
--dump-po-strings understands only its deprecated $"..." syntax)
- can extract comments placed by the programmer, intended to be read
by the translator.
This shell code is then not specific to Bash any more; it works the
same way with Bash 1.x and other /bin/sh implementations.
Now, build a language.po file for each language that the script will
be translated into, specifying the msgstr. Alfredo Pironti gives the
following example:
fr.po:
: a:6
msgid "Can't cd to $var."
msgstr "Impossible de se positionner dans le repertoire $var."
: a:7
msgid "Enter the value: "
msgstr "Entrez la valeur : "
The string are dumped with the variable names, not with the %s syntax,
- This is a very cool feature if the programmer uses
- variable names that make sense!
Then, run msgfmt.
msgfmt -o localized.sh.mo fr.po
Place the resulting localized.sh.mo file in the
/usr/local/share/locale/fr/LC_MESSAGES directory, and at the
beginning of the script, insert the lines:
TEXTDOMAINDIR=/usr/local/share/locale
TEXTDOMAIN=localized.sh
If a user on a French system runs the script, she will get French
messages.
Note
With older versions of Bash or other shells, localization requires
gettext, using the -s option. In this case, the script becomes:
!/bin/bash
localized.sh
E_CDERROR=65
error() {
local format=$1
shift
printf "$(gettext -s "$format")" "$@" >&2
exit $E_CDERROR
}
cd $var || error "Can't cd to %s." "$var"
read -p "$(gettext -s "Enter the value: ")" var
...
The TEXTDOMAIN and TEXTDOMAINDIR variables need to be set and
exported to the environment. This should be done within the script
itself.
This appendix written by Stéphane Chazelas, with modifications
suggested by Alfredo Pironti, and by Bruno Haible, maintainer of GNU
gettext.
________________________________________________________________
Appendix L. History Commands
The Bash shell provides command-line tools for editing and
manipulating a user's command history. This is primarily a
convenience, a means of saving keystrokes.
Bash history commands:
1. history
2. fc
bash$ history
1 mount /mnt/cdrom
2 cd /mnt/cdrom
3 ls
...
Internal variables associated with Bash history commands:
1. $HISTCMD
2. $HISTCONTROL
3. $HISTIGNORE
4. $HISTFILE
5. $HISTFILESIZE
6. $HISTSIZE
7. $HISTTIMEFORMAT (Bash, ver. 3.0 or later)
8. !!
9. !$
- !#
- !N
- !-N
- !STRING
- !?STRING?
- ^STRING^string^
Unfortunately, the Bash history tools find no use in scripting.
!/bin/bash
history.sh
A (vain) attempt to use the 'history' command in a script.
history # No output.
var=$(history); echo "$var" # $var is empty.
History commands are, by default, disabled within a script.
However, as dhw points out,
- enables the history mechanism.
set -o history
var=$(history); echo "$var" # 1 var=$(history)
bash$ ./history.sh
(no output)
The Advancing in the Bash Shell site gives a good introduction to the
use of history commands in Bash.
________________________________________________________________
Appendix M. Sample .bashrc and .bash_profile Files
The ~/.bashrc file determines the behavior of interactive shells. A
good look at this file can lead to a better understanding of Bash.
Emmanuel Rouat contributed the following very elaborate .bashrc file,
written for a Linux system. He welcomes reader feedback on it.
Study the file carefully, and feel free to reuse code snippets and
functions from it in your own .bashrc file or even in your scripts.
Example M-1. Sample .bashrc file
=============================================================== #
PERSONAL $HOME/.bashrc FILE for bash-3.0 (or later)
By Emmanuel Rouat [no-email]
Last modified: Tue Nov 20 22:04:47 CET 2012
This file is normally read by interactive shells only.
- Here is the place to define your aliases, functions and
- other interactive features like your prompt.
The majority of the code here assumes you are on a GNU
- system (most likely a Linux box) and is often based on code
- found on Usenet or Internet.
See for instance:
http://tldp.org/LDP/abs/html/index.html
http://www.caliban.org/bash
http://www.shelldorado.com/scripts/categories.html
http://www.dotfiles.org
The choice of colors was done for a shell with a dark background
- (white on black), and this is usually also suited for pure text-mode
- consoles (no X server available). If you use a white background,
- you'll have to do some other choices for readability.
This bashrc file is a bit overcrowded.
Remember, it is just just an example.
Tailor it to your needs.
=============================================================== #
--> Comments added by HOWTO author.
If not running interactively, don't do anything
[ -z "$PS1" ] && return
Source global definitions (if any)
if [ -f /etc/bashrc ]; then
. /etc/bashrc # --> Read /etc/bashrc, if present.
fi
Automatic setting of $DISPLAY (if not set already).
This works for me - your mileage may vary. . . .
The problem is that different types of terminals give
- different answers to 'who am i' (rxvt in particular can be
- troublesome) - however this code seems to work in a majority
function get_xserver ()
{
case $TERM in
xterm )
XSERVER=$(who am i | awk '{print $NF}' | tr -d ')''(' )
# Ane-Pieter Wieringa suggests the following alternative:
# I_AM=$(who am i)
# SERVER=${I_AM#*(}
# SERVER=${SERVER%*)}
XSERVER=${XSERVER%%:*}
;;
aterm | rxvt)
# Find some code that works here. ...
;;
esac
}
if [ -z ${DISPLAY:=""} ]; then
get_xserver
if [[ -z ${XSERVER} || ${XSERVER} == $(hostname) ||
${XSERVER} == "unix" ]]; then
DISPLAY=":0.0" # Display on local host.
else
DISPLAY=${XSERVER}:0.0 # Display on remote host.
fi
fi
export DISPLAY
Some settings
set -o nounset # These two options are useful for debugging.
set -o xtrace
alias debug="set -o nounset; set -o xtrace"
ulimit -S -c 0 # Don't want coredumps.
set -o notify
set -o noclobber
set -o ignoreeof
Enable options:
shopt -s cdspell
shopt -s cdable_vars
shopt -s checkhash
shopt -s checkwinsize
shopt -s sourcepath
shopt -s no_empty_cmd_completion
shopt -s cmdhist
shopt -s histappend histreedit histverify
shopt -s extglob # Necessary for programmable completion.
Disable options:
shopt -u mailwarn
unset MAILCHECK # Don't want my shell to warn me of incoming mail.
Greeting, motd etc. ...
Color definitions (taken from Color Bash Prompt HowTo).
Some colors might look different of some terminals.
For example, I see 'Bold Red' as 'orange' on my screen,
hence the 'Green' 'BRed' 'Red' sequence I often use in my prompt.
Normal Colors
Black='\e[0;30m' # Black
Red='\e[0;31m' # Red
Green='\e[0;32m' # Green
Yellow='\e[0;33m' # Yellow
Blue='\e[0;34m' # Blue
Purple='\e[0;35m' # Purple
Cyan='\e[0;36m' # Cyan
White='\e[0;37m' # White
Bold
BBlack='\e[1;30m' # Black
BRed='\e[1;31m' # Red
BGreen='\e[1;32m' # Green
BYellow='\e[1;33m' # Yellow
BBlue='\e[1;34m' # Blue
BPurple='\e[1;35m' # Purple
BCyan='\e[1;36m' # Cyan
BWhite='\e[1;37m' # White
Background
On_Black='\e[40m' # Black
On_Red='\e[41m' # Red
On_Green='\e[42m' # Green
On_Yellow='\e[43m' # Yellow
On_Blue='\e[44m' # Blue
On_Purple='\e[45m' # Purple
On_Cyan='\e[46m' # Cyan
On_White='\e[47m' # White
NC="\e[m" # Color Reset
ALERT=${BWhite}${On_Red} # Bold White on red background
echo -e "${BCyan}This is BASH ${BRed}${BASH_VERSION%.*}${BCyan}\
- DISPLAY on ${BRed}$DISPLAY${NC}\n"
date
if [ -x /usr/games/fortune ]; then
/usr/games/fortune -s # Makes our day a bit more fun.... :-)
fi
function _exit() # Function to run upon exit of shell.
{
echo -e "${BRed}Hasta la vista, baby${NC}"
}
trap _exit EXIT
Shell Prompt - for many examples, see:
http://www.debian-administration.org/articles/205
http://www.askapache.com/linux/bash-power-prompt.html
http://tldp.org/HOWTO/Bash-Prompt-HOWTO
https://github.com/nojhan/liquidprompt
Current Format: [TIME USER@HOST PWD] >
TIME:
Green == machine load is low
Orange == machine load is medium
Red == machine load is high
ALERT == machine load is very high
USER:
Cyan == normal user
Orange == SU to user
Red == root
HOST:
Cyan == local session
Green == secured remote connection (via ssh)
Red == unsecured remote connection
PWD:
Green == more than 10% free disk space
Orange == less than 10% free disk space
ALERT == less than 5% free disk space
Red == current user does not have write privileges
Cyan == current filesystem is size zero (like /proc)
:
White == no background or suspended jobs in this shell
Cyan == at least one background job in this shell
Orange == at least one suspended job in this shell
Command is added to the history file each time you hit enter,
so it's available to all shells (using 'history -a').
Test connection type:
if [ -n "${SSH_CONNECTION}" ]; then
CNX=${Green} # Connected on remote machine, via ssh (good).
elif [[ "${DISPLAY%%:0*}" != "" ]]; then
CNX=${ALERT} # Connected on remote machine, not via ssh (bad).
else
CNX=${BCyan} # Connected on local machine.
fi
Test user type:
if [[ ${USER} == "root" ]]; then
SU=${Red} # User is root.
elif [[ ${USER} != $(logname) ]]; then
SU=${BRed} # User is not login user.
else
SU=${BCyan} # User is normal (well ... most of us are).
fi
NCPU=$(grep -c 'processor' /proc/cpuinfo) # Number of CPUs
SLOAD=$(( 100*${NCPU} )) # Small load
MLOAD=$(( 200*${NCPU} )) # Medium load
XLOAD=$(( 400*${NCPU} )) # Xlarge load
Returns system load as percentage, i.e., '40' rather than '0.40)'.
function load()
{
local SYSLOAD=$(cut -d " " -f1 /proc/loadavg | tr -d '.')
# System load of the current host.
echo $((10#$SYSLOAD)) # Convert to decimal.
}
Returns a color indicating system load.
function load_color()
{
local SYSLOAD=$(load)
if [ ${SYSLOAD} -gt ${XLOAD} ]; then
echo -en ${ALERT}
elif [ ${SYSLOAD} -gt ${MLOAD} ]; then
echo -en ${Red}
elif [ ${SYSLOAD} -gt ${SLOAD} ]; then
echo -en ${BRed}
else
echo -en ${Green}
fi
}
Returns a color according to free disk space in $PWD.
function disk_color()
{
if [ ! -w "${PWD}" ] ; then
echo -en ${Red}
# No 'write' privilege in the current directory.
elif [ -s "${PWD}" ] ; then
local used=$(command df -P "$PWD" |
awk 'END {print $5} {sub(/%/,"")}')
if [ ${used} -gt 95 ]; then
echo -en ${ALERT} # Disk almost full (>95%).
elif [ ${used} -gt 90 ]; then
echo -en ${BRed} # Free disk space almost gone.
else
echo -en ${Green} # Free disk space is ok.
fi
else
echo -en ${Cyan}
# Current directory is size '0' (like /proc, /sys etc).
fi
}
Returns a color according to running/suspended jobs.
function job_color()
{
if [ $(jobs -s | wc -l) -gt "0" ]; then
echo -en ${BRed}
elif [ $(jobs -r | wc -l) -gt "0" ] ; then
echo -en ${BCyan}
fi
}
Adds some text in the terminal frame (if applicable).
Now we construct the prompt.
PROMPT_COMMAND="history -a"
case ${TERM} in
*term | rxvt | linux)
PS1="\[\$(load_color)\][\A\[${NC}\] "
# Time of day (with load info):
PS1="\[\$(load_color)\][\A\[${NC}\] "
# User@Host (with connection type info):
PS1=${PS1}"\[${SU}\]\u\[${NC}\]@\[${CNX}\]\h\[${NC}\] "
# PWD (with 'disk space' info):
PS1=${PS1}"\[\$(disk_color)\]\W]\[${NC}\] "
# Prompt (with 'job' info):
PS1=${PS1}"\[\$(job_color)\]>\[${NC}\] "
# Set title of current xterm:
PS1=${PS1}"\[\e]0;[\u@\h] \w\a\]"
;;
*)
PS1="(\A \u@\h \W) > " # --> PS1="(\A \u@\h \w) > "
# --> Shows full pathname of current dir.
;;
esac
export TIMEFORMAT=$'\nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n'
export HISTIGNORE="&:bg:fg:ll:h"
export HISTTIMEFORMAT="$(echo -e ${BCyan})[%d/%m %H:%M:%S]$(echo -e ${NC}) "
export HISTCONTROL=ignoredups
export HOSTFILE=$HOME/.hosts # Put a list of remote hosts in ~/.hosts
============================================================
ALIASES AND FUNCTIONS
Arguably, some functions defined here are quite big.
If you want to make this file smaller, these functions can
- be converted into scripts and removed from here.
============================================================
Personnal Aliases
alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'
-> Prevents accidentally clobbering files.
alias mkdir='mkdir -p'
alias h='history'
alias j='jobs -l'
alias which='type -a'
alias ..='cd ..'
Pretty-print of some PATH variables:
alias path='echo -e ${PATH//:/\n}'
alias libpath='echo -e ${LD_LIBRARY_PATH//:/\n}'
alias du='du -kh' # Makes a more readable output.
alias df='df -kTh'
The 'ls' family (this assumes you use a recent GNU ls).
Add colors for filetype and human-readable sizes by default on 'ls':
alias ls='ls -h --color'
alias lx='ls -lXB' # Sort by extension.
alias lk='ls -lSr' # Sort by size, biggest last.
alias lt='ls -ltr' # Sort by date, most recent last.
alias lc='ls -ltcr' # Sort by/show change time,most recent last.
alias lu='ls -ltur' # Sort by/show access time,most recent last.
The ubiquitous 'll': directories first, with alphanumeric sorting:
alias ll="ls -lv --group-directories-first"
alias lm='ll |more' # Pipe through 'more'
alias lr='ll -R' # Recursive ls.
alias la='ll -A' # Show hidden files.
alias tree='tree -Csuh' # Nice alternative to 'recursive ls' ...
Tailoring 'less'
alias more='less'
export PAGER=less
export LESSCHARSET='latin1'
export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&-'
# Use this if lesspipe.sh exists.
export LESS='-i -N -w -z-4 -g -e -M -X -F -R -P%t?f%f \
:stdin .?pb%pb%:?lbLine %lb:?bbByte %bb:-...'
LESS man page colors (makes Man pages more readable).
export LESS_TERMCAP_mb=$'\E[01;31m'
export LESS_TERMCAP_md=$'\E[01;31m'
export LESS_TERMCAP_me=$'\E[0m'
export LESS_TERMCAP_se=$'\E[0m'
export LESS_TERMCAP_so=$'\E[01;44;33m'
export LESS_TERMCAP_ue=$'\E[0m'
export LESS_TERMCAP_us=$'\E[01;32m'
Spelling typos - highly personnal and keyboard-dependent :-)
alias xs='cd'
alias vf='cd'
alias moer='more'
alias moew='more'
alias kk='ll'
A few fun ones
Adds some text in the terminal frame (if applicable).
function xtitle()
{
case "$TERM" in
*term* | rxvt)
echo -en "\e]0;$*\a" ;;
*) ;;
esac
}
Aliases that use xtitle
alias top='xtitle Processes on $HOST && top'
alias make='xtitle Making $(basename $PWD) ; make'
.. and functions
function man()
{
for i ; do
xtitle The $(basename $1|tr -d .[:digit:]) manual
command man -a "$i"
done
}
Make the following commands run in background automatically:
function te() # wrapper around xemacs/gnuserv
{
if [ "$(gnuclient -batch -eval t 2>&-)" == "t" ]; then
gnuclient -q "$@";
else
( xemacs "$@" &);
fi
}
function soffice() { command soffice "$@" & }
function firefox() { command firefox "$@" & }
function xpdf() { command xpdf "$@" & }
File & strings related functions:
Find a file with a pattern in name:
function ff() { find . -type f -iname ''"$"'*' -ls ; }
Find a file with pattern $1 in name and Execute $2 on it:
function fe() { find . -type f -iname ''"${1:-}"'' \
-exec ${2:-file} {} ; ; }
Find a pattern in a set of files and highlight them:
- (needs a recent version of egrep).
function fstr()
{
OPTIND=1
local mycase=""
local usage="fstr: find string in files.
Usage: fstr [-i] "pattern" ["filename pattern"] "
while getopts :it opt
do
case "$opt" in
i) mycase="-i " ;;
*) echo "$usage"; return ;;
esac
done
shift $(( $OPTIND - 1 ))
if [ "$#" -lt 1 ]; then
echo "$usage"
return;
fi
find . -type f -name "${2:-*}" -print0 | \
xargs -0 egrep --color=always -sn ${case} "$1" 2>&- | more
}
function swap()
{ # Swap 2 filenames around, if they exist (from Uzi's bashrc).
local TMPFILE=tmp.$$
[ $# -ne 2 ] && echo "swap: 2 arguments needed" && return 1
[ ! -e $1 ] && echo "swap: $1 does not exist" && return 1
[ ! -e $2 ] && echo "swap: $2 does not exist" && return 1
mv "$1" $TMPFILE
mv "$2" "$1"
mv $TMPFILE "$2"
}
function extract() # Handy Extract Program
{
if [ -f $1 ] ; then
case $1 in
*.tar.bz2) tar xvjf $1 ;;
*.tar.gz) tar xvzf $1 ;;
*.bz2) bunzip2 $1 ;;
*.rar) unrar x $1 ;;
*.gz) gunzip $1 ;;
*.tar) tar xvf $1 ;;
*.tbz2) tar xvjf $1 ;;
*.tgz) tar xvzf $1 ;;
*.zip) unzip $1 ;;
*.Z) uncompress $1 ;;
*.7z) 7z x $1 ;;
*) echo "'$1' cannot be extracted via >extract<" ;;
esac
else
echo "'$1' is not a valid file!"
fi
}
Creates an archive (*.tar.gz) from given directory.
function maketar() { tar cvzf "${1%%/}.tar.gz" "${1%%/}/"; }
Create a ZIP archive of a file or folder.
function makezip() { zip -r "${1%%/}.zip" "$1" ; }
Make your directories and files access rights sane.
function sanitize() { chmod -R u=rwX,g=rX,o= "$@" ;}
Process/system related functions:
function my_ps() { ps $@ -u $USER -o pid,%cpu,%mem,bsdtime,command ; }
function pp() { my_ps f | awk '!/awk/ && $0~var' var=${1:-".*"} ; }
function killps() # kill by process name
{
local pid pname sig="-TERM" # default signal
if [ "$#" -lt 1 ] || [ "$#" -gt 2 ]; then
echo "Usage: killps [-SIGNAL] pattern"
return;
fi
if [ $# = 2 ]; then sig=$1 ; fi
for pid in $(my_ps| awk '!/awk/ && $0~pat { print $1 }' pat=${!#} )
do
pname=$(my_ps | awk '$1~var { print $5 }' var=$pid )
if ask "Kill process $pid <$pname> with signal $sig?"
then kill $sig $pid
fi
done
}
function mydf() # Pretty-print of 'df' output.
{ # Inspired by 'dfc' utility.
for fs ; do
if [ ! -d $fs ]
then
echo -e $fs" :No such file or directory" ; continue
fi
local info=( $(command df -P $fs | awk 'END{ print $2,$3,$5 }') )
local free=( $(command df -Pkh $fs | awk 'END{ print $4 }') )
local nbstars=$(( 20 * ${info[1]} / ${info[0]} ))
local out="["
for ((j=0;j<20;j++)); do
if [ ${j} -lt ${nbstars} ]; then
out=$out"*"
else
out=$out"-"
fi
done
out=${info[2]}" "$out"] ("$free" free on "$fs")"
echo -e $out
done
}
function my_ip() # Get IP adress on ethernet.
{
MY_IP=$(/sbin/ifconfig eth0 | awk '/inet/ { print $2 } ' |
sed -e s/addr://)
echo ${MY_IP:-"Not connected"}
}
function ii() # Get current host related info.
{
echo -e "\nYou are logged on ${BRed}$HOST"
echo -e "\n${BRed}Additionnal information:$NC " ; uname -a
echo -e "\n${BRed}Users logged on:$NC " ; w -hs |
cut -d " " -f1 | sort | uniq
echo -e "\n${BRed}Current date :$NC " ; date
echo -e "\n${BRed}Machine stats :$NC " ; uptime
echo -e "\n${BRed}Memory stats :$NC " ; free
echo -e "\n${BRed}Diskspace :$NC " ; mydf / $HOME
echo -e "\n${BRed}Local IP Address :$NC" ; my_ip
echo -e "\n${BRed}Open connections :$NC "; netstat -pan --inet;
echo
}
Misc utilities:
function repeat() # Repeat n times command.
{
local i max
max=$1; shift;
for ((i=1; i <= max ; i++)); do # --> C-like syntax
eval "$@";
done
}
function ask() # See 'killps' for example of use.
{
echo -n "$@" '[y/n] ' ; read ans
case "$ans" in
y*|Y*) return 0 ;;
*) return 1 ;;
esac
}
function corename() # Get name of app that created a corefile.
{
for file ; do
echo -n $file : ; gdb --core=$file --batch | head -1
done
}
=========================================================================
PROGRAMMABLE COMPLETION SECTION
Most are taken from the bash 2.05 documentation and from Ian McDonald's
'Bash completion' package (http://www.caliban.org/bash/#completion)
You will in fact need bash more recent then 3.0 for some features.
Note that most linux distributions now provide many completions
'out of the box' - however, you might need to make your own one day,
so I kept those here as examples.
=========================================================================
if [ "${BASH_VERSION%.*}" < "3.0" ]; then
echo "You will need to upgrade to version 3.0 for full \
programmable completion features"
return
fi
shopt -s extglob # Necessary.
complete -A hostname rsh rcp telnet rlogin ftp ping disk
complete -A export printenv
complete -A variable export local readonly unset
complete -A enabled builtin
complete -A alias alias unalias
complete -A function function
complete -A user su mail finger
complete -A helptopic help # Currently same as builtins.
complete -A shopt shopt
complete -A stopped -P '%' bg
complete -A job -P '%' fg jobs disown
complete -A directory mkdir rmdir
complete -A directory -o default cd
Compression
complete -f -o default -X '*.+(zip|ZIP)' zip
complete -f -o default -X '!*.+(zip|ZIP)' unzip
complete -f -o default -X '*.+(z|Z)' compress
complete -f -o default -X '!*.+(z|Z)' uncompress
complete -f -o default -X '*.+(gz|GZ)' gzip
complete -f -o default -X '!*.+(gz|GZ)' gunzip
complete -f -o default -X '*.+(bz2|BZ2)' bzip2
complete -f -o default -X '!*.+(bz2|BZ2)' bunzip2
complete -f -o default -X '!*.+(zip|ZIP|z|Z|gz|GZ|bz2|BZ2)' extract
Documents - Postscript,pdf,dvi.....
complete -f -o default -X '!*.+(ps|PS)' gs ghostview ps2pdf ps2ascii
complete -f -o default -X \
'!*.+(dvi|DVI)' dvips dvipdf xdvi dviselect dvitype
complete -f -o default -X '!*.+(pdf|PDF)' acroread pdf2ps
complete -f -o default -X '!*.@(@(?(e)ps|?(E)PS|pdf|PDF)?\
(.gz|.GZ|.bz2|.BZ2|.Z))' gv ggv
complete -f -o default -X '!.texi' makeinfo texi2dvi texi2html texi2pdf
complete -f -o default -X '!*.tex' tex latex slitex
complete -f -o default -X '!*.lyx' lyx
complete -f -o default -X '!.+(htm|HTM*)' lynx html2ps
complete -f -o default -X \
'!*.+(doc|DOC|xls|XLS|ppt|PPT|sx?|SX?|csv|CSV|od?|OD?|ott|OTT)' soffice
Multimedia
complete -f -o default -X \
'!.+(gif|GIF|jpg|JP*G|bmp|BMP|xpm|XPM|png|PNG)' xv gimp ee gqview
complete -f -o default -X '!*.+(mp3|MP3)' mpg123 mpg321
complete -f -o default -X '!*.+(ogg|OGG)' ogg123
complete -f -o default -X \
'!*.@(mp[23]|MP[23]|ogg|OGG|wav|WAV|pls|\
m3u|xm|mod|s[3t]m|it|mtm|ult|flac)' xmms
complete -f -o default -X '!*.@(mp?(e)g|MP?(E)G|wma|avi|AVI|\
asf|vob|VOB|bin|dat|vcd|ps|pes|fli|viv|rm|ram|yuv|mov|MOV|qt|\
QT|wmv|mp3|MP3|ogg|OGG|ogm|OGM|mp4|MP4|wav|WAV|asx|ASX)' xine
complete -f -o default -X '!*.pl' perl perl5
This is a 'universal' completion function - it works when commands have
- a so-called 'long options' mode , ie: 'ls --all' instead of 'ls -a'
Needs the '-o' option of grep
- (try the commented-out version if not available).
First, remove '=' from completion word separators
- (this will allow completions like 'ls --color=auto' to work correctly).
COMP_WORDBREAKS=${COMP_WORDBREAKS/=/}
_get_longopts()
{
#$1 --help | sed -e '/--/!d' -e 's/.--([^[:space:].,]).*/--\1/'| \
#grep ^"$2" |sort -u ;
$1 --help | grep -o -e "--[^[:space:].,]*" | grep -e "$2" |sort -u
}
_longopts()
{
local cur
cur=${COMP_WORDS[COMP_CWORD]}
case "${cur:-*}" in
-*) ;;
*) return ;;
esac
case "$1" in
\~*) eval cmd="$1" ;;
*) cmd="$1" ;;
esac
COMPREPLY=( $(_get_longopts ${1} ${cur} ) )
}
complete -o default -F _longopts configure bash
complete -o default -F _longopts wget id info a2ps ls recode
_tar()
{
local cur ext regex tar untar
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
# If we want an option, return the possible long options.
case "$cur" in
-*) COMPREPLY=( $(_get_longopts $1 $cur ) ); return 0;;
esac
if [ $COMP_CWORD -eq 1 ]; then
COMPREPLY=( $( compgen -W 'c t x u r d A' -- $cur ) )
return 0
fi
case "${COMP_WORDS[1]}" in
?(-)c*f)
COMPREPLY=( $( compgen -f $cur ) )
return 0
;;
+([^Izjy])f)
ext='tar'
regex=$ext
;;
*z*f)
ext='tar.gz'
regex='t\(ar\.\)\(gz\|Z\)'
;;
*[Ijy]*f)
ext='t?(ar.)bz?(2)'
regex='t\(ar\.\)bz2\?'
;;
*)
COMPREPLY=( $( compgen -f $cur ) )
return 0
;;
esac
if [[ "$COMP_LINE" == tar*.$ext' '* ]]; then
# Complete on files in tar file.
#
# Get name of tar file from command line.
tar=$( echo "$COMP_LINE" | \
sed -e 's|^.* \([^ ]*'$regex'\) .*$|\1|' )
# Devise how to untar and list it.
untar=t${COMP_WORDS[1]//[^Izjyf]/}
COMPREPLY=( $( compgen -W "$( echo $( tar $untar $tar \
2>/dev/null ) )" -- "$cur" ) )
return 0
else
# File completion on relevant files.
COMPREPLY=( $( compgen -G $cur\*.$ext ) )
fi
return 0
}
complete -F _tar -o default tar
_make()
{
local mdef makef makef_dir="." makef_inc gcmd cur prev i;
COMPREPLY=();
cur=${COMP_WORDS[COMP_CWORD]};
prev=${COMP_WORDS[COMP_CWORD-1]};
case "$prev" in
-*f)
COMPREPLY=($(compgen -f $cur ));
return 0
;;
esac;
case "$cur" in
-*)
COMPREPLY=($(_get_longopts $1 $cur ));
return 0
;;
esac;
# ... make reads
# GNUmakefile,
# then makefile
# then Makefile ...
if [ -f ${makef_dir}/GNUmakefile ]; then
makef=${makef_dir}/GNUmakefile
elif [ -f ${makef_dir}/makefile ]; then
makef=${makef_dir}/makefile
elif [ -f ${makef_dir}/Makefile ]; then
makef=${makef_dir}/Makefile
else
makef=${makef_dir}/*.mk # Local convention.
fi
# Before we scan for targets, see if a Makefile name was
#+ specified with -f.
for (( i=0; i < ${#COMP_WORDS[@]}; i++ )); do
if [[ ${COMP_WORDS[i]} == -f ]]; then
# eval for tilde expansion
eval makef=${COMP_WORDS[i+1]}
break
fi
done
[ ! -f $makef ] && return 0
# Deal with included Makefiles.
makef_inc=$( grep -E '^-?include' $makef |
sed -e "s,^.* ,"$makef_dir"/," )
for file in $makef_inc; do
[ -f $file ] && makef="$makef $file"
done
# If we have a partial word to complete, restrict completions
#+ to matches of that word.
if [ -n "$cur" ]; then gcmd='grep "^$cur"' ; else gcmd=cat ; fi
COMPREPLY=( $( awk -F':' '/^[a-zA-Z0-9][^$#\/\t=]*:([^=]|$)/ \
{split($1,A,/ /);for(i in A)print A[i]}' \
$makef 2>/dev/null | eval $gcmd ))
}
complete -F _make -X '+($|.[cho])' make gmake pmake
_killall()
{
local cur prev
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
# Get a list of processes
#+ (the first sed evaluation
#+ takes care of swapped out processes, the second
#+ takes care of getting the basename of the process).
COMPREPLY=( $( ps -u $USER -o comm | \
sed -e '1,1d' -e 's#[]\[]##g' -e 's#^.*/##'| \
awk '{if ($0 ~ /^'$cur'/) print $0}' ))
return 0
}
complete -F _killall killall killps
Local Variables:
mode:shell-script
sh-shell:bash
End:
And, here is a snippet from Andrzej Szelachowski's instructive
.bash_profile file.
Example M-2. .bash_profile file
From Andrzej Szelachowski's ~/.bash_profile:
Note that a variable may require special treatment
DARKGRAY='\e[1;30m'
LIGHTRED='\e[1;31m'
GREEN='\e[32m'
YELLOW='\e[1;33m'
LIGHTBLUE='\e[1;34m'
NC='\e[m'
PCT="`if [[ $EUID -eq 0 ]]; then T='$LIGHTRED' ; else T='$LIGHTBLUE'; fi;
echo $T `"
For "literal" command substitution to be assigned to a variable,
- use escapes and double quotes:
Otherwise, the value of PCT variable is assigned only once,
- when the variable is exported/read from .bash_profile,
- and it will not change afterwards even if the user ID changes.
PS1="\n$GREEN[\w] \n$DARKGRAY($PCT\t$DARKGRAY)-($PCT\u$DARKGRAY)-($PCT!
$DARKGRAY)$YELLOW-> $NC"
Escape a variables whose value changes:
if [[ $EUID -eq 0 ]],
Otherwise the value of the EUID variable will be assigned only once,
When a variable is assigned, it should be called escaped:
Otherwise the value of the T variable is taken from the moment the PCT
- variable is exported/read from .bash_profile.
So, in this example it would be null.
When a variable's value contains a semicolon it should be strong quoted:
T='$LIGHTRED',
Otherwise, the semicolon will be interpreted as a command separator.
Variables PCT and PS1 can be merged into a new PS1 variable:
PS1="`if [[ $EUID -eq 0 ]]; then PCT='$LIGHTRED';
else PCT='$LIGHTBLUE'; fi;
echo '\n$GREEN[\w] \n$DARKGRAY('$PCT'\t$DARKGRAY)-\
('$PCT'\u$DARKGRAY)-('$PCT'!$DARKGRAY)$YELLOW-> $NC'`"
The trick is to use strong quoting for parts of old PS1 variable.
________________________________________________________________
Appendix N. Converting DOS Batch Files to Shell Scripts
Quite a number of programmers learned scripting on a PC running DOS.
Even the crippled DOS batch file language allowed writing some fairly
powerful scripts and applications, though they often required
extensive kludges and workarounds. Occasionally, the need still
arises to convert an old DOS batch file to a UNIX shell script. This
is generally not difficult, as DOS batch file operators are only a
limited subset of the equivalent shell scripting ones.
Table N-1. Batch file keywords / variables / operators, and their
shell equivalents
Batch File Operator Shell Script Equivalent Meaning
% $ command-line parameter prefix
/ - command option flag
\ / directory path separator
== = (equal-to) string comparison test
!==! != (not equal-to) string comparison test
| | pipe
@ set +v do not echo current command
file redirection (overwrite)
file redirection (append)
< < redirect stdin
%VAR% $VAR environmental variable
REM # comment
NOT ! negate following test
NUL /dev/null "black hole" for burying command output
ECHO echo echo (many more option in Bash)
ECHO. echo echo blank line
ECHO OFF set +v do not echo command(s) following
FOR %%VAR IN (LIST) DO for var in [list]; do "for" loop
:LABEL none (unnecessary) label
GOTO none (use a function) jump to another location in the script
PAUSE sleep pause or wait an interval
CHOICE case or select menu choice
IF if if-test
IF EXIST FILENAME if [ -e filename ] test if file exists
IF !%N==! if [ -z "$N" ] if replaceable parameter "N" not present
CALL source or . (dot operator) "include" another script
COMMAND /C source or . (dot operator) "include" another script (same
as CALL)
SET export set an environmental variable
SHIFT shift left shift command-line argument list
SGN -lt or -gt sign (of integer)
ERRORLEVEL $? exit status
CON stdin "console" (stdin)
PRN /dev/lp0 (generic) printer device
LPT1 /dev/lp0 first printer device
COM1 /dev/ttyS0 first serial port
Batch files usually contain DOS commands. These must be translated
into their UNIX equivalents in order to convert a batch file into a
shell script.
Table N-2. DOS commands and their UNIX equivalents
DOS Command UNIX Equivalent Effect
ASSIGN ln link file or directory
ATTRIB chmod change file permissions
CD cd change directory
CHDIR cd change directory
CLS clear clear screen
COMP diff, comm, cmp file compare
COPY cp file copy
Ctl-C Ctl-C break (signal)
Ctl-Z Ctl-D EOF (end-of-file)
DEL rm delete file(s)
DELTREE rm -rf delete directory recursively
DIR ls -l directory listing
ERASE rm delete file(s)
EXIT exit exit current process
FC comm, cmp file compare
FIND grep find strings in files
MD mkdir make directory
MKDIR mkdir make directory
MORE more text file paging filter
MOVE mv move
PATH $PATH path to executables
REN mv rename (move)
RENAME mv rename (move)
RD rmdir remove directory
RMDIR rmdir remove directory
SORT sort sort file
TIME date display system time
TYPE cat output file to stdout
XCOPY cp (extended) file copy
Note
Virtually all UNIX and shell operators and commands have many more
options and enhancements than their DOS and batch file counterparts.
Many DOS batch files rely on auxiliary utilities, such as ask.com, a
crippled counterpart to read.
DOS supports only a very limited and incompatible subset of filename
wild-card expansion, recognizing just the * and ? characters.
Converting a DOS batch file into a shell script is generally
straightforward, and the result ofttimes reads better than the
original.
Example N-1. VIEWDATA.BAT: DOS Batch File
REM VIEWDATA
REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS"
REM BY PAUL SOMERSON
@ECHO OFF
IF !%1==! GOTO VIEWDATA
REM IF NO COMMAND-LINE ARG...
FIND "%1" C:\BOZO\BOOKLIST.TXT
GOTO EXIT0
REM PRINT LINE WITH STRING MATCH, THEN EXIT.
:VIEWDATA
TYPE C:\BOZO\BOOKLIST.TXT | MORE
REM SHOW ENTIRE FILE, 1 PAGE AT A TIME.
:EXIT0
The script conversion is somewhat of an improvement. [152]
Example N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
!/bin/bash
viewdata.sh
Conversion of VIEWDATA.BAT to shell script.
DATAFILE=/home/bozo/datafiles/book-collection.data
ARGNO=1
@ECHO OFF Command unnecessary here.
if [ $# -lt "$ARGNO" ] # IF !%1==! GOTO VIEWDATA
then
less $DATAFILE # TYPE C:\MYDIR\BOOKLIST.TXT | MORE
else
grep "$1" $DATAFILE # FIND "%1" C:\MYDIR\BOOKLIST.TXT
fi
exit 0 # :EXIT0
GOTOs, labels, smoke-and-mirrors, and flimflam unnecessary.
The converted script is short, sweet, and clean,
- which is more than can be said for the original.
Ted Davis' Shell Scripts on the PC site had a set of comprehensive
tutorials on the old-fashioned art of batch file programming.
Unfortunately the page has vanished without a trace.
________________________________________________________________
Appendix O. Exercises
The exercises that follow test and extend your knowledge of
scripting. Think of them as a challenge, as an entertaining way to
take you further along the stony path toward UNIX wizardry.
On a dingy side street in a run-down section of Hoboken, New Je
rsey,
there sits a nondescript squat two-story brick building with an
inscription
incised on a marble plate in its wall:
Bash Scripting Hall of Fame.
Inside, among various dusty uninteresting exhibits is a corrodi
ng,
cobweb-festooned brass plaque inscribed with a short, very shor
t
list of those few persons who have successfully mastered the ma
terial
in the Advanced Bash Scripting
Guide, as evidenced by their performance
on the following Exercise sections.
(Alas, the author of the ABS
Guide is not represented among the exhibits.
This is possibly due to malicious rumors about lack of
credentials and
deficient scripting skills.)
________________________________________________________________
O.1. Analyzing Scripts
Examine the following script. Run it, then explain what it does.
Annotate the script and rewrite it in a more compact and elegant
manner.
!/bin/bash
MAX=10000
for((nr=1; nr<$MAX; nr++))
do
let "t1 = nr % 5"
if [ "$t1" -ne 3 ]
then
continue
fi
let "t2 = nr % 7"
if [ "$t2" -ne 4 ]
then
continue
fi
let "t3 = nr % 9"
if [ "$t3" -ne 5 ]
then
continue
fi
break # What happens when you comment out this line? Why?
done
echo "Number = $nr"
exit 0
Explain what the following script does. It is really just a
parameterized command-line pipe.
!/bin/bash
DIRNAME=/usr/bin
FILETYPE="shell script"
LOGFILE=logfile
file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l
exit 0
Examine and explain the following script. For hints, you might refer
to the listings for find and stat.
!/bin/bash
Author: Nathan Coulter
This code is released to the public domain.
The author gave permission to use this code snippet in the ABS Guide.
find -maxdepth 1 -type f -printf '%f\000' | {
while read -d $'\000'; do
mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'
)-$REPLY"
done
}
Warning: Test-drive this script in a "scratch" directory.
It will somehow affect all the files there.
A reader sent in the following code snippet.
while read LINE
do
echo $LINE
done < tail -f /var/log/messages
He wished to write a script tracking changes to the system log file,
/var/log/messages. Unfortunately, the above code block hangs and does
nothing useful. Why? Fix this so it does work. (Hint: rather than
redirecting the stdin of the loop, try a pipe.)
Analyze the following "one-liner" (here split into two lines for
clarity) contributed by Rory Winston:
export SUM=0; for f in $(find src -name "*.java");
do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM
Hint: First, break the script up into bite-sized sections. Then,
carefully examine its use of double-parentheses arithmetic, the
export command, the find command, the wc command, and awk.
Analyze Example A-10, and reorganize it in a simplified and more
logical style. See how many of the variables can be eliminated, and
try to optimize the script to speed up its execution time.
Alter the script so that it accepts any ordinary ASCII text file as
input for its initial "generation". The script will read the first
$ROW*$COL characters, and set the occurrences of vowels as "living"
cells. Hint: be sure to translate the spaces in the input file to
underscore characters.
________________________________________________________________
O.2. Writing Scripts
Write a script to carry out each of the following tasks.
EASY
Self-reproducing Script
Write a script that backs itself up, that is, copies itself to
a file named backup.sh.
Hint: Use the cat command and the appropriate positional
parameter.
Home Directory Listing
Perform a recursive directory listing on the user's home
directory and save the information to a file. Compress the
file, have the script prompt the user to insert a USB flash
drive, then press ENTER. Finally, save the file to the flash
drive after making certain the flash drive has properly
mounted by parsing the output of df. Note that the flash drive
must be unmounted before it is removed.
Converting for loops to while and until loops
Convert the for loops in Example 11-1 to while loops. Hint:
store the data in an array and step through the array
elements.
Having already done the "heavy lifting," now convert the loops
in the example to until loops.
Changing the line spacing of a text file
Write a script that reads each line of a target file, then
writes the line back to stdout, but with an extra blank line
following. This has the effect of double-spacing the file.
Include all necessary code to check whether the script gets
the necessary command-line argument (a filename), and whether
the specified file exists.
When the script runs correctly, modify it to triple-space the
target file.
Finally, write a script to remove all blank lines from the
target file, single-spacing it.
Backwards Listing
Write a script that echoes itself to stdout, but backwards.
Automatically Decompressing Files
Given a list of filenames as input, this script queries each
target file (parsing the output of the file command) for the
type of compression used on it. Then the script automatically
invokes the appropriate decompression command (gunzip,
bunzip2, unzip, uncompress, or whatever). If a target file is
not compressed, the script emits a warning message, but takes
no other action on that particular file.
Unique System ID
Generate a "unique" 6-digit hexadecimal identifier for your
computer. Do not use the flawed hostid command. Hint: md5sum
/etc/passwd, then select the first 6 digits of output.
Backup
Archive as a "tarball" (*.tar.gz file) all the files in your
home directory tree (/home/your-name) that have been modified
in the last 24 hours. Hint: use find.
Optional: you may use this as the basis of a backup script.
Checking whether a process is still running
Given a process ID (PID) as an argument, this script will
check, at user-specified intervals, whether the given process
is still running. You may use the ps and sleep commands.
Primes
Print (to stdout) all prime numbers between 60000 and 63000.
The output should be nicely formatted in columns (hint: use
printf).
Lottery Numbers
One type of lottery involves picking five different numbers,
in the range of 1 - 50. Write a script that generates five
pseudorandom numbers in this range, with no duplicates. The
script will give the option of echoing the numbers to stdout
or saving them to a file, along with the date and time the
particular number set was generated. (If your script
consistently generates winning lottery numbers, then you can
retire on the proceeds and leave shell scripting to those of
us who have to work for a living.)
INTERMEDIATE
Integer or String
Write a script function that determines if an argument passed
to it is an integer or a string. The function will return TRUE
(0) if passed an integer, and FALSE (1) if passed a string.
Hint: What does the following expression return when $1 is not
an integer?
expr $1 + 0
ASCII to Integer
The atoi function in C converts a string character to an
integer. Write a shell script function that performs the same
operation. Likewise, write a shell script function that does
the inverse, mirroring the C itoa function which converts an
integer into an ASCII character.
Managing Disk Space
List, one at a time, all files larger than 100K in the
/home/username directory tree. Give the user the option to
delete or compress the file, then proceed to show the next
one. Write to a logfile the names of all deleted files and the
deletion times.
Banner
Simulate the functionality of the deprecated banner command in
a script.
Removing Inactive Accounts
Inactive accounts on a network server waste disk space and may
become a security risk. Write an administrative script (to be
invoked by root or the cron daemon) that checks for and
deletes user accounts that have not been accessed within the
last 90 days.
Enforcing Disk Quotas
Write a script for a multi-user system that checks users' disk
usage. If a user surpasses a preset limit (500 MB, for
example) in her /home/username directory, then the script
automatically sends her a "pigout" warning e-mail.
The script will use the du and mail commands. As an option, it
will allow setting and enforcing quotas using the quota and
setquota commands.
Logged in User Information
For all logged in users, show their real names and the time
and date of their last login.
Hint: use who, lastlog, and parse /etc/passwd.
Safe Delete
Implement, as a script, a "safe" delete command, sdel.sh.
Filenames passed as command-line arguments to this script are
not deleted, but instead gzipped if not already compressed
(use file to check), then moved to a ~/TRASH directory. Upon
invocation, the script checks the ~/TRASH directory for files
older than 48 hours and permanently deletes them. (An better
alternative might be to have a second script handle this,
periodically invoked by the cron daemon.)
Extra credit: Write the script so it can handle files and
directories recursively. This would give it the capability of
"safely deleting" entire directory structures.
Making Change
What is the most efficient way to make change for $1.68, using
only coins in common circulations (up to 25c)? It's 6
quarters, 1 dime, a nickel, and three cents.
Given any arbitrary command-line input in dollars and cents
($*.??), calculate the change, using the minimum number of
coins. If your home country is not the United States, you may
use your local currency units instead. The script will need to
parse the command-line input, then change it to multiples of
the smallest monetary unit (cents or whatever). Hint: look at
Example 24-8.
Quadratic Equations
Solve a quadratic equation of the form Ax^2 + Bx + C = 0. Have
a script take as arguments the coefficients, A, B, and C, and
return the solutions to five decimal places.
Hint: pipe the coefficients to bc, using the well-known
formula, x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A.
Table of Logarithms
Using the bc and printf commands, print out a nicely-formatted
table of eight-place natural logarithms in the interval
between 0.00 and 100.00, in steps of .01.
Hint: bc requires the -l option to load the math library.
Unicode Table
Using Example T-1 as a template, write a script that prints to
a file a complete Unicode table.
Hint: Use the -e option to echo: echo -e '\uXXXX', where XXXX
is the Unicode numerical character designation. This requires
version 4.2 or later of Bash.
Sum of Matching Numbers
Find the sum of all five-digit numbers (in the range 10000 -
99999) containing exactly two out of the following set of
digits: { 4, 5, 6 }. These may repeat within the same number,
and if so, they count once for each occurrence.
Some examples of matching numbers are 42057, 74638, and 89515.
Lucky Numbers
A lucky number is one whose individual digits add up to 7, in
successive additions. For example, 62431 is a lucky number (6
+ 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the lucky numbers
between 1000 and 10000.
Craps
Borrowing the ASCII graphics from Example A-40, write a script
that plays the well-known gambling game of craps. The script
will accept bets from one or more players, roll the dice, and
keep track of wins and losses, as well as of each player's
bankroll.
Tic-tac-toe
Write a script that plays the child's game of tic-tac-toe
against a human player. The script will let the human choose
whether to take the first move. The script will follow an
optimal strategy, and therefore never lose. To simplify
matters, you may use ASCII graphics:
o | x |
| x |
| o |
Your move, human (row, column)?
Alphabetizing a String
Alphabetize (in ASCII order) an arbitrary string read from the
command-line.
Parsing
Parse /etc/passwd, and output its contents in nice,
easy-to-read tabular form.
Logging Logins
Parse /var/log/messages to produce a nicely formatted file of
user logins and login times. The script may need to run as
root. (Hint: Search for the string "LOGIN.")
Pretty-Printing a Data File
Certain database and spreadsheet packages use save-files with
the fields separated by commas, commonly referred to as
comma-separated values or CSVs. Other applications often need
to parse these files.
Given a data file with comma-separated fields, of the form:
Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989
Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612
...
Reformat the data and print it out to stdout in labeled,
evenly-spaced columns.
Justification
Given ASCII text input either from stdin or a file, adjust the
word spacing to right-justify each line to a user-specified
line-width, then send the output to stdout.
Mailing List
Using the mail command, write a script that manages a simple
mailing list. The script automatically e-mails the monthly
company newsletter, read from a specified text file, and sends
it to all the addresses on the mailing list, which the script
reads from another specified file.
Generating Passwords
Generate pseudorandom 8-character passwords, using characters
in the ranges [0-9], [A-Z], [a-z]. Each password must contain
at least two digits.
Monitoring a User
You suspect that one particular user on the network has been
abusing her privileges and possibly attempting to hack the
system. Write a script to automatically monitor and log her
activities when she's signed on. The log file will save
entries for the previous week, and delete those entries more
than seven days old.
You may use last, lastlog, and lastcomm to aid your
surveillance of the suspected fiend.
Checking for Broken Links
Using lynx with the -traversal option, write a script that
checks a Web site for broken links.
DIFFICULT
Testing Passwords
Write a script to check and validate passwords. The object is
to flag "weak" or easily guessed password candidates.
A trial password will be input to the script as a command-line
parameter. To be considered acceptable, a password must meet
the following minimum qualifications:
+ Minimum length of 8 characters
+ Must contain at least one numeric character
+ Must contain at least one of the following non-alphabetic
characters: @, #, $, %, &, *, +, -, =
Optional:
+ Do a dictionary check on every sequence of at least four
consecutive alphabetic characters in the password under
test. This will eliminate passwords containing embedded
"words" found in a standard dictionary.
+ Enable the script to check all the passwords on your system.
These do not reside in /etc/passwd.
This exercise tests mastery of Regular Expressions.
Cross Reference
Write a script that generates a cross-reference (concordance)
on a target file. The output will be a listing of all word
occurrences in the target file, along with the line numbers in
which each word occurs. Traditionally, linked list constructs
would be used in such applications. Therefore, you should
investigate arrays in the course of this exercise. Example
16-12 is probably not a good place to start.
Square Root
Write a script to calculate square roots of numbers using
Newton's Method.
The algorithm for this, expressed as a snippet of Bash
pseudo-code is:
(Isaac) Newton's Method for speedy extraction
guess = $argument
$argument is the number to find the square root of.
$guess is each successive calculated "guess" -- or trial solution --
Our first "guess" at a square root is the argument itself.
oldguess = 0
$oldguess is the previous $guess.
tolerance = .000001
To how close a tolerance we wish to calculate.
loopcnt = 0
Let's keep track of how many times through the loop.
Some arguments will require more loop iterations than others.
while [ ABS( $guess $oldguess ) -gt $tolerance ]
^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.
"ABS" is a (floating point) function to find the absolute value
So, as long as difference between current and previous
do
oldguess = $guess # Update $oldguess to previous $guess.
=======================================================
guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0
= 1/2 ( ($oldguess **2 + $argument) / $oldguess )
equivalent to:
= 1/2 ( $oldguess + $argument / $oldguess )
that is, "averaging out" the trial solution and
- the proportion of argument deviation
- (in effect, splitting the error in half).
This converges on an accurate solution
- with surprisingly few loop iterations . . .
- for arguments > $tolerance, of course.
=======================================================
(( loopcnt++ )) # Update loop counter.
done
It's a simple enough recipe, and seems at first glance easy
enough to convert into a working Bash script. The problem,
though, is that Bash has no native support for floating point
numbers. So, the script writer needs to use bc or possibly awk
to convert the numbers and do the calculations. It could get
rather messy . . .
Logging File Accesses
Log all accesses to the files in /etc during the course of a
single day. This information should include the filename, user
name, and access time. If any alterations to the files take
place, that will be flagged. Write this data as tabular
(tab-separated) formatted records in a logfile.
Monitoring Processes
Write a script to continually monitor all running processes
and to keep track of how many child processes each parent
spawns. If a process spawns more than five children, then the
script sends an e-mail to the system administrator (or root)
with all relevant information, including the time, PID of the
parent, PIDs of the children, etc. The script appends a report
to a log file every ten minutes.
Strip Comments
Strip all comments from a shell script whose name is specified
on the command-line. Note that the initial #! line must not be
stripped out.
Strip HTML Tags
Strip all the HTML tags from a specified HTML file, then
reformat it into lines between 60 and 75 characters in length.
Reset paragraph and block spacing, as appropriate, and convert
HTML tables to their approximate text equivalent.
XML Conversion
Convert an XML file to both HTML and text format.
Optional: A script that converts Docbook/SGML to XML.
Chasing Spammers
Write a script that analyzes a spam e-mail by doing DNS
lookups on the IP addresses in the headers to identify the
relay hosts as well as the originating ISP. The script will
forward the unaltered spam message to the responsible ISPs. Of
course, it will be necessary to filter out your own ISP's IP
address, so you don't end up complaining about yourself.
As necessary, use the appropriate network analysis commands.
For some ideas, see Example 16-41 and Example A-28.
Optional: Write a script that searches through a list of
e-mail messages and deletes the spam according to specified
filters.
Creating man pages
Write a script that automates the process of creating man
pages.
Given a text file which contains information to be formatted
into a man page, the script will read the file, then invoke
the appropriate groff commands to output the corresponding man
page to stdout. The text file contains blocks of information
under the standard man page headings, i.e., NAME, SYNOPSIS,
DESCRIPTION, etc.
Example A-39 is an instructive first step.
Hex Dump
Do a hex(adecimal) dump on a binary file specified as an
argument to the script. The output should be in neat tabular
fields, with the first field showing the address, each of the
next 8 fields a 4-byte hex number, and the final field the
ASCII equivalent of the previous 8 fields.
The obvious followup to this is to extend the hex dump script
into a disassembler. Using a lookup table, or some other
clever gimmick, convert the hex values into 80x86 op codes.
Emulating a Shift Register
Using Example 27-15 as an inspiration, write a script that
emulates a 64-bit shift register as an array. Implement
functions to load the register, shift left, shift right, and
rotate it. Finally, write a function that interprets the
register contents as eight 8-bit ASCII characters.
Calculating Determinants
Write a script that calculates determinants [153] by
recursively expanding the minors. Use a 4 x 4 determinant as a
test case.
Hidden Words
Write a "word-find" puzzle generator, a script that hides 10
input words in a 10 x 10 array of random letters. The words
may be hidden across, down, or diagonally.
Optional: Write a script that solves word-find puzzles. To
keep this from becoming too difficult, the solution script
will find only horizontal and vertical words. (Hint: Treat
each row and column as a string, and search for substrings.)
Anagramming
Anagram 4-letter input. For example, the anagrams of word are:
do or rod row word. You may use /usr/share/dict/linux.words as
the reference list.
Word Ladders
A "word ladder" is a sequence of words, with each successive
word in the sequence differing from the previous one by a
single letter.
For example, to "ladder" from mark to vase:
mark --> park --> part --> past --> vast --> vase
^ ^ ^ ^ ^
Write a script that solves word ladder puzzles. Given a
starting and an ending word, the script will list all
intermediate steps in the "ladder." Note that all words in the
sequence must be legitimate dictionary words.
Fog Index
The "fog index" of a passage of text estimates its reading
difficulty, as a number corresponding roughly to a school
grade level. For example, a passage with a fog index of 12
should be comprehensible to anyone with 12 years of schooling.
The Gunning version of the fog index uses the following
algorithm.
1. Choose a section of the text at least 100 words in length.
2. Count the number of sentences (a portion of a sentence
truncated by the boundary of the text section counts as
one).
3. Find the average number of words per sentence.
AVE_WDS_SEN = TOTAL_WORDS / SENTENCES
4. Count the number of "difficult" words in the segment --
those containing at least 3 syllables. Divide this quantity
by total words to get the proportion of difficult words.
PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS
5. The Gunning fog index is the sum of the above two
quantities, multiplied by 0.4, then rounded to the nearest
integer.
G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )
Step 4 is by far the most difficult portion of the exercise.
There exist various algorithms for estimating the syllable
count of a word. A rule-of-thumb formula might consider the
number of letters in a word and the vowel-consonant mix.
A strict interpretation of the Gunning fog index does not
count compound words and proper nouns as "difficult" words,
but this would enormously complicate the script.
Calculating PI using Buffon's Needle
The Eighteenth Century French mathematician de Buffon came up
with a novel experiment. Repeatedly drop a needle of length n
onto a wooden floor composed of long and narrow parallel
boards. The cracks separating the equal-width floorboards are
a fixed distance d apart. Keep track of the total drops and
the number of times the needle intersects a crack on the
floor. The ratio of these two quantities turns out to be a
fractional multiple of PI.
In the spirit of Example 16-50, write a script that runs a
Monte Carlo simulation of Buffon's Needle. To simplify
matters, set the needle length equal to the distance between
the cracks, n = d.
Hint: there are actually two critical variables: the distance
from the center of the needle to the nearest crack, and the
inclination angle of the needle to that crack. You may use bc
to handle the calculations.
Playfair Cipher
Implement the Playfair (Wheatstone) Cipher in a script.
The Playfair Cipher encrypts text by substitution of digrams
(2-letter groupings). It is traditional to use a 5 x 5 letter
scrambled-alphabet key square for the encryption and
decryption.
C O D E S
A B F G H
I K L M N
P Q R T U
V W X Y Z
Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all
the rest of the alphabet, in order from left to right, skipping letters
already used.
To encrypt, separate the plaintext message into digrams (2-letter
groups). If a group has two identical letters, delete the second, and
form a new group. If there is a single letter left over at the end,
insert a "null" character, typically an "X."
THIS IS A TOP SECRET MESSAGE
TH IS IS AT OP SE CR ET ME SA GE
For each digram, there are three possibilities.
- Both letters will be on the same row of the key square:
For each letter, substitute the one immediately to the right, in that
row. If necessary, wrap around left to the beginning of the row.
or
- Both letters will be in the same column of the key square:
For each letter, substitute the one immediately below it, in that
row. If necessary, wrap around to the top of the column.
or
- Both letters will form the corners of a rectangle within the key square:
For each letter, substitute the one on the other corner the rectangle
which lies on the same row.
The "TH" digram falls under case #3.
G H
M N
T U (Rectangle with "T" and "H" at corners)
T --> U
H --> G
The "SE" digram falls under case #1.
C O D E S (Row containing "S" and "E")
S --> C (wraps around left to beginning of row)
E --> S
=> =======================================================================
To decrypt encrypted text, reverse the above procedure under cases #1
and #2 (move in opposite direction for substitution). Under case #3,
just take the remaining two corners of the rectangle.
Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
fairly detailed description of the Playfair Cipher and its solution methods.
This script will have three main sections
I. Generating the key square, based on a user-input keyword.
II. Encrypting a plaintext message.
III. Decrypting encrypted text.
The script will make extensive use of arrays and functions. You may
use Example A-56 as an inspiration.
--
Please do not send the author your solutions to these exercises.
There are more appropriate ways to impress him with your cleverness,
such as submitting bugfixes and suggestions for improving the book.
________________________________________________________________
Appendix P. Revision History
This document first appeared as a 60-page HOWTO in the late spring
of 2000. Since then, it has gone through quite a number of updates
and revisions. This book could not have been written without the
assistance of the Linux community, and especially of the volunteers
of the [http://www.tldp.org] Linux Documentation Project.
Here is the e-mail to the LDP requesting permission to submit version
0.1.
From thegrendel@theriver.com Sat Jun 10 09:05:33 2000 -0700
Date: Sat, 10 Jun 2000 09:05:28 -0700 (MST)
From: "M. Leo Cooper" thegrendel@theriver.com
X-Sender: thegrendel@localhost
To: ldp-discuss@lists.linuxdoc.org
Subject: Permission to submit HOWTO
Dear HOWTO Coordinator,
I am working on and would like to submit to the LDP a HOWTO on the subject
of "Bash Scripting" (shell scripting, using 'bash'). As it happens,
I have been writing this document, off and on, for about the last eight
months or so, and I could produce a first draft in ASCII text format in
a matter of just a few more days.
I began writing this out of frustration at being unable to find a
decent book on shell scripting. I managed to locate some pretty good
articles on various aspects of scripting, but nothing like a complete,
beginning-to-end tutorial. Well, in keeping with my philosophy, if all
else fails, do it yourself.
As it stands, this proposed "Bash-Scripting HOWTO" would serve as a
combination tutorial and reference, with the heavier emphasis on the
tutorial. It assumes Linux experience, but only a very basic level
of programming skills. Interspersed with the text are 79 illustrative
example scripts of varying complexity, all liberally commented. There
are even exercises for the reader.
At this stage, I'm up to 18,000+ words (124k), and that's over 50 pages of
text (whew!).
I haven't mentioned that I've previously authored an LDP HOWTO, the
"Software-Building HOWTO", which I wrote in Linuxdoc/SGML. I don't know
if I could handle Docbook/SGML, and I'm glad you have volunteers to do
the conversion. You people seem to have gotten on a more organized basis
these last few months. Working with Greg Hankins and Tim Bynum was nice,
but a professional team is even nicer.
Anyhow, please advise.
Mendel Cooper
thegrendel@theriver.com
Table P-1. Revision History
Release Date Comments
0.1 14 Jun 2000 Initial release.
0.2 30 Oct 2000 Bugs fixed, plus much additional material and more
example scripts.
0.3 12 Feb 2001 Major update.
0.4 08 Jul 2001 Complete revision and expansion of the book.
0.5 03 Sep 2001 Major update: Bugfixes, material added, sections
reorganized.
1.0 14 Oct 2001 Stable release: Bugfixes, reorganization, material
added.
1.1 06 Jan 2002 Bugfixes, material and scripts added.
1.2 31 Mar 2002 Bugfixes, material and scripts added.
1.3 02 Jun 2002 TANGERINE release: A few bugfixes, much more material
and scripts added.
1.4 16 Jun 2002 MANGO release: A number of typos fixed, more material
and scripts.
1.5 13 Jul 2002 PAPAYA release: A few bugfixes, much more material
and scripts added.
1.6 29 Sep 2002 POMEGRANATE release: Bugfixes, more material, one
more script.
1.7 05 Jan 2003 COCONUT release: A couple of bugfixes, more material,
one more script.
1.8 10 May 2003 BREADFRUIT release: A number of bugfixes, more
scripts and material.
1.9 21 Jun 2003 PERSIMMON release: Bugfixes, and more material.
2.0 24 Aug 2003 GOOSEBERRY release: Major update.
2.1 14 Sep 2003 HUCKLEBERRY release: Bugfixes, and more material.
2.2 31 Oct 2003 CRANBERRY release: Major update.
2.3 03 Jan 2004 STRAWBERRY release: Bugfixes and more material.
2.4 25 Jan 2004 MUSKMELON release: Bugfixes.
2.5 15 Feb 2004 STARFRUIT release: Bugfixes and more material.
2.6 15 Mar 2004 SALAL release: Minor update.
2.7 18 Apr 2004 MULBERRY release: Minor update.
2.8 11 Jul 2004 ELDERBERRY release: Minor update.
3.0 03 Oct 2004 LOGANBERRY release: Major update.
3.1 14 Nov 2004 BAYBERRY release: Bugfix update.
3.2 06 Feb 2005 BLUEBERRY release: Minor update.
3.3 20 Mar 2005 RASPBERRY release: Bugfixes, much material added.
3.4 08 May 2005 TEABERRY release: Bugfixes, stylistic revisions.
3.5 05 Jun 2005 BOXBERRY release: Bugfixes, some material added.
3.6 28 Aug 2005 POKEBERRY release: Bugfixes, some material added.
3.7 23 Oct 2005 WHORTLEBERRY release: Bugfixes, some material added.
3.8 26 Feb 2006 BLAEBERRY release: Bugfixes, some material added.
3.9 15 May 2006 SPICEBERRY release: Bugfixes, some material added.
4.0 18 Jun 2006 WINTERBERRY release: Major reorganization.
4.1 08 Oct 2006 WAXBERRY release: Minor update.
4.2 10 Dec 2006 SPARKLEBERRY release: Important update.
4.3 29 Apr 2007 INKBERRY release: Bugfixes, material added.
5.0 24 Jun 2007 SERVICEBERRY release: Major update.
5.1 10 Nov 2007 LINGONBERRY release: Minor update.
5.2 16 Mar 2008 SILVERBERRY release: Important update.
5.3 11 May 2008 GOLDENBERRY release: Minor update.
5.4 21 Jul 2008 ANGLEBERRY release: Major update.
5.5 23 Nov 2008 FARKLEBERRY release: Minor update.
5.6 26 Jan 2009 WORCESTERBERRY release: Minor update.
6.0 23 Mar 2009 THIMBLEBERRY release: Major update.
6.1 30 Sep 2009 BUFFALOBERRY release: Minor update.
6.2 17 Mar 2010 ROWANBERRY release: Minor update.
6.3 30 Apr 2011 SWOZZLEBERRY release: Major update.
6.4 30 Aug 2011 VORTEXBERRY release: Minor update.
6.5 05 Apr 2012 TUNGSTENBERRY release: Minor update.
6.6 27 Nov 2012 YTTERBIUMBERRY release: Minor update.
10 10 Mar 2014 YTTERBIUMBERRY release: License change.
________________________________________________________________
Appendix Q. Download and Mirror Sites
The latest update of this document, as an archived, bzip2-ed
"tarball" including both the SGML source and rendered HTML, may be
downloaded from the author's home site). A
[http://bash.deta.in/abs-guide.pdf] pdf version is also available
(mirror site). There is likewise an
[http://bash.deta.in/abs-guide.epub] epub version, courtesy of Craig
Barnes and Michael Satke. The change log gives a detailed revision
history. The ABS Guide even has its own freshmeat.net/freecode page
to keep track of major updates, user comments, and popularity ratings
for the project.
The legacy hosting site for this document is the
[http://www.tldp.org/LDP/abs/] Linux Documentation Project, which
maintains many other Guides and HOWTOs as well.
Many thanks to Ronny Bangsund for donating [http://bash.deta.in/]
server space to host this project.
________________________________________________________________
Appendix R. To Do List
* A comprehensive survey of incompatibilities between Bash and the
classic Bourne shell.
* Same as above, but for the Korn shell (ksh).
________________________________________________________________
Appendix S. Copyright
The Advanced Bash Scripting Guide is herewith granted to the PUBLIC
DOMAIN. This has the following implications and consequences.
A. All previous releases of the Advanced Bash Scripting Guide
are as well granted to the Public Domain.
A1. All printed editions, whether authorized by the author or not,
are as well granted to the Public Domain. This legally overrides
any stated intention or wishes of the publishers. Any statement
of copyright is void and invalid.
THERE ARE NO EXCEPTIONS TO THIS.
A2. Any release of the Advanced Bash Scripting Guide, whether in
electronic or print form is granted to the Public Domain by the
express directive of the author and previous copyright holder, Mendel
Cooper. No other person(s) or entities have ever held a valid copyright.
B. As a Public Domain document, unlimited copying and distribution rights
are granted. There can be NO restrictions. If anyone has published or will
in the future publish an original or modified version of this document,
then only additional original material may be copyrighted. The core
work will remain in the Public Domain.
By law, distributors and publishers (including on-line publishers)
are prohibited from imposing any conditions, strictures, or
provisions on this document, any previous versions, or any derivative
versions. The author asserts that he has not entered into any
contractual obligations that would alter the foregoing declarations.
Essentially, you may freely distribute this book or any derivative
thereof in electronic or printed form. If you have previously
purchased or are in possession of a printed copy of a current or
previous edition, you have the LEGAL RIGHT to copy and/or
redistribute it, regardless of any copyright notice. Any copyright
notice is void.
Additionally, the author wishes to state his intention that:
If you copy or distribute this book, kindly DO NOT
use the materials within, or any portion thereof, in a patent or copyright
lawsuit against the Open Source community, its developers, its
distributors, or against any of its associated software or documentation
including, but not limited to, the Linux kernel, Open Office, Samba,
and Wine. Kindly DO NOT use any of the materials within
this book in testimony or depositions as a plaintiff's "expert witness" in
any lawsuit against the Open Source community, any of its developers, its
distributors, or any of its associated software or documentation.
A Public Domain license essentially does not restrict ANY legitimate
distribution or use of this book. The author especially encourages
its (royalty-free!) use for classroom and instructional purposes.
To date, limited print rights (Lulu edition) have been granted to one
individual and to no one else. Neither that individual nor Lulu holds
or ever has held a valid copyright.
Warning
It has come to the attention of the author that unauthorized
electronic and print editions of this book are being sold
commercially on itunes®, amazon.com and elsewhere. These are illegal
and pirated editions produced without the author's permission, and
readers of this book are strongly urged not to purchase them. In
fact, these pirated editions are now legal, but necessarily fall into
the Public Domain, and any copyright notices contained within them
are invalid and void.
The author produced this book in a manner consistent with the spirit
of the LDP Manifesto.
Linux is a trademark registered to Linus Torvalds.
Fedora is a trademark registered to Red Hat.
Unix and UNIX are trademarks registered to the Open Group.
MS Windows is a trademark registered to the Microsoft Corp.
Solaris is a trademark registered to Oracle, Inc.
OSX is a trademark registered to Apple, Inc.
Yahoo is a trademark registered to Yahoo, Inc.
Pentium is a trademark registered to Intel, Inc.
Thinkpad is a trademark registered to Lenovo, Inc.
Scrabble is a trademark registered to Hasbro, Inc.
Librie, PRS-500, and PRS-505 are trademarks registered to Sony, Inc.
All other commercial trademarks mentioned in the body of this work
are registered to their respective owners.
Hyun Jin Cha has done a Korean translation of version 1.0.11 of this
book. Spanish, Portuguese, [http://abs.traduc.org/] French, German,
[http://it.tldp.org/guide/abs/index.html] Italian,
[http://gazette.linux.ru.net/rus/articles/index-abs-guide.html]
Russian, [http://premekvihan.net/bash] Czech,
[http://www.linuxsir.org/bbs/showthread.php?t=256887] Chinese,
Indonesian, Dutch, Romanian, Bulgarian, and Turkish translations are
also available or in progress. If you wish to translate this document
into another language, please feel free to do so, subject to the
terms stated above. The author wishes to be notified of such efforts.
Those generous readers desiring to make a donation to the author may
contribute a small amount via Paypal to my e-mail address,
thegrendel.abs@gmail.com. (An Honor Roll of Supporters is given at
the beginning of the [http://bash.deta.in/Change.log] Change Log.)
This is not a requirement. The ABS Guide is a free and freely
distributed document for the use and enjoyment of the Linux
community. However, in these difficult times, showing support for
voluntary projects and especially to authors of limited means is more
critically important than ever.
________________________________________________________________
Appendix T. ASCII Table
Traditionally, a book of this sort has an ASCII Table appendix. This
book does not. Instead, here are several short scripts, each of which
generates a complete ASCII table.
Example T-1. A script that generates an ASCII table
!/bin/bash
ascii.sh
ver. 0.2, reldate 26 Aug 2008
Patched by ABS Guide author.
Original script by Sebastian Arming.
Used with permission (thanks!).
exec >ASCII.txt # Save stdout to file,
#+ as in the example scripts
#+ reassign-stdout.sh and upperconv.sh.
MAXNUM=256
COLUMNS=5
OCT=8
OCTSQU=64
LITTLESPACE=-3
BIGSPACE=-5
i=1 # Decimal counter
o=1 # Octal counter
while [ "$i" -lt "$MAXNUM" ]; do # We don't have to count past 400 octal.
paddi=" $i"
echo -n "${paddi: $BIGSPACE} " # Column spacing.
paddo="00$o"
echo -ne "\${paddo: $LITTLESPACE}" # Original.
echo -ne "\\0${paddo: $LITTLESPACE}" # Fixup.
^
echo -n " "
if (( i % $COLUMNS == 0)); then # New line.
echo
fi
((i++, o++))
# The octal notation for 8 is 10, and 64 decimal is 100 octal.
(( i % $OCT == 0)) && ((o+=2))
(( i % $OCTSQU == 0)) && ((o+=20))
done
exit $?
Compare this script with the "pr-asc.sh" example.
This one handles "unprintable" characters.
Exercise:
Rewrite this script to use decimal numbers, rather than octal.
Example T-2. Another ASCII table script
!/bin/bash
Script author: Joseph Steinhauser
Lightly edited by ABS Guide author, but not commented.
Used in ABS Guide with permission.
-- File: ascii.sh Print ASCII chart, base 10/8/16 (JETS-2012)
-- Usage: ascii [oct|dec|hex|help|8|10|16]
--
-- This script prints out a summary of ASCII char codes from Zero to 127.
-- Numeric values may be printed in Base10, Octal, or Hex.
--
-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default.
-- For more detail, man ascii . . .
[ -n "$BASH_VERSION" ] && shopt -s extglob
case "$1" in
oct|[Oo]?([Cc][Tt])|8) Obase=Octal; Numy=3o;;
hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex; Numy=2X;;
help|?(-)[h?]) sed -n '2,/^[ ]*$/p' $0;exit;;
code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p' $0;exit;;
*) Obase=Decimal
esac # CODE is actually shorter than the chart!
printf "\t\t## $Obase ASCII Chart ##\n\n"; FM1="|%0${Numy:-3d}"; LD=-1
AB="nul soh stx etx eot enq ack bel bs tab nl vt np cr so si dle"
AD="dc1 dc2 dc3 dc4 nak syn etb can em sub esc fs gs rs us sp"
for TOK in $AB $AD; do ABR[$((LD+=1))]=$TOK; done;
ABR[127]=del
IDX=0
while [ $IDX -le 127 ] && CHR="${ABR[$IDX]}"
do ((${#CHR}))&& FM2='%-3s'|| FM2=printf '\\\\%o ' $IDX
printf "$FM1 $FM2" "$IDX" $CHR; (( (IDX+=1)%8))||echo '|'
done
exit $?
Example T-3. A third ASCII table script, using awk
!/bin/bash
ASCII table script, using awk.
Author: Joseph Steinhauser
Used in ABS Guide with permission.
-- File: ascii Print ASCII chart, base 10/8/16 (JETS-2010)
-- Usage: ascii [oct|dec|hex|help|8|10|16]
--
-- This script prints a summary of ASCII char codes from Zero to 127.
-- Numeric values may be printed in Base10, Octal, or Hex (Base16).
--
-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default.
-- For more detail, man ascii
[ -n "$BASH_VERSION" ] && shopt -s extglob
case "$1" in
oct|[Oo]?([Cc][Tt])|8) Obase=Octal; Numy=3o;;
hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex; Numy=2X;;
help|?(-)[h?]) sed -n '2,/^[ ]*$/p' $0;exit;;
code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p' $0;exit;;
*) Obase=Decimal
esac
export Obase # CODE is actually shorter than the chart!
awk 'BEGIN{print "\n\t\t## "ENVIRON["Obase"]" ASCII Chart ##\n"
ab="soh,stx,etx,eot,enq,ack,bel,bs,tab,nl,vt,np,cr,so,si,dle,"
ad="dc1,dc2,dc3,dc4,nak,syn,etb,can,em,sub,esc,fs,gs,rs,us,sp"
split(ab ad,abr,",");abr[0]="nul";abr[127]="del";
fm1="|%0'"${Numy:- 4d}"' %-3s"
for(idx=0;idx<128;idx++){fmt=fm1 (++colz%8?"":"|\n")
printf(fmt,idx,(idx in abr)?abr[idx]:sprintf("%c",idx))} }'
exit $?
________________________________________________________________
Index
This index / glossary / quick-reference lists many of the important
topics covered in the text. Terms are arranged in approximate ASCII
sorting order, modified as necessary for enhanced clarity.
Note that commands are indexed in Part 4.
^ (caret)
* Beginning-of-line, in a Regular Expression
* ^
^^
Uppercase conversion in parameter substitution
~ Tilde
* ~ home directory, corresponds to $HOME
* ~/ Current user's home directory
* ~+ Current working directory
* ~- Previous working directory
= Equals sign
* = Variable assignment operator
* = String comparison operator
== String comparison operator
* =~ Regular Expression match operator
Example script
< Left angle bracket
* Is-less-than
String comparison
Integer comparison within double parentheses
* Redirection
< stdin
<< Here document
<<< Here string
<> Opening a file for both reading and writing
Right angle bracket
* Is-greater-than
String comparison
Integer comparison, within double parentheses
* Redirection
> Redirect stdout to a file
>> Redirect stdout to a file, but append
i>&j Redirect file descriptor i to file descriptor j
>&j Redirect stdout to file descriptor j
>&2 Redirect stdout of a command to stderr
2>&1 Redirect stderr to stdout
&> Redirect both stdout and stderr of a command to a file
:> file Truncate file to zero length
| Pipe, a device for passing the output of a command to another
command or to the shell
|| Logical OR test operator
* Prefix to default parameter, in parameter substitution
* Prefix to option flag
* Indicating redirection from stdin or stdout
* -- (double-dash)
Prefix to long command options
C-style variable decrement within double parentheses
; (semicolon)
* As command separator
* \; Escaped semicolon, terminates a find command
* ;; Double-semicolon, terminator in a case option
Required when ...
do keyword is on the first line of loop
terminating curly-bracketed code block
* ;;& ;& Terminators in a case option (version 4+ of Bash).
: Colon
* :> filename Truncate file to zero length
* null command, equivalent to the true Bash builtin
* Used in an anonymous here document
* Used in an otherwise empty function
* Used as a function name
! Negation operator, inverts exit status of a test or command
* != not-equal-to String comparison operator
? (question mark)
* Match zero or one characters, in an Extended Regular Expression
* Single-character wild card, in globbing
* In a C-style Trinary operator
// Double forward slash, behavior of cd command toward
. (dot / period)
* . Load a file (into a script), equivalent to source command
* . Match single character, in a Regular Expression
* . Current working directory
./ Current working directory
* .. Parent directory
' ... ' (single quotes) strong quoting
" ... " (double quotes) weak quoting
* Double-quoting the backslash (\) character
,
* Comma operator
* ,
,,
Lowercase conversion in parameter substitution
() Parentheses
* ( ... ) Command group; starts a subshell
* ( ... ) Enclose group of Extended Regular Expressions
* >( ... )
<( ... ) Process substitution
* ... ) Terminates test-condition in case construct
* (( ... )) Double parentheses, in arithmetic expansion
[ Left bracket, test construct
[ ]Brackets
* Array element
* Enclose character set to match in a Regular Expression
* Test construct
[[ ... ]] Double brackets, extended test construct
$ Anchor, in a Regular Expression
$ Prefix to a variable name
$( ... ) Command substitution, setting a variable with output of a
command, using parentheses notation
... Command substitution, using backquotes notation
$[ ... ] Integer expansion (deprecated)
${ ... } Variable manipulation / evaluation
* ${var} Value of a variable
* ${#var} Length of a variable
* ${#@}
${#*} Number of positional parameters
* ${parameter?err_msg} Parameter-unset message
* ${parameter-default}
${parameter:-default}
${parameter=default}
${parameter:=default} Set default parameter
* ${parameter+alt_value}
${parameter:+alt_value}
Alternate value of parameter, if set
* ${!var}
Indirect referencing of a variable, new notation
* ${!#}
Final positional parameter. (This is an indirect reference to
$#.)
* ${!varprefix*}
${!varprefix@}
Match names of all previously declared variables beginning with
varprefix
* ${string:position}
${string:position:length} Substring extraction
* ${var#Pattern}
${var##Pattern} Substring removal
* ${var%Pattern}
${var%%Pattern} Substring removal
* ${string/substring/replacement}
${string//substring/replacement}
${string/#substring/replacement}
${string/%substring/replacement} Substring replacement
$' ... ' String expansion, using escaped characters.
\ Escape the character following
* \< ... \> Angle brackets, escaped, word boundary in a Regular
Expression
* \{ N \} "Curly" brackets, escaped, number of character sets to
match in an Extended RE
* \; Semicolon, escaped, terminates a find command
* \$$ Indirect reverencing of a variable, old-style notation
* Escaping a newline, to write a multi-line command
&
* &> Redirect both stdout and stderr of a command to a file
* >&j Redirect stdout to file descriptor j
>&2 Redirect stdout of a command to stderr
* i>&j Redirect file descriptor i to file descriptor j
2>&1 Redirect stderr to stdout
* Closing file descriptors
n<&- Close input file descriptor n
0<&-, <&- Close stdin
n>&- Close output file descriptor n
1>&-, >&- Close stdout
* && Logical AND test operator
* Command & Run job in background
Hashmark, special symbol beginning a script comment
#! Sha-bang, special string starting a shell script
* Wild card, in globbing
* Any number of characters in a Regular Expression
* ** Exponentiation, arithmetic operator
* ** Extended globbing file-match operator
% Percent sign
* Modulo, division-remainder arithmetic operation
* Substring removal (pattern matching) operator
* Character match, in an extended Regular Expression
* Prefix to alternate parameter, in parameter substitution
* ++ C-style variable increment, within double parentheses
Shell Variables
$_ Last argument to previous command
$- Flags passed to script, using set
$! Process ID of last background job
$? Exit status of a command
$@ All the positional parameters, as separate words
$* All the positional parameters, as a single word
$$ Process ID of the script
$# Number of arguments passed to a function, or to the script itself
$0 Filename of the script
$1 First argument passed to script
$9 Ninth argument passed to script
Table of shell variables
-a Logical AND compound comparison test
Address database, script example
Advanced Bash Scripting Guide, where to download
Alias
* Removing an alias, using unalias
Anagramming
And list
* To supply default command-line argument
And logical operator &&
Angle brackets, escaped, < . . . > word boundary in a Regular
Expression
Anonymous here document, using :
Archiving
* rpm
* tar
Arithmetic expansion
* exit status of
* variations of
Arithmetic operators
* combination operators, C-style
+= -= *= /= %=
Note
In certain contexts, += can also function as a string concatenation
operator.
Arrays
* Associative arrays
more efficient than conventional arrays
* Bracket notation
* Concatenating, example script
* Copying
* Declaring
declare -a array_name
* Embedded arrays
* Empty arrays, empty elements, example script
* Indirect references
* Initialization
array=( element1 element2 ... elementN)
Example script
Using command substitution
* Loading a file into an array
* Multidimensional, simulating
* Nesting and embedding
* Notation and usage
* Number of elements in
${#array_name[@]}
${#array_name[*]}
* Operations
* Passing an array to a function
* As return value from a function
* Special properties, example script
* String operations, example script
* unset deletes array elements
Arrow keys, detecting
ASCII
* Definition
* Scripts for generating ASCII table
awk field-oriented text processing language
* rand(), random function
* String manipulation
* Using export to pass a variable to an embedded awk script
Backlight, setting the brightness
Backquotes, used in command substitution
Base conversion, example script
Bash
* Bad scripting practices
* Basics reviewed, script example
* Command-line options
Table
* Features that classic Bourne shell lacks
* Internal variables
* Version 2
* Version 3
* Version 4
Version 4.1
Version 4.2
.bashrc
$BASH_SUBSHELL
Basic commands, external
Batch files, DOS
Batch processing
bc, calculator utility
* In a here document
* Template for calculating a script variable
Bibliography
Bison utility
Bitwise operators
* Example script
Block devices
* testing for
Blocks of code
* Iterating / looping
* Redirection
Script example: Redirecting output of a a code block
Bootable flash drives, creating
Brace expansion
* Extended, {a..z}
* Parameterizing
* With increment and zero-padding (new feature in Bash, version 4)
Brackets, [ ]
* Array element
* Enclose character set to match in a Regular Expression
* Test construct
Brackets, curly, {}, used in
* Code block
* find
* Extended Regular Expressions
* Positional parameters
* xargs
break loop control command
* Parameter (optional)
Builtins in Bash
* Do not fork a subprocess
case construct
* Command-line parameters, handling
* Globbing, filtering strings with
cat, concatentate file(s)
* Abuse of
* cat scripts
* Less efficient than redirecting stdin
* Piping the output of, to a read
* Uses of
Character devices
* testing for
Checksum
Child processes
Colon, : , equivalent to the true Bash builtin
Colorizing scripts
* Cycling through the background colors, example script
* Table of color escape sequences
* Template, colored text on colored background
Comma operator, linking commands or operations
Command-line options
command_not_found_handle () builtin error-handling function (version
4+ of Bash)
Command substitution
* $( ... ), preferred notation
* Backquotes
* Extending the Bash toolset
* Invokes a subshell
* Nesting
* Removes trailing newlines
* Setting variable from loop output
* Word splitting
Comment headers, special purpose
Commenting out blocks of code
* Using an anonymous here document
* Using an if-then construct
Communications and hosts
Compound comparison operators
Compression utilities
* bzip2
* compress
* gzip
* zip
continue loop control command
Control characters
* Control-C, break
* Control-D, terminate / log out / erase
* Control-G, BEL (beep)
* Control-H, rubout
* Control-J, newline
* Control-M, carriage return
Coprocesses
cron, scheduling daemon
C-style syntax , for handling variables
Crossword puzzle solver
Cryptography
Curly brackets {}
* in find command
* in an Extended Regular Expression
* in xargs
Daemons, in UNIX-type OS
date
dc, calculator utility
dd, data duplicator command
* Conversions
* Copying raw data to/from devices
* File deletion, secure
* Keystrokes, capturing
* Options
* Random access on a data stream
* Raspberry Pi, script for preparing a bootable SD card
* Swapfiles, initializing
* Thread on www.linuxquestions.org
Debugging scripts
* Tools
* Trapping at exit
* Trapping signals
Decimal number, Bash interprets numbers as
declare builtin
* options
case-modification options (version 4+ of Bash)
Default parameters
/dev directory
* /dev/null pseudo-device file
* /dev/urandom pseudo-device file, generating pseudorandom numbers
with
* /dev/zero, pseudo-device file
Device file
dialog, utility for generating dialog boxes in a script
$DIRSTACK directory stack
Disabled commands, in restricted shells
do keyword, begins execution of commands within a loop
done keyword, terminates a loop
DOS batch files, converting to shell scripts
DOS commands, UNIX equivalents of (table)
dot files, "hidden" setup and configuration files
Double brackets [[ ... ]] test construct
* and evaluation of octal/hex constants
Double parentheses (( ... )) arithmetic expansion/evaluation
construct
Double quotes " ... " weak quoting
* Double-quoting the backslash (\) character
Double-spacing a text file, using sed
-e File exists test
echo
* Feeding commands down a pipe
* Setting a variable using command substitution
* /bin/echo, external echo command
elif, Contraction of else and if
else
Encrypting files, using openssl
esac, keyword terminating case construct
Environmental variables
-eq , is-equal-to integer comparison test
Eratosthenes, Sieve of, algorithm for generating prime numbers
Escaped characters, special meanings of
* Within $' ... ' string expansion
* Used with Unicode characters
/etc/fstab (filesystem mount) file
/etc/passwd (user account) file
$EUID, Effective user ID
eval, Combine and evaluate expression(s), with variable expansion
* Effects of, Example script
* Forces reevaluation of arguments
* And indirect references
* Risk of using
* Using eval to convert array elements into a command list
* Using eval to select among variables
Evaluation of octal/hex constants within [[ ... ]]
exec command, using in redirection
Exercises
Exit and Exit status
* exit command
* Exit status (exit code, return status of a command)
Table, Exit codes with special meanings
Anomalous
Out of range
Pipe exit status
Specified by a function return
Successful, 0
/usr/include/sysexits.h, system file listing C/C++ standard exit
codes
Export, to make available variables to child processes
* Passing a variable to an embedded awk script
expr, Expression evaluator
* Substring extraction
* Substring index (numerical position in string)
* Substring matching
Extended Regular Expressions
* ? (question mark) Match zero / one characters
* ( ... ) Group of expressions
* \{ N \} "Curly" brackets, escaped, number of character sets to
match
* + Character match
factor, decomposes an integer into its prime factors
* Application: Generating prime numbers
false, returns unsuccessful (1) exit status
Field, a group of characters that comprises an item of data
Files / Archiving
File descriptors
* Closing
n<&- Close input file descriptor n
0<&-, <&- Close stdin
n>&- Close output file descriptor n
1>&-, >&- Close stdout
* File handles in C, similarity to
File encryption
find
* {} Curly brackets
* \; Escaped semicolon
Filter
* Using - with file-processing utility as a filter
* Feeding output of a filter back to same filter
Floating point numbers, Bash does not recognize
fold, a filter to wrap lines of text
Forking a child process
for loops
Functions
* Arguments passed referred to by position
* Capturing the return value of a function using echo
* Colon as function name
* Definition must precede first call to function
* Exit status
* Local variables
and recursion
* Passing an array to a function
* Passing pointers to a function
* Positional parameters
* Recursion
* Redirecting stdin of a function
* return
Multiple return values from a function, example script
Returning an array from a function
Return range limits, workarounds
* Shift arguments passed to a function
* Unusual function names
Games and amusements
* Anagrams
* Anagrams, again
* Bingo Number Generator
* Crossword puzzle solver
* Crypto-Quotes
* Dealing a deck of cards
* Fifteen Puzzle
* Horse race
* Knight's Tour
* "Life" game
* Magic Squares
* Music-playing script
* Nim
* Pachinko
* Perquackey
* Petals Around the Rose
* Podcasting
* Poem
* Speech generation
* Towers of Hanoi
Graphic version
Alternate graphic version
getopt, external command for parsing script command-line arguments
* Emulated in a script
getopts, Bash builtin for parsing script command-line arguments
* $OPTIND / $OPTARG
Global variable
Globbing, filename expansion
* Handling filenames correctly
* Wild cards
* Will not match dot files
Golden Ratio (Phi)
-ge , greater-than or equal integer comparison test
-gt , greater-than integer comparison test
groff, text markup and formatting language
Gronsfeld cipher
$GROUPS, Groups user belongs to
gzip, compression utility
Hashing, creating lookup keys in a table
* Example script
head, echo to stdout lines at the beginning of a text file
help, gives usage summary of a Bash builtin
Here documents
* Anonymous here documents, using :
Commenting out blocks of code
Self-documenting scripts
* bc in a here document
* cat scripts
* Command substitution
* ex scripts
* Function, supplying input to
* Here strings
Calculating the Golden Ratio
Prepending text
As the stdin of a loop
Using read
* Limit string
! as a limit string
Closing limit string may not be indented
Dash option to limit string, <<-LimitString
* Literal text output, for generating program code
* Parameter substitution
Disabling parameter substitution
* Passing parameters
* Temporary files
* Using vi non-interactively
History commands
$HOME, user's home directory
Homework assignment solver
$HOSTNAME, system host name
$Id parameter, in rcs (Revision Control System)
if [ condition ]; then ... test construct
* if-grep, if and grep in combination
Fixup for if-grep test
$IFS, Internal field separator variable
* Defaults to whitespace
Integer comparison operators
in, keyword preceding [list] in a for loop
Initialization table, /etc/inittab
Inline group, i.e., code block
Interactive script, test for
I/O redirection
Indirect referencing of variables
* New notation, introduced in version 2 of Bash ( example script)
iptables, packet filtering and firewall utility
* Usage example
* Example script
Iteration
Job IDs, table
jot, Emit a sequence of integers. Equivalent to seq.
* Random sequence generation
Just another Bash hacker!
Keywords
* error, if missing
kill, terminate a process by process ID
* Options (-l, -9)
killall, terminate a process by name
killall script in /etc/rc.d/init.d
lastpipe shell option
-le , less-than or equal integer comparison test
let, setting and carrying out arithmetic operations on variables
* C-style increment and decrement operators
Limit string, in a here document
$LINENO, variable indicating the line number where it appears in a
script
Link, file (using ln command)
* Invoking script with multiple names, using ln
* symbolic links, ln -s
List constructs
* And list
* Or list
Local variables
* and recursion
Localization
Logical operators (&&, ||, etc.)
Logout file, the ~/.bash_logout file
Loopback device, mounting a file on a block device
Loops
* break loop control command
* continue loop control command
* C-style loop within double parentheses
for loop
while loop
* do (keyword), begins execution of commands within a loop
* done (keyword), terminates a loop
* for loops
for arg in [list]; do
Command substitution to generate [list]
Filename expansion in [list]
Multiple parameters in each [list] element
Omitting [list], defaults to positional parameters
Parameterizing [list]
Redirection
* in, (keyword) preceding [list] in a for loop
* Nested loops
* Running a loop in the background, script example
* Semicolon required, when do is on first line of loop
for loop
while loop
* until loop
until [ condition-is-true ]; do
* while loop
while [ condition ]; do
Function call inside test brackets
Multiple conditions
Omitting test brackets
Redirection
while read construct
* Which type of loop to use
Loopback devices
* In /dev directory
* Mounting an ISO image
-lt , less-than integer comparison test
m4, macro processing language
$MACHTYPE, Machine type
Magic number, marker at the head of a file indicating the file type
Makefile, file containing the list of dependencies used by make
command
man, manual page (lookup)
* Man page editor (script)
mapfile builtin, loads an array with a text file
Math commands
Meta-meaning
Morse code training script
Modulo, arithmetic remainder operator
* Application: Generating prime numbers
Mortgage calculations, example script
-n String not null test
Named pipe, a temporary FIFO buffer
* Example script
nc, netcat, a network toolkit for TCP and UDP ports
-ne, not-equal-to integer comparison test
Negation operator, !, reverses the sense of a test
netstat, Network statistics
Network programming
nl, a filter to number lines of text
Noclobber, -C option to Bash to prevent overwriting of files
NOT logical operator, !
null variable assignment, avoiding
-o Logical OR compound comparison test
Obfuscation
* Colon as function name
* Homework assignment
* Just another Bash hacker!
octal, base-8 numbers
od, octal dump
$OLDPWD Previous working directory
openssl encryption utility
Operator
* Definition of
* Precedence
Options, passed to shell or script on command line or by set command
Or list
Or logical operator, ||
Parameter substitution
* ${parameter+alt_value}
${parameter:+alt_value}
Alternate value of parameter, if set
* ${parameter-default}
${parameter:-default}
${parameter=default}
${parameter:=default}
Default parameters
* ${!varprefix*}
${!varprefix@}
Parameter name match
* ${parameter?err_msg}
Parameter-unset message
* ${parameter}
Value of parameter
* Case modification (version 4+ of Bash).
* Script example
* Table of parameter substitution
Parent / child process problem, a child process cannot export
variables to a parent process
Parentheses
* Command group
* Enclose group of Extended Regular Expressions
* Double parentheses, in arithmetic expansion
$PATH, the path (location of system binaries)
* Appending directories to $PATH using the += operator.
Pathname, a filename that incorporates the complete path of a given
file.
* Parsing pathnames
Perl, programming language
* Combined in the same file with a Bash script
* Embedded in a Bash script
Perquackey-type anagramming game (Quackey script)
Petals Around the Rose
PID, Process ID, an identification number assigned to a running
process.
Pipe, | , a device for passing the output of a command to another
command or to the shell
* Avoiding unnecessary commands in a pipe
* Comments embedded within
* Exit status of a pipe
* Pipefail, set -o pipefail option to indicate exit status within a
pipe
* $PIPESTATUS, exit status of last executed pipe
* Piping output of a command to a script
* Redirecting stdin, rather than using cat in a pipe
Pitfalls
* - (dash) is not redirection operator
* // (double forward slash), behavior of cd command toward
* #!/bin/sh script header disables extended Bash features
* Abuse of cat
* CGI programming, using scripts for
* Closing limit string in a here document, indenting
* DOS-type newlines (\r\n) crash a script
* Double-quoting the backslash (\) character
* eval, risk of using
* Execute permission lacking for commands within a script
* Exit status, anomalous
* Exit status of arithmetic expression not equivalent to an error
code
* Export problem, child process to parent process
* Extended Bash features not available
* Failing to quote variables within test brackets
* GNU command set, in cross-platform scripts
* let misuse: attempting to set string variables
* Multiple echo statements in a function whose output is captured
* null variable assignment
* Numerical and string comparison operators not equivalent
= and -eq not interchangeable
* Omitting terminal semicolon, in a curly-bracketed code block
* Piping
echo to a loop
echo to read (however, this problem can be circumvented)
tail -f to grep
* Preserving whitespace within a variable, unintended consequences
* suid commands inside a script
* Undocumented Bash features, danger of
* Updates to Bash breaking older scripts
* Uninitialized variables
* Variable names, inappropriate
* Variables in a subshell, scope limited
* Subshell in while-read loop
* Whitespace, misuse of
Pointers
* and file descriptors
* and functions
* and indirect references
* and variables
Portability issues in shell scripting
* Setting path and umask
* A test suite script (Bash versus classic Bourne shell)
* Using whatis
Positional parameters
* $@, as separate words
* $*, as a single word
* in functions
POSIX, Portable Operating System Interface / UNIX
* --posix option
* 1003.2 standard
* Character classes
$PPID, process ID of parent process
Precedence, operator
Prepending lines at head of a file, script example
Prime numbers
* Generating primes using the factor command
* Generating primes using the modulo operator
* Sieve of Eratosthenes, example script
printf, formatted print command
/proc directory
* Running processes, files describing
* Writing to files in /proc, warning
Process
* Child process
* Parent process
* Process ID (PID)
Process substitution
* To compare contents of directories
* To supply stdin of a command
* Template
* while-read loop without a subshell
Programmable completion (tab expansion)
Prompt
* $PS1, Main prompt, seen at command line
* $PS2, Secondary prompt
Pseudo-code, as problem-solving method
$PWD, Current working directory
Quackey, a Perquackey-type anagramming game (script)
Question mark, ?
* Character match in an Extended Regular Expression
* Single-character wild card, in globbing
* In a C-style Trinary (ternary) operator
Quoting
* Character string
* Variables
within test brackets
* Whitespace, using quoting to preserve
Random numbers
* /dev/urandom
* rand(), random function in awk
* $RANDOM, Bash function that returns a pseudorandom integer
* Random sequence generation, using date command
* Random sequence generation, using jot
* Random string, generating
Raspberry Pi (single-board computer)
* Script for preparing a bootable SD card
rcs
read, set value of a variable from stdin
* Detecting arrow keys
* Options
* Piping output of cat to read
* "Prepending" text
* Problems piping echo to read
* Redirection from a file to read
* $REPLY, default read variable
* Timed input
* while read construct
readline library
Recursion
* Demonstration of
* Factorial
* Fibonacci sequence
* Local variables
* Script calling itself recursively
* Towers of Hanoi
Redirection
* Code blocks
* exec <filename,
to reassign file descriptors
* Introductory-level explanation of I/O redirection
* Open a file for both reading and writing
<>filename
* read input redirected from a file
* stderr to stdout
2>&1
* stdin / stdout, using -
* stdinof a function
* stdout to a file
> ... >>
* stdout to file descriptor j
>&j
* file descriptori to file descriptor j
i>&j
* stdout of a command to stderr
>&2
* stdout and stderr of a command to a file
&>
* tee, redirect to a file output of command(s) partway through a
pipe
Reference Cards
* Miscellaneous constructs
* Parameter substitution/expansion
* Special shell variables
* String operations
* Test operators
Binary comparison
Files
Regular Expressions
* ^ (caret) Beginning-of-line
* $ (dollar sign) Anchor
* . (dot) Match single character
* * (asterisk) Any number of characters
* [ ] (brackets) Enclose character set to match
* \ (backslash) Escape, interpret following character literally
* \< ... \> (angle brackets, escaped) Word boundary
* Extended REs
+ Character match
\{ \} Escaped "curly" brackets
[: :] POSIX character classes
$REPLY, Default value associated with read command
Restricted shell, shell (or script) with certain commands disabled
return, command that terminates a function
run-parts
* Running scripts in sequence, without user intervention
Scope of a variable, definition
Script options, set at command line
Scripting routines, library of useful definitions and functions
Secondary prompt, $PS2
Security issues
* nmap, network mapper / port scanner
* sudo
* suid commands inside a script
* Viruses, trojans, and worms in scripts
* Writing secure scripts
sed, pattern-based programming language
* Table, basic operators
* Table, examples of operators
select, construct for menu building
* in list omitted
Semaphore
Semicolon required, when do keyword is on first line of loop
* When terminating curly-bracketed code block
seq, Emit a sequence of integers. Equivalent to jot.
set, Change value of internal script variables
* set -u, Abort script with error message if attempting to use an
undeclared variable.
Shell script, definition of
Shell wrapper, script embedding a command or utility
shift, reassigning positional parameters
$SHLVL, shell level, depth to which the shell (or script) is nested
shopt, change shell options
Signal, a message sent to a process
Simulations
* Brownian motion
* Galton board
* Horserace
* Life, game of
* PI, approximating by firing cannonballs
* Pushdown stack
Single quotes (' ... ') strong quoting
Socket, a communication node associated with an I/O port
Sorting
* Bubble sort
* Insertion sort
source, execute a script or, within a script, import a file
* Passing positional parameters
Spam, dealing with
* Example script
* Example script
* Example script
* Example script
Special characters
Stack
* Definition
* Emulating a push-down stack, example script
Standard Deviation, example script
Startup files, Bash
stdin and stdout
Stopwatch, example script
Strings
* =~ String match operator
* Comparison
* Length
${#string}
* Manipulation
* Manipulation, using awk
* Null string, testing for
* Protecting strings from expansion and/or reinterpretation, script
example
Unprotecting strings, script example
* strchr(), equivalent of
* strlen(), equivalent of
* strings command, find printable strings in a binary or data file
* Substring extraction
${string:position}
${string:position:length}
Using expr
* Substring index (numerical position in string)
* Substring matching, using expr
* Substring removal
${var#Pattern}
${var##Pattern}
${var%Pattern}
${var%%Pattern}
* Substring replacement
${string/substring/replacement}
${string//substring/replacement}
${string/#substring/replacement}
${string/%substring/replacement}
Script example
* Table of string/substring manipulation and extraction operators
Strong quoting ' ... '
Stylesheet for writing scripts
Subshell
* Command list within parentheses
* Variables, $BASH_SUBSHELL and $SHLVL
* Variables in a subshell
scope limited, but ...
... can be accessed outside the subshell?
su Substitute user, log on as a different user or as root
suid (set user id) file flag
* suid commands inside a script, not advisable
Symbolic links
Swapfiles
Tab completion
Table lookup, script example
tail, echo to stdout lines at the (tail) end of a text file
tar, archiving utility
tee, redirect to a file output of command(s) partway through a pipe
Terminals
* setserial
* setterm
* stty
* tput
* wall
test command
* Bash builtin
* external command, /usr/bin/test (equivalent to /usr/bin/[)
Test constructs
Test operators
* -a Logical AND compound comparison
* -e File exists
* -eq is-equal-to (integer comparison)
* -f File is a regular file
* -ge greater-than or equal (integer comparison)
* -gt greater-than (integer comparison)
* -le less-than or equal (integer comparison)
* -lt less-than (integer comparison)
* -n not-zero-length (string comparison)
* -ne not-equal-to (integer comparison)
* -o Logical OR compound comparison
* -u suid flag set, file test
* -z is-zero-length (string comparison)
* = is-equal-to (string comparison)
== is-equal-to (string comparison)
* < less-than (string comparison)
* < less-than, (integer comparison, within double parentheses)
* <= less-than-or-equal, (integer comparison, within double
parentheses)
* > greater-than (string comparison)
* > greater-than, (integer comparison, within double parentheses)
* >= greater-than-or-equal, (integer comparison, within double
parentheses)
* || Logical OR
* && Logical AND
* ! Negation operator, inverts exit status of a test
!= not-equal-to (string comparison)
* Tables of test operators
Binary comparison
File
Text and text file processing
Time / Date
Timed input
* Using read -t
* Using stty
* Using timing loop
* Using $TMOUT
Tips and hints for Bash scripts
* Array, as return value from a function
Associative array more efficient than a numerically-indexed array
* Capturing the return value of a function, using echo
* CGI programming, using scripts for
* Comment blocks
Using anonymous here documents
Using if-then constructs
* Comment headers, special purpose
* C-style syntax , for manipulating variables
* Double-spacing a text file
* Filenames prefixed with a dash, removing
* Filter, feeding output back to same filter
* Function return value workarounds
* if-grep test fixup
* Library of useful definitions and functions
* null variable assignment, avoiding
* Passing an array to a function
* $PATH, appending to, using the += operator.
* Prepending lines at head of a file
* Progress bar template
* Pseudo-code
* rcs
* Redirecting a test to /dev/null to suppress output
* Running scripts in sequence without user intervention, using
run-parts
* Script as embedded command
* Script portability
Setting path and umask
Using whatis
* Setting script variable to a block of embedded sed or awk code
* Speeding up script execution by disabling unicode
* Subshell variable, accessing outside the subshell
* Testing a variable to see if it contains only digits
* Testing whether a command exists, using type
* Tracking script usage
* while-read loop without a subshell
* Widgets, invoking from a script
$TMOUT, Timeout interval
Token, a symbol that may expand to a keyword or command
tput, terminal-control command
tr, character translation filter
* DOS to Unix text file conversion
* Options
* Soundex, example script
* Variants
Trap, specifying an action upon receipt of a signal
Trinary (ternary) operator, C-style, var>10?88:99
* in double-parentheses construct
* in let construct
true, returns successful (0) exit status
typeset builtin
* options
$UID, User ID number
unalias, to remove an alias
uname, output system information
Unicode, encoding standard for representing letters and symbols
* Disabling unicode to optimize script
Uninitialized variables
uniq, filter to remove duplicate lines from a sorted file
unset, delete a shell variable
until loop
until [ condition-is-true ]; do
Variables
* Array operations on
* Assignment
Script example
Script example
Script example
* Bash internal variables
* Block of sed or awk code, setting a variable to
* C-style increment/decrement/trinary operations
* Change value of internal script variables using set
* declare, to modify the properties of variables
* Deleting a shell variable using unset
* Environmental
* Expansion / Substring replacement operators
* Indirect referencing
eval variable1=\$$variable2
Newer notation
${!variable}
* Integer
* Integer / string (variables are untyped)
* Length
${#var}
* Lvalue
* Manipulating and expanding
* Name and value of a variable, distinguishing between
* Null string, testing for
* Null variable assignment, avoiding
* Quoting
within test brackets
to preserve whitespace
* rvalue
* Setting to null value
* In subshell not visible to parent shell
* Testing a variable if it contains only digits
* Typing, restricting the properties of a variable
* Undeclared, error message
* Uninitialized
* Unquoted variable, splitting
* Unsetting
* Untyped
wait, suspend script execution
* To remedy script hang
Weak quoting " ... "
while loop
while [ condition ]; do
* C-style syntax
* Calling a function within test brackets
* Multiple conditions
* Omitting test brackets
* while read construct
Avoiding a subshell
Whitespace, spaces, tabs, and newline characters
* $IFS defaults to
* Inappropriate use of
* Preceding closing limit string in a here document, error
* Preceding script comments
* Quoting, to preserve whitespace within strings or variables
* [:space:], POSIX character class
who, information about logged on users
* w
* whoami
* logname
Widgets
Wild card characters
* Asterisk *
* In [list] constructs
* Question mark ?
* Will not match dot files
Word splitting
* Definition
* Resulting from command substitution
Wrapper, shell
xargs, Filter for grouping arguments
* Curly brackets
* Limiting arguments passed
* Options
* Processes arguments one at a time
* Whitespace, handling
yes
* Emulation
-z String is null
Zombie, a process that has terminated, but not yet been killed by its
parent
Notes
[1]
These are referred to as builtins, features internal to the shell.
[2]
Although recursion is possible in a shell script, it tends to be slow
and its implementation is often an ugly kludge.
[3]
An acronym is an ersatz word formed by pasting together the initial
letters of the words into a tongue-tripping phrase. This morally
corrupt and pernicious practice deserves appropriately severe
punishment. Public flogging suggests itself.
[4]
Many of the features of ksh88, and even a few from the updated ksh93
have been merged into Bash.
[5]
By convention, user-written shell scripts that are Bourne shell
compliant generally take a name with a .sh extension. System scripts,
such as those found in /etc/rc.d, do not necessarily conform to this
nomenclature.
[6]
More commonly seen in the literature as she-bang or sh-bang. This
derives from the concatenation of the tokens sharp (#) and bang (!).
[7]
Some flavors of UNIX (those based on 4.2 BSD) allegedly take a
four-byte magic number, requiring a blank after the ! -- #! /bin/sh.
[http://www.in-ulm.de/~mascheck/various/shebang/#details] According
to Sven Mascheck this is probably a myth.
[8]
The #! line in a shell script will be the first thing the command
interpreter (sh or bash) sees. Since this line begins with a #, it
will be correctly interpreted as a comment when the command
interpreter finally executes the script. The line has already served
its purpose - calling the command interpreter.
If, in fact, the script includes an extra #! line, then bash will
interpret it as a comment.
!/bin/bash
echo "Part 1 of script."
a=1
!/bin/bash
This does not launch a new script.
echo "Part 2 of script."
echo $a # Value of $a stays at 1.
[9]
This allows some cute tricks.
!/bin/rm
Self-deleting script.
Nothing much seems to happen when you run this... except that the file disap
pears.
WHATEVER=85
echo "This line will never print (betcha!)."
exit $WHATEVER # Doesn't matter. The script will not exit here.
# Try an echo $? after script termination.
# You'll get a 0, not a 85.
Also, try starting a README file with a #!/bin/more, and making it
executable. The result is a self-listing documentation file. (A here
document using cat is possibly a better alternative -- see Example
19-3).
[10]
Portable Operating System Interface, an attempt to standardize
UNIX-like OSes. The POSIX specifications are listed on the Open Group
site.
[11]
To avoid this possibility, a script may begin with a #!/bin/env bash
sha-bang line. This may be useful on UNIX machines where bash is not
located in /bin
[12]
If Bash is your default shell, then the #! isn't necessary at the
beginning of a script. However, if launching a script from a
different shell, such as tcsh, then you will need the #!.
[13]
Caution: invoking a Bash script by sh scriptname turns off
Bash-specific extensions, and the script may therefore fail to
execute.
[14]
A script needs read, as well as execute permission for it to run,
since the shell needs to be able to read it.
[15]
Why not simply invoke the script with scriptname? If the directory
you are in ($PWD) is where scriptname is located, why doesn't this
work? This fails because, for security reasons, the current directory
(./) is not by default included in a user's $PATH. It is therefore
necessary to explicitly invoke the script in the current directory
with a ./scriptname.
[16]
An operator is an agent that carries out an operation. Some examples
are the common arithmetic operators, + - * /. In Bash, there is some
overlap between the concepts of operator and keyword.
[17]
This is more commonly known as the ternary operator. Unfortunately,
ternary is an ugly word. It doesn't roll off the tongue, and it
doesn't elucidate. It obfuscates. Trinary is by far the more elegant
usage.
[18]
American Standard Code for Information Interchange. This is a system
for encoding text characters (alphabetic, numeric, and a limited set
of symbols) as 7-bit numbers that can be stored and manipulated by
computers. Many of the ASCII characters are represented on a standard
keyboard.
[19]
A PID, or process ID, is a number assigned to a running process. The
PIDs of running processes may be viewed with a ps command.
Definition: A process is a currently executing command (or program),
sometimes referred to as a job.
[20]
The shell does the brace expansion. The command itself acts upon the
result of the expansion.
[21]
Exception: a code block in braces as part of a pipe may run as a
subshell.
ls | { read firstline; read secondline; }
Error. The code block in braces runs as a subshell,
- so the output of "ls" cannot be passed to variables within the block.
echo "First line is $firstline; second line is $secondline" # Won't work.
Thanks, S.C.
[22]
Even as in olden times a philtre denoted a potion alleged to have
magical transformative powers, so does a UNIX filter transform its
target in (roughly) analogous fashion. (The coder who comes up with a
"love philtre" that runs on a Linux machine will likely win accolades
and honors.)
[23]
Bash stores a list of commands previously issued from the
command-line in a buffer, or memory space, for recall with the
builtin history commands.
[24]
A linefeed (newline) is also a whitespace character. This explains
why a blank line, consisting only of a linefeed, is considered
whitespace.
[25]
Technically, the name of a variable is called an lvalue, meaning that
it appears on the left side of an assignment statment, as in
VARIABLE=23. A variable's value is an rvalue, meaning that it appears
on the right side of an assignment statement, as in VAR2=$VARIABLE.
A variable's name is, in fact, a reference, a pointer to the memory
location(s) where the actual data associated with that variable is
kept.
[26]
Note that functions also take positional parameters.
[27]
The process calling the script sets the $0 parameter. By convention,
this parameter is the name of the script. See the manpage (manual
page) for execv.
From the command-line, however, $0 is the name of the shell.
bash$ echo $0
bash
tcsh% echo $0
tcsh
[28]
If the the script is sourced or symlinked, then this will not work.
It is safer to check $BASH_Source.
[29]
Unless there is a file named first in the current working directory.
Yet another reason to quote. (Thank you, Harald Koenig, for pointing
this out.
[30]
Encapsulating "!" within double quotes gives an error when used from
the command line. This is interpreted as a history command. Within a
script, though, this problem does not occur, since the Bash history
mechanism is disabled then.
Of more concern is the apparently inconsistent behavior of \ within
double quotes, and especially following an echo -e command.
bash$ echo hello!
hello!
bash$ echo "hello!"
hello!
bash$ echo \
bash$ echo ""
bash$ echo \a
a
bash$ echo "\a"
\a
bash$ echo x\ty
xty
bash$ echo "x\ty"
x\ty
bash$ echo -e x\ty
xty
bash$ echo -e "x\ty"
x y
Double quotes following an echo sometimes escape . Moreover, the -e
option to echo causes the "\t" to be interpreted as a tab.
(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and
Daniel Barclay for explaining it.)
[31]
"Word splitting," in this context, means dividing a character string
into separate and discrete arguments.
[32]
In those instances when there is no return terminating the function.
[33]
A token is a symbol or short string with a special meaning attached
to it (a meta-meaning). In Bash, certain tokens, such as [ and .
(dot-command), may expand to keywords and commands.
[34]
Per the 1913 edition of Webster's Dictionary:
Deprecate
...
To pray against, as an evil;
to seek to avert by prayer;
to desire the removal of;
to seek deliverance from;
to express deep regret for;
to disapprove of strongly.
[35]
Be aware that suid binaries may open security holes. The suid flag
has no effect on shell scripts.
[36]
On Linux systems, the sticky bit is no longer used for files, only on
directories.
[37]
As S.C. points out, in a compound test, even quoting the string
variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause
an error with some versions of Bash if $string is empty. The safe way
is to append an extra character to possibly empty variables, [
"x$string" != x -o "x$a" = "x$b" ] (the "x's" cancel out).
[38]
In a different context, += can serve as a string concatenation
operator. This can be useful for modifying environmental variables.
[39]
Side effects are, of course, unintended -- and usually undesirable --
consequences.
[40]
Precedence, in this context, has approximately the same meaning as
priority
[41]
A stack register is a set of consecutive memory locations, such that
the values stored (pushed) are retrieved (popped) in reverse order.
The last value stored is the first retrieved. This is sometimes
called a LIFO (last-in-first-out) or pushdown stack.
[42]
The PID of the currently running script is $$, of course.
[43]
Somewhat analogous to recursion, in this context nesting refers to a
pattern embedded within a larger pattern. One of the definitions of
nest, according to the 1913 edition of Webster's Dictionary,
illustrates this beautifully: "A collection of boxes, cases, or the
like, of graduated size, each put within the one next larger."
[44]
The words "argument" and "parameter" are often used interchangeably.
In the context of this document, they have the same precise meaning:
a variable passed to a script or function.
[45]
Within a script, inside a subshell, $$ returns the PID of the script,
not the subshell.
[46]
In this context, typing a variable means to classify it and restrict
its properties. For example, a variable declared or typed as an
integer is no longer available for string operations.
declare -i intvar
intvar=23
echo "$intvar" # 23
intvar=stringval
echo "$intvar" # 0
[47]
True "randomness," insofar as it exists at all, can only be found in
certain incompletely understood natural phenomena, such as
radioactive decay. Computers only simulate randomness, and
computer-generated sequences of "random" numbers are therefore
referred to as pseudorandom.
[48]
The seed of a computer-generated pseudorandom number series can be
considered an identification label. For example, think of the
pseudorandom series with a seed of 23 as Series #23.
A property of a pseurandom number series is the length of the cycle
before it starts repeating itself. A good pseurandom generator will
produce series with very long cycles.
[49]
This applies to either command-line arguments or parameters passed to
a function.
[50]
Note that $substring and $replacement may refer to either literal
strings or variables, depending on context. See the first usage
example.
[51]
If $parameter is null in a non-interactive script, it will terminate
with a 127 exit status (the Bash error code for "command not found").
[52]
Iteration: Repeated execution of a command or group of commands,
usually -- but not always, while a given condition holds, or until a
given condition is met.
[53]
These are shell builtins, whereas other loop commands, such as while
and case, are keywords.
[54]
Pattern-match lines may also start with a ( left paren to give the
layout a more structured appearance.
case $( arch ) in # $( arch ) returns machine architecture.
( i386 ) echo "80386-based machine";;
^ ^
( i486 ) echo "80486-based machine";;
( i586 ) echo "Pentium-based machine";;
( i686 ) echo "Pentium2+-based machine";;
( * ) echo "Other type of machine";;
esac
[55]
For purposes of command substitution, a command may be an external
system command, an internal scripting builtin, or even a script
function.
[56]
In a more technically correct sense, command substitution extracts
the stdout of a command, then assigns it to a variable using the =
operator.
[57]
In fact, nesting with backticks is also possible, but only by
escaping the inner backticks, as John Default points out.
word_count= wc -w \echo * | awk '{print $8}'` `
[58]
As Nathan Coulter points out, "while forking a process is a low-cost
operation, executing a new program in the newly-forked child process
adds more overhead."
[59]
An exception to this is the time command, listed in the official Bash
documentation as a keyword ("reserved word").
[60]
Note that let cannot be used for setting string variables.
[61]
To Export information is to make it available in a more general
context. See also scope.
[62]
An option is an argument that acts as a flag, switching script
behaviors on or off. The argument associated with a particular option
indicates the behavior that the option (flag) switches on or off.
[63]
Technically, an exit only terminates the process (or shell) in which
it is running, not the parent process.
[64]
Unless the exec is used to reassign file descriptors.
[65]
Hashing is a method of creating lookup keys for data stored in a
table. The data items themselves are "scrambled" to create keys,
using one of a number of simple mathematical algorithms (methods, or
recipes).
An advantage of hashing is that it is fast. A disadvantage is that
collisions -- where a single key maps to more than one data item --
are possible.
For examples of hashing see Example A-20 and Example A-21.
[66]
The readline library is what Bash uses for reading input in an
interactive shell.
[67]
This only applies to child processes, of course.
[68]
The C source for a number of loadable builtins is typically found in
the /usr/share/doc/bash-?.??/functions directory.
Note that the -f option to enable is not portable to all systems.
[69]
The same effect as autoload can be achieved with typeset -fu.
[70]
The -v option also orders the sort by upper- and lowercase prefixed
filenames.
[71]
Dotfiles are files whose names begin with a dot, such as
~/.Xdefaults. Such filenames do not appear in a normal ls listing
(although an ls -a will show them), and they cannot be deleted by an
accidental rm -rf *. Dotfiles are generally used as setup and
configuration files in a user's home directory.
[72]
This particular feature may not yet be implemented in the version of
the ext2/ext3 filesystem installed on your system. Check the
documentation for your Linux distro.
[73]
And even when xargs is not strictly necessary, it can speed up
execution of a command involving batch-processing of multiple files.
[74]
This is only true of the GNU version of tr, not the generic version
often found on commercial UNIX systems.
[75]
An archive, in the sense discussed here, is simply a set of related
files stored in a single location.
[76]
A tar czvf ArchiveName.tar.gz * will include dotfiles in
subdirectories below the current working directory. This is an
undocumented GNU tar "feature."
[77]
The checksum may be expressed as a hexadecimal number, or to some
other base.
[78]
For even better security, use the sha256sum, sha512, and sha1pass
commands.
[79]
This is a symmetric block cipher, used to encrypt files on a single
system or local network, as opposed to the public key cipher class,
of which pgp is a well-known example.
[80]
Creates a temporary directory when invoked with the -d option.
[81]
A daemon is a background process not attached to a terminal session.
Daemons perform designated services either at specified times or
explicitly triggered by certain events.
The word "daemon" means ghost in Greek, and there is certainly
something mysterious, almost supernatural, about the way UNIX daemons
wander about behind the scenes, silently carrying out their appointed
tasks.
[82]
This is actually a script adapted from the Debian Linux distribution.
[83]
The print queue is the group of jobs "waiting in line" to be printed.
[84]
Large mechanical line printers printed a single line of type at a
time onto joined sheets of greenbar paper, to the accompaniment of a
great deal of noise. The hardcopy thusly printed was referred to as a
printout.
[85]
For an excellent overview of this topic, see Andy Vaught's article,
Introduction to Named Pipes, in the September, 1997 issue of Linux
Journal.
[86]
EBCDIC (pronounced "ebb-sid-ick") is an acronym for Extended Binary
Coded Decimal Interchange Code, an obsolete IBM data format. A
bizarre application of the conv=ebcdic option of dd is as a quick 'n
easy, but not very secure text file encoder.
cat $file | dd conv=swab,ebcdic > $file_encrypted
Encode (looks like gibberish).
Might as well switch bytes (swab), too, for a little extra obscurity.
cat $file_encrypted | dd conv=swab,ascii > $file_plaintext
Decode.
[87]
A macro is a symbolic constant that expands into a command string or
a set of operations on parameters. Simply put, it's a shortcut or
abbreviation.
[88]
This is the case on a Linux machine or a UNIX system with disk
quotas.
[89]
The userdel command will fail if the particular user being deleted is
still logged on.
[90]
For more detail on burning CDRs, see Alex Withers' article, Creating
CDs, in the October, 1999 issue of Linux Journal.
[91]
The -c option to mke2fs also invokes a check for bad blocks.
[92]
Since only root has write permission in the /var/lock directory, a
user script cannot set a lock file there.
[93]
Operators of single-user Linux systems generally prefer something
simpler for backups, such as tar.
[94]
As of the version 4 update of Bash, the -f and -c options take a
block size of 512 when in POSIX mode. Additionally, there are two new
options: -b for socket buffer size, and -T for the limit on the
number of threads.
[95]
NAND is the logical not-and operator. Its effect is somewhat similar
to subtraction.
[96]
In Bash and other Bourne shell derivatives, it is possible to set
variables in a single command's environment.
var1=value1 var2=value2 commandXXX
$var1 and $var2 set in the environment of 'commandXXX' only.
[97]
The killall system script should not be confused with the killall
command in /usr/bin.
[98]
A meta-meaning is the meaning of a term or expression on a higher
level of abstraction. For example, the literal meaning of regular
expression is an ordinary expression that conforms to accepted usage.
The meta-meaning is drastically different, as discussed at length in
this chapter.
[99]
Since sed, awk, and grep process single lines, there will usually not
be a newline to match. In those cases where there is a newline in a
multiple line expression, the dot will match the newline.
!/bin/bash
sed -e 'N;s/.*/[&]/' << EOF # Here Document
line1
line2
EOF
OUTPUT:
[line1
line2]
echo
awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
line 1
line 2
EOF
OUTPUT:
line
1
Thanks, S.C.
exit 0
[100]
Filename expansion means expanding filename patterns or templates
containing special characters. For example, example.??? might expand
to example.001 and/or example.txt.
[101]
A wild card character, analogous to a wild card in poker, can
represent (almost) any other character.
[102]
Filename expansion can match dotfiles, but only if the pattern
explicitly includes the dot as a literal character.
~/[.]bashrc # Will not expand to ~/.bashrc
~/?bashrc # Neither will this.
# Wild cards and metacharacters will NOT
#+ expand to a dot in globbing.
~/.[b]ashrc # Will expand to ~/.bashrc
~/.ba?hrc # Likewise.
~/.bashr* # Likewise.
Setting the "dotglob" option turns this off.
Thanks, S.C.
[103]
Except, as Dennis Benzinger points out, if using <<- to suppress
tabs.
[104]
By convention in UNIX and Linux, data streams and peripherals (device
files) are treated as files, in a fashion analogous to ordinary
files.
[105]
A file descriptor is simply a number that the operating system
assigns to an open file to keep track of it. Consider it a simplified
type of file pointer. It is analogous to a file handle in C.
[106]
Using file descriptor 5 might cause problems. When Bash creates a
child process, as with exec, the child inherits fd 5 (see Chet
Ramey's archived e-mail,
[http://groups.google.com/group/gnu.bash.bug/browse_thread/thread/139
55daafded3b5c/18c17050087f9f37] SUBJECT: RE: File descriptor 5 is
held open). Best leave this particular fd alone.
[107]
An external command invoked with an exec does not (usually) fork off
a subprocess / subshell.
[108]
This has the same effect as a named pipe (temp file), and, in fact,
named pipes were at one time used in process substitution.
[109]
The return command is a Bash builtin.
[110]
However, as Thomas Braunberger points out, a local variable declared
in a function is also visible to functions called by the parent
function.
!/bin/bash
function1 ()
{
local func1var=20
echo "Within function1, $func1var = $func1var."
function2
}
function2 ()
{
echo "Within function2, $func1var = $func1var."
}
function1
exit 0
Output of the script:
Within function1, $func1var = 20.
Within function2, $func1var = 20.
This is documented in the Bash manual:
"Local can only be used within a function; it makes the variable name
have a visible scope restricted to that function and its children."
[emphasis added] The ABS Guide author considers this behavior to be a
bug.
[111]
Otherwise known as redundancy.
[112]
Otherwise known as tautology.
[113]
Otherwise known as a metaphor.
[114]
Otherwise known as a recursive function.
[115]
Too many levels of recursion may crash a script with a segfault.
!/bin/bash
Warning: Running this script could possibly lock up your system!
If you're lucky, it will segfault before using up all available memory.
recursive_function ()
{
echo "$1" # Makes the function do something, and hastens the segfault.
(( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
As long as 1st parameter is less than 2nd,
- increment 1st and recurse.
}
recursive_function 1 50000 # Recurse 50,000 levels!
Most likely segfaults (depending on stack size, set by ulimit -m).
Recursion this deep might cause even a C program to segfault,
- by using up all the memory allotted to the stack.
echo "This will probably not print."
exit 0 # This script will not exit normally.
Thanks, Stéphane Chazelas.
[116]
... as the first word of a command string. Obviously, an alias is
only meaningful at the beginning of a command.
[117]
However, aliases do seem to expand positional parameters.
[118]
The entries in /dev provide mount points for physical and virtual
devices. These entries use very little drive space.
Some devices, such as /dev/null, /dev/zero, and /dev/urandom are
virtual. They are not actual physical devices and exist only in
software.
[119]
A block device reads and/or writes data in chunks, or blocks, in
contrast to a character device, which acesses data in character
units. Examples of block devices are hard drives, CDROM drives, and
flash drives. Examples of character devices are keyboards, modems,
sound cards.
[120]
Of course, the mount point /mnt/flashdrive must exist. If not, then,
as root, mkdir /mnt/flashdrive.
To actually mount the drive, use the following command: mount
/mnt/flashdrive
Newer Linux distros automount flash drives in the /media directory
without user intervention.
[121]
Certain system commands, such as procinfo, free, vmstat, lsdev, and
uptime do this as well.
[122]
By convention, signal 0 is assigned to exit.
[123]
Setting the suid permission on the script itself has no effect in
Linux and most other UNIX flavors.
[124]
In this context, "magic numbers" have an entirely different meaning
than the magic numbers used to designate file types.
[125]
Quite a number of Linux utilities are, in fact, shell wrappers. Some
examples are /usr/bin/pdf2ps, /usr/bin/batch, and /usr/bin/xmkmf.
[126]
ANSI is, of course, the acronym for the American National Standards
Institute. This august body establishes and maintains various
technical and industrial standards.
[127]
This usually means liberal use of functions.
[128]
See Marius van Oers' article, Unix Shell Scripting Malware, and also
the Denning reference in the bibliography.
[129]
Or, better yet, #!/bin/env sh.
[130]
To be more specific, Bash 4+ has limited support for associative
arrays. It's a bare-bones implementation, and it lacks the much of
the functionality of such arrays in other programming languages.
Note, however, that associative arrays in Bash seem to execute faster
and more efficiently than numerically-indexed arrays.
[131]
Copyright 1995-2009 by Chester Ramey.
[132]
This only works with pipes and certain other special files.
[133]
But only in conjunction with readline, i.e., from the command-line.
[134]
And while you're at it, consider fixing the notorious piped read
problem.
[135]
This is the notorious flog it to death technique that works so well
with slow learners, eccentrics, odd ducks, fools and geniuses.
[136]
In fact, he has no credentials or special qualifications. He's a
school dropout with no formal credentials or professional experience
whatsoever. None. Zero. Nada. Aside from the ABS Guide, his major
claim to fame is a First Place in the sack race at the Colfax
Elementary School Field Day in June, 1958.
[137]
Those who can, do. Those who can't . . . get an MCSE.
[138]
Sometimes it seems as if he has spent his entire life flouting
conventional wisdom and defying the sonorous Voice of Authority:
"Hey, you can't do that!"
[139]
Well, if you absolutely insist, you can try modifying Example A-44 to
suit your purposes.
[140]
It was hard to resist the obvious pun. No slight intended, since the
book is a pretty decent introduction to the basic concepts of shell
scripting.
[141]
Sed executes without user intervention.
[142]
If no address range is specified, the default is all lines.
[143]
Its name derives from the initials of its authors, Aho, Weinberg, and
Kernighan.
[144]
Out of range exit values can result in unexpected exit codes. An exit
value greater than 255 returns an exit code modulo 256. For example,
exit 3809 gives an exit code of 225 (3809 % 256 = 225).
[145]
An update of /usr/include/sysexits.h allocates previously unused exit
codes from 64 - 78. It may be anticipated that the range of
unallotted exit codes will be further restricted in the future. The
author of this document will not do fixups on the scripting examples
to conform to the changing standard. This should not cause any
problems, since there is no overlap or conflict in usage of exit
codes between compiled C/C++ binaries and shell scripts.
[146]
This does not apply to csh, tcsh, and other shells not related to or
descended from the classic Bourne shell (sh).
[147]
In older versions of UNIX, passwords were stored in /etc/passwd, and
that explains the name of the file.
[148]
Some early UNIX systems had a fast, small-capacity fixed disk
(containing /, the root partition), and a second drive which was
larger, but slower (containing /usr and other partitions). The most
frequently used programs and utilities therefore resided on the
small-but-fast drive, in /bin, and the others on the slower drive, in
/usr/bin.
This likewise accounts for the split between /sbin and /usr/sbin,
/lib and /usr/lib, etc.
[149]
This works only from the command line, of course, and not within a
script.
[150]
Normally the default parameter completion files reside in either the
/etc/profile.d directory or in /etc/bash_completion. These autoload
on system startup. So, after writing a useful completion script, you
might wish to move it (as root, of course) to one of these
directories.
[151]
It has been extensively documented that programmers are willing to
put in long hours of effort in order to save ten minutes of
"unnecessary" labor. This is known as optimization.
[152]
Various readers have suggested modifications of the above batch file
to prettify it and make it more compact and efficient. In the opinion
of the ABS Guide author, this is wasted effort. A Bash script can
access a DOS filesystem, or even an NTFS partition (with the help of
[http://www.ntfs-3g.org] ntfs-3g) to do batch or scripted operations.
[153]
For all you clever types who failed intermediate algebra, a
determinant is a numerical value associated with a multidimensional
matrix (array of numbers).
For the simple case of a 2 x 2 determinant:
|a b|
|b a|
The solution is aa - bb, where "a" and "b" represent numbers.
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