Common LISP awk macro for easy text file operations

• Author: Solène
• Date: 04 February 2020
• Tags: awk lisp

NILI like Common LISP and I also like awk. Dealing with text files in Common LISP

is often painful. So I wrote a small awk like common lisp macro, which helps a

lot dealing with text files.

Here is the implementation, I used the uiop package for split-string function,

it comes with sbcl. But it's possible to write your own split-string or reused

the infamous split-str function shared on the Internet.

(defmacro awk(file separator &body code)

  "allow running code for each line of a text file,

   giving access to NF and NR variables, and also to

   fields list containing fields, and line containing $0"

    `(progn

       (let ((stream (open ,file :if-does-not-exist nil)))

         (when stream

           (loop for line = (read-line stream nil)

              counting t into NR

              while line do

                (let* ((fields (uiop:split-string line :separator ,separator))

                       (NF (length fields)))

                  ,@code))))))

It's interesting that the "do" in the loop could be replaced with a "collect",

allowing to reuse awk output as a list into another function, a quick example I

have in mind is this:

;; equivalent of awk '{ print NF }' file | sort | uniq

;; for counting how many differents fields long line we have

(uniq (sort (awk "file" " " NF)))

Now, here are a few examples of usage of this macro, I've written the original

awk command in the comments in comparison:

;; numbering lines of a text file with NR

;; awk '{ print NR": "$0 }' file.txt

;;

(awk "file.txt" " "

     (format t "~a: ~a~%" NR line))

;; display NF-1 field (yes it's -2 in the example because -1 is last field in the list)

;; awk -F ';' '{ print NF-1 }' file.csv

;;

(awk "file.csv" ";"

     (print (nth (- NF 2) fields)))

;; filtering lines (like grep)

;; awk '/unbound/ { print }' /var/log/messages

;;

(awk "/var/log/messages" " "

     (when (search "unbound" line)

       (print line)))

;; printing 4nth field

;; awk -F ';' '{ print $4 }' data.csv

;;

(awk "data.csv" ";"

     (print (nth 4 fields)))