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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)))


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