Shell Scripting Using Sed

May 8, 2021

Finding and Replacing Strings

The best part about sed is that it uses regex, which makes it super easy to match patterns.

The general form of searching and replacing text using sed takes the following form:

sed -i 's/SEARCH_REGEX/REPLACEMENT/g' INPUTFILE

-i - By default, sed writes its output to the standard output. This option tells sed to edit files in place. If an extension is supplied, a backup of the original file is created.
s - The substitute command, probably the most used command in sed.
/ / / - Delimiter character. It can be any character but usually the slash (/) character is used.
SEARCH_REGEX - Normal string or a regular expression to search for.
REPLACEMENT - The replacement string.
g - Global replacement flag. By default, sed reads the file line by line and changes only the first occurrence of the SEARCH_REGEX on a line. When the replacement flag is provided, all occurrences are replaced.
INPUTFILE - The name of the file on which you want to run the command.

All other use cases are similar to this one, so I recommend you learn the basics of regex before learning sed.

Scripting

This is a script I wrote using sed some time ago. It replaces some strings in different files between the desktop and the web versions of an app. Notice that the last line uses sed to do that task like this: sed -i "$scripts" $files.

#!/bin/bash

files="
  ./markup/onboarding.html
  ./markup/popover.html
  ./scripts/onboarding.js
  ./scripts/popover.js
"

paths=(
  "UI_FONT/:../fonts/"
  "UI_IMG/:../images/"
  "JS_POLYFILL/:../scripts/vendor/polyfill/"
  "JS_JQUERY/:../scripts/vendor/jquery/"
  "JS_CHART/:../scripts/vendor/chartjs/"
  "JS_APP/:../scripts/"
)

[ $# != 1 ] && echo "Usage: $0 {web|desktop}" && exit

case "$1" in
  web)
    for path in "${paths[@]}" ; do
      key="${path%%:*}"
      value="${path##*:}"

      scripts+="
        s|$key|$value|g;
      "
    done
    ;;

  desktop)
    for path in "${paths[@]}" ; do
      key="${path%%:*}"
      value="${path##*:}"

      scripts+="
        s|$value|$key|g;
      "
    done
    ;;

  *)
    echo "Usage: $0 {web|desktop}"
    ;;
esac

sed -i "$scripts" $files