The Memory Bank

While I'm the guy that effectively lives on the command-line, I still rarely use sed. As I've been trying to use it more, I've run into some gotchas, mostly around the regular expression syntax and doing multiple replacements at one time.

Fortunately, both are easy enough to accomplish.

Doing multiple replacements can be done simply by chaining commands together:

sed --in-place='.bak' 's/match1/replace1/g; s/match2/replace2/g' /tmp/some-file

If you want to replace multiple strings with one value, you can use an extremely escaped version of a regular expression:

sed --in-place='.bak' 's/\(first\|second\)/next/g' /tmp/some-file

The general syntax for this operation is:

sed -i '/pattern/ s/$/value_to_append/' filename

Explanation:

  sed -i:\\
  This option enables in-place editing, meaning the changes are directly applied to the original filename. If you want to see the changes before modifying the file, remove -i to print the output to standard output.\\
  /pattern/:\\
  This specifies the regular expression pattern to search for within each line. Only lines matching this pattern will be affected.
  s/$/value_to_append/:\\
  This is the substitution command:\\
      s: Indicates a substitution operation.\\
      $: This is a special regular expression character that matches the end of a line.\\
      value_to_append: This is the text or value you want to add to the end of the matched line.\\

Example:
To find lines containing “example_string” in my_file.txt and append “_NEW” to the end of those lines:

sed -i '/example_string/ s/$/_NEW/' my_file.txt

Using a variable for the value to append:
You can also use a shell variable to store the value you want to append:

append_value="_ADDITION"
sed -i "/example_string/ s/\$/${append_value}/" my_file.txt

Note: When using variables in the substitution part of sed, it's important to use double quotes around the sed command to allow for variable expansion.
Also, the $ in s/$/ needs to be escaped with a backslash (\$) when using double quotes to prevent the shell from interpreting it as a variable.