sed: substitute text with a string that contains special characters

Question

I have a lot of text file with specific tag called "Comment", example I have a file called 3dchess.sh,

I need change the tag:

Comment=3D chess for X11 

to

Comment='<span size="xx-large">3D chess for X11</b>'

The script is:

#!/bin/sh
Package=3dchess
Popcon=48
Section=universe

Comment=3D chess for X11
Exec=3Dc

I have tried this but it doesn't work in my case:

sed -i ':Comment=:s:Comment=:Comment='<span size="xx-large">:g;/Comment=/s/$/</b>'/' $HOME/3dchess.sh

Answer 1

Personally, I like the -r option of sed that allows extended regular expressions so parentheses don't need to be escaped:

sed -r "s#(Comment=)(.*)#\1'<span size=\"xx-large\">\2</b>'#" 3dchess.sh 

Note that since you want to use ' in the substitution, you cannot use sed 's///' but need to use sed "s///" instead. Also, since you need to have / as part of the pattern, you can't use that as a delimiter so you need to choose another character. I picked # but you can use anything you like. As Graeme said, once you're sure this works OK, add the -i switch to make it edit the file in place.

Answer 2

sed 's:^Comment=\(.*$\):Comment='\''<span size="xx-large">\1</b>'\': \
  $HOME/3dchess.sh

Add -i when you are sure you have what you want.

Breakdown:

s:x:y: - This is the overall replacement pattern, basically replace x with y. Obviously we have a lot more between the colons above! Usually : is / but sed takes whatever comes after the s and using / here would mean you need to escape it in the pattern.

^Comment= - regex match Comment= at the start of a line ( ^ ). Good practice since it is possible (though unlikely) to get this elsewhere, which would cause problems.

\(.*$\) - match anything ( .* ) up to the end of the line ( $ ). Surrounding with \( \) creates a back reference for use later.

'\'' - close the '' quotes, put a literal ' and reopen quotes. Alternative method to @terdon's.

Comment=<span size="xx-large">\1</b> - replacement string, \1 is the backreference from before.

'\': - close quotes, put a literal ' then the final colon.