How to use an environment variable inside a quoted string in Bash

Question

I\'ve tried various forms of the following in a bash script:

#!/bin/bash
svn diff $@ --diff-cmd /usr/bin/diff -x \"-y -w -p -W $COLUMNS\"

B

Answer 1

Note that COLUMNS is:

1. NOT an environment variable. It is an ordinary bash parameter that is set by bash itself.
2. Set automatically upon receipt of a SIGWINCH signal.

That second point usually means that your COLUMNS variable will only be set in your interactive shell, not in a bash script.

If your script's stdin is connected to your terminal you can manually look up the width of your terminal by asking your terminal:

tput cols

And to use this in your SVN command:

svn diff "$@" --diff-cmd /usr/bin/diff -x "-y -w -p -W $(tput cols)"

(Note: you should quote "$@" and stay away from eval ;-))