How to use environment variable inside a quoted string in BASH script

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"

But I can't get the syntax to correctly expand the COLUMNS environment variable.

I've tried various forms of the following:

svn diff $@ --diff-cmd /usr/bin/diff -x '-y -w -p -W $COLUMNS'

and

svn diff $@ --diff-cmd /usr/bin/diff -x '-y -w -p -W ${COLUMNS}'

and

eval svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W $COLUMNS"

Suggestions?

Answer 1

The following script works for me for multiple values of $COLUMNS. I wonder if you are not setting COLUMNS prior to this call?

#!/bin/bash
COLUMNS=30
svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W $COLUMNS"

Can you echo $COLUMNS inside your script to see if it set correctly?

Answer 2

You are doing it right, so I guess something else is at fault (not export-ing COLUMNS ?).

A trick to debug these cases is to make a specialized command (a closure for programming language guys). Create a shell script named diff-columns doing:

exec /usr/bin/diff -x -y -w -p -W "$COLUMNS" "$@"

and just use

svn diff "$@" --diff-cmd  diff-columns

This way your code is cleaner to read and more modular (top-down approach), and you can test the diff-columns code thouroughly separately (bottom-up approach).

Answer 3

If unsure, you might use the 'cols' request on the terminal, and forget COLUMNS:

COLS=$(tput cols)

Answer 4

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 ;-))