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

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?

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 ;-))

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?

Just a quick note/summary for any who came here via Google looking for the answer to the general question asked in the title (as I was). Any of the following should work for getting access to shell variables inside quotes:

echo "$VARIABLE"
echo "${VARIABLE}"

Use of single quotes is the main issue. According to the Bash Reference Manual:

Enclosing characters in single quotes (') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. [...] Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed. The special parameters * and @ have special meaning when in double quotes (see Shell Parameter Expansion).

In the specific case asked in the question, $COLUMNS is a special variable which has nonstandard properties (see lhunath's answer above).

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).