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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?

codeforester
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Jamie
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  • so what do those examples produce in your case? And what do you want them to produce? –  May 08 '09 at 16:14
  • the command outside the script is working? – dfa May 08 '09 at 16:24
  • Have you tried `svn diff $@ --diff-cmd /usr/bin/diff -x "-y -w -p -W ""$COLUMNS"` – Qian Chen Nov 30 '15 at 13:08
  • BTW, using `$@` unquoted makes it exactly the same as `$*` -- which is to say that it breaks `"foo bar"` into two separate arguments, `foo` and `bar`. If you want to preserve the original argument list just as it was given to you, you want `"$@"`. – Charles Duffy Sep 21 '17 at 04:47
  • See [Difference between single and double quotes in Bash](https://stackoverflow.com/questions/6697753/difference-between-single-and-double-quotes-in-bash). – codeforester Dec 21 '17 at 18:50

5 Answers5

477

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).

Lucas Wiman
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    In the case of single quotes, maybe this workaround helps: `'before'"$variable"'after'` (as stated in this answer: http://stackoverflow.com/a/13802438/2254346) – MakisH Feb 05 '16 at 00:01
  • Great answer! I had the same issue of referring my env variable within single quotes. Once I changed my single quote to double quotes, my env variable was being expanded as expected. – Binita Bharati Jun 19 '18 at 12:23
20

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

COLS=$(tput cols)
TheBonsai
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  • For whatever reason, this is the only solution I was able to get working off this page (as of 11/May 10:38 ET). Thanks – Jamie May 11 '09 at 14:38
15

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 ;-))

lhunath
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  • Thanks for the information; knowing whats going on brings a sense of catharsis to the issue. – Jamie May 11 '09 at 14:44
2

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?

Alex B
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  • Boo, hiss re: unquoted `$@` -- that splits a single parameter `"two words"` up into two separate parameters, `two` and `words`. Should be `"$@"` to pass the argument list through exactly as it was given. – Charles Duffy Sep 21 '17 at 04:48
0

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).

Colas Nahaboo
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