bash: remove empty parameter

Question

I wonder if it's possible to reduce the last four lines in the following example:

#!/bin/bash

function xxx {
  echo $#
  echo $1
}

[[ -n $SOME_ENV ]] && P="-Dabc=$SOME_ENV"

if [[ -n $P ]]; then
  xxx "$P" abc
else
  xxx abc
fi

Replacing the if-else-fi by just

xxx "$P" abc

does work if $P is not empty, otherwise not because "" still counts as empty parameter. Omitting the quotes around $P does not work if SOME_ENV contains spaces. I know that it's possible with "eval", but eval is evil: http://mywiki.wooledge.org/BashFAQ/048

Any other solution?

Community · Accepted Answer · 2017-05-23 12:32:38Z

9

I like using an array the best. An alternative, for what it's worth, is to use alternate value substitution.

xxx ${P:+"$P"} abc

This will be substituted with "$P" if $P is set and not blank. If it is empty, the entire word is removed. The neat thing is you could get rid of $P altogether.

xxx ${SOME_ENV:+"-Dabc=$SOME_ENV"} abc

edited May 23, 2017 at 12:32

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answered Jul 18, 2016 at 20:27

John Kugelman

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Comments

John1024 · Accepted Answer · 2016-07-18 20:23:50Z

4

If you want to use an options variable, p, make it an array:

p=()
[[ -n $SOME_ENV ]] && p+=("-Dabc=$SOME_ENV")

xxx "${p[@]}" abc

If p is empty, then "${p[@]}" expands to nothing, just as you want.

For example, with SOME_ENV unset, $# is 1 as it should be:

$ bash script
1
abc

With SOME_ENV set, then $# is 2 as it should be:

$ SOME_ENV='1 2 3' bash script
2
-Dabc=1 2 3

answered Jul 18, 2016 at 20:23

John1024

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Comments

axiac · Accepted Answer · 2016-07-18 20:30:19Z

2

You already test the length of $SOME_ENV and set $P only when $SOME_ENV is not empty. $P and $SOME_ENV are either both empty or both non-empty. Do you have any good reason to test $P?

I would merge the two tests and then just make $P disappear (it is just a temporary variable in the code you posted). Something like this:

if [[ -n $SOME_ENV ]]; then
  xxx -Dabc="$SOME_ENV" abc
else
  xxx abc
fi

answered Jul 18, 2016 at 20:30

axiac

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1 Comment

Henning Over a year ago

Your solution is perfect if one would keep the last 4 lines. But i want to get rid of it. My intention is to have only one line starting with "xxx". The real example contains much more that just a common "abc". I want to avoid that redundancy.

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