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Test if a variable is set in bash when using "set -o nounset"

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Original source (stackoverflow.com)
Tags: bash shell-scripting
Clipped on: 2016-05-05

The following code exits with a unbound variable error. How to fix this, while still using the set -o nounset option?

#!/bin/bash

set -o nounset

if [ ! -z ${WHATEVER} ];
 then echo "yo"
fi

echo "whatever"
asked Oct 20 '11 at 6:44
Image (Asset 3/8) alt=
vinodkone
6801918
up vote 28 down vote accepted
#!/bin/bash

set -o nounset


VALUE=${WHATEVER:-}

if [ ! -z ${VALUE} ];
 then echo "yo"
fi

echo "whatever"

In this case, VALUE ends up being an empty string if WHATEVER is not set. We're using the {parameter:-word} expansion, which you can look up in man bash under "Parameter Expansion".

answered Oct 20 '11 at 6:52
Image (Asset 5/8) alt=
Angelom
1,20977
1 upvote
  flag
awesome thanx! looks like -z ${WHATEVER:-} works too... – vinodkone Oct 20 '11 at 7:00
2 upvote
  flag
just replace if [ ! -z ${VALUE} ]; with if [ ! -z ${WHATEVER:-} ]; – Angelom Oct 20 '11 at 7:00
11 upvote
  flag
:- checks whether the variable is unset or empty. If you want to check only whether it's unset, use -: VALUE=${WHATEVER-}. Also, a more readable way to check whether a variable is empty: if [ "${WHATEVER+defined}" = defined ]; then echo defined; else echo undefined; fi – l0b0 Oct 24 '11 at 11:04
   upvote
  flag
Also, this won't work if $WHATEVER contains only whitespace - See my answer. – l0b0 Mar 22 '12 at 15:07

You need to quote the variables if you want to get the result you expect:

check() {
    if [ -n "${WHATEVER-}" ]
    then
        echo 'not empty'
    elif [ "${WHATEVER+defined}" = defined ]
    then
        echo 'empty but defined'
    else
        echo 'unset'
    fi
}

Test:

$ unset WHATEVER
$ check
unset
$ WHATEVER=
$ check
empty but defined
$ WHATEVER='   '
$ check
not empty
answered Mar 22 '12 at 15:03
Image (Asset 6/8) alt=
l0b0
17.7k560114
   upvote
  flag
I tried this and I'm surprised this works... Everything is correct except according to "info bash", "${WHATEVER-}" should have a ":" (colon) before the "-" (dash) like: "${WHATEVER:-}", and "${WHATEVER+defined}" should have a colon before the "+" (plus) like: "${WHATEVER:+defined}". For me, it works either way, with or without the colon. On some versions of 'nix it probably won't work without including the colon, so it should probably be added. – Kevin Fegan Jan 7 '14 at 20:58
1 upvote
  flag
Nope, -, +, :+, and :- are all supported. The former detect whether the variable is set, and the latter detect whether it is set or empty. From man bash: "Omitting the colon results in a test only for a parameter that is unset." – l0b0 Jan 7 '14 at 21:00
   upvote
  flag
Nevermind =). You are correct. I don't know how I missed that. – Kevin Fegan Jan 7 '14 at 21:11
   upvote
  flag
From the docs: Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence. – A-B-B Apr 3 '14 at 0:25
$ echo $SHELL
/bin/bash
$ /bin/bash --version | head -1
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
$ set -o nounset

If you want a non-interactive script to print an error and exit if a variable is null or not set:

$ [[ "${HOME:?}" ]]

$ [[ "${IAMUNBOUND:?}" ]]
bash: IAMUNBOUND: parameter null or not set

$ IAMNULL=""
$ [[ "${IAMNULL:?}" ]]
bash: IAMNULL: parameter null or not set

If you don't want the script to exit:

$ [[ "${HOME:-}" ]] || echo "Parameter null or not set."

$ [[ "${IAMUNBOUND:-}" ]] || echo "Parameter null or not set."
Parameter null or not set.

$ IAMNULL=""
$ [[ "${IAMUNNULL:-}" ]] || echo "Parameter null or not set."
Parameter null or not set.

You can even use [ and ] instead of [[ and ]] above, but the latter is preferable in Bash.

Note what the colon does above. From the docs:

Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

There is apparently no need for -n or -z.

In summary, I may typically just use [[ "${VAR:?}" ]]. Per the examples, this prints an error and exits if a variable is null or not set.

answered Apr 3 '14 at 16:41
Image (Asset 7/8) alt=
A-B-B
6,66923026

How about a oneliner?

[ -z "${VAR:-}" ] && echo "VAR is not set or is empty" || echo "VAR is set to $VAR"

-z checks both for empty or unset variable

answered Jun 29 '12 at 14:56
Image (Asset 8/8) alt=
NublaII
693
   upvote
  flag
No, -z only checks if the next parameter is empty. -z is is just an argument of the [ command. Variable expansion happens before [ -z can do anything. – dolmen Mar 30 '15 at 21:39

You can use

if [[ ${WHATEVER:+$WHATEVER} ]]; then

but

if [[ "${WHATEVER:+isset}" == "isset" ]]; then

might be more readable.

answered Mar 22 '12 at 14:49
   upvote
  flag
String comparisons should use the standard (POSIX) = operator, not == to aid in portability, and [ instead of [[ if possible. – Jens Oct 26 '12 at 9:30

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