CODEWITHSUNDEEP
bash
Rick Smith
Rick Smith

Reputation: 9251

What does ${VARIABLE+set} mean?

I'm looking through someone else's code and am not sure what this means. It is either a variable call VARIABLE+set which is a strange variable name since is has a +, or is is evaluated and is hard to Google because it has ${} in it ;)

Upvotes: 7

Views: 1746

Answers (1)

Rick Smith
Rick Smith

Reputation: 9251

It took be some time, but I found a link explaining what this does. It is a form of bash parameter-substitution that will evaluate to "set" if $VARIABLE has been set and null otherwise. This allows you to check if a variable is set by doing the following:

if [ -z "${VARIABLE+set}" ] ; then
    echo "VARIABLE is not set"
fi

It is also interesting to note that ${VARIABLE+set} can just as easily be ${VARIABLE+anything}. The only reason for using +set is because it is slightly more self-documenting (although not enough to keep me from asking this question).

Upvotes: 12

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