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

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"

Solution 1:

#!/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".

Solution 2:

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

Solution 3:

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