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

#!/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".

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