Answer by MikeyB for How to determine if a bash variable is empty?

If you're interested in distinguishing the cases of set-empty versus unset status, look at the -u option for bash:

$ set -u
$ echo $BAR
bash: BAR: unbound variable
$ [ -z "$BAR" ] && echo true
bash: BAR: unbound variable
$ BAR=""
$ echo $BAR

$ [ -z "$BAR" ] && echo true
true