How do I check if a variable is unset AND use set -u in my bash prelude? [duplicate]

Question

I know that set -u will fail the script if there are any unbound variables referenced, but in my bash script, I'm checking to see if a certain variable is unset within an if statement before I attempt to do something with it, like so:

if [[ -z "${SOME_VARIABLE}" ]] ; then
    echo '$SOME_VARIABLE' is not set
else
    do_stuff_with_SOME_VARIABLE
fi

However, if I try to run the above with set -eu in my prelude, I get the following error, which seems a bit counterintuitive given what I'm trying to do:

-bash: SOME_VARIABLE: unbound variable

[Process completed]

As you can see, I'm not actually trying to use $SOME_VARIABLE when it's unset, so what I'd like to know is if there is some way to fail the script when I'm actually trying to use unset variables (like passing them as arguments or applying string manipulations to them) but not when I'm merely checking to see if they're unset?

Answer 1

You could use an expansion:

if [ -z "${SOME_VARIABLE:-}" ]; then
    echo '$SOME_VARIABLE' is not set
else
    do_stuff_with_SOME_VARIABLE
fi

This expands $SOME_VARIABLE to null. Read more about parameter substitution here (see the section on ${parameter:-default} near the top)