Which will expand to “x” if THEVAR is set and not empty.
Alternatively, this test can be used:
if [[ ! -v THEVAR ]]; then echo "THEVAR is not set" fi
But that will work even if THEVAR is set but empty.
I work as a freelancer, so if you don’t want to do that kind of things yourself or don’t have the time, just drop me a line to hire me.