[OJFord]

I don't recall which one it is (u for unset perhaps?) but I cover this by starting every (bash, E and pipefail are not POSIX) script with `set -eEuo pipefail`. All should be the default IMO, I don't understand - other than back-compat - why you wouldn't want them.

[SAI_Peregrinus]

It's -u.

You can also prevent this by never using $VAR, and instead always using ${VAR:?} to exit with error if VAR is unset (or use one of the other options to provide a default).