Question

How can I write a shell script that checks for an environment variable and writes to a log file if the variable is unset?

Answer 1

If you only want a message when it is unset, then:

if [ -z "${CHOSEN_ENV_VAR}" ]
then echo "CHOSEN_ENV_VAR was not set but should have been" >> log.file
fi

If you simply want the script to stop and report on stderr, then:

: ${CHOSEN_ENV_VAR:?'was not set but should have been'}

(You can test that in an interactive shell, but the interactive shell won’t exit. Put it in a script and the script is exited.)