Error if variable is empty or unset

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The semantics for this are similar to that of default value substitution, but instead of substituting a default value, it errors out with the provided error message. The forms are ${VARNAME?ERRMSG} and ${VARNAME:?ERRMSG}. The form with : will error our if the variable is unset or empty, whereas the form without will only error out if the variable is unset. If an error is thrown, the ERRMSG is output and the exit code is set to 1.

# ./ line 4: FOO: EMPTY
echo "FOO is ${FOO:?EMPTY}"
# FOO is 
echo "FOO is ${FOO?UNSET}"
# ./ line 8: BAR: EMPTY
echo "BAR is ${BAR:?EMPTY}"
# ./ line 10: BAR: UNSET
echo "BAR is ${BAR?UNSET}"

The run the full example above each of the erroring echo statements needs to be commented out to proceed.

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