Make `envsubst` fail if a variable is not defined

Question

I have a file with a large number of variables in it.

$ cat message.txt
Hello, ${LOCATION}! You too, ${PERSON} ;)

If PERSON is not defined, envsubst replaces it with nothing:

$ LOCATION=World envsubst < message.txt
Hello, World! You too,  ;)

How can I make envsubst fail with a non-zero exit code (or something reliable) if any of the many environment variables in the file isn't defined?

Answer 1

This is not ideal, in that it doesn't alter the behaviour of envsubst as requested, but it is able to identify unset variables. The user must ensure that the delimiter EOF does not appear in the text. If it does, then choose a different delimiter.

#!/usr/bin/env bash

msg="$( printf 'cat << EOF\n%s\nEOF\n' "$(cat)" )"

bash -u <<< "$msg"

Output:

$ ./test.sh < message.txt || echo fail
bash: line 1: LOCATION: unbound variable
fail
$ LOCATION=World ./test.sh < message.txt || echo fail
bash: line 1: PERSON: unbound variable
fail
$ LOCATION=World PERSON=Ralph ./test.sh < message.txt || echo fail
Hello, World! You too, Ralph ;)

Here is a lengthier version which will list all the unset variables in one go, instead of disclosing them one at a time:

#!/usr/bin/env bash

check_vars() {

# pass a list of variable names on stdin, one to a line

rc=0

while read v
do
        if [[ ! "${!v}" ]]
        then
                printf '%s\n' "$v"
                rc=1
        fi
done

return $rc

}

envsubst -v "$(cat)" | check_vars

This version will output a list of unset (or null) variables, one to a line, and exit with 0 if and only if the list is empty.

Output:

$ ./test2.sh < message.txt || echo fail
LOCATION
PERSON
fail
$ PERSON=Ralph ./test2.sh < message.txt || echo fail
LOCATION
fail
$ LOCATION=World ./test2.sh < message.txt || echo fail
PERSON
fail
$ LOCATION=World PERSON=Ralph ./test2.sh < message.txt || echo fail
$

Answer 2

You can easily to the equivalent in perl where you can exit with an error if the env var is not defined:

perl -pe 's{\$(?|\{(\w+)\}|(\w+))}{$ENV{$1} // die "$& not defined\n"}ge'