Makefile variable as prerequisite

Question

In a Makefile, a deploy recipe needs a environment variable ENV to be set to properly execute itself, whereas others don\'t care, e.g.:

Answer 1

You can create an implicit guard target, that checks that the variable in the stem is defined, like this:

guard-%:
    @ if [ "${${*}}" = "" ]; then \
        echo "Environment variable $* not set"; \
        exit 1; \
    fi

You then add a guard-ENVVAR target anywhere you want to assert that a variable is defined, like this:

change-hostname: guard-HOSTNAME
        ./changeHostname.sh ${HOSTNAME}

If you call make change-hostname, without adding HOSTNAME=somehostname in the call, then you'll get an error, and the build will fail.