How to check a mandatory variable in a Makefile

#tooltip #makefile

When you use a variable in a Makefile task and you want to make it mandatory and check it before run the effective task,

You can guard it.

task-who-need-SPECIFIC_ENVVAR: guard-SPECIFIC_ENVVAR
    @echo ${SPECIFIC_ENVVAR}

You only need to add the following task

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

So when you run it, the guard will prevent to run the task

$ make task-who-need-SPECIFIC_ENVVAR
Environment variable SPECIFIC_ENVVAR not set
make: *** [guard-SPECIFIC_ENVVAR] Error 1

$ export SPECIFIC_ENVVAR=value
$ make task-who-need-SPECIFIC_ENVVAR
value

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