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

I've found with the best answer cannot be used as a requirement, except for other PHONY targets. If used as a dependency for a target that is an actual file, using check-env will force that file target to be rebuilt.

Other answers are global (e.g. the variable is required for all targets in the Makefile) or use the shell, e.g. if ENV was missing make would terminate regardless of target.

A solution I found to both issues is

ndef = $(if $(value $(1)),,$(error $(1) not set))

.PHONY: deploy
deploy:
    $(call ndef,ENV)
    echo "deploying $(ENV)"

.PHONY: build
build:
    echo "building"

The output looks like

$ make build
echo "building"
building
$ make deploy
Makefile:5: *** ENV not set.  Stop.
$ make deploy ENV="env"
echo "deploying env"
deploying env
$

value has some scary caveats, but for this simple use I believe it is the best choice.