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

Inline variant

In my makefiles, I normally use an expression like:

deploy:
    test -n "$(ENV)"  # $$ENV
    rsync . $(ENV).example.com:/var/www/myapp/

The reasons:

• it's a simple one-liner
• it's compact
• it's located close to the commands which use the variable

Don't forget the comment which is important for debugging:

test -n ""
Makefile:3: recipe for target 'deploy' failed
make: *** [deploy] Error 1

... forces you to lookup the Makefile while ...

test -n ""  # $ENV
Makefile:3: recipe for target 'deploy' failed
make: *** [deploy] Error 1

... explains directly what's wrong

Global variant (for completeness, but not asked)

On top of your Makefile, you could also write:

ifeq ($(ENV),)
  $(error ENV is not set)
endif

Warnings:

• don't use tab in that block
• use with care: even the clean target will fail if ENV is not set. Otherwise see Hudon's answer which is more complex