How to ensure Makefile variable is set as a prerequisite

makefile

A Makefile deploy recipe needs an environment variable ENV to be set to properly execute itself, whereas other recipes don't care, e.g.,

ENV = 

.PHONY: deploy hello

deploy:
    rsync . $(ENV).example.com:/var/www/myapp/

hello:
    echo "I don't care about ENV, just saying hello!"

How can I make sure this ENV variable is set? Is there a way to declare this makefile variable as a prerequisite of the deploy recipe? e.g.,

deploy: make-sure-ENV-variable-is-set

Best Answer

This will cause a fatal error if ENV is undefined and something needs it (in GNUMake, anyway).

.PHONY: deploy check-env

deploy: check-env
	...

other-thing-that-needs-env: check-env
	...

check-env:
ifndef ENV
	$(error ENV is undefined)
endif

(Note that ifndef and endif are not indented - they control what make "sees", taking effect before the Makefile is run. "$(error" is indented with a tab so that it only runs in the context of the rule.)

Lazy Set

VARIABLE = value

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

Lazy Set If Absent

VARIABLE ?= value

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif

See the documentation for more details.

Append

VARIABLE += value

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

Gcc makefile error: “No rule to make target …”

That's usually because you don't have a file called vertex.cpp available to make. Check that:

that file exists.
you're in the right directory when you make.

Other than that, I've not much else to suggest. Perhaps you could give us a directory listing of that directory.

Best Answer

Related Solutions

The difference between the GNU Makefile variable assignments =, ?=, := and +=

Lazy Set

Immediate Set

Lazy Set If Absent

Append

Gcc makefile error: “No rule to make target …”

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