Assign variable once when first used in Makefile

Question

How do I only invoke a make shell function once in a variable expansion, and only when the variable is first used? I don't want to use := to expand at declaration time (ie: simple expansion) because the expansion is expensive and only some of my targets need the variable.

I tried using conditional variable assignment but it invokes the shell every time, for example, the following invokes shell ls twice:

.PHONY: test

FILES ?= $(warning Invoking the shell)$(shell ls)

test:
    echo $(FILES) one
    echo $(FILES) two

A longer writeup is here: http://make.mad-scientist.net/deferred-simple-variable-expansion/ — MadScientist, Nov 18 '18 at 14:23

score -1 · Answer 1 · answered Nov 18 '18 at 07:21

-1

Move the assignment inside the recipe

test:
    FILES=$$(ls) ;\
    echo $$FILES one ;\
    echo $$FILES two

answered Nov 18 '18 at 07:21

user657267

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Assign variable once when first used in Makefile

1 Answers1