Makefile variable initialization and export

Question

somevar := apple
export somevar
update := $(shell echo "v=$$somevar")

all:
    @echo $(update)

I was hoping to apple as output of command, however it's empty, which makes me think export and := variable expansion taking place on different phases. how to overcome this?

Answer 1

Running the makefile

foo:=apple
export foo
all:
        @echo ">"$(shell echo "$$foo")
        @echo ">""$$foo"

gives for me (with foo undefined in the environment)

$ make
>
>apple

$ make foo=bar
>
>apple

$ export foo=bar; make
>bar
>apple

$ export foo=bar; make foo=bar
>bar
>bar

Try using the quoted form (update := "v=$$somevar") and let the shell handle expansion when a command is run (you'll still need the export)

Answer 2

I think this is because of the double-quotes. This is working:

somevar := apple
export somevar
update := $(shell echo 'v=$$somevar')

all:
    @echo $(update)

Answer 3

The problem is that export exports the variable to the subshells used by the commands; it is not available for expansion in other assignments. So don't try to get it from the environment outside a rule.

somevar := apple
export somevar

update1 := $(shell perl -e 'print method 1 "$$ENV{somevar}\n"')
# Make runs the shell command, the shell does not know somevar, so update1 is "method 1 ".

update2 := perl -e 'print method 2 "$$ENV{somevar}\n"'
# Now update2 is perl -e 'print method 2 "$$ENV{somevar}\n"'

# Lest we forget:
update3 := method 3 $(somevar)

all:
    echo $(update1)
    $(update2)
    $echo $(update3)
    perl -e 'print method 4 "$$ENV{somevar}\n"'