Makefile变量的初始化和导出 [英] Makefile variable initialization and export

问题描述

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

all:
    @echo $(update)

我希望将Apple作为命令输出，但是它为空，这使我认为导出和:=变量扩展发生在不同的阶段.如何克服这个问题?

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?

推荐答案

问题是export将变量导出到命令所使用的子shell中.在其他作业中无法扩展.因此，不要试图从规则之外的环境中获取它.

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"'

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