Makefile variable initialization and export

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)

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

The output is:

echo method 1 
method 1
perl -e 'print "method 2 $ENV{somevar}\n"'
method 2 apple
echo method 3 apple
method 3 apple
perl -e 'print "method 4 $ENV{somevar}\n"'
method 4 apple

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