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Editorial Team
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Asked: 2026-05-31T12:23:18+00:00

I want to do immediate expansion of a shell command within a Makefile, but

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I want to do immediate expansion of a shell command within a Makefile, but I want the shell command to have access to the environment variables within the Makefile. If I use the $(shell …), it expands immediately, but there is no access to the variables. If I use the backquotes, the expansion is not immediate, and it causes problems for me later in the Makefile. I’m wondering if there is any way to make the backquotes expand immediately, or to pass the current environment to a $(shell) command.

For example, the following makefile:

SOME_VAR := some_val
export SOME_VAR

VAR1 := `echo $$SOME_VAR`
export VAR1
VAR2 := `echo $$VAR1`

all:
      @echo VAR1=$(VAR1)
      @echo VAR2=$(VAR2)

Will output:

~/tmp/t2> make
VAR1=some_val
VAR2=`echo $SOME_VAR`

Where I want it to print “VAR2=some_val”. The real example is a bit more complicated (environment variables are inherited from parent makefiles, and I’m trying to use a perl script to edit the variables), but the principle is the same.

Any help is appreciated.

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1 Answer

  1. Editorial Team

    As I mentioned in some of the comments, my actual goal was to make the script generate filenames based on the settings the object was being compiled with. I then need another script to generate a specially formatted list of all the filenames generated (the target is an embedded system which doesn’t have a JIT compiler on it). At any given time, there are over thirty settings which can potentially effect the binary, and this may be used on more than one module in the future, so I’d like something scalable.

    My solution is as follows. Instead of passing the variables in, I modified my script to output a makefile-parsable string based on the settings:

    -include $(SOME_MK_FILE)

$(SOME_MK_FILE) : .phony
    script.pl $(SETTINGS_OF_INTEREST_LIST) > $(SOME_MK_FILE)

someFilename := $(shell script2.pl $(VAR1))

    script.pl outputs a string that looks something like:

    VAR1 := CONFIG_X1=$(CONFIG_X1) CONFIG_X2=$(CONFIG_X2) CONFIG_X33=$(CONFIG_X33)

    and script2 outputs a filename that looks something like ‘someFilename.X1_y.X2_n.elf’

    and then, later on, in another rule, I have:

    someobj: somedep
    script3.pl $(someFilename) >> builtfiles.txt

    which properly builds builtfiles.txt (which in turn is the input for yet another script…). In the end this is a workaround to the fact that make cannot pass its environement to $(shell). It’s not overly pretty but it works.

    John

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