Assigning variables in Makefile recipe

January 2, 2022

In one of my Makefile recipes, I want to create a temporary file, pass the name of that file to a shell command and assign the output of that command to a make variable so that I can use that subsequently. For the life of me, I cannot get it to work.

For the purpose of debugging I have tried to boil down the problem to the most simple target I could come up with:

.PHONY: foo
foo:
    $(eval TMPFILE = $(shell mktemp -p ./))
    dd if=/dev/random of=${TMPFILE} bs=1 count=512
    $(eval FOO = $(shell wc -c ${TMPFILE}))
    @echo FOO: ${FOO}

Here is what happens:

❯ make foo
dd if=/dev/random of=./tmp.K1au4WrZ76 bs=1 count=512
512+0 records in
512+0 records out
512 bytes copied, 0.00287818 s, 178 kB/s
FOO: 0 ./tmp.K1au4WrZ76

So somehow, wc thinks the file is empty. But when I check the TMPFILE it has 512 bytes, as expected:

❯ wc -c tmp.K1au4WrZ76
512 tmp.K1au4WrZ76

Can someone, please enlighten me what is going on here and how to do that correctly?

Thanks
Phil

Update: Based on the answer I put together this target which works as desired:

.PHONEY: foo
.ONESHELL:
foo:
    set -e
    TMPFILE=`mktemp -p ./`
    dd if=/dev/random of=$$TMPFILE bs=1 count=512
    FOO=`wc -c $$TMPFILE`
    @echo FOO: $$FOO

Thanks!

>Solution :

Make always expands the entire recipe (all lines of the recipe) first, before it starts any shell commands. So all your eval, etc. operations are invoked before any shell command, such as dd, is run.