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Define make variable at rule execution time

In my GNUmakefile, I would like to have a rule that uses a temporary directory. For example:

out.tar: TMP := $(shell mktemp -d)
        echo hi $(TMP)/hi.txt
        tar -C $(TMP) cf $@ .
        rm -rf $(TMP)

As written, the above rule creates the temporary directory at the time that the rule is parsed. This means that, even I don't make out.tar all the time, many temporary directories get created. I would like to avoid my /tmp being littered with unused temporary directories.

Is there a way to cause the variable to only be defined when the rule is fired, as opposed to whenever it is defined?

My main thought is to dump the mktemp and tar into a shell script but that seems somewhat unsightly.

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Just as a reminder, GNU make has a syntax for order-only prerequisites which may be necessary to complement this approach.
– anol
Commented Jun 5, 2014 at 19:24
21

Be careful with this solution! ruleA: TMP = $(shell mktemp -d testruleA_XXXX) and ruleB: TMP = $(shell mktemp -d testruleB_XXXX) will happen when the Makefile is first evaluated. In other words, ruleA: TMP = $(shell ... happens sooner than you think. While this might work for this particular case, this method will cause problems for some sequential operations.
– JamesThomasMoon
Commented Jun 11, 2015 at 2:55
1

That's not my experience - rule-local variables are expanded only when the particular rule is requested (either explicitly or as dependcy)
– Zaar Hai
Commented Oct 23, 2020 at 9:54
1

This should still work if you do ruleA: private TMP = .... See target-specific variables.
– Victor Sergienko
Commented Jan 22, 2021 at 0:23

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Define make variable at rule execution time

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