How can I create a time-based Makefile rule?

Question

I would like to have a Makefile target that gets rebuilt only if the target file is older than some time interval.

As an example, say that I have some way of generating a key that is valid for one day, but generating it takes a non-trivial amount of time. I could just regenerate it each time I needed it:

.PHONY: key
key:
    sleep 5 && echo generated > key
foo: key
    echo foo
bar: key
    echo bar

But, over the course of the day, I might type make foo or make bar quite a few times. Waiting each time is annoying, and I would rather just eat this cost once per day.

Answer 1

Have the generated file depend on some dummy file like key-timestamp, then have a cron job touch that file every day.