telling 'make' to ignore dependencies when the top target has been created

Question

I\'m running the following kind of pipeline:

digestA: hugefileB hugefileC
    cat $^ > $@
    rm $^

hugefileB:
    touch $@

hugefileC:
    touch $@
<         


        

Answer 1

See also .PRECIOUS:

.PRECIOUS : hugefileA hugefileB

.PRECIOUS

The targets which .PRECIOUS depends on are given the following special treatment: if make is killed or interrupted during the execution of their recipes, the target is not deleted. See Interrupting or Killing make. Also, if the target is an intermediate file, it will not be deleted after it is no longer needed, as is normally done. See Chains of Implicit Rules. In this latter respect it overlaps with the .SECONDARY special target.

You can also list the target pattern of an implicit rule (such as ‘%.o’) as a prerequisite file of the special target .PRECIOUS to preserve intermediate files created by rules whose target patterns match that file’s name.

Edit: On re-reading the question, I see that you don't want to keep the hugefiles; maybe do this:

digestA : hugefileA hugefileB
    grep '^Subject:' %^ > $@
    for n in $^; do echo > $$n; done
    sleep 1; touch $@

It truncates the hugefiles after using them, then touches the output file a second later, just to ensure that the output is newer than the input and this rule won't run again until the empty hugefiles are removed.

Unfortunately, if only the digest is removed, then running this rule will create an empty digest. You'd probably want to add code to block that.