Make and last modification times of directories on Linux

Question

Consider the following makefile:

foo :
    mkdir foo

foo/a.out : foo a.in
    cp a.in foo/a.out

foo/b.out : foo b.in
    cp b.in foo/b.out

an

Answer 1

I like @Beta's answer, but it is not portable. For a simple portable workaround, create a sentinel file when you create the directory, and depend on the sentinel file instead.

foo/.dir:
        mkdir -p foo
        touch $@
foo/a.out: a.in foo/.dir
        cp $< $@
foo/b.out: b.in foo/.dir
        cp $< $@

This can be further simplified with a pattern rule:

foo/%.out: %.in foo/.dir
        cp $< $@