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 $< $@

Answer 2

Instead of making the targets dependent on the directory, you can simply create the directory unconditionally in their build rules:

foo/a.out: a.in
        mkdir -p foo
        cp a.in foo/a.out

foo/b.out: b.in
        mkdir -p foo
        cp b.in foo/b.out

This avoids problems with using a directory as a prerequisite.

Answer 3

As an authority on Make recently pointed out:

"The one thing you must NEVER do is use a directory as a simple prerequisite. The rules the filesystem uses to update the modified time on directories do not work well with make."

But I think this will do what you want:

foo :
    mkdir foo

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

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