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

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