Recursively making directories

[Norman Wilson]

It's a common pattern to do something like this when you want a
directory as a prerequisite of a target:

dir/foo: dir/.dummy

%/.dummy:
        mkdir -p $(@D) && touch $(@)

The .dummy file provides a pattern you can write a rule to match. It's
also there to prevent dir/foo being out of date whenever dir changes.

It's recently occurred to me that you don't actually need the dummy file
since 1) you can make the directory an order only dependency, 2) you can
use %/. as the directory creation pattern. (This works on Unix based
system. I've no idea if there is an equivalent in dos world.) So you end
up with the more natural looking:

dir/foo: | dir/.

%/.:
        mkdir -p $(@D)

After working this out I had another flash of inspiration. I can make
each directory depend on it's parent and thus do away with the -p flag
to mkdir:

%/.: | %/../.
        mkdir $(@D)

However this doesn't work. So is this a bug or a feature. If it's a
feature can someone explain the rules.

Cheers.




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