Execute Permissions on Directories

Tom Schneider, who does cool stuff at the NIH, just passed me this question:

> Ok I used chmod a+X regulon. Didn't know that directories had to be
> executable, that's kinda odd.

I agree, it is odd. It would be interesting to know why that is the case.

I'll cc this to Jeff Haemer, my friend who may know.

Sho 'nuff.

It doesn't make sense to "execute" a directory, so Unix used the extra bit for something else: searchability. Watch:

$ mkdir /tmp/foo
$ echo Molecular machines > /tmp/foo/toms
$ chmod -r /tmp/foo
$ ls -l /tmp/foo
ls: cannot open directory /tmp/foo: Permission denied
$ chmod +r-x /tmp/foo
$ ls -l /tmp/foo
ls: cannot access /tmp/foo/toms: Permission denied
total 0
-????????? ? ? ? ? ? toms