Here's the scenario: I'm using Ubuntu Linux (12.10, if it matters), and I've got a directory whose permissions are thus:
drwxrws--- paul www-data myfolder/
You can see the
setgid bit is set. If I change any permissions on this folder (e.g.,
chmod o+x myfolder) it unsets the
chmod preserves a directory's
set-group-IDbits unless you explicitly specify otherwise.
Elsewhere, it says that it unsets the
setgid bit if the file's group doesn't match the user's effective group ID, but only on regular files (i.e., not folders). So, because my group is
paul rather than
www-data, I'd expect the above to happen if
myfolder/ were a regular file, but it ain't -- it's a directory.
So either I'm understanding the manual wrong, or
chmod is not obeying its own instructions. Does anyone know which?