Wonders of chmod: set executeable bit only on directories

On some systems, the default file permissions are rather restrictive. I.e. at our university the policy is umask=002, which means that any new file created or copied from a remote computer will only have owner read/write access enabled and read/write/executable to directories.

This is sometimes annoying when files are being uploaded to the ~/public_html/ directory which were meant to be accessed by external users. Usually one forgets to set the correct permissions, leaving the files inaccessible to general public.

This morning I got an email from my PhD student (he is currently in Australia, which explains why he was up that early) - he had to turn in an exam project via his web page, but he had forgotten to set the permission right.

His WIFI access did not allow ssh (lame!), so he gave me his password so I could fix this for him. Entering his directory, I encountered a large directory structure with several sub-directories and files.

Issue a recursive chmod

chmod -R a+r *

from the base directory would still leave the sub-directories inaccessible.

chmod -R a+rx *

is an ugly thing to do. Sure it works, but messes up auto completion as bash/dash will think all files can be executed. Also on a coloured console this would produce psychedelic output when listing files. Manually setting permissions on all directories would be a very stupid thing to do.

That is when I realized that chmod can also explicitly be told only to set the executable bit on directories using capital X. Awesome, I thought, so here we go:

chmod -R a+rX *

Yeah, this may be trivial, but I was not aware of this option before.