Trouble understanding chmod & group permissions
 

I have now read many tutorials about Linux user and group administration, but I still don't completely understand the group permissions. For example, I can give a group write rights by

but what I don't understand is:

- to which group does it apply?
- can I specify one specific group or does it apply to all groups (that wouldn't make any sense, would it)?
- how do I give a specific group a right to create new directories under a specific directory?

jmcnama is the owner
prog is the group

You're changing the access for whatever group the owner of the file belongs to.

You can change the group of the file as well, if you want the file to be accessible to a group that is different from the one the owenr belongs to.

and now the "bin" group has group access.

So for example if I want to give access to the group "squid" to a directory "cache" and all its subdirectories I have to do

? And root still has all access rights to everything no matter what I do?