Every UNIX file has both an owner and a “group owner.” The owner of the file enjoys one special privilege that is not shared with everyone on the system: the ability to modify the permissions of the file. In particular, the owner can set the permissions on a file so restrictively that no one else can access it.1 We take up the subject of file permissions in Chapter 5, The Filesystem.
While the owner of a file is always a single person, many people may be group owners of the file, so long as they are all part of a single UNIX group. Groups are defined in the /etc/group file.
See page 79 for more information about groups.
The owner of a file gets to specify what the group owners can do with it. This scheme ...