Because the UNIX operating system is a multi-user system, you usually do not work alone in the file system. System users can follow pathnames to various directories and read and use files belonging to one another, as long as they have permission to do so.
If you own a file, you can decide who has the right to read it, write in it (make changes to it), or, if it is a program, to execute it. You can also restrict permissions for directories. When you grant execute permission for a directory, you allow the specified users to change directory to it and list its contents with the ls command (see ls(1)). Only the owner or a privileged user can define the following:
This section introduces access-permissions for files and discusses file protection.