Linux Mac File permissions, POSIX and ACLs
Traditionally, systems that support the POSIX (Portable Operating System Interface) family of standards [11,2] share a simple yet powerful file system permission model: Every file system object is associated with three sets of permissions that define access for the owner, the owning group, and for others. Each set may contain Read (r), Write (w), and Execute (x) permissions. This scheme is implemented using only nine bits for each object. In addition to these nine bits, the Set User Id, Set Group Id, and Sticky bits are used for a number of special cases. Many introductory and advanced texts on the UNIX operating system describe this model [19].
