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 .
But there are obviously times this is insufficient. Like, say Bob is working in a group with Alice, so he wants to share files with her, but they don’t want to share files with everybody else. I guess root could go and create a group, add Bob and Alice to it, then they could set the file to the new group or whatever, but this becomes unmanagable with very large systems.
A short definition from the man page of what an ACL is
This manual page describes POSIX Access Control Lists, which are used to define more fine-grained discretionary access rights for files and directories.”
The following tags are defined
ACL_USER_OBJ The ACL_USER_OBJ entry denotes access rights for the file owner.
ACL_USER ACL_USER entries denote access rights for users identified by the entry’s qualifier.ACL_GROUP_OBJ The
ACL_GROUP_OBJ entry denotes access rights for the file group.
ACL_GROUP ACL_GROUP entries denote access rights for groups identified by the entry’s qualifier.
ACL_MASK The ACL_MASK entry denotes the maximum access rights that can be granted by entries of type ACL_USER, ACL_GROUP_OBJ, or ACL_GROUP.
ACL_OTHER The ACL_OTHER entry denotes access rights for processes that do not match any other entry in the ACL.
When an access check is performed, the ACL_USER_OBJ and ACL_USER entries are tested against the effective user ID. The effective group ID, as well as all supplementary group IDs are tested against the ACL_GROUP_OBJ and ACL_GROUP entries.
I won’t go into the not-really-that-gory details of how to implement it here, other than to say ACLs can be easily modified with the getfacl and setfacl commands.
So for example, if Alice wanted to give Bob read access to a file, she could type
$ setfacl -m u:Bob:r myfile