The conventional POSIX permission concept uses three
classes of users for assigning permissions
in the file system: the owner, the owning group, and other users.
Three permission bits can be set for each user class, giving
permission to read (
w), and execute (
Default ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.
Each ACL consists of a set of ACL entries. An ACL entry contains a type, a qualifier for the user or group to which the entry refers, and a set of permissions. For some entry types, the qualifier for the group or users is undefined.