User identifier

In Unix-like operating systems, a user identifier (often abbreviated to user ID or UID) is a value used to identify a user. The UID, along with the group identifier (GID) and other access control criteria, is used to determine which system resources a user can access. The password file maps textual user names to UIDs. UIDs are stored in the inodes of the Unixfile system, running processes, tar archives, and the now-obsolete Network Information Service. In POSIX-compliant environments, the shell command id gives the current user's UID, as well as more information such as the user name and the primary and supplementary group identifiers (GIDs) and group names.