Login user IDs

The operating system identifies users by their login user ID.

The login user ID allows the system to trace all user actions to their source. After a user logs in to the system but before running the initial user program, the system sets the login ID of the process to the user ID found in the user database. All subsequent processes during the login session are tagged with this ID. These tags provide a trail of all activities performed by the login user ID. The user can reset the effective user ID, real user ID, effective group ID, real group ID, and supplementary group ID during the session, but cannot change the login user ID.