The UID of a process. Each process has three user IDs: the real user ID (RUID), effective user ID (EUID), and saved user ID (SUID). The RUID is always inherited from the user or process that executes the process. The EUID is normally the same as the RUID but can differ in special circumstances. It is the EUID that BSD cheques to determine permissions. The SUID is used by BSD to enable a privileged process to switch in and out of privileged mode.