On Unix-like systems, users are represented by a user identifier, often abbreviated UID. The range of values for a UID varies amongst different systems; at the very least, a UID can be between 0 and 32767, with some restrictions:

• The Superuser must always have a UID of zero (0).
• On systems where the highest possible UID is greater than 32767 the special user called "nobody " may either be assigned that maximum value or 32767 for compatibility. On systems for which 32767 is the highest possible UID, it is usually assigned to the "nobody" user.
• UIDs from 1 to 100 are otherwise reserved for system use by convention; some manuals recommend that UIDs from 101 to 499 be reserved as well.

The UID value references users in the /etc/passwd file. Shadow password files and Network Information Service also refer to numeric UIDs. The user identifier is a necessary component of Unix file systems and processes.

See also

• User name
• Group identifier (Unix)
• I-node
• File system permissions