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 65535, with some restrictions:

• The Superuser must always have a UID of zero (0).
• The user "nobody" was traditionally assigned the largest possible UID (as the opposite of the Superuser), 32767. More recently, the user is assigned a UID in the system range (1–100, see below) or between 65530–65535.
• UIDs from 1 to 100 are otherwise reserved for system use by convention; some manuals recommend that UIDs from 101 to 499 (RedHat) or even 999 (Debian) 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. Some operating systems might have support for 16-bit UIDs, making 65536 unique IDs possible, though a modern system with 32-bit UIDs will potentially make 4,294,967,296 (2³²) distinct values available.

See also

• id (Unix)
• setuid
• Sticky bit
• User name
• Group identifier (Unix)
• Process identifier
• inode
• File system permissions