User Identifier

Unix-like operating systems identify users within the kernel by an unsigned integer value called a user identifier, often abbreviated to UID or User ID. The range of UID values varies amongst different systems; at the very least, a UID represents a 15-bit integer, ranging between 0 and 32767, with the following restrictions:

• The superuser must always have a UID of zero (0).
• The user “nobody” traditionally got the largest possible UID (as the opposite of the superuser): 32767. More recently, systems have assigned the user a UID in the system range (1–100, see below) or in the range 65530–65535.
• Convention reserves UIDs from 1 to 100 for system use; some manuals recommend reserving UIDs from 101 up to 499 (Red Hat Enterprise Linux) or even up to 999 (Debian) 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 has become a necessary component of Unix file systems and processes. Some operating systems support 16-bit UIDs, making 65536 unique IDs possible; a modern system with 32-bit UIDs will potentially make 4,294,967,296 (232) distinct values available.

In POSIX-compliant environments the command-line command id gives the user's user identifier number as well as more information on the user account like the user name, primary user group and group identifier (GID).