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: A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. ...

• 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 or even 511 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 (2 raised to the 32nd power) distinct values available. On many computer operating systems, superuser is the term used for the special user account that is controlled by the system administrator. ... In many Unix variants, nobody is the conventional name of a user account which owns no files, is in no privileged groups, and has no abilities over and above those which every user has. ... The process of shadowing passwords is used to increase the security level of passwords on Unix systems, by hiding the encrypted passwords from ordinary users. ... The Network Information Service or NIS is Sun Microsystems Yellow Pages (YP) client-server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network. ... Unix or UNIX is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie, and Douglas McIlroy. ... In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. ... In computing, a process is a running instance of a program, including all variables and other states. ...

See also

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