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Chapter 5. Users, Groups, and the Superuser > Users and Groups
5.1. Users and Groups
Although every Unix user has a username consisting of one or more characters, inside the computer Unix represents the identity of each user by a single number: the user identifier (UID). Under most circumstances, each user is assigned his own unique ID.
Unix also uses special usernames for a variety of system functions. As with usernames associated with human users, system usernames usually have their own UIDs as well. Here are some common "users" on various versions of Unix:
