Chapter 8. Users and Groups

Every user has a unique login name and an associated numeric user identifier (UID). Users can belong to one or more groups. Each group also has a unique name and a group identifier (GID).

The primary purpose of user and group IDs is to determine ownership of various system resources and to control the permissions granted to processes accessing those resources. For example, each file belongs to a particular user and group, and each process has a number of user and group IDs that determine who owns the process and what permissions it has when accessing a file (see Chapter 9 for details).