Essential System Administration, 3rd Edition

Unix Users and Groups

From the system’s point of view, auser isn’t necessarily an individual person. Technically, to the operating system, a user is an entity that can execute programs or own files. For example, some user accounts exist only to execute the processes required by a specific subsystem or service (and own the files associated with it); such users are sometimes referred to as pseudo users . In most cases, however, a user means a particular individual who can log in, edit files, run programs, and otherwise make use of the system.

Each user has a username that identifies him. When adding a new user account to the system, the administrator assigns the username a user identification number (UID). Internally, the UID is the system’s way of identifying a user. The username is just mapped to the UID. The administrator also assigns each new user to one or more groups : a named collection of users who generally share a similar function (for example, being members of the same department or working on the same project). Each group has a group identification number (GID) that is analogous to the UID: it is the system’s internal way of defining and identifying a group. Everyuser is a member of one or more groups. Taken together, a user’s UID and group memberships determine what access rights he has to files and other system resources.

User account information is stored in several ASCII configuration files:

/etc/passwd

User accounts.

/etc/shadow

Encoded passwords and password ...