2.1 Users

A “user” is the finest grain in terms of access control in Linux. Furthermore, a user is the finest grain in terms of authentication. You can use adduser to add additional users to a system.

A user in Linux has several associated properties:

• login name: This is the symbolic name of a user. For example, tak and root are both valid login names.
• encrypted password: Because a user can be authenticated, it also potentially has an associated password. To make it harder for hackers to gain access, this password is encrypted. If a user has no associated password, it cannot log in at all. This means that in Linux, it is impossible to set up a user account with no password for authentication purposes.
• user ID: Although a user has a symbolic name so that a person can remember the user name, a user is merely a number for all system applications.
• group ID: a user belongs to a default group that has the same symbolic name as the user. The group ID distinguishes this group from all other groups. Note that even though in most situations the user ID and group ID are the same, they do not have to be the same.
• user name: This field is strictly for commenting or description use. It is optional. Most users enter their first (given) names and last (family) names.
• user home directory: This is a path specification to the “home” of a user. The “home” directory is the default folder that serves as the “CWD” (current working directory) when a user logs in. Furthermore, the home directory is also configured so that the associated user has full permission to read, write and modify files and subdirectories.
• user command interpreter (shell): This specifies the default “shell” for this user. A user usually cannot change his/her shell. This option can also be used to disallow a user from shell access.