6.3.2. Files Controlling User Accounts and Groups

On Red Hat Enterprise Linux, information about user accounts and groups are stored in several text files within the /etc/ directory. When a system administrator creates new user accounts, these files must either be edited manually or applications must be used to make the necessary changes.
The following section documents the files in the /etc/ directory that store user and group information under Red Hat Enterprise Linux.

6.3.2.1. /etc/passwd

The /etc/passwd file is world-readable and contains a list of users, each on a separate line. On each line is a colon delimited list containing the following information:
  • Username — The name the user types when logging into the system.
  • Password — Contains the encrypted password (or an x if shadow passwords are being used — more on this later).
  • User ID (UID) — The numerical equivalent of the username which is referenced by the system and applications when determining access privileges.
  • Group ID (GID) — The numerical equivalent of the primary group name which is referenced by the system and applications when determining access privileges.
  • GECOS — Named for historical reasons, the GECOS[25] field is optional and is used to store extra information (such as the user's full name). Multiple entries can be stored here in a comma delimited list. Utilities such as finger access this field to provide additional user information.
  • Home directory — The absolute path to the user's home directory, such as /home/juan/.
  • Shell — The program automatically launched whenever a user logs in. This is usually a command interpreter (often called a shell). Under Red Hat Enterprise Linux, the default value is /bin/bash. If this field is left blank, /bin/sh is used. If it is set to a non-existent file, then the user will be unable to log into the system.
Here is an example of a /etc/passwd entry: 
 root:x:0:0:root:/root:/bin/bash
This line shows that the root user has a shadow password, as well as a UID and GID of 0. The root user has /root/ as a home directory, and uses /bin/bash for a shell.
For more information about /etc/passwd, see the passwd(5) man page.

[25] GECOS stands for General Electric Comprehensive Operating Supervisor. This field was used at Bell Labs, in the original UNIX implementation. The lab had many different computers, including one running GECOS. This field was used to store information when the UNIX system sent batch and print jobs to the GECOS system.