Chapter 6. Users and Groups
The control of users and groups is a core element of Red Hat Enterprise
Linux system administration.
Users can be either people, meaning
accounts tied to physical users, or accounts which exist for
specific applications to use.
Groups are logical expressions of
organization, tying users together for a common purpose. Users
within a group can read, write, or execute files owned by that
Each user and group has a unique numerical identification number
called a userid (UID) and a groupid
A user who creates a file is also the owner and group owner of
that file. The file is assigned separate read, write, and execute
permissions for the owner, the group, and everyone else. The file
owner can be changed only by the root user as well as access
permissions can be changed by both the root user and the owner of
Red Hat Enterprise Linux supports access
control lists (ACLs) for files and
directories which allow permissions for specific users outside of
the owner to be set. For more information about using ACLs, refer
to the chapter titled Access Control Lists
in the Red Hat Enterprise Linux System
Proper management of users and groups as well as the effective
management of file permissions are among the most important tasks a
system administrator undertakes. For a detailed look at strategies
for managing users and groups, refer to the chapter titled
Managing User Accounts and Resource Access
in the Red Hat Enterprise Linux Introduction
to System Administration guide.
Managing users and groups can be a tedious task, but Red Hat
Enterprise Linux provides tools and conventions to make their
The easiest way to manage users and groups is through the
graphical application, User Manager
(system-config-users). For more
information on User Manager, refer to
the chapter titled User and Group
Configuration in the Red Hat Enterprise
Linux System Administration Guide.
The following command line tools can also be used to manage
users and groups:
useradd, usermod, and userdel
— Industry-standard methods of adding, deleting and modifying
groupadd, groupmod, and groupdel
— Industry-standard methods of adding, deleting, and
modifying user groups.
gpasswd — Industry-standard
method of administering the /etc/group
— Tools used for the verification of the password, group, and
associated shadow files.
pwconv, pwunconv — Tools used for the conversion of
passwords to shadow passwords and back to standard passwords.
For an overview of users and group management, refer to the
Red Hat Enterprise Linux Introduction to
System Administration. For a detailed look at command line
tools for managing users and groups, see the chapter titled
User and Group Configuration in the
Red Hat Enterprise Linux System Administration