Red Hat Enterprise Linux 5 Administration Unleashed

206CHAPTER 9Managing Users and GroupsWhat Are Users and Groups?In addition to a Red Hat Enterprise Linux system having a username for each user allowedaccess to a system, each system has user groups. A user group is a group of one or moreusers. A user can be a member of more than one group. As discussed in the section “FilePermissions” of Chapter 4, “Understanding Linux Concepts,” file and directory permissionscan be granted for the owner of the file, the group associated with the file, and allusers on the system.User groups can be any grouping of users on which you decide: groups of users in a functionaldepartment, groups in the same physical location, or groups based on securityaccess. It is important to plan the user groups for your company carefully before implementingthem because changing them means changing the groups associated with files,which can sometimes lead to incorrect group permissions if they are not changed correctly.Managing UsersEach user on a Red Hat Enterprise Linux system is assigned a unique user identificationnumber, also known as a UID. UIDs below 500 are reserved for system users such as the rootuser. System users also include those added for a specific service such as the nfsnobody, rpc,and rpcuser users for the NFS service.By default in Red Hat Enterprise Linux, when a user is added, a private user group iscreated—meaning that a user group of the same name is created and that the new user isthe sole user in that group.Red Hat Enterprise Linux includes a graphical program for managing users and groups.The system-config-users package is required to do so. Install it using the RHN website orYUM as described in Chapter 3, “Operating System Updates.” Start the user and grouptool from the System menu on the top panel of the desktop by selecting Administration,Users and Groups or execute the command system-config-users. If the program is runas a non-root user, enter the root password when prompted. As shown in Figure 9.1, allexisting users are listed on the Users tab.By default, system users are not shown in the list of users. To show system users in thelist, select Edit, Preferences from the pull-down menu. In the Preferences dialog, unselectthe Hide system users and groups option, and click Close.Adding and Modifying UsersTo add a new user, click Add User to display the dialog window in Figure 9.2.Configure the username, full name, and password for the new user. The default loginshell for new users is bash. By default, the directory /home// is created as theuser’s home directory, and a private group is created for the user. These options can bemodified as shown in Figure 9.2. A UID above 500 is automatically selected for the user.To manually set the UID, select the Specify user ID manually option and then select aUID. Click OK to add the user to the system. The user immediately appears on the Userstab of the main window, and is also added to the system.