An X display manager presents a graphical login that requires a username and password to be entered before access is granted to the X desktop. It also allows you to choose a different desktop for your X session. Whether or not an X display manager is presented after you boot Linux is controlled by a runlevela system state entry in /etc/inittab. The following runlevels are defined in the file:
# Runlevel 0 is halt.
# Runlevel 1 is single-user.
# Runlevels 2-5 are multi-user.
# Runlevel 6 is reboot.
Runlevels 2-5 are used for multiuser mode with a graphical X login via a display manager; booting to runlevel 1 provides a single-user, or text-based, login. The initdefault setting in the /etc/inittab file determines the default runlevel:
In this example, Linux boots and then runs X.
The gdm display manager is part of the GNOME library and client distribution included with Ubuntu and provides a graphical login when a system boots directly to X. Its login (which is actually displayed by the gdmlogin client) hosts pop-up menus of window managers, languages, and system options for shutting down (halting) or rebooting the workstation. Although you can edit (as root) gdm.conf under the /etc/gdm directory to configure gdm, a much better way to configure GNOME's display manager is to use the gdmsetup client.
You can use the gdmsetup client to configure many aspects and features of the login display. You launch this client from the GNOME desktop panel's System, Administration, Login Screen menu item, or from the command line, like this:
$ gksudo gdmsetup &
After you press Enter, you see the GDM Setup window, as shown in Figure 6.1.
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