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20.5.2. Runlevel 5

When the system boots into runlevel 5, a special X client application called a display manager is launched. A user must authenticate using the display manager before any desktop environment or window managers are launched.
Depending on the desktop environments installed on the system, three different display managers are available to handle user authentication.
When booting into runlevel 5, the prefdm script determines the preferred display manager by referencing the /etc/sysconfig/desktop file. A list of options for this file is available in this file:

where <version-number> is the version number of the initscripts package.
Each of the display managers reference the /etc/X11/xdm/Xsetup_0 file to set up the login screen. Once the user logs into the system, the /etc/X11/xdm/GiveConsole script runs to assign ownership of the console to the user. Then, the /etc/X11/xdm/Xsession script runs to accomplish many of the tasks normally performed by the xinitrc script when starting X from runlevel 3, including setting system and user resources, as well as running the scripts in the /etc/X11/xinit/xinitrc.d/ directory.
Users can specify which desktop environment they want to utilize when they authenticate using the GNOME or KDE display managers by selecting it from the Sessions menu item (accessed by selecting System (on the panel) > Preferences > More Preferences > Sessions). If the desktop environment is not specified in the display manager, the /etc/X11/xdm/Xsession script checks the .xsession and .Xclients files in the user's home directory to decide which desktop environment to load. As a last resort, the /etc/X11/xinit/Xclients file is used to select a desktop environment or window manager to use in the same way as runlevel 3.
When the user finishes an X session on the default display (:0) and logs out, the /etc/X11/xdm/TakeConsole script runs and reassigns ownership of the console to the root user. The original display manager, which continues running after the user logged in, takes control by spawning a new display manager. This restarts the X server, displays a new login window, and starts the entire process over again.
The user is returned to the display manager after logging out of X from runlevel 5.
For more information on how display managers control user authentication, refer to the /usr/share/doc/gdm-<version-number>/README (where <version-number> is the version number for the gdm package installed) and the xdm man page.