Red Hat LINUX 7.2 Reference Manual page 123

Section 6.4:Runlevels 123
the .Xclients-default file. If .Xclients does not exist in the user's home directory, the
standard /etc/X11/init/Xclients script attempts to start another desktop environment, try-
ing GNOME first and then KDE. If a desktop environment cannot be found by this point, Xclients
cycles through a list of window managers to find the one to start, after attempting the default window
manager listed in the .wm_style file in the user's home directory.
By this point, the preferred X client applications should be started, along with the XFree86 server. If
you need more details concerning starting X in runlevel 3, refer to the startx and xinit man pages
and read through the scripts named above.
6.4.2 Runlevel 5: prefdm
Runlevel 5 uses a slightly different method to start X. When the system starts, no one is logged into
the system by default. In order for a user to start a session, they must log in to the system. In runlevel
5, users authenticating themselves at the console use a display manager, a special X client that allows
the user to submit their login name and password.
Depending on the desktop environments installed on your specific Red Hat Linux system, three dif-
ferent display managers are available to handle the user authentication. The xdm display manager
is the original X authentication tool. xdm only allows you to log in and start an X session, nothing
more. The gdm display manager, designed to work with the GNOME desktop environment, and kdm
display manager, used with the KDE desktop environment, allow you to set the desktop environment,
or session, you would like to use after authentication. Additionally, you can restart or halt the system
from the login screen. The gdm display manager also allows you to configure the language you would
like to use.
When the system enters runlevel 5, a line in the /etc/inittab file specifies that the prefdm script
is executed in order to determine the preferred display manager to bring up for user authentication.
The prefdm script uses the preferences stated in the /etc/sysconfig/desktop file to find the
proper display manager. If no desktop environment is specified, prefdm cycles through the gdm,
kdm, and xdm display managers to find one to use. Once one is found, prefdm launches it to handle
the user login.
Each of the display managers look to 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 own-
ership of the console to the user. Then, the /etc/X11/xdm/Xsession script runs to accomplish
many of the tasks normally done by the xinitrc script when starting X in runlevel 3, including
setting system and user resources, as well as running the scripts in the /etc/X11/xinit/xini-
trc.d directory.
The user can specify which desktop environment they want to utilize when they authenticate using
the gdm or kdm display managers by selecting it from the menu. If the desktop environ-
Session
ment is not specified in the display manager, the /etc/X11/xdm/Xsession script will check the