In most cases, the Red Hat Enterprise Linux installer configures a machine to boot into a graphical login environment, known as Runlevel 5. It is possible, however, to boot into a text-only multi-user mode called Runlevel 3 and begin an X session from there.
The following subsections review how X starts up in both runlevel 3 and runlevel 5.
When in runlevel 3, the best way to start an X session is to log in and type
command is a front-end to the
command, which launches the X server (
) and connects X client applications to it. Because the user is already logged into the system at runlevel 3,
does not launch a display manager or authenticate users. Refer to Section 35.5.2, “Runlevel 5”
for more information about display managers.
startx command is executed, it searches for the
.xinitrc file in the user's home directory to define the desktop environment and possibly other X client applications to run. If no
.xinitrc file is present, it uses the system default
/etc/X11/xinit/xinitrc file instead.
xinitrc script then searches for user-defined files and default system files, including
.Xkbmap in the user's home directory, and
Xkbmap in the
/etc/X11/ directory. The
Xkbmap files, if they exist, are used by the
xmodmap utility to configure the keyboard. The
Xresources file is read to assign specific preference values to applications.
After setting these options, the
xinitrc script executes all scripts located in the
/etc/X11/xinit/xinitrc.d/ directory. One important script in this directory is
xinput.sh, which configures settings such as the default language.
xinitrc script attempts to execute
.Xclients in the user's home directory and turns to
/etc/X11/xinit/Xclients if it cannot be found. The purpose of the
Xclients file is to start the desktop environment or, possibly, just a basic window manager. The
.Xclients script in the user's home directory starts the user-specified desktop environment in the
.Xclients-default file. If
.Xclients does not exist in the user's home directory, the standard
/etc/X11/xinit/Xclients script attempts to start another desktop environment, trying GNOME first and then KDE followed by
When in runlevel 3, the user is returned to a text mode user session after ending an X session.
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.
GNOME — The default display manager for Red Hat Enterprise Linux,
GNOME allows the user to configure language settings, shutdown, restart or log in to the system.
KDE — KDE's display manager which allows the user to shutdown, restart or log in to the system.
xdm — A very basic display manager which only lets the user log in to the system.
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
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
Users can specify which desktop environment they want to utilize when they authenticate using the
KDE display managers by selecting it from the menu item (accessed by selecting System (on the panel) > > > ). If the desktop environment is not specified in the display manager, the
/etc/X11/xdm/Xsession script checks the
.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.