In most cases, the default installation of Red Hat Enterprise Linux configures a machine to boot into a graphical login environment, known as runlevel 5. It is possible, however, to boot into the text-only multi-user mode called runlevel 3 and begin an X session from there.
For more information about runlevels, refer to Section 1.4 SysV Init Runlevels.
This section reviews how XFree86 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 startx. The startx command is a front-end to the xinit command, which launches the XFree86 server and connects X clients applications to it. Because the user is already logged into the system at runlevel 3, startx does not launch a display manager or authenticate users. Refer to Section 7.5.2 Runlevel 5 for more information about display managers.
When the startx command is executed, it searches for a .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.
The default xinitrc script then looks for user-defined files and default system files, including .Xresources, .Xmodmap, and .Xkbmap in the user's home directory, and Xresources, Xmodmap, and Xkbmap in the /etc/X11/ directory. The Xmodmap and 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, which configures settings such as the default language.
Next, the xinitrc script tries 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 twm.
The user is returned to a text mode user session after logging out of X from runlevel 3.
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 environments or window managers are launched.
Depending on the desktop environments installed on the system, three different display managers are available to handle user authentication.
gdm — The default display manager for Red Hat Enterprise Linux, gdm allows the user to configure language settings, shutdown, restart or log in to the system.
kdm — 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 within the /usr/share/doc/initscripts-<version-number>/sysconfig.txt file (where <version-number> is the version number of the initscripts package).
Each of the display managers references 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.
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 Sessions menu item (accessed by selecting Main Menu Button [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 continued running after the user logged in, takes control by spawning a new display manager. This restarts the XFree86 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 about 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.