Runlevels
Most users run X from runlevels 3 or 5. Runlevel 3 places
your system in multi-user mode with full networking capabilities. The
machine will boot to a text-based login prompt with all necessary
preconfigured services started. Most servers are run in runlevel 3, as X
is not necessary to provide any services utilized by most
users. Runlevel 5 is similar to 3, except that it automatically starts X
and provides a graphical login screen. Many workstation users prefer
this method, because it never forces them to see a command prompt.
The default runlevel used when your system boots can be found in the
/etc/inittab file. If you have a line in that file
that looks like id:3:initdefault:, then
your system will boot to runlevel 3. If you have a line that looks like
id:5:initdefault:, your system is set to
boot into runlevel 5. As root, change the runlevel number in this file
to set a different default. Save the file and restart your system to
verify that it boots to the correct runlevel. More information on
runlevels can be found in the Section called Init Runlevels in Chapter 3.
Runlevel 3: startx
When in runlevel 3, the preferred way to start an X session is to type
the startx command. startx, a
front-end to the xinit program, launches the
XFree86 server and connects the X clients to it. Because you must
already be logged into the system at runlevel 3 to be able to type
commands, startx is only designed to bring up
certain X clients, such as a desktop environment, in a particular
manner. It does not provide any user authentication.
When startx begins, it looks for a
.xinitrc file in the user's home directory to
define the X clients to run. If it does not find that file present, it
will run the system default
/etc/X11/xinit/xinitrc script instead. The
startx script then does the same thing with the
.xserverrc file, looking for it in the user's
home directory and then running the default
/etc/X11/xinit/xserverrc script if it cannot be
found. Since many different X clients exist, the
xinitrc files are very important. The
xserverrc script is less important. It only sets
the X server to connect to the X clients. Since the default X server
is already configured with the /etc/X11/X link,
Red Hat Linux does not install a default xserverrc.
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 files are read to assign specific
preference values to particular 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 to use and the
desktop environment to start from
(/etc/sysconfig/desktop).
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 or window
manager in 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, trying GNOME
first and then KDE. If a desktop environment cannot be found by this
point, Xclients attempts the default window
manager listed in the .wm_style file in the
user's home directory. If this fails, it cycles through a predefined
list of window managers.
The preferred X client applications should now have started by this
point, 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.
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 different 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
ownership 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/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
menu. If the desktop environment is not
specified in the display manager, the
/etc/X11/xdm/Xsession script will check 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.
For more information about how display managers control user
authentication, read the xdm man page.
