Setting up a Graphical Login

Setting up your Linux workstation to display a graphical login screen is easy-all you have to do is make the appropriate selection during the installation. Even if you have not initially elected to have a graphical login screen, you can easily set it up once you know the details. The following sections explain how your CentOS Linux system displays the graphical login screen.

Understanding How init Starts the Display Manager

A process named init starts the initial set of processes on your CentOS Linux system. What init starts depends on the current run level, the contents of the /etc/inittab file, and the shell scripts located in the /etc/rc.d directory and its subdirectories. For now, you don't need to understand the details; you must know, however, that the graphical login screen starts at run level 5.

Cross Ref

Chapter 20 describes in detail the init process, the /etc/inittab file, and the CentOS Linux boot process.

The last line of the /etc/inittab file is responsible for starting the graphical login process with the following entry (the number 5 denotes run level 5):

x:5:respawn:/etc/X11/prefdm -nodaemon

This command runs /etc/X11/prefdm, a shell script that contains the following lines:

#!/bin/sh
PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/X11R6/bin
# Shut down any graphical boot that might exist.
if [ -x /usr/bin/rhgb-client ]; then
    /usr/bin/rhgb-client -quit
fi

# We need to source this so that the login screens are translated.
[ -f /etc/profile.d/lang.sh ] && . /etc/profile.d/lang.sh
# Run preferred X display manager
# Try autologin first, if wanted...
if [ -f /etc/sysconfig/autologin -a -x /usr/sbin/autologin ]; then
     if /usr/sbin/autologin; then
            exit 0
     fi
     # If autologin fails (bad permissions, etc.), we get here -
     # time to start a real display manager.
fi
preferred=
if [ -f /etc/sysconfig/desktop ]; then
     . /etc/sysconfig/desktop
     if [ "$DISPLAYMANAGER" = GNOME ]; then
            preferred=gdm
     elif [ "$DISPLAYMANAGER" = KDE ]; then
            preferred=kdm
     elif [ "$DISPLAYMANAGER" = XDM ]; then
            preferred=xdm
     fi
fi
if [ -z "$preferred" ]; then
     if which gdm >/dev/null 2>&1; then
            preferred=gdm
     elif which kdm >/dev/null 2>&1; then
            preferred=kdm
     elif which xdm >/dev/null 2>&1; then
            preferred=xdm
     fi
fi
if [ -n "$preferred" ] && which $preferred >/dev/null 2>&1; then 
     exec `which $preferred` $* >/dev/null 2>&1
fi
if which gdm >/dev/null 2>&1; then
     exec `which gdm` $* >/dev/null 2>&1
elif which kdm >/dev/null 2>&1; then
     exec `which kdm` $* >/dev/null 2>&1
elif which xdm >/dev/null 2>&1; then
     exec `which xdm` $* >/dev/null 2>&1
fi
# Catch all exit error
exit 1

This script starts a specific display manager-gdm, kdm, or xdm-depending on the setting of the DISPLAYMANAGER variable in the /etc/sysconfig/desktop file. The display manager is a program responsible for displaying the graphical login window, authenticating users who log in, running initialization scripts at the start of a session, and cleaning up after the session. The display manager process manages the display, making it available to the users and cleaning up after the user finishes a session. Note that of the three display managers-gdm, kdm, or xdm-xdm is the most generic.

If the DISPLAYMANAGER variable is not defined, the /etc/X11/prefdm script checks for gdm, kdm, and xdm-in that order-and runs whichever display manager it finds first. The end result is that you get gdm if you installed GNOME and kdm if you installed KDE as your sole desktop. If you install both GNOME and KDE, you get gdm by default because the prefdm script looks for gdm first. If you do have both GNOME and KDE installed on your system and you want the kdm as the display manager, define the DISPLAYMANAGER variable in /etc/sysconfig/desktop like this:

DISPLAYMANAGER="KDE"

Then, the next time you reboot the system, the kdm display manager will run.

To summarize, regardless of your choice of GUI, at run level 5 init starts a display manager. The display manager, in turn, displays the graphical login dialog box, which enables you to log in to the system.

If you have not enabled the graphical login screen during Red Hat installation, you can do so by editing the /etc/inittab file. Locate the line containing initdefault, and make sure that it reads as follows:

id:5:initdefault:

Caution

Before you edit the /etc/inittab file, you should know that any errors in this file may prevent Linux from starting up to a point at which you can log in. If you cannot log in, you cannot use your system.

After editing the default run level in /etc/inittab, you can either reboot the system or type the following command to switch to run level 5:

telinit 5

Learning the GNOME Display Manager (gdm)

The gdm program is a display manager similar to xdm, the X display manager. Like xdm, gdm starts an X server for each local display, displays a login dialog box, and enables the user to log in to the system. Figure 9-1 shows a typical graphical login screen displayed by gdm.

Figure 9-1: Graphical Login Screen Managed by gdm.

When gdm runs, it reads various configuration parameters from the configuration file /etc/X11/gdm/gdm.conf, which has a structure similar to a Windows INI file. For example, here is a typical gdm.conf file:

[daemon]
AlwaysRestartServer=false
AutomaticLogin=
AutomaticLoginEnable=false
Chooser=/usr/bin/gdmchooser
Configurator=/usr/sbin/gdmsetup --disable-sound --disable-crash-dialog
DefaultPath=/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin
DisplayInitDir=/etc/X11/gdm/Init
FailsafeXServer=
FirstVT=7
FlexibleXServers=5
GnomeDefaultSession=/usr/share/gnome/default.session
Greeter=/usr/bin/gdmgreeter
Group=gdm
HaltCommand=/usr/bin/poweroff;/sbin/poweroff;/sbin/shutdown -h now;/usr/sbin/shutdown -h now
KillInitClients=true
LocalNoPasswordUsers=
LogDir=/var/log/gdm
PidFile=/var/run/gdm.pid
PostSessionScriptDir=/etc/X11/gdm/PostSession/
PreSessionScriptDir=/etc/X11/gdm/PreSession/
RebootCommand=/sbin/shutdown -r now;/usr/sbin/shutdown -r now
RemoteGreeter=/usr/bin/gdmlogin
RootPath=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin
ServAuthDir=/var/gdm
SessionDir=/etc/X11/gdm/Sessions/
StandardXServer=/usr/X11R6/bin/X
SuspendCommand=
TimedLogin=
TimedLoginDelay=30
TimedLoginEnable=false
User=gdm
UserAuthDir=
UserAuthFBDir=/tmp
UserAuthFile=.Xauthority
VTAllocation=true
XKeepsCrashing=/etc/X11/gdm/XKeepsCrashing
Xnest=/usr/X11R6/bin/Xnest -name Xnest
[security]
AllowRemoteAutoLogin=false
AllowRemoteRoot=true
AllowRoot=true
RelaxPermissions=0
RetryDelay=1
SessionMaxFile=524388
UserMaxFile=65536
[xdmcp]
DisplaysPerHost=1
Enable=false
HonorIndirect=true
MaxPending=4
MaxPendingIndirect=4
MaxSessions=16
MaxWait=15
MaxWaitIndirect=15
Port=177
Willing=/etc/X11/gdm/Xwilling
[gui]
GtkRC=/usr/share/themes/Bluecurve/gtk-2.0/gtkrc
MaxIconHeight=128
MaxIconWidth=128
[greeter]
BackgroundColor=#27408b
BackgroundImage=
BackgroundProgram=
BackgroundRemoteOnlyColor=true
BackgroundScaleToFit=true
BackgroundType=0
Browser=false
ConfigAvailable=false
DefaultFace=/usr/share/pixmaps/nobody.png
Exclude= bin,daemon,adm,lp,sync,shutdown,halt,mail,news,uucp,operator,nobody,gdm,postgres,pvm,rpm
GlobalFaceDir=/usr/share/faces/
GraphicalTheme=Bluecurve
GraphicalThemeDir=/usr/share/gdm/themes/
Icon=/usr/share/pixmaps/gdm.png
LocaleFile=/etc/X11/gdm/locale.alias
LockPosition=true
Logo=
MinimalUID=500
PositionX=0
PositionY=0
Quiver=true
RunBackgroundProgramAlways=false
SetPosition=false
ShowGnomeChooserSession=false
ShowGnomeFailsafeSession=false
ShowXtermFailsafeSession=false
SystemMenu=true
TitleBar=false
Use24Clock=false
UseCirclesInEntry=false
Welcome=Welcome to %n
XineramaScreen=0
[chooser]
Broadcast=true
DefaultHostImg=/usr/share/pixmaps/nohost.png
HostImageDir=/usr/share/hosts/
Hosts=
ScanTime=3
[debug]
Enable=false
[servers]
0=Standard
[server-Standard]
command=/usr/X11R6/bin/X
flexible=true
name=Standard server

The [servers] section of the file specifies the displays on which gdm displays the login dialog box. Each line of this section starts with a display number and shows the command needed to start the X server.

You see the term greeter used often in the display manager configuration file. Greeter refers to the login screen-the user interface through which the display manager prompts the user to log in to the system.

The different sections in the configuration file control different aspects of the graphical login screen as well as the tasks the user can perform from that screen. Table 9-1 describes the gdm configuration options, organized by sections.

Insider Insight

Edit the Welcome= line to change the welcome message on the graphical login screen. The default message says welcome to the hostname, which can be somewhat cryptic. You can edit that line and change the message to something more meaningful with the syntax Welcome=Welcome to XYZ Corp.

Table 9-1: The gdm Configuration Options
Configuration Option	Description
[daemon] section
`AlwaysRestartServer=true or false`	If set to true, gdm always kills the existing X server and starts a new one.
`AutomaticLoginEnable=true or false`	If set to true, automatically log in the user specified in the `AutomaticLogin` option without asking for any password.
`AutomaticLogin=username`	If a user name is specified and `AutomaticLogin Enable` is true, that user is logged without asking for a password.
`Chooser=chooserprogram`	Full path and name of the `gdmchooser` executable file (typically, this should be `/usr/bin/gdmchooser`)
`Configurator=chooserprogram`	Full path and name of the `gdmconfig` executable file (typically, this should be `/usr/bin/gdmconfig`)
`DefaultPath=pathname`	Path (names of directories separated by colons) to be used for the user's session
`DisplayInitDir=pathname`	Directory containing the initialization scripts (the default is `/etc/X11/gdm/Init`)
`FailsafeXServer = safex-script`	Full path name of script that gdm should run to provide a failsafe GUI login screen
`FirstVT =number`	The virtual terminal (VT) number where X should be run (default is 7)
`FlexibleXServers =count`	Maximum number of X servers to run
`GnomeDefaultSession= sessionfile`	Full path name of file that gdm should read if there are no user GNOME session files (the default is `/usr/share/gnome/default.session`)
`Greeter=greeterprogram`	Full path and name of the greeter executable file, followed by arguments (typically, this should be `/usr/ bin/gdmlogin`)
`Group=groupname`	Group name under which `gdmlogin` and `gdmchooser` are run (the default is `gdm`)
`HaltCommand=haltprogram`	Full path and arguments to the commands to be executed when the user selects Halt from the System menu (typically, this should be `/sbin/shutdown -h now`)
`KillInitClients=true or false`	When set to true, gdm kills the X clients started by the `Init` scripts after the user logs in (the default is true).
`LocalNoPasswordUsers = userlist`	Comma-separated list of users that are allowed to log in without password
`LogDir=pathname`	Directory containing the log files for the displays (the default is `/var/log/gdm`)
`PidFile=pidfile`	Name of the file where the process ID (pid) of the gdm process is stored (the default is `/var/run/gdm.pid`)
`PostSessionScriptDir=pathname`	Directory that contains the PostSession scripts-scripts run after the user logs out (the default is `/etc/X11/gdm/ PostSession/`)
`PreSessionScriptDir=pathname`	Directory that contains the `PreSession` scripts-scripts run right after the user logs in (the default is `/etc/X11/ gdm/PreSession/`)
`RebootCommand=rebootprogram`	Full path and arguments to the command to be executed when the user selects Reboot from the System menu (typically, this should be `/sbin/shutdown -r now`)
`RemoteGreeter =pathname`	Full path name of the greeter executable file for remote displays
`RootPath=pathname`	Path (names of directories separated by colons) to be used for the `root` user's session
`ServAuthDir=pathname`	Directory (the default is `/var/gdm`) containing the X authentication files for the displays (this directory should be owned by user `gdm`, group `gdm`, with permission 750)
`SessionDir=pathname`	Directory containing scripts for all session types available on the system (the default is `/etc/X11/gdm/Sessions`)
`StandardXServer =pathname`	Path name of standard X server (the default is `/usr/X11R6/bin/X`)
`TimedLogin=username`	If a user name is specified and `TimedLoginEnable` is true, then that user is logged without asking for any password after the login screen is inactive for the number of seconds specified in the `TimedLoginDelay` option
`TimedLoginDelay=N`	The number of seconds delay before the user specified in `TimedLogin` is automatically logged in
`TimedLoginEnable=true or false`	If set to true, automatically logs in the user specified in the `TimedLogin` option without asking for any password
`User=username`	User name under which `gdmchooser` and `gdmlogin` are run (the default is `gdm`)
`UserAuthDir=pathname`	Directory where the user's `.Xauthority` file should be saved (if this field is empty, the user's home directory is used, which is the default)
`UserAuthFBDir=pathname`	Directory where a fallback cookie (which simply refers to the small amount of authentication information) is created if gdm fails to update the user's `.Xauthority` file (the default is `/tmp`)
`UserAuthFile=filename`	Name of the file for storing the user's authentication cookies (the default is `.Xauthority`)
`VTAllocation =true or false`	If true, automatically allocate virtual terminal (VT) for use by X server (default is `true`)
`XKeepsCrashing=sessionfile`	Full path name of a script that gdm should run if X keeps crashing (the default script is `/etc/X11/gdm/ XKeepsCrashing`)
`Xnest = pathname`	Full path name of the nested X server (`Xnest`) that runs as an X client, but also manages graphics on behalf of the X server (default is `/usr/X11R6/bin/Xnest -name Xnest`)
[security] section
`AllowRemoteAutoLogin =true or false`	When set to true, allows remote timed login (the default is `false`)
`AllowRemoteRoot=true or false`	When set to true, the root user can log in remotely (the default is `true`)
`AllowRoot=true or false`	When set to true, the root user can log in at the console (the default is `true`)
`RelaxPermissions=0, 1, or 2`	0 = `gdm` only accepts files and directories owned by the user; 1 = `gdm` allows group writeable files and directories; 2 = `gdm` allows world writeable files and directories (the default is `1`)
`RetryDelay=N`	Number of seconds to wait before gdm displays the login window again after a failed login (the default is 3 seconds)
`UserMaxFile=N`	Maximum size (in bytes) of files that gdm will read or write (the default is 64KB, or 65,536 bytes)
[xdmcp] section
`DisplaysPerHost =number`	Number of displays per host (the default is `1`)
`Enable=true or false`	When set to true, `gdm` listens on port 177 and supports the X Display Manager Control Protocol (XDMCP) that enables users to log in to other hosts on the network (the default is `false`)
`HonorIndirect=true or false`	When set to true, gdm allows remote execution of the `gdmchooser` client on X terminals (the default is `true`)
`MaxPending=N`	Maximum number of pending connections (the default is `4`) that gdm should allow (this helps avoid any denial of service attacks)
`MaxPendingIndirect=N`	Maximum number of `gdmchooser` clients that `gdm` will run simultaneously on remote displays (the default is `4`)
`MaxSessions=N`	Maximum number of remote display connections that gdm will accept (the default is `16`)
`MaxWait=N`	Maximum number of seconds that gdm waits for a response from a display before assuming that the display is not active anymore (the default is 15 seconds)
`MaxWaitIndirect=N`	Maximum number of seconds that gdm waits for a remote display to complete protocol negotiations after the display requests a chooser (the default is 15 seconds)
`Port=177`	The UDP port number where gdm listens for XDMCP requests (the default is `177`)
`Willing =pathname`	Full path name of script to run to generate replies to XDMCP BroadcastQuery requests (the default is `/etc/X11/gdm/Xwilling`)
[gui] section
`GtkRC=pathname`	Path name of file that contains the theme to be used by the `gdmlogin` and `gdmchooser` programs (the default is the BlueCurve theme, defined in the file `/usr/ share/themes/Bluecurve/gtk-2.0/gtkrc`)
`MaxIconWidth=N`	Maximum width (the default is 128 pixels) of icons that the face browser will display (gdmlogin can display a face browser containing icons for all the users on a system; these icons can be installed globally by the system administrator or in the users' home directories)
`MaxIconHeight=N`	Maximum height (the default is 128 pixels) of icons that the face browser will display
[greeter] section
`BackgroundColor =#colornum`	Background color
`BackgroundImage =pathname`	Pathname of image to use as background (when `BackgroundType`=1)
`BackgroundProgram =pathname`	Pathname of program to run to draw the background
`BackgroundRemoteOnlyColor = true or false`	When set to `true`, displays a solid color background for remote displays (the default is `true`)
`BackgroundScaleToFit = true or false`	When set to `true`, scales image to fit screen (the default is `true`)
`BackgroundType =0, 1, or 2`	Type of background for the greeter, `0` = None, `1` = Image, `2` = Color (the default is 0)
`Browser=true or false`	When set to `true`, displays a browser, called the face browser, where the faces of all system users are displayed (the default is `false`)
`ConfigAvailable =true or false`	When set to `true`, configuration option is available from the System menu (default is `false`)
`DefaultFace=imagefile`	A file, readable by the `gdm` user, containing an image that is to be displayed if a user does not have a personal picture in the `~/gnome/photo` directory (which refers to the `gnome/photo` subdirectory of the user's home directory). The default is the `/usr/share/pixmaps/ nobody.png` file.
`Exclude=usernames`	Comma-separated list of user names to be excluded from the face browser; the excluded users will still be able to log in
`GlobalFaceDir=pathname`	Directory where face files (each containing an icon showing a user's face) are located (the default is `/usr/share/faces/`)
`GraphicalTheme =themename`	Name of graphical theme to use (default is `BlueCurve`)
`GraphicalThemeDir =pathname`	Directory where graphical theme files are located (the default is `/usr/share/gdm/themes/`)
`Icon=imagefile`	A file, readable by the `gdm` user, containing the image to be displayed when the login window is in iconified state (the default is `/usr/share/pixmaps/gdm.xpm`)
`LocaleFile=localefile`	Full pathname for the file in GNU locale format, with entries for all languages supported on the system (the default is `/etc/X11/gdm/locale.alias`)
`LockPosition =true or false`	When set to `true`, the user cannot move the greeter window (the default is `true`)
`Logo=imagefile`	A file, readable by the `gdm` user, with the image to be displayed in the logo area (the default is `/usr/share/ pixmaps/gnome-logo-large.png`).
`MinimalUID =number`	Users with this user ID or higher can log in (the default is 500)
`PositionX =xpos`	Horizontal and vertical position of greeter window `PositionY=ypos`
`Quiver=true or false`	When set to `true`, the gdmlogin program shakes the display (not physically, but by moving the graphics output) when a user enters an incorrect password (the default is false)
`RunBackgroundProgramAlways = true or false`	When set to `true`, runs the background program specified by `BackgroundProgram` (the default is `false`)
`SetPosition =true or false`	When set to `true`, the gdmlogin program shakes the display (not physically, but by moving the graphics output) when a user enters an incorrect password (the default is `false`)
`ShowGnomeChooserSession = true or false`	When set to `true`, shows previous GNOME sessions in the chooser (the default is `false`)
`ShowGnomeFailsafeSession = true or false`	When set to `true`, shows the GNOME failsafe session in the chooser (the default is `false`)
`ShowXtermFailsafeSession = true or false`	When set to `true`, show the failsafe session that runs an xterm window (the default is `false`)
`SystemMenu =true or false`	When set to `true`, show the System menu on the greeter (the default is `true`)
`TitleBar =true or false`	When set to true, show a title bar on the greeter window (the default is `false`)
`Use24Clock =true or false`	When set to `true`, use a 24-hour clock no matter what the locale (the default is `false`)
`UseCirclesInEntry =true or false`	When set to `true`, echo the typed password as circles (the default is `false`)
`Welcome=message`	The English welcome message that the `gdmlogin` program displays next to the logo on the login screen (the default message is `Welcome to %n`, where `%n` means the node name-the hostname without the domain)
`XineramaScreen =number`	The Xinerama screen where greeter is displayed (default is `0`)
[chooser] section
`Broadcast =true or false`	When set to `true`, an XDMCP query is broadcast to get the names of all the hosts that accept login (the default is `true`)
`DefaultHostImg=filename`	File containing an image to be displayed for those hosts that do not have a unique icon file (the default is `/usr/ share/pixmaps/nohost.png`)
`HostImageDir=pathname`	Directory (the default is `/usr/share/hosts/`) that contains host icon files; each filename matches the host system's fully qualified domain name (such as `yourhost.yourdomain.com`).
`Hosts=hostnames`	Comma-separated names of hosts to list
`ScanTime=N`	Number of seconds the chooser should wait for replies to its XDMCP broadcast query (the default is 3 seconds)
[debug] section
`Enable=true or false`	When set to true, gdm saves information that can help isolate any bugs in gdm (the default is `false`)
[servers] section
`N=ServerName`	Name of the X server to start on local display number N (if your system has a single monitor, N is 0). Default is Standard
[server-Standard] section
`command=ServerPathName`	Full pathname of the Standard X server (the default is `/usr/X11R6/bin/X`)
`flexible=true or false`	When set to `true`, the Standard server is considered a flexible server (the default is `true`)
`name=servername`	A descriptive name of the Standard server (default is Standard server)

For the default configuration (after you install CentOS Linux and X from the companion CD-ROMs), gdm manages the login session as follows:

The gdm program starts the X server and looks for a script named after the local display's name (for the first display, the filename is :0) in the /etc/X11/gdm/ Init directory. If it does not find that file, gdm executes the file named Default from that directory. The /etc/X11/gdm/Init/Default script in CentOS Linux contains the following commands:

    #!/bin/sh
    # Xsetup for CentOS Linux
    # Copyright (c) 1999, 2000, 2001 Red Hat, Inc.
    sysresources=/etc/X11/Xresources
    # merge in defaults
    if [ -f "$sysresources" ]; then
        xrdb -merge "$sysresources"
    fi
    /usr/X11R6/bin/xsetroot -solid "#5477A0"
    if [ -z "$RUNNING_UNDER_GDM" ] ; then
      /sbin/pidof -s kdm &> /dev/null && [ $? -eq 0 -a -x      /usr/bin/kdmdesktop ] && /usr/bin/kdmdesktop && sleep 1s
      # Only set the background if the xsri program is installed.
      if [ -x /usr/bin/xsri -a -f /etc/X11/xsrirc ]; then
          /usr/bin/xsri --redhat-login --set
      fi
    fi

In this case, the script sets the background to a solid color and displays the Red Hat logo. The gdm program runs this script as root and waits until the commands finish.

The gdm program runs the /usr/bin/gdmlogin program, which displays a dialog box that contains fields in which the user can enter a name and a password.
After a user enters a name and password and presses Enter, gdm verifies the password and tries to execute the startup script /etc/X11/gdm/PreSession/ DisplayName, where DisplayName is :0 for the first display. If that file is not found, gdm executes the /etc/X11/gdm/PreSession/Default script, if it exists.
The gdm program runs the session script from the /etc/X11/gdm/Sessions directory. The exact script that gdm runs depends on the session the user selects in the Sessions menu in the login dialog box gdmlogin is displayed. If the user has not explicitly selected a specific type of session (GNOME or KDE), gdm runs the /etc/X11/gdm/Default script, which, in turn, executes the /etc/ X11/xdm/Xsession script. This script looks for another script file, named .xsession, in the user's home directory and executes that script, if it exists. Next, it looks for the .Xclients script file in the user's home directory and executes that file, if it exists. Otherwise, Xsession executes the /etc/X11/ xinit/Xclients script. When you follow through these scripts, you find that the Xclients script executes the /usr/bin/gnome-session program if the /etc/sysconfig/desktop file contains the line DESKTOP="GNOME". The gnome-session program then starts the GNOME session (which includes a set of applications and a window manager). Initially, gnome-session uses the session file /usr/share/gnome/default.session (a text file) to start the applications and the window manager. Later, the GNOME session is restored to the values saved in a file in the ~/.metacity/sessions directory (this refers to a subdirectory in the user's home directory). The default session file runs a script named /usr/bin/gnome-wm, which, in turn, starts an appropriate window manager. The default window manager is metacity, but you can specify a different window manager by defining the WINDOW_MANAGER environment value to the name of a window manager you want.
From this point on, the user can interact with the system through the X display and through the window manager (started by the gnome-session program).
When the user ends the GNOME session (by selecting Log out from the Main Menu button), gdm attempts to run the script /etc/X11/gdm/PostSession/ DisplayName, where DisplayName is :0 for the first display. If that file does not exist, gdm executes the /etc/X11/gdm/PostSession/Default script, if it exists.
Although this brief description of gdm does not show you all the details, gdm provides a user-friendly interface for user login and for starting a GNOME session. Moreover, like many other X programs, gdm is highly configurable. For example, the names of the script files in the preceding list are the typical ones, but you can specify other names through the file named gdm.conf, which, by default, also resides in the /etc/X11/gdm directory.

Switching from GNOME to KDE

GNOME and KDE are both capable GUI environments. CentOS Linux comes with both of these GUIs, but you get GNOME as the default GUI. However, Red Hat also includes the utility script /usr/bin/switchdesk, which enables you easily to switch from one GUI to another, and vice versa. To switch from GNOME to KDE, follow these steps:

From your GNOME desktop, select Main Menu>System Settings>More System Settings>Desktop Switching Tool, or type switchdesk in a terminal window. The Desktop Switcher dialog box appears.
In the Desktop Switcher dialog box, click the KDE radio button to select it (see Figure 9-2), then click the OK button.

Figure 9-2: Switching to KDE by Using the Desktop Switching Tool.
A message informs you that the desktop configuration has been changed, but you must restart X for the change to take effect. Click the OK button to dismiss the dialog box.

Although the message in Step 3 states that you must restart X, all you need to do is log out of the session and log back in. To log out, select Main Menu>Log Out. When you log in again, you should get the KDE GUI.

The switchdesk utility changes the GUI by creating the appropriate scripts in your home directory. It leaves the display manager unchanged. This means that you use the GNOME display manager, gdm, for login, even when you switch to the KDE GUI. After you log in, gdm executes a session script that eventually executes the .Xclients script in your home directory, if that script exists.

When you run the switchdesk utility, you execute a shell script in the /usr/bin/ switchdesk file that contains the following lines:

#!/bin/sh
echo "CentOS Linux `basename $0` 3.9"
echo "Copyright (C) 1999-2001 Red Hat, Inc"
echo "Redistributable under the terms of the GNU General Public License"
kicker=`/sbin/pidof "kdeinit: kicker"`
[ -z "$kicker" ] && kicker=`/sbin/pidof kicker`
[ -z "$kicker" ] && kicker=`/sbin/pidof kpanel`
[ -z "$kicker" ] && kicker=`/sbin/pidof kpanel1`
good=1
if [ "$DISPLAY" != "" -a "$1" = "" ]; then
    if [ -n "$kicker" -a -x /usr/bin/switchdesk-kde ]; then
        exec switchdesk-kde
    elif [ -x /usr/bin/switchdesk-gnome ]; then
        exec switchdesk-gnome
    elif [ -x /usr/bin/switchdesk-kde ]; then
        exec switchdesk-kde
    else
        echo "Desktop Switcher GUI unavailable."
        echo "Please install the switchdesk-kde or switchdesk-gnome packages or use"
        echo "switchdesk desktopname."
        good=0;
    fi
else
    good=0;
fi
if [ "$good" = "0" ]; then
    exec switchdesk-helper $1
fi

If you are using GNOME, the switchdesk script runs the switchdesk-gnome application that displays the GUI from which you change the desktop. In KDE, this script runs the switchdesk-kde GUI tool. The switchdesk-gnome GUI tool creates two scripts-.Xclients and .Xclients-default-in the home directory of the user. The .Xclients script contains the following lines:

#!/bin/bash
# Created by Red Hat Desktop Switcher
if [ -e "$HOME/.Xclients-$HOSTNAME$DISPLAY" ]; then
    exec $HOME/.Xclients-$HOSTNAME$DISPLAY
else
    exec $HOME/.Xclients-default
fi

The .Xclients-default script is simpler; it contains the following commands:

# Created by Red Hat Desktop Switcher
exec startkde

The result of these changes is that when you log in the next time, gdm executes the .Xclients script in your home directory, which, in turn, executes the startkde program (/usr/bin/startkde) to start a KDE session.

Because the switchdesk utility changes the GUI by creating the .Xclients and .Xclients-default scripts in a user's home directory, these changes only affect that particular user.

To switch from KDE to GNOME, log in as root, open a terminal window, and type switchdesk at the shell prompt, then select GNOME as the desktop. After you log out of KDE and log back in, you should get GNOME as your desktop.

Note that if you want to use GNOME for one session only (as opposed to making it your default GUI), simply click Session at the bottom of the login screen, and then select Gnome from the login window menu in the initial login screen, as shown in Figure 9-3.

Figure 9-3: Switching to GNOME for the Current Session Only.

You should get the GNOME desktop for this session. The next time you log in, the system reverts to KDE.

Learning the KDE Display Manager (kdm)

If you install only the KDE desktop or set the DISPLAYMANAGER variable in the /etc/ sysconfig/desktop file to KDE, the kdm program starts the X server for the local display and displays the graphical login window through which you log in to the system. Figure 9-4 shows the graphical login screen that you see when you run the kdm display manager.

When kdm runs, it reads various configuration parameters from the configuration file /usr/share/config/kdm/kdmrc, which has a structure similar to a Windows INI file. For example, here is a typical kdmrc file:

# KDM configuration example.
[General]
Xservers=/usr/share/config/kdm/Xservers
PidFile=/var/run/kdm.pid
[Xdmcp]
Enable=false
Xaccess=/usr/share/config/kdm/Xaccess
Willing=/usr/share/config/kdm/Xwilling
[Shutdown]
HaltCmd=/sbin/poweroff
[X-*-Core]
# Core config for all displays
Setup=/usr/share/config/kdm/Xsetup
Startup=/usr/share/config/kdm/Xstartup
Reset=/usr/share/config/kdm/Xreset
Session=/usr/share/config/kdm/Xsession
AutoReLogin=false
AllowRootLogin=true
AllowNullPasswd=true
AllowShutdown=Root
[X-*-Greeter]
# Greeter config for all displays
SessionTypes=default,kde,failsafe,gnome
GUIStyle=Bluecurve
LogoArea=Clock
GreetFont=Sans,12,-1,5,75,0,0,0,0,0
StdFont=Sans,10,-1,5,50,0,0,0,0,0
FailFont=Sans,10,-1,5,75,0,0,0,0,0
AntiAliasing=true
ShowUsers=None
MinShowUID=500
SortUsers=true
PreselectUser=None
FocusPasswd=false
EchoMode=OneStar
[X-:*-Core]
# Core configurations for local displays
# place local display core configurations here
[X-:*-Greeter]
# Greeter configurations for local displays
# place local display greeter configurations here

Figure 9-4: Graphical Login Screen Managed by kdm.

As you can see, the file is organized into sections, with each section identified by labels within square brackets. Each section specifies a related set of options that affects some aspect of the GUI login process.

You can configure the look and feel of the kdm greeter-the login dialog box-on each X display individually. The configuration of each display is specified by a section name that has the following format:

[X-host:number_class-subsection]

where host refers to the hostname of the system to which the display is attached (can be empty for a local display), number is the display number (0, if you have only one display), class is a display class (it's optional and ignored in most cases), and subsection identifies the element of the display manager to which the options apply. For example, if the options apply to the desktop, subsection is Desktop. If the options are for the greeter, the subsection is Greeter. You can use an asterisk (*) for any of these elements-host, number, and class-as a wildcard.

Thus, to specify the options for the greeter for all displays, place the options in a section named [X-*-Greeter]. To change some of these greeter options for the local displays, place the changed options in the [X-:*-Greeter] section. On the other hand, if you want to customize some aspect of the greeter for the first local display (which is display number 0), place those options in the section labeled [X-:0-Greeter].

Table 9-2 summarizes some common kdm configuration options, organized by sections. I show the X display options for all displays. To customize these for a specific display, you can add a section with a name that reflects the X display to which the options apply.

Table 9-2: Common kdm Configuration Options
Configuration Option	Description
[General] section
`LockPidFile=true or false`	If set to true, kdm locks the `PidFile` to prevent multiple instances of kdm from running simultaneously
`PidFile=/var/run/kdm.pid`	File where kdm should store its process ID (ID)
`Xservers= /usr/share/config/kdm/Xservers`	File containing list of X servers to start (one per display)
[Xdmcp] section
`ChoiceTimeout=seconds`	Number of seconds to wait for display to respond after the user has selected a host from the chooser (default is 15 seconds)
`Enable=true or false`	If set to `true`, kdm listens on port 177 and supports the X Display Manager Control Protocol (XDMCP) that enables users to log in to other hosts on the network (the default is `true`)
`Port=177`	The UDP port number where kdm listens for XDMCP requests (default is `177`)
`RemoveDomainname=true or false`	If set to true, strips domain name from remote display names if it is equal to the local domain (default is `true`)
`SourceAddress=true or false`	Use the numeric IP address of the incoming connection instead of the hostname (useful on multihomed hosts, the default is `false`)
`Willing= /usr/share/config/kdm/Xwilling`	Program to run to generate replies to XDMCP BroadcastQuery requests (the Xwilling program is a shell script that displays the number of users and current load average)
[Shutdown] section
`HaltCmd=/sbin/poweroff`	Full pathname of command to run to halt the system
`RebootCmd=/sbin/reboot`	Full pathname of command to run to reboot the system (default is /`sbin/reboot`)
*[X--Core] section (core configuration for all displays)**
`AllowNullPasswd=true or false`	If set to `true`, `kdm` allows users with empty passwords to log in (default is `true`)
`AllowRootLogin=true or false`	If set to true, kdm allows logins by `root` (default is `true`)
`AllowSdForceNow=Root or All`	Specifies who can abort all currently running programs when shutting down system; choices are `All` for anyone or `Root` for `root` user (default is `All`)
`AllowShutdown=Root or All`	Specifies who can shut down the system; possible values are `All` to enable anyone to shut down the system, `None` to prevent shutdown from the login screen, and `Root` to require `root` password for shutdown (default is `All`)
`AutoReLogin=true or false`	If set to true, kdm will automatically restart a session after the X server crashes or if it is killed by pressing Ctrl-Alt-Backspace (default is `false`)
`DefaultSdMode=ForceNow or ForceNow` `TryNow or Schedule`	The default mode for shutdown; choices are to shut down immediately, `Schedule` to shut down after the session exits, or `TryNow` to shutdown if no session is open and do nothing otherwise (default is `Schedule`)
`OpenDelay=seconds`	Number of seconds to wait before retrying to open the display after any error (default is 15 seconds)
`OpenRepeat=seconds`	How many seconds to wait before trying to open display again (default is 5 seconds)
`OpenTimeout=seconds`	Number of seconds to wait before timing out on the attempt to open the display (default is 120 seconds)
`PingInterval=seconds`	How many minutes between successive pinging of remote displays (ping checks if network connection is up and running; default interval is 5 minutes)
`Reset= /usr/share/config/`	Program to run after a user session ends `kdm/Xreset`
`ResetSignal=1`	The signal to send to reset the local X server (default is `1`, which stands for SIGHUP)
`Session= /usr/share/config/kdm/Xsession`	Program that accepts the `SessionTypes` argument and starts an appropriate session
`SessSaveFile=.wmrc`	Filename (in user's home directory) where the last session type is stored (default is `.wmrc`)
`Setup= usr/share/config/`	Program to run before showing the greeter window `kdm/Xsetup`
`StartAttempts=number`	The maximum number of times to try opening a display before the display is disabled (default is `4`)
`StartInterval=seconds`	How many seconds to wait before resetting the `StartAttempts` counter (default is 30 seconds)
`Startup= usr/share/config/`	Program to run before starting a user session `kdm/Xstartup`
`TerminateServer=true or false`	If set to true, restart the local X server instead of resetting after session ends (default is false)
`TermSignal=signum`	The signal to send to terminate the local X server (default is 15, which stands for SIGTERM)
*[X--Greeter] section (greeter configuration for all displays)**
`AntiAliasing=true or false`	If set to true, fonts shown in the greeter should be antialiased (default is `false`)
`DefaultUser=username`	User preselected for login if `PreselectUser`=`Default`
`EchoMode=OneStar or ThreeStar or NoEcho`	What to show as user types the password; choices are `NoEcho` to show nothing at all, `OneStar` to show a `` for each letter typed, or `ThreeStars` to show `**` for each letter typed (default is `OneStar`)
`FailFont= Sans,10,-1,5,75,`	Font used for the "Login Failed" message `0,0,0,0,0`
`FocusPasswd=true or false`	If set to true, the password input line is automatically selected to receive the keyboard input (default is `false`)
`GreeterPosFixed=true or false`	If set to `true`, the position of the greeter window is fixed and specified by `GreeterPosX` and `GreeterPosY` (default is `false`)
`GreeterPosX=pixels`	Horizontal position, in pixels, of greeter window
`GreeterPosY=pixels`	Vertical position, in pixels, of greeter window
`GreetFont=Sans,12,-1,5,75,`	The font to use for the welcome text `0,0,0,0,0`
`GreetString= welcometext`	Welcome text in greeter window (default is `"Welcome to %s at %n"`); use following special characters strings: `%d` = current display `%h` = hostname, possibly with domain name `%n` = node name, most probably the hostname without domain name `%s` = operating system (`Linux`) `%r` = operating system's version `%m` = machine (hardware) type `%%` = a single `%`
`GUIStyle=Bluecurve`	The look and feel of the greeter (other choices are `Default`, `Windows`, `Motif`, `CDE`, and any other styles installed on the system)
`HiddenUsers=user1,user2,...`	List of user names not to be listed in the greeter when `ShowUsers` is `NotHidden`
`Language=en_US`	Language to use in the greeter (default is `en_US`)
`LogoArea=Clock`	What to show in the area to the right of the text input line (other options are `Logo` to display an image specified by `LogoPixmap`, `Clock` to display an analog clock, and `None` to display nothing)
`LogoPixmap= /usr/share/apps/`	The image to show when `LogoArea` is set to Logo `kdm/pics/kdelogo.png`
`MinShowUID=500`	Users with UID less than the specified number are not shown
`NumLock=Off`	What to do with the Num Lock key during the time when the greeter is running; choices are `On` to turn it on, `Off` to turn it off, and `Keep` to leave it at its previous state
`PreselectUser=None or Previous or Default`	User name to be preselected for login; choices are `Previous` to preselect the previous user, `Default` to select user listed in `DefaultUser` field, or `None` to not preselect at all (default is `None`)
`SelectedUsers=user1,user2,...`	List of users to show when `ShowUsers`=`Selected`
`SessionTypes= default,kde, failsafe,gnome`	Names of session types that the user can select from the GUI login window
`ShowUsers=None`	Which user names, along with pictures, to show in the greeter; choices are `NotHidden` to show all users except those listed in `HiddenUsers`, Selected to show only those users listed in `SelectedUsers`, and `None` to show no users at all (default is `NotHidden`)
`SortUsers=true or false`	If set to `true`, sort user names alphabetically (default is `true`)
`StdFont= Sans,10,-1,5,50,`	Normal font used in the greeter `0,0,0,0,0`