Starting an X session is typically done in one of two ways: the X session is started via a display manager (like xdm), and the user logs in at a GUI screen. Or, the user starts X manually after logging in to a text console. The latter is typically done with the startx command, which is a simple shell script wrapper for xinit. X runs with root privileges in either case, since it needs raw access to hardware devices.
Typically, which method is used, is determined by the system "runlevel". The default runlevel to launch at boot is generally set in /etc/inittab on Linux:
# Run xdm in runlevel 5 x:5:respawn:/etc/X11/xdm -nodaemon
That would start xdm, and thus X, at runlevel 5. It will "respawn", if it dies or is stopped for any reason. You can also use the "init" command to change runlevels without rebooting (see man page).
Let's look briefly at both approaches, and then some additional configuration to set up the user's working environment.
startx will start X by first invoking xinit. By itself, this would put you at a blank, fuzzy looking, bare-bones desktop with no Window Manager loaded. xinit basically takes two sets of command line arguments: client specifications (programs to run, etc), and server specifications (X server options), separated by "--". If no client program is specified on the command line, xinit will look for a .xinitrc file in the user's home directory, to run as a shell script. If found, this then would in turn run whatever user specified commands to set up the environment, or launch programs that the file contained. If this file does not exist, xinit will use the following initial command:
xterm -geometry +1+1 -n login -display :0
If no .xserverrc is found in the user's home directory, X itself will be started with the following command:
As you see, this is not overly helpful as it just launches one xterm. The startx shell wrapper provides additional functionality and flexibility to xinit. startx will invoke xinit for us, and provide some simple configuration options as well. You can also issue commands such as the following, for instance:
startx -- -dpi 100 -depth 16 #force X to 100 dots per inch #and colordepth of 16 (X v4 syntax)
Anything after the double dashes are passed as arguments directly to the X server via xinit. In this example, you can force X to the resolution of your preference, and still have it use the configuration files we will cover later in this document. See the Xserver man page for more command line options.
Instead of issuing the same command line every time, it is easier to use the configuration files to store this type of information for us.
If you take a look at the startx script (/usr/X11R6/bin/startx on my system), you see it uses two default configuration files to help set up the X environment: xinitrc and xserverrc. It looks first in /etc/X11/xinit/, for the system wide files. It then checks the user's home directory for similar files, which will take precedence if found. Note that the latter are Unix style "dot" files (e.g. ~/.xinitrc), and are executable shell scripts.
You normally would not want to edit the system wide files, but you can freely copy these to your home directory as a starting point, or just start from scratch. As you can tell by the names, one helps set up the X server, and one sets up xinit by executing commands, preparing the environment and possibly starting client programs like xterm or a Window Manager (yes, it's a client too).
As with all XFree86 configuration files, this is a plain text file, and is usually a simple, one line statement to start the X server. It can include any valid command line options supported by your X installation. If you always start X with your own options, this should be easier than typing the options each time. One possible ~/.xserverrc:
exec X :0 -dpi 100 -nolisten tcp
This will start X on display :0, the first "display", at a dots-per-inch resolution of 100, and disables TCP connections. See the Xserver man page for other valid options. This is just an example.
xinitrc is used to set up a suitable X environment, and to launch other programs, a.k.a "clients" that we may want available as soon as X is started. You likely have a system wide xinitrc to start a predefined set off programs. To customize this, create your own in your home directory. Name it .xinitrc, make sure it is an executable script, and chmod +x. An example (slightly modified from the original on my system):
#!/bin/sh # $XConsortium: xinitrc.cpp,v 1.4 91/08/22 11:41:34 rws Exp $ userresources=$HOME/.Xresources usermodmap=$HOME/.Xmodmap # merge in defaults and keymaps if [ -f $userresources ]; then xrdb -merge $userresources fi if [ -f $usermodmap ]; then xmodmap $usermodmap fi if [ -z "$BROWSER" ] ; then # we need to find a browser on this system BROWSER=`which netscape` if [ -z "$BROWSER" ] || [ ! -e "$BROWSER" ] ; then # not found yet BROWSER= fi fi if [ -z "$BROWSER" ] ; then # we need to find a browser on this system BROWSER=`which lynx` if [ -z "$BROWSER" ] || [ ! -e "$BROWSER" ] ; then # not found yet BROWSER= else BROWSER="xterm -font 9x15 -e lynx" fi fi export BROWSER # start some nice programs if [ -f $HOME/.Xclients ]; then exec $HOME/.Xclients else xclock -geometry 50x50-1+1 & xterm -geometry 80x50+494+51 & if [ -f /usr/X11R6/bin/fvwm ]; then exec fvwm else exec twm fi fi #eof
Briefly, what this script does, is set up our working environment, with xmodmap (keyboard) and xrdb (application resource settings). More on these below. Then the shell variable $BROWSER is set for a GUI environment (Netscape in this example) so that any applications that might expect this, have a reasonable choice available. Then the presence of the file Xclients is checked, both as a system wide file and in the user's home directory. In this particular example, this is where any client applications are to be started, including a Window Manager (see below). These could just have as easily been started here if we had wanted to. If an Xclients file can't be found, then a Window Manager is started for us. Either fvwm, if available, or XFree86's minimalist twm if not. If for some reason, neither of these can be started, the script would exit, and X would fail to start.
Everything up to this point has followed pretty much a standard and predictable sequence of events. To summarize, we have invoked startx, which in turn invoked xinit, which has parsed xinitrc for initial settings. Most Linuxes should follow this same sequence, though the various values and settings may differ.
We now are at the last link in the chain where the user normally would specify his or her preferences, including the Window Manager and/or desktop environment to be used. The system will provide sane, though possibly uninteresting, defaults if the user has not done so. Presumably, this is why you are here ;-)
The Window Manager, or desktop environment, is typically the last application started. If you want other programs (like xterm) started, they should be started before the Window Manager and "backgrounded" with an "&". This can all be done in the user's ~/.xinitrc. Or as in the above example, the actual applications are started from yet another script. Let's look at one short, hypothetical such script, .Xclients:
#!/bin/bash # ~/.Xclients, start my programs. xset s off s noblank xset m 30/10 4 xset r rate 200 40 xscreensaver & rxvt -geometry 80x50-50+150 & echo Starting Window Manager... if [ -x /usr/X11R6/bin/wmaker ]; then echo `date`: Trying /usr/X11R6/bin/wmaker... |tee -a ~/.wm-errors 2>&1 exec /usr/X11R6/bin/wmaker >> ~/.wm-errors 2>&1 fi echo `date`: Failed, trying fvwm... |tee -a ~/.wm-errors 2>&1 # let's try regular fvwm (AnotherLevel doesn't work with fvwm1). if [ -n "$(type -path fvwm)" ]; then # if this works, we stop here exec fvwm >> ~/.wm-errors 2>&1 fi echo `date`: Failed, trying twm... |tee -a ~/.wm-errors 2>&1 # wow, fvwm isn't here either ... # use twm as a last resort. exec twm >> ~/.wm-errors 2>&1 # Dead in the water here, X will exit as well, sigh... echo `date`: Unable to start a Window Manager ... |tee -a ~/.wm-errors 2>&1 # eof
This really isn't so different than what xinitrc was doing at all. We added a few wrinkles, including starting a screen saver, a different terminal emulator that this user prefers (rxvt), with even more setting up of the environment (monitor, mouse and keyboard) using xset this time, and a different Window Manager than was available with the system defaults. This is in the user's home directory, so it will not be overwritten during upgrades too.
Actually, X has already started at this point, and we are just putting the finishing touches on the configuration. Notice the Window Managers are not "backgrounded" with "&" here. This is important! Something has to run in the foreground, or X will exit. We didn't start a desktop environment in this example, like KDE or GNOME, but if we did, this final application would have to be gnome-session or startkde instead. Since we are rolling our own here, if we wanted to change Window Managers, all we have to do is edit this file, and restart X. Vendor supplied configurations may be more complex than this, but the same principles apply.
As an afterword, do not think that any initial client applications must be started as we've done here. This is how it has been traditionally done, and some may prefer this approach. Most window managers have their own built-in ways to start initial programs, as do KDE and GNOME. See the respective documentation.
The other, more common, approach is the "GUI log-in", where X is running before log-in. This is done with the help of a "display manager", of which there are various implementations. XFree86 includes xdm (X Display Manager) for this purpose, though your distribution may use one of the others such as gdm (GNOME) or kdm (KDE).
Display managers really do much more than enable GUI style log-ins. They are also used to manage local as well as remote "displays" on a network. We shall not get into details on this here, but it is nicely covered in the Remote X Apps Mini HOWTO and the XDMCP HOWTO (see the links section). For our purposes here, they provide similar services to getty and login, which allow users to log into a system and start their default shell, but in a GUI environment.
Here is an example of a more advanced usage of what else a display manager might be used for, from Diego Zamboni:
I have two X sessions running with different resolutions. I switch between them depending on whether my laptop is connected to an external monitor or using its own LCD display.
Here's my /usr/lib/X11/xdm/Xservers file that initiates both displays:
:1 local /usr/X11R6/bin/X :1 -layout 1024x768 :0 local /usr/X11R6/bin/X :0 -layout 1600x1200
Then I have "1024x768" and "1600x1200" defined as "server layouts" in my /etc/X11/XF86Config-4, as follows:
Section "ServerLayout" Identifier "1600x1200" Screen "Screen0" 0 0 InputDevice "Mouse0" "CorePointer" InputDevice "Keyboard0" "CoreKeyboard" EndSection Section "ServerLayout" Identifier "1024x768" Screen "Screen1" 0 0 InputDevice "Mouse0" "CorePointer" InputDevice "Keyboard0" "CoreKeyboard" EndSection ## snip ... Section "Screen" Identifier "Screen0" Device "S3 Savage/MX" Monitor "Monitor0" DefaultDepth 16 Subsection "Display" Depth 16 Modes "1600x1200" "1280x1024" "1024x768" EndSubsection EndSection Section "Screen" Identifier "Screen1" Device "S3 Savage/MX" Monitor "Monitor0" DefaultDepth 16 Subsection "Display" Depth 16 Modes "1024x768" "800x600" EndSubsection EndSection
Note the use of "Identifiers" here. Diego is starting two separate "displays" here. Then he can choose which one he wants when he logs in.
Most display managers are derived from XFree86's venerable xdm, and add their own enhancements. Let's look at the most popular ones briefly.
xdm can be configured with configuration files located in /etc/X11/xdm/, /usr/X11R6/lib/X11/xdm, or similar locations depending on your system. These are system wide files. The file xdm-config is the main configuration file, and mostly describes where to find secondary configuration files:
! $XConsortium: xdm-conf.cpp /main/3 1996/01/15 15:17:26 gildea $ DisplayManager.errorLogFile: /var/log/xdm-errors DisplayManager.servers: /etc/X11/xdm/Xservers DisplayManager.accessFile: /etc/X11/xdm/Xaccess ! All displays should use authorization, but we cannot be sure ! X terminals will be configured that way, so by default ! use authorization only for local displays :0, :1, etc. DisplayManager._0.authorize: true DisplayManager._1.authorize: true ! The following three resources set up display :0 as the console. DisplayManager._0.setup: /etc/X11/xdm/Xsetup_0 DisplayManager._0.startup: /etc/X11/xdm/GiveConsole DisplayManager._0.reset: /etc/X11/xdm/TakeConsole ! DisplayManager*resources: /etc/X11/xdm/Xresources DisplayManager*session: /etc/X11/xdm/Xsession ! ! SECURITY: do not listen for XDMCP or Chooser requests ! Comment out this line if you want to manage X terminals with xdm DisplayManager.requestPort: 0
The "!" denotes comments. The command that starts the X server is in /etc/X11/xdm/Xservers in this particular example as defined by "DisplayManager.servers", and is the equivalent to xserverrc that was used for startx X server start up commands, but the syntax is slightly different here. The contents of /etc/X11/xdm/Xservers on my system are simply:
:0 local /usr/X11R6/bin/X
This starts X on the first local display (designated by 0). Any special command line arguments that you want to add go here at the end.
Below is a sample /etc/X11/xdm/Xsetup_0 which is used to configure the log-in screen only. Notice that we're using a shell script here, and it's calling xv (a graphics display program) to set the background to a nice image (instead of the boring black and white background pattern), and if that fails, xsetroot is then invoked to at least try to set the background to a nicer blue color. This does not configure the login widget itself -- just other things that might be wanted on the screen during login.
#!/bin/sh xconsole -geometry 480x100-0-0 -daemon -notify -verbose -fn \ '-schumacher-clean-medium-r-*-*-10-*-*-*-*-*-*-*' -exitOnFail & /usr/X11R6/bin/xv -quit -root /usr/share/pixmaps/Backgrounds/InDreams.jpg \ || xsetroot -solid darkblue
/etc/X11/xdm/Xresources controls the X "resources" used during log in. In this context, "resources" are user preferences for such items as fonts and colors (described in more detail below). Below is a snippet that sets up fonts for the log-in widget:
#if WIDTH > 800 xlogin*greetFont: -adobe-helvetica-bold-o-normal--24-240-75-75-p-138-iso8859-1 xlogin*font: -adobe-helvetica-medium-r-normal--18-180-75-75-p-103-iso8859-1 xlogin*promptFont: -adobe-helvetica-bold-r-normal--18-180-75-75-p-103-iso8859-1 xlogin*failFont: -adobe-helvetica-bold-r-normal--18-180-75-75-p-103-iso8859-1 #else xlogin*greetFont: -adobe-helvetica-bold-o-normal--17-120-100-100-p-92-iso8859-1 xlogin*font: -adobe-helvetica-medium-r-normal--12-120-75-75-p-69-iso8859-1 xlogin*promptFont: -adobe-helvetica-bold-r-normal--12-120-75-75-p-69-iso8859-1 xlogin*failFont: -adobe-helvetica-bold-o-normal--14-140-75-75-p-82-iso8859-1 #endif
As you can see this is using helvetica as the preferred font, with different point sizes and dots per inch depending on the screen size. This is customizable to suit individual needs. (See below for more on understanding X font naming conventions.) Various other aspects can similarly be configured.
/etc/X11/xdm/Xsession is the rough equivalent to xinitrc for startx. It will similarly set up a default environment for keyboard, etc. And can also start either KDE or GNOME, and other X client programs. This is the system wide configuration file. It should also check the user's home directory for ~/.xsession, and possibly ~/.Xclients, which would contain the user's preferred environment and start up programs, just as ~/.xinitrc did with startx. Again, the files in a user's home directory may be created or modified by the user any time and must be executable shell scripts.
We shall not include an ~/.xsession example here, since it would be very similar to the ~/.xinitrc and ~/.Xclients examples above.
We've looked only briefly at the main xdm configuration files. Be sure to read the man page, and look at what is installed locally, for more information. Let's look now at gdm and kdm. We'll just highlight significant differences, since they essentially provide the same functionality.
gdm is the default display manager for GNOME. gdm was written from scratch, but functions similarly to xdm. The main configuration file is gdm.conf, typically located as /etc/X11/gdm/gdm.conf. This is quite different looking than xdm-config. Comments are denoted with a "#", and the file has sections, with section headers enclosed in square brackets. The command to start X is in the "[servers]" section:
[servers] 0=/usr/bin/X11/X #1=/usr/bin/X11/X
Notice this has potentially two displays set up, but the second one is commented out. Add any additional X startup options here, e.g. "-dpi 100". The log-in screen and log-in widget are configured in the "[greeter]" section.
Start up clients and programs are determined by the "SessionDir" statement in the "[daemon]" section. On my installation, this points to /etc/X11/gdm/Sessions/, which contains several short scripts. If I look at my Default script, it actually executes /etc/X11/xdm/Xsession, which in turn would execute ~/.xsession, if present. So at this final stage, gdm acts very much like xdm.
GNOME includes the gdmconfig utility to control many aspects of gdm behavior.
kdm is the display manager from KDE. The main configuration file for kdm is kdmrc and is typically installed as /etc/kde/kdm/kdmrc. As is the case with gdm.conf, kdmrc uses "#" for comments, and has sections with section headers in similar square brackets. kdm configuration can also be edited with the kcontrol utility.
The visible desktop is configured in the "[Desktop*]" section(s), and by the "Setup" directive which should point to a file like /usr/share/config/kdm/Xsetup or /etc/X11/xdm/Xsetup_0. This will accomplish the same thing as xdm's Xsetup_0 does: namely running any programs the user might want such as xconsole.
The command to launch the X server is the "Xservers" directive in the "[General]". Again, this should point to a file such as /etc/X11/xdm/Xservers, and uses the same syntax as xdm:
:0 local /usr/X11R6/bin/X
Any command line options for the X server, go here.
The login widget itself is configured in the "[X-*-Greeter]" section(s). Compiled in defaults are used if the user does not specify any.
KDE includes the kdmdesktop utility to control some aspects of kdm behavior, mostly just the login background.