X Window System

Basic concepts

X uses a client/server model, with an X server and client applications such as browsers, editors, etc
The client's usually run the same machine as the server but could be anywhere in the network
(From Wikipedia)
X is old: over 30 years. Some distros are replacing it with Wayland
Note: many of the files or their contents keep changing as distros evolve

Starting X

You get the most control by running Linux in runlevel 3 instead of runlevel 5. Set this in Fedora by establishing links as decribed in /etc/inittab
In Ubuntu, edit /etc/default grub and change
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash text"
Then run
sudo update-grub
(http://www.cebuntu.com/how-to/how-to-boot-your-ubuntu-desktop-in-terminal-mode/)
Once logged in, run X to get just an X server running
Run an X server plus an xterm by starting xinit
Run a window/desktop manager by startx
As root, run a display manager (which controls logins) by gdm (for Gnome display manager), kdm (for KDE display manager), wdm (for Wings display manager) lightdm (for Ubuntu 11.10 display manager) or xdm (the original X display manager)

X libraries

The basic library for clients is Xlib. This allows you to build applications such as xterm, xclock, etc
The first O/O application library was Xaw (Athena widgets) which is used to build e.g. xedit
Other early libraries are Motif and Tk
Modern libraries include Qt (for KDE) and GTK+ (for Gnome)
All these different client-side libraries have different appearances, interaction styles and configuration mechanisms e.g. a KDE client doesn't know about the Gnome gconf system

xinit

This is the program that actually runs the server and applications
It can be run with an application on the command line
```
xinit twm
      
```
If it is run with no arguments, it tries to execute the commands in ~/.xinitrc e.g
```
#!/bin/bash

xsetroot -solid blue
xterm &
twm
      
```
With no arguments and no ~/.xinitrc it runs a default xterm

Startx

This is the more common way of starting X
It uses ~/.xinitrc as startup script if it exists
If it doesn't, it uses a system-wide startup script /etc/X11/xinit/xinitrc
The script /etc/X11/xinit/xinitrc is different for Fedora and Ubuntu
Ubuntu will execute user commands in ~/.xsessionrc
Fedora will execute user commands in ~/.Xclients

Window managers

A window manager controls many aspects of application windows on the screen

Initial location - may be done automatically or left to the user
Movement and resizing of windows
Iconfication, restore and zoom
Window menus
Screen menus

Common window managers

TWM - very old and basic
Mwm - Motif and CDE
Metacity - default for Gnome
KWin - default for KDE
Compiz - lots of fancy effects such as Wobbly Windows
Matchbox - small footprint, for embedded systems

Desktops

A desktop is a window under the control of a window manager
It usually takes up the full screen and all other windows sit "inside" it
As well as application windows it also supplies additional windows such as a task bar; application startup menus; a tool bar, and so on
Major ones are KDE and Gnome but there are also Enlightenment, Blackbox, etc
Each desktop is usually built from an extensive library which also supports inter-process communication, audio/video handling, configuration tools etc
Major libraries for desktops are Qt (for KDE) and GTK+ (for Gnome),
Other widget libraries include Xaw (for Athena widgets) Motif/Lesstif and XUL (for Firefox)

Fedora desktop choice

startx reads ~/.xinitrc or /etc/X11/xinit/xinitrc

For Fedora 16 the default is (simplified)

	
if [ -f $HOME/.Xclients ]; then
    exec $HOME/.Xclients || \
elif [ -f /etc/X11/xinit/Xclients ]; then
    exec /etc/X11/xinit/Xclients || \
fi

/etc/X11/xinit/Xclients contains

	
GSESSION=gnome-session
STARTKDE=startkde

# check to see if the user has a preferred desktop
PREFERRED=
if [ -f /etc/sysconfig/desktop ]; then
    . /etc/sysconfig/desktop
    if [ "$DESKTOP" = "GNOME" ]; then
        PREFERRED="$GSESSION"
    elif [ "$DESKTOP" = "KDE" ]; then
        PREFERRED="$STARTKDE"
    fi
fi
if [ -n "$PREFERRED" ]; then
    exec "$PREFERRED"
fi

So
- System-wide settings can be made in /etc/sysconfig/desktop
- A user can override these in ~/.Xclients
on my system, ~/.Xclients says to run startkde

Display manager

Using runlevel 3, Linux boots into text mode and you get a login prompt. After login, you can start X by startx
In runlevel 5, Linux boots into X. You then need to do a graphical login
A display manager gives you a GUI login dialog and then logs you in, setting USER, HOME etc
Common display managers are xdm, gdm (for Gnome) and kdm (for KDE)
Ubuntu sets the default display manager in /etc/X11/default-display-manager. In Ubuntu 11.10 it is lightdm
Fedora uses the value of DISPLAYMANAGER which you can set in /etc/sysconfig/desktop

kdm

kdm is the display manager for KDE
Its configuration file is typically /etc/kde3/kdm/kdmrc - rarely edited by you

gdm

gdm is the display manager for Gnome
The configuration file is /etc/gdm.conf - rarely edited by you

Ubuntu

Most Unix/Linux systems use the runlevel to decide if they run X or a text system
Debian/Ubuntu just treat X as yest another application
Debian/Ubuntu boot into runlevel 2
Ubuntu is in transition mode;
- It has replaced Gnome with its own desktop Unity
- While many distros are moving away from X to Wayland, Ubunti has forked Wayland and will instead use Mir

EEE PC

The EEE PC is based on a Xandros distro. What does it use?

The window manager is icewm
It uses the kdm display manager, set in /etc/X11/default-display-manager
The file /etc/X11/xinit/xinitrc calls /etc/X11/Xsession which sources all the files in /etc/X11/Xsession.d

The file 50x11-common-determine-startup in Xsession.d contains the test

if [ -x /usr/bin/x-session-manager ]; then 
    STARTUP=x-session-manager
elif [ -x /usr/bin/x-window-manager ]; then
    STARTUP=x-window-manager
fi

and the file 99x11-common_start contains

exec $STARTUP

The file /usr/bin/x-session-manager is a symbolic link to /etc/alternatives/x-session-manager which is a symbolic link to /usr/bin/startsimple.sh
startsimple.sh is specially built for the Asus EEE and sets a mixture of processes (such as networkmonitor and powermonitor) and then calls icewm and
There is no desktop manager, just the icewm window manager
You can replace lots of files in this setup to get a different GUI

X servers

There are two major X servers, XFree86 and Xorg
The configuration files for both live in /etc/X11 as XF86.conf or xorg.conf
Nowadays you rarely have to configure the servers yourself
There are a number of ways of generating a configuration file such as Xorg -configure, XF86config etc

Distributed X

An application on one machine can connect to an X server on a different machine
A machine may have several displays

You can set the DISPLAY variable

export DISPLAY=hostname:0

for all applications, or

xedit -display hostname:0

An X server on one machine can let applications from another by allowing xhost +hostname

Wayland

X is now over 20 years old, and PC hardware has changed a lot since then
The s/w model has also changed, with processing being moved from the X server to X clients (e.g. wobbly windows can't be done by the server)
A new lightweight server is being developed, called Wayland
It is expected to replace X over the next few years...
... except for Ubunti which has its own replacement, Mir
Window managers, display managers, application libraries will all be moved to Wayland/Mir so that users and most programmers won't notice

Conclusion

X is designed to implement the minimum needed in the simplest way - so it just gives you a server
Clients talk to the server
A window manager handles clients - and there are many different window managers
A desktop manager may handle desktop effects - and there are several desktop managers
Fedora and Ubuntu/Debian have different ways of starting X and the clients :-(
X is reaching "end of life" and will be replaced over the next few years