We come now to the X Window System--one of the most powerful and important software packages available for Linux. If you've ever used X on a Unix system before, you're in luck; running X under Linux is almost no different from Unix systems. And, if you've never had the occasion to use it before, never fear: salvation is at hand.
It's difficult to describe the X Window System in a nutshell. X is a complete windowing graphics interface for Unix systems. It provides a huge number of options to both the programmer and the user. For instance, there are at least half a dozen window managers available for X, each one offering a different interface for manipulating windows. By customizing the attributes of the window manager, you have complete control over how windows are placed on the screen, the colors and borders used to decorate them, and so forth.
X was originally developed by Project Athena at MIT and Digital Equipment Corporation. The current version of X is Version 11 revision 6 (X11R6), which was first released in April 1994. Since the release of Version 11, X has virtually taken over as the de facto standard for Unix graphical environments. It is now developed and distributed by The Open Group, an association that is composed of many large computer manufacturers.
Despite its commercial use, the X Window System remains distributable under a liberal license from the X Consortium. As such, a complete implementation of X is freely available for Linux systems. XFree86, an implementation of X, originally for i386 Unix systems, is the version most often used by Linux. Today, this version supports not only Intel-based systems, but also Alpha AXP, MicroSPARC, PowerPC, and other architectures. Further architectures will follow. XFree86 is based on X386-1.2, which was part of the official X11R5 sources, but is no longer maintained and is therefore outdated. The current versions now have only a very little part in common with their ancestors. Support for innumerable graphics boards and many other operating systems (including Linux) has been added--and XFree86 implements the latest version X11R6.3.
In this chapter, we will tell you how to install and configure the X Window System, and in the next chapter, we will explore how to use X.
Linux distributions automatically install X (if you ask them to). If you're lucky, you won't need this chapter at all. But a large percentage of users aren't lucky--the distribution doesn't recognize some graphics hardware, writes a file to the wrong location so the X server can't start up, or has some other problem. One of the big advantages of this book is that we take you down to the depths of X configuration so you can get it running no matter what your distribution does. You may not need to read this chapter, but if you do need it, you'll appreciate everything that's here.
X is based on a client-server model in which the X server is a program that runs on your system and handles all access to the graphics hardware. An X client is an applications program that communicates with the server, sending it requests such as "draw a line" or "pay attention to keyboard input." The X server takes care of servicing these requests by drawing a line on the display or sending user input (via the keyboard, mouse, or whatever) to the client application. Examples of X clients are xterm (which emulates a terminal within a window) or xman (an X-based manual-page reader).
It is important to note that X is a network-oriented graphics system. That is, X clients can run either locally (on the same system that the server is running) or remotely (on a system somewhere on a TCP/IP network). The X server listens to both local and remote network sockets for requests from clients. This feature is obviously quite powerful. If you have a connection to a TCP/IP network, you can log in to another system over the network and run an X application there, directing it to display on your local X server.
Further advantages of X are security (if the user so desires), the modular separation of functions, and the support for many different architectures. All this makes the X Window System technically superior by far to all other window systems.
The X Window System makes a distinction between application behavior and window management. Clients running under X are displayed within one or more windows on your screen. However, how windows are manipulated (placed on the display, resized, and so forth) and how they are decorated (the appearance of the window frames) is not controlled by the X server. Instead, it is handled by another X client called a window manager that runs concurrently with the other X clients. Your choice of window manager will decide to some extent how X as a whole looks and feels. Most window managers are utterly flexible and configurable; the user can select the look of the window decoration, the focus policy, the meaning of the mouse buttons when the mouse is on the background part of the screen rather than on an application window, and many other things by editing the configuration files of the window manager. More modern systems even let you configure those aspects over a graphical user interface.
In order to fully understand the concept of window managers, you need to know that the window manager does not affect the presentation of the window created by the client. The window manager is only in charge of painting the window decoration, that is, the frame and the buttons that let you close, move, and resize windows.
There can be only one window manager on any X server. Theoretically, it is even possible to completely do away with window managers, but then you would not be able to move windows around the screen; put a hidden window on top; or minimize, maximize, or resize windows unless the programs themselves provide this functionality.
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