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Appendix C. The X Window System

While the heart of Red Hat Enterprise Linux is the kernel, for many users, the face of the operating system is the graphical environment provided by the X Window System, also called X.
Other windowing environments have existed in the UNIX world, including some that predate the release of the X Window System in June 1984. Nonetheless, X has been the default graphical environment for most UNIX-like operating systems, including Red Hat Enterprise Linux, for many years.
The graphical environment for Red Hat Enterprise Linux is supplied by the X.Org Foundation, an open source organization created to manage development and strategy for the X Window System and related technologies. X.Org is a large-scale, rapid-developing project with hundreds of developers around the world. It features a wide degree of support for a variety of hardware devices and architectures, and runs on myriad operating systems and platforms.
The X Window System uses a client-server architecture. Its main purpose is to provide network transparent window system, which runs on a wide range of computing and graphics machines. The X server (the Xorg binary) listens for connections from X client applications via a network or local loopback interface. The server communicates with the hardware, such as the video card, monitor, keyboard, and mouse. X client applications exist in the user space, creating a graphical user interface (GUI) for the user and passing user requests to the X server.

C.1. The X Server

Red Hat Enterprise Linux 6 uses X server version, which includes several video drivers, EXA, and platform support enhancements over the previous release, among others. In addition, this release includes several automatic configuration features for the X server, as well as the generic input driver, evdev, that supports all input devices that the kernel knows about, including most mice and keyboards.
X11R7.1 was the first release to take specific advantage of making the X Window System modular. This release split X into logically distinct modules, which make it easier for open source developers to contribute code to the system.
In the current release, all libraries, headers, and binaries live under the /usr/ directory. The /etc/X11/ directory contains configuration files for X client and server applications. This includes configuration files for the X server itself, the X display managers, and many other base components.
The configuration file for the newer Fontconfig-based font architecture is still /etc/fonts/fonts.conf. For more information on configuring and adding fonts, see Section C.4, “Fonts”.
Because the X server performs advanced tasks on a wide array of hardware, it requires detailed information about the hardware it works on. The X server is able to automatically detect most of the hardware that it runs on and configure itself accordingly. Alternatively, hardware can be manually specified in configuration files.
The Red Hat Enterprise Linux system installer, Anaconda, installs and configures X automatically, unless the X packages are not selected for installation. If there are any changes to the monitor, video card or other devices managed by the X server, most of the time, X detects and reconfigures these changes automatically. In rare cases, X must be reconfigured manually.