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Introduction to Linux

By Machtelt Garrels

10.3.3. The X Window System

10.3.3.1. X features

As we already explained in Chapter 7 (see Section 7.3.3 ), the X Window system comes with an X server which serves graphics to clients that need a display.

It is important to realize the distinction between the X server and the X client application(s). The X server controls the display directly and is responsible for all input and output via keyboard, mouse and display. The X client, on the other hand, does not access the input and output devices directly. It communicates with the X server which handles input and output. It is the X client which does the real work, like computing values, running applications and so forth. The X server only opens windows to handle input and output for the specified client.

In normal operation (runlevel five, graphical mode), every Linux workstation is an X server to itself, even if it only runs client applications. All the applications you are running (for example, Gimp, a terminal window, your browser, your office application, your CD playing tool, and so on) are clients to your X server. Server and client are running on the same machine in this case.

This client/server nature of the X system makes it an ideal environment for remote execution of applications and programs. Because the process is actually being executed on the remote machine, very little CPU power is needed on the local host. Such machines, purely acting as servers for X, are called X terminals and were once very popular. More information may be found in the Remote X applications mini-HOWTO .

10.3.3.2. Telnet and X

If you would want to use telnet to display graphical applications running on a remote machine, you first need to give the remote machine access to your display (to your X server!) using the xhost command, by typing a command similar to the one below in a terminal window on your local machine:


   

 davy:~> xhost +remote.machine.com
 

After that, connect to the remote host and tell it to display graphics on the local machine by setting the environment variable DISPLAY :


   

 [davy@remote ~] export DISPLAY="local.host.com:0.0"
 

After completing this step, any application started in this terminal window will be displayed on your local desktop, using remote resources for computing, but your local graphical resources (your X server) for displaying the application.

This procedure assumes that you have some sort of X server (XFree86, Exceed, Cygwin) already set up on the machine where you want to display images. The architecture and operating system of the client machine are not important as long as they allow you to run an X server on it.

Mind that displaying a terminal window from the remote machine is also considered to be a display of an image.

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