One of the first things people see when looking at Linux is the desktop, or Graphical User Interface (GUI). Unlike most other operating systems such as MS Windows, BeOS and MacOS, with Linux (and UNIX) you have a choice of which GUI to use. Additionally, you have the ability to change the look and feel of any of the GUIs.
First, a pronunciation lesson. GUI is commonly pronounced "gooey", and almost never "G-U-I". Now that you know how to be cool, let's move on to what you get in terms of GUIs with Linux.
Unlike MS Windows, BeOS and MacOS, where the GUI is an integral part of the operating system, the graphical environment in Linux is made up of a bunch of pieces.
The bottom-level piece is called the X Window System, or X for short. X itself can be divided into two parts: the client and the server. The client is the part that connects the applications program. On the other end is the server, which is the part that connects to your graphical display. This may sound backward--think of the server as the part that serves the display.
At the next level, we have what is called the window manager. This is the part that adds decorations to the windows, allowing you to perform window-related tasks such as resizing, moving and killing.
Finally, there is a development environment. This is the software that allows you to write application programs that take advantage of the graphical environment. Let me point out that you could do all your development using the functionality offered by X, but it would be a lot harder than using one of the higher-level toolkits.
Two advantages of the multi-piece setup are:
You can run your display on a system other than where the program is running.
You can pick and choose what the GUI looks like.
The first point may sound a little obscure, but it is actually pretty simple and very useful. Let's suppose that some pictures you want to view, using the xv utility, are located on a friend's workstation. Assuming you and your friend are on the same network, your options for accessing the pictures are:
Copy the files to your local system using ftp, rcp or scp.
NFS-mount the directory containing the pictures, so you can access them from your system.
Go to their workstation and view the pictures there.
Take advantage of the client-server design of the X Window System.
The obvious question is how to take advantage of the client-server design of X. The first step is to acquaint yourself with the xhost command. By entering
on your local computer, you allow other systems to write to your display. See the man page on xhost for details on other options.
Next, remotely log on to the system with the pictures on them. You can do this with one of several commands, including rsh and ssh. (Find out more about rsh and ssh at our on-line man pages.)
Now you need to tell the X client on this remote system that you want it to use the X server on your desktop to display images. You do this by setting the environment variable named DISPLAY equal to the name or IP address of your desktop system, followed by :0. For example, if the name of your local system is sugarmag.ssc.com and you are running the bash or korn shell, you would enter
Now you can invoke xv just like you normally would, except you are doing it on the remote system. For example, to display the picture coolpic.jpg, just enter
on the remote system. The image will be displayed on your local system.
The window manager is simply a helper program that gives you the ability to control the windows on your desktop. Unlike other platforms, you can pick and choose the window manager.
The two most popular choices today are GNOME and KDE. Both offer a sophisticated user interface and their own development environments. GNOME came into existence for political reasons. The licensing on the Qt toolkit, the software that KDE is based on, was rather restrictive. Today, Qt licensing has been relaxed, and the choice is much more on the level of personal preference.
If performance is an issue--for example, you want to run Linux on a 486 or other slow machine, or have only 16MB of RAM--you can still run Linux with a GUI. You are best off looking at FVWM, a completely capable window manager that is much smaller than KDE or GNOME. What does FVWM stand for? Well, the Virtual Window Manager part is clear enough, but the "F"? Consult the FVWM FAQ for some of the (often humorous) details. Another alternative is Motif, or its free relative, Lesstif.
Both GNOME and KDE are a lot more than just window managers. They dictate a look and feel. You can offer that same look and feel with your application by using their related development environment. In the case of KDE, the development environment is Qt. In the case of GNOME, it is GTK+. Again, the choice is really a combination of personal preference and compatibility with other applications.
The compatibility issue isn't about whether you can run GTK-based and Qt-based applications on the same system (you can). It is strictly look-and-feel, again. For example, while the GIMP, a bitmap graphics manipulation program similar to Adobe's PhotoShop, is based on GTK, it runs perfectly under KDE.