Introduction to UNIX - Part 3: The GUI

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Graphical User Interfaces in UNIX

It is important to recognize that in UNIX the GUI is separate from the operating system itself. This is unlike Windows or MacOS9 and earlier which won't run without the GUI (or at the very least are extremely crippled if you do so). The GUI on UNIX is a separate layer that sits on top of the core operating system. In principle, you can uninstall the GUI and still have a completely functional UNIX system sans any graphical programs.

The X Window System

Version 11 of something called the X Window System is the underlying graphics layer on all modern UNIX systems (Except for MacOSX where it is an optional add-on). "X Windows", or just "X11" or "X" as it is commonly called, provides the glue between the underlying operating system, graphical applications, and graphics hardware. There are many thousands of software applications written for the X window system.

X by itself is not a GUI. Rather, X provides the facilities that a GUI requires - namely the ability to accept input from devices like a mouse and keyboard, and the ability to draw graphical objects to a computer display. So, despite its name, the X Window System cannot draw "windows", at least not by the modern definition of "window". This is the job of another piece of software called the window manager. There are many different window managers available for X. Some of these are proprietary and will only run on a particular type of UNIX. Some of these are open and will run on any system that has X installed.

Window Managers

For the most part, the window manager is what defines the "look and feel" of an X-based Graphical User Interface. It provides features such as window frames and buttons, window titles, window resizing (as well as maximize and minimize features), icons, toolbars, desktop backgrounds/menus and many other user interface elements. Some window managers also ship with their own set of desktop applications like file browsers, mail clients and other common utilities.

Because the window manager is just a piece of software that sits on top of X, there may be several different GUIs to choose from on a given UNIX system. Not all systems come preinstalled with multiple window managers to choose from, but this is something to keep in mind. Linux systems, in particular, often have a number of different GUIs to choose from at the console login screen (or via some other mechanism once you've logged in).

The window manager has been a somewhat fragmented space across different UNIX platforms. Some groups have attempted to unify the look and feel of all UNIX desktops by creating window managers that are open standards but in the end nothing has emerged as a de-facto standard that is adopted uniformly across all platforms. Below is a list of some of the more prevalent window managers that you are likely to encounter.

A list of common window managers

Name	Description	Availability
twm	The Tab Window Manager. It is the default window manager that ships with the X Window System. Very simple.	This is available on all UNIX/X implementations.
mwm	The Motif Window Manager. At one time this was fairly ubiquitous. It is based on the Motif toolkit. Many modern commercial window managers are deriviatives of mwm.	This is available for all UNIX/X implementations. Requires Motif.
4dwm	The IRIS 4d window manager. This is the default window manager for Silicon Graphics (SGI) systems. It is based on mwm and Motif.	This is SGI proprietary software and therefore is only available for SGI systems. There has been an attempt to create an open source lookalike called 5dwm.
CDE	The Common Desktop Environment - CDE is an open standard that has had some commercial success. Parts of CDE grew out of mwm and is based on Motif.	CDE ships, or has shipped, with Sun, HP, Compaq and IBM workstations. CDE is available from commercial sources for other operating systems such as Linux.
KDE	The K Desktop Environment. KDE is an open source desktop environment for UNIX workstations. It ships as the default window manager on some Linux distributions	KDE is open source and can in principle be used on any UNIX/X11 system. Due to its open source roots, KDE is primarily used on free UNIX operating systems such as Linux and FreeBSD.
Gnome	Gnome is an open source desktop environment for UNIX workstations. It competes directly with KDE. It ships as the default window manager on some Linux distributions	Like KDE, Gnome is open source and can be used on any UNIX/X11 system. Due to its open source roots, Gnome has up to now been primarily used on free UNIX operating systems. Sun has recently announced that Gnome will ship on Solaris as the default operating environment.

Getting to Know X

The X Window System has some very unique and useful features. These things are provided by X itself and as such are common to all X window managers and all UNIX/X11 systems:

Copying and pasting text

You may have noticed that in addition to the typical left and right mouse buttons, your UNIX workstation also sports a middle mouse button. Here is why: Most X applications allow you to highlight text with the left mouse button, making this text immediately available to be pasted to a new location. To paste the highlighted text, place the mouse in the new location and single-click the middle button. This is an extremely useful and time-saving technique, especially when working at the command line.

Note: In order to paste text, it must remain selected. Once a selection has been de-activated (usually by left-clicking inside of selectable area) the current selection is "forgotten" and is no longer available to X.

Displaying a remote program locally
This is perhaps the most useful and powerful feature of the X window system. X is a "client-server" environment. The way to think of this is the following: Every X program is a client, and every X display device is a server. Any X application (client) can display its contents over the network to any X display (server) as long as the permissions on the X server allow it. What does this mean? You can log in to a remote machine, run an X application, and have the application's contents show up on the console of the local system. There are typically five steps to this:

Login to the console of an X Windows system. For this example, we will assume the console machine's name is "moray.structbio.vanderbilt.edu". This is where your X applications will be displayed .
Start two UNIX shells. Use the first one to remotely log in to another system (See part 2 for an example). For this example, we will assume the remote machine's name is reef.structbio.vanderbilt.edu
Once you are on the machine reef, set your DISPLAY environment variable to moray's default display:
reef% setenv DISPLAY moray.structbio.vanderbilt.edu:0
In the second UNIX shell on moray, give reef permission to write to moray's X server:
moray% xhost +reef.structbio.vanderbilt.edu
Run an X application on reef. The application will run on reef, but it's graphical output will be output over the network to moray's X server:
reef% xterm &

You will now have the program xterm running on reef, but displaying on moray.
Note 1: In some instances, the ssh program will automatically set up the X network connection for you. In these cases, you can skip the "setenv DISPLAY" and "xhost" commands (steps 3 and 4 above).
Note 2: You are not limited to one remote X connection. You can have multiple remote applications from multiple remote machines simultaneously running and displaying their contents onto a single console's display.
Useful X programs
As mentioned above, there are thousands of X applications. There are a few useful X programs distributed as part of the standard X Window System distribution, and should be available on all machines which have a copy of X installed. Here is a partial listing of these:
- xhost - sets permissions for the console's X display
- xdpyinfo - reports lots of useful technical information about the console's X display
- xterm - a terminal program for X (UNIX shell)
- xcalc - a graphical calculator
- xclock - a graphical clock
- xeyes - a cute program to help keep track of your mouse pointer
- xfontsel - a font browser
- xman - a graphical manual page viewer
- xset - sets various parameters such as keyboard repeat rate, system bell volume, and screen blank timings.
You can learn more about these programs in their respective UNIX manual pages (read about the UNIX manual pages in the next part).

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