For each window manager, the relevant package therefore registers the appropriate command as a possible choice for x-window-manager along with an associated priority. Barring explicit configuration by the administrator, this priority allows picking the best installed window manager when the generic command is run.
Both the registration of commands and the explicit configuration involve the update-alternatives script. Choosing where a symbolic command points at is a simple matter of running update-alternatives --config symbolic-command. The update-alternatives script creates (and maintains) symbolic links in the /etc/alternatives/ directory, which in turn references the location of the executable. As time passes, packages are installed or removed, and/or the administrator makes explicit changes to the configuration. When a package providing an alternative is removed, the alternative automatically goes to the next best choice among the remaining possible commands.
Not all symbolic commands are explicitly listed by the Debian policy; some Debian package maintainers deliberately chose to use this mechanism in less straightforward cases where it still brings interesting flexibility (examples include x-www-browser, www-browser, cc, c++, awk, and so on).
13.2.3. Menu Management
Modern desktop environments and many window managers provide menus listing the available applications for the user. In order to keep menus up-to-date in relation to the actual set of available applications, Debian created a centralized database registering all installed applications. A newly installed package registers itself in that database, and tells the system to update the menus accordingly. This infrastructure is handled in the menu package.
When a package provides an application that should appear in the menu system, it stores a file in the /usr/share/menu/ directory. That file describes some of the application features (including whether it's a graphical application or not), and the best location for it in the menu hierarchy. The post-installation script for this package then runs the update-menus command, which in turn updates all the required files. This command cannot know all the menu types used by installed applications. As a consequence, packages able to display a menu must provide an executable script that will be invoked with all the required information from the menu file; the script should then turn this information into elements that the application with the menu can use. These filter scripts are installed in the /etc/menu-methods/ directory.
GOING FURTHER Menus standardization
Debian provides its own menu system, but both GNOME and KDE developed their own menu management solutions as well. The two projects agreed on a format for these menus — more precisely, a common format for the .desktop files that represent menu elements — under the FreeDesktop.org umbrella project.
→ http://www.freedesktop.org/
The Debian developers have kept a close eye on this project and .desktop files can be generated from the Debian menu system. However, neither GNOME nor KDE use the Debian menu. They both prefer keeping complete control over their menus. Still, it is possible to enable a “Debian” submenu containing the official menu as maintained by Debian: in GNOME, the menu editor (in the alacarte package) is available by right-clicking on the panel menu, then choosing “Edit menus”.
The administrator can also have a say in the process and in the resulting generated menus. First, they can delete a menu element even when the matching application is installed, by simply storing in /etc/menu/ an empty file named according to the package providing the entries to be disabled. Second, the menu can be reorganized and sections renamed or grouped. The /etc/menu-methods/translate_menus file is where this reorganization is defined and contains commented examples. Last, new elements can be added to the menu, for example to start programs installed outside the packaging system, or to run a particular command such as starting a web browser on a particular page. These extra elements are specified in /etc/menu/local.element files, which have the same format as other menu files available under /usr/share/menu/.
13.3. Graphical Desktops
The free graphical desktop field is dominated by two large software collections: GNOME and KDE. Both of them are very popular. This is rather a rare instance in the free software world; the Apache web server, for instance, has very few peers.
This diversity is rooted in history. KDE was the first graphical desktop project, but it chose the Qt graphical toolkit and that choice wasn't acceptable for a large number of developers. Qt was not free software at the time, and GNOME was started based on the GTK+ toolkit. Qt became free software in the interval, but the projects haven't merged and evolved in parallel instead.
GNOME and KDE still work together: under the FreeDesktop.org umbrella, the projects collaborated in defining standards for interoperability across applications.
Choosing “the best” graphical desktop is a sensitive topic which we prefer to steer clear of. We will merely describe the many possibilities and give a few pointers for further thoughts. The best choice will be the one you make after some experimentation.
13.3.1. GNOME
Debian Squeeze includes GNOME version 2.30, which can be installed by a simple apt-get install gnome (it can also be installed by selecting the “Graphical desktop environment” task).
GNOME is noteworthy for its efforts in usability and accessibility. Design professionals have been involved in writing standards and recommendations. This has helped developers to create satisfying graphical user interfaces. The project also gets encouragement from the big players of computing, such as Intel, IBM, Oracle, Novell, and of course, various Linux distributions. Finally, many programming languages can be used in developing applications interfacing to GNOME.
It took quite some time for the GNOME project to build up this infrastructure, which can account for a seeminly less mature desktop than KDE. The usability and accessibility efforts, in particular, are recent, and the benefits have only started to show in the latest versions of the environment.
Figure 13.1. The GNOME desktop
For administrators, GNOME seems to be better prepared for massive deployments. Application configuration is handled by GConf, a kind of registry that can be queried and edited with the gconftool-2 command-line tool. The administrator can therefore change users' configuration with a simple script. The following webside lists all information of interest to an administrator tasked to manage GNOME workstations:
→ http://library.gnome.org/admin/system-admin-guide/stable/
→ http://library.gnome.org/admin/deployment-guide/
13.3.2. KDE
Debian Squeeze includes version 4.4.5 of KDE, which can be installed with apt-get install kde.
KDE has had a rapid evolution based on a very hands-on approach. Its authors quickly got very good results, which allowed them to grow a large user-base. These factors contributed to the overall project quality. KDE is a perfectly mature desktop environment with a wide range of applications.
Figure 13.2. The KDE desktop
Since the Qt 4.0 release, the last remaining license problem with KDE is no more. This version was released under the GPL both for Linux and Windows (whereas the Windows version was previously released under a non-free license). Note that KDE applications must be developed using the C++ language.