13.2. Customizing the Graphical Interface

13.2.1. Choosing a Display Manager

The graphical interface only provides display space. Running the X server by itself only leads to an empty screen, which is why most installations use a display manager to display a user authentication screen and start the graphical desktop once the user has authenticated. The three most popular display managers in current use are gdm3 (GNOME Display Manager), kdm (KDE Display Manager) and xdm (X Display Manager). Since the Falcot Corp administrators have opted to use the GNOME desktop environment, they logically picked gdm3 as a display manager too. The /etc/gdm3/daemon.conf configuration file has many options, some of which can also be set by gdmsetup, a graphical interface for gdm3 configuration that can be run from the System → Administration → Display screen menu in the GNOME desktop.

ALTERNATIVE gdm vs. gdm3

A fresh installation of Squeeze sets up gdm3. On the other hand, a system upgraded to Squeeze from a previous version of Debian will usually have gdm (the 2.x version). Version 3 is a full rewrite, but it is still rather young: many of the options provided by previous versions have been removed, and the gdmsetup configuration interface is much more limited in scope. This explains why both versions are available in Squeeze. Note, however, the 2.x versions will be removed from the next stable Debian release. It makes sense to anticipate the migration and install gdm3 straight away if the missing features are not important to you.

13.2.2. Choosing a Window Manager

Since each graphical desktop provides its own window manager, choosing the former usually implies software selections from the latter. GNOME uses the metacity window manager, KDE uses kwin, and Xfce (which we present later) has xfwm. The Unix philosophy always allows using one's window manager of choice, but following the recommendations allows an administrator to best take advantage of the integration efforts led by each project.

BACK TO BASICS Window manager

True to the Unix tradition of doing one thing only but doing it well, the window manager displays the “decorations” around the windows belonging to the currently running applications, which includes frames and the title bar. It also allows reducing, restoring, maximizing, and hiding windows. Most window managers also provide a menu that pops up when the desktop is clicked in a specific way. This menu provides the means to close the window manager session, starting new applications, and in some cases, change to another window manager (if installed).

Older computers may, however, have a hard time running heavyweight graphical desktop environments. In these cases, a lighter configuration should be used. "Light" (or small footprint) window managers include WindowMaker (in the wmaker package), Afterstep, fvwm, icewm, blackbox, fluxbox, or openbox. In these cases, the system should be configured so that the appropriate window manager gets precedence; the standard way is to change the x-window-manager alternative with the update-alternatives --config x-window-manager command.

DEBIAN SPECIFICITY Alternatives

The Debian policy lists a number of standardized commands able to perform a particular action. For example, the x-window-manager command invokes a window manager. But Debian does not assign this command to a fixed window manager. The administrator can choose which manager it should invoke.

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/.