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The Debian GNU/Linux FAQ
Chapter 11 - Customizing your installation of Debian GNU/Linux

11.1 How can I ensure that all programs use the same paper size?

Install the libpaper1 package, and it will ask you for a system-wide default paper size. This setting will be kept in the file /etc/papersize.

Users can override the paper size setting using the PAPERSIZE environment variable. For details, see the manual page papersize(5).

11.2 How can I provide access to hardware peripherals, without compromising security?

Many device files in the /dev directory belong to some predefined groups. For example, /dev/fd0 belongs to the floppy group, and /dev/dsp belongs to the audio group.

If you want a certain user to have access to one of these devices, just add the user to the group the device belongs to, i.e. do:

     adduser user group

This way you won't have to change the file permissions on the device.

If you do this from within a user's shell or a GUI environment you have to logout and login again to become an effective member of that group. To check which groups you belong to run groups.

Notice that, since the introduction of udev if you change the permissions of a hardware peripheral they might be be adjusted for some devices when the system starts, if this happens to the hardware peripherals you are instered in you will have to adjust the rules at /etc/udev.

11.3 How do I load a console font on startup the Debian way?

The kbd and console-tools packages support this, edit /etc/kbd/config or /etc/console-tools/config files.

11.4 How can I configure an X11 program's application defaults?

Debian's X programs will install their application resource data in the /etc/X11/app-defaults/ directory. If you want to customize X applications globally, put your customizations in those files. They are marked as configuration files, so their contents will be preserved during upgrades.

11.5 Every distribution seems to have a different boot-up method. Tell me about Debian's.

Like all Unices, Debian boots up by executing the program init. The configuration file for init (which is /etc/inittab) specifies that the first script to be executed should be /etc/init.d/rcS. This script runs all of the scripts in /etc/rcS.d/ by sourcing or forking subprocess depending on their file extension to perform initialization such as to check and to mount file systems, to load modules, to start the network services, to set the clock, and to perform other initialization. Then, for compatibility, it runs the files (except those with a `.'in the filename) in /etc/rc.boot/ too. Any scripts in the latter directory are usually reserved for system administrator use, and using them in packages is deprecated.

After completing the boot process, init executes all start scripts in a directory specified by the default runlevel (this runlevel is given by the entry for id in /etc/inittab). Like most System V compatible Unices, Linux has 7 runlevels:

Debian systems come with id=2, which indicates that the default runlevel will be '2' when the multi-user state is entered, and the scripts in /etc/rc2.d/ will be run.

In fact, the scripts in any of the directories, /etc/rcN.d/ are just symbolic links back to scripts in /etc/init.d/. However, the names of the files in each of the /etc/rcN.d/ directories are selected to indicate the way the scripts in /etc/init.d/ will be run. Specifically, before entering any runlevel, all the scripts beginning with 'K' are run; these scripts kill services. Then all the scripts beginning with 'S' are run; these scripts start services. The two-digit number following the 'K' or 'S' indicates the order in which the script is run. Lower numbered scripts are executed first.

This approach works because the scripts in /etc/init.d/ all take an argument which can be either `start', `stop', `reload', `restart' or `force-reload' and will then do the task indicated by the argument. These scripts can be used even after a system has been booted, to control various processes.

For example, with the argument `reload' the command

     /etc/init.d/sendmail reload

sends the sendmail daemon a signal to reread its configuration file. (BTW, Debian supplies invoke-rc.d as a wrapper for invoking the scripts in /etc/init.d/.)

11.6 It looks as if Debian does not use rc.local to customize the boot process; what facilities are provided?

Suppose a system needs to execute script foo on start-up, or on entry to a particular (System V) runlevel. Then the system administrator should:

One might, for example, cause the script foo to execute at boot-up, by putting it in /etc/init.d/ and running update-rc.d foo defaults 19. The argument `defaults' refers to the default runlevels, which means (at least in absence of any LSB comment block to the contrary) to start the service in runlevels 2 through 5, and to stop the service in runlevels 0, 1 and 6. (Any LSB Default-Start and Default-Stop directives in foo take precedence when using the sysv-rc version of update-rc.d, but are ignored by the current (v0.8.10) file-rc version of update-rc.d.) The argument `19' ensures that foo is called after all scripts whose number is less than 19 have completed, and before all scripts whose number is 20 or greater.

11.7 How does the package management system deal with packages that contain configuration files for other packages?

Some users wish to create, for example, a new server by installing a group of Debian packages and a locally generated package consisting of configuration files. This is not generally a good idea, because dpkg will not know about those configuration files if they are in a different package, and may write conflicting configurations when one of the initial "group" of packages is upgraded.

Instead, create a local package that modifies the configuration files of the "group" of Debian packages of interest. Then dpkg and the rest of the package management system will see that the files have been modified by the local "sysadmin" and will not try to overwrite them when those packages are upgraded.

11.8 How do I override a file installed by a package, so that a different version can be used instead?

Suppose a sysadmin or local user wishes to use a program "login-local" rather than the program "login" provided by the Debian login package.

Do not:

The package management system will not know about this change, and will simply overwrite your custom /bin/login whenever login (or any package that provides /bin/login) is installed or updated.

Rather, do

Run dpkg-divert --list to see which diversions are currently active on your system.

Details are given in the manual page dpkg-divert(8).

11.9 How can I have my locally-built package included in the list of available packages that the package management system knows about?

Execute the command:

     dpkg-scanpackages BIN_DIR OVERRIDE_FILE [PATHPREFIX] > my_Packages


Once you have built the file my_Packages, tell the package management system about it by using the command:

     dpkg --merge-avail my_Packages

If you are using APT, you can add the local repository to your sources.list(5) file, too.

11.10 Some users like mawk, others like gawk; some like vim, others like elvis; some like trn, others like tin; how does Debian support diversity?

There are several cases where two packages provide two different versions of a program, both of which provide the same core functionality. Users might prefer one over another out of habit, or because the user interface of one package is somehow more pleasing than the interface of another. Other users on the same system might make a different choice.

Debian uses a "virtual" package system to allow system administrators to choose (or let users choose) their favorite tools when there are two or more that provide the same basic functionality, yet satisfy package dependency requirements without specifying a particular package.

For example, there might exist two different versions of newsreaders on a system. The news server package might 'recommend' that there exist some news reader on the system, but the choice of tin or trn is left up to the individual user. This is satisfied by having both the tin and trn packages provide the virtual package news-reader. Which program is invoked is determined by a link pointing from a file with the virtual package name /etc/alternatives/news-reader to the selected file, e.g., /usr/bin/trn.

A single link is insufficient to support full use of an alternate program; normally, manual pages, and possibly other supporting files must be selected as well. The Perl script update-alternatives provides a way of ensuring that all the files associated with a specified package are selected as a system default.

For example, to check what executables provide `x-window-manager', run:

     update-alternatives --display x-window-manager

If you want to change it, run:

     update-alternatives --config x-window-manager

And follow the instructions on the screen (basically, press the number next to the entry you'd like better).

If a package doesn't register itself as a window manager for some reason (file a bug if it's in error), or if you use a window manager from /usr/local directory, the selections on screen won't contain your preferred entry. You can update the link through command line options, like this:

     update-alternatives --install /usr/bin/x-window-manager \
       x-window-manager /usr/local/bin/wmaker-cvs 50

The first argument to `--install' option is the symlink that points to /etc/alternatives/NAME, where NAME is the second argument. The third argument is the program to which /etc/alternatives/NAME should point to, and the fourth argument is the priority (larger value means the alternative will more probably get picked automatically).

To remove an alternative you added, simply run:

     update-alternatives --remove x-window-manager /usr/local/bin/wmaker-cvs

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The Debian GNU/Linux FAQ

version 5.0, 27 August 2011

Authors are listed at Debian FAQ Authors