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# The Debian GNU/Linux FAQ
Chapter 6 - Basics of the Debian package management system

## 6.1 What is a Debian package?

Packages generally contain all of the files necessary to implement a set of related commands or features. There are two types of Debian packages:

- Binary packages, which contain executables, configuration files, man/info pages, copyright information, and other documentation. These packages are distributed in a Debian-specific archive format (see What is the format of a Debian binary package?, Section 6.2); they are usually distinguished by having a '.deb' file extension. Binary packages can be unpacked using the Debian utility `dpkg`; details are given in its manual page.

- Source packages, which consist of a `.dsc` file describing the source package (including the names of the following files), a `.orig.tar.gz` file that contains the original unmodified source in gzip-compressed tar format and usually a `.diff.gz` file that contains the Debian-specific changes to the original source. The utility `dpkg-source` packs and unpacks Debian source archives; details are provided in its manual page.

Installation of software by the package system uses "dependencies" which are carefully designed by the package maintainers. These dependencies are documented in the `control` file associated with each package. For example, the package containing the GNU C compiler (`gcc`) "depends" on the package `binutils` which includes the linker and assembler. If a user attempts to install `gcc` without having first installed `binutils`, the package management system (dpkg) will send an error message that it also needs `binutils`, and stop installing `gcc`. (However, this facility can be overridden by the insistent user, see `dpkg(8)`.) See more in What is meant by saying that a package Depends, Recommends, Suggests, Conflicts, Replaces or Provides another package?, Section 6.9 below.

Debian's packaging tools can be used to:

- manipulate and manage packages or parts of packages,

- aid the user in the break-up of packages that must be transmitted through a limited-size medium such as floppy disks,

- aid developers in the construction of package archives, and

- aid users in the installation of packages which reside on a remote FTP site.

## 6.2 What is the format of a Debian binary package?

A Debian "package", or a Debian archive file, contains the executable files, libraries, and documentation associated with a particular suite of program or set of related programs. Normally, a Debian archive file has a filename that ends in `.deb`.

The internals of this Debian binary packages format are described in the `deb(5)` manual page. This internal format is subject to change (between major releases of Debian GNU/Linux), therefore please always use `dpkg-deb(1)` for manipulating `.deb` files.

## 6.3 Why are Debian package file names so long?

The Debian binary package file names conform to the following convention: <foo>_<VersionNumber>-<DebianRevisionNumber>.deb

Note that `foo` is supposed to be the package name. As a check, one can learn the package name associated with a particular Debian archive file (.deb file) in one of these ways:

- inspect the "Packages" file in the directory where it was stored at a Debian FTP archive site. This file contains a stanza describing each package; the first field in each stanza is the formal package name.

- use the command `dpkg --info foo_VVV-RRR.deb` (where VVV and RRR are the version and revision of the package in question, respectively). This displays, among other things, the package name corresponding to the archive file being unpacked.

The `VVV` component is the version number specified by the upstream developer. There are no standards in place here, so the version number may have formats as different as "19990513" and "1.3.8pre1".

The `RRR` component is the Debian revision number, and is specified by the Debian developer (or an individual user if he chooses to build the package himself). This number corresponds to the revision level of the Debian package, thus, a new revision level usually signifies changes in the Debian Makefile (`debian/rules`), the Debian control file (`debian/control`), the installation or removal scripts (`debian/p*`), or in the configuration files used with the package.

## 6.4 What is a Debian control file?

Specifics regarding the contents of a Debian control file are provided in the Debian Policy Manual, section 5, see What other documentation exists on and for a Debian system?, Section 11.1.

Briefly, a sample control file is shown below for the Debian package hello:

         Package: hello
         Priority: optional
         Section: devel
         Installed-Size: 45
         Maintainer: Adam Heath <doogie@debian.org>
         Architecture: i386
         Version: 1.3-16
         Depends: libc6 (>= 2.1)
         Description: The classic greeting, and a good example
          The GNU hello program produces a familiar, friendly greeting.  It
          allows nonprogrammers to use a classic computer science tool which
          would otherwise be unavailable to them.
          .
          Seriously, though: this is an example of how to do a Debian package.
          It is the Debian version of the GNU Project's `hello world' program
          (which is itself an example for the GNU Project).

The Package field gives the package name. This is the name by which the package can be manipulated by the package tools, and usually similar to but not necessarily the same as the first component string in the Debian archive file name.

The Version field gives both the upstream developer's version number and (in the last component) the revision level of the Debian package of this program as explained in Why are Debian package file names so long?, Section 6.3.

The Architecture field specifies the chip for which this particular binary was compiled.

The Depends field gives a list of packages that have to be installed in order to install this package successfully.

The Installed-Size indicates how much disk space the installed package will consume. This is intended to be used by installation front-ends in order to show whether there is enough disk space available to install the program.

The Section line gives the "section" where this Debian package is stored at the Debian FTP sites. This is the name of a subdirectory (within one of the main directories, see What are all those directories at the Debian FTP archives?, Section 5.1) where the package is stored.

The Priority indicates how important is this package for installation, so that semi-intelligent software like dselect or console-apt can sort the package into a category of e.g. packages optionally installed. See What is an Essential Required, Important, Standard, Optional, or Extra package?, Section 6.7.

The Maintainer field gives the e-mail address of the person who is currently responsible for maintaining this package.

The Description field gives a brief summary of the package's features.

For more information about all possible fields a package can have, please see the Debian Policy Manual, section 5., "Control files and their fields".

## 6.5 What is a Debian conffile?

Conffiles is a list of configuration files (usually placed in `/etc`) that the package management system will not overwrite when the package is upgraded. This ensures that local values for the contents of these files will be preserved, and is a critical feature enabling the in-place upgrade of packages on a running system.

To determine exactly which files are preserved during an upgrade, run:

         dpkg --status package

And look under "Conffiles:".

## 6.6 What is a Debian preinst, postinst, prerm, and postrm script?

These files are executable scripts which are automatically run before or after a package is installed. Along with a file named `control`, all of these files are part of the "control" section of a Debian archive file.

The individual files are:

preinst

  This script executes before that package will be unpacked from its Debian archive (".deb") file. Many 'preinst' scripts stop services for packages which are being upgraded until their installation or upgrade is completed (following the successful execution of the 'postinst' script).

postinst

  This script typically completes any required configuration of the package `foo` once `foo` has been unpacked from its Debian archive (".deb") file. Often, 'postinst' scripts ask the user for input, and/or warn the user that if he accepts default values, he should remember to go back and re-configure that package as the situation warrants. Many 'postinst' scripts then execute any commands necessary to start or restart a service once a new package has been installed or upgraded.

prerm

  This script typically stops any daemons which are associated with a package. It is executed before the removal of files associated with the package.

postrm

  This script typically modifies links or other files associated with `foo`, and/or removes files created by the package. (Also see What is a Virtual Package?, Section 6.8.)

Currently all of the control files can be found in directory `/var/lib/dpkg/info`. The files relevant to package `foo` begin with the name "foo" and have file extensions of "preinst", "postinst", etc., as appropriate. The file `foo.list` in that directory lists all of the files that were installed with the package `foo`. (Note that the location of these files is a dpkg internal; you should not rely on it.)

## 6.7 What is an Essential Required, Important, Standard, Optional, or Extra package?

Each Debian package is assigned a priority by the distribution maintainers, as an aid to the package management system. The priorities are:

- Required: packages that are necessary for the proper functioning of the system.

  This includes all tools that are necessary to repair system defects. You must not remove these packages or your system may become totally broken and you may probably not even be able to use dpkg to put things back. Systems with only the Required packages are probably unusable, but they do have enough functionality to allow the sysadmin to boot and install more software.

- Important packages should be found on any Unix-like system.

  Other packages which the system will not run well or be usable without will be here. This does NOT include Emacs or X11 or TeX or any other large applications. These packages only constitute the bare infrastructure.

- Standard packages are standard on any Linux system, including a reasonably small but not too limited character-mode system. Tools are included to be able to browse the web (using w3m), send e-mail (with mutt) and download files from FTP servers.

  This is what will install by default if users do not select anything else. It does not include many large applications, but it does include the Python interpreter and some server software like OpenSSH (for remote administration), Exim (for mail delivery, although it can be configured for local delivery only), an identd server (pidentd) and the RPC portmapper (`portmap`). It also includes some common generic documentation that most users will find helpful.

- Optional packages include all those that you might reasonably want to install if you did not know what it was, or do not have specialized requirements.

  This includes X11, a full TeX distribution, and lots of applications.

- Extra: packages that either conflict with others with higher priorities, are only likely to be useful if you already know what they are, or have specialized requirements that make them unsuitable for "Optional".

If you do a default Debian installation all the packages of priority Standard or higher will be installed in your system. If you select pre-defined tasks you will get lower priority packages too.

Additionally, some packages are marked as Essential since they are absolutely necessary for the proper functioning of the system. The package management tools will refuse to remove these.

## 6.8 What is a Virtual Package?

A virtual package is a generic name that applies to any one of a group of packages, all of which provide similar basic functionality. For example, both the `tin` and `trn` programs are news readers, and should therefore satisfy any dependency of a program that required a news reader on a system, in order to work or to be useful. They are therefore both said to provide the "virtual package" called `news-reader`.

Similarly, `smail` and `sendmail` both provide the functionality of a mail transport agent. They are therefore said to provide the virtual package, "mail transport agent". If either one is installed, then any program depending on the installation of a `mail-transport-agent` will be satisfied by the existence of this virtual package.

Debian provides a mechanism so that, if more than one package which provide the same virtual package is installed on a system, then system administrators can set one as the preferred package. The relevant command is `update-alternatives`, and is described further in Some users like mawk, others like gawk; some like vim, others like elvis; some like trn, others like tin; how does Debian support diversity?, Section 10.10.

## 6.9 What is meant by saying that a package Depends, Recommends, Suggests, Conflicts, Replaces or Provides another package?

The Debian package system has a range of package "dependencies" which are designed to indicate (in a single flag) the level at which Program A can operate independently of the existence of Program B on a given system:

- Package A depends on Package B if B absolutely must be installed in order to run A. In some cases, A depends not only on B, but on a version of B. In this case, the version dependency is usually a lower limit, in the sense that A depends on any version of B more recent than some specified version.

- Package A recommends Package B, if the package maintainer judges that most users would not want A without also having the functionality provided by B.

- Package A suggests Package B if B contains files that are related to (and usually enhance) the functionality of A.

- Package A conflicts with Package B when A will not operate if B is installed on the system. Most often, conflicts are cases where A contains files which are an improvement over those in B. "Conflicts" are often combined with "replaces".

- Package A replaces Package B when files installed by B are removed and (in some cases) over-written by files in A.

- Package A provides Package B when all of the files and functionality of B are incorporated into A. This mechanism provides a way for users with constrained disk space to get only that part of package A which they really need.

More detailed information on the use of each these terms can be found in the Policy manual.

## 6.10 What is meant by Pre-Depends?

"Pre-Depends" is a special dependency. In the case of most packages, `dpkg` will unpack its archive file (i.e., its `.deb` file) independently of whether or not the files on which it depends exist on the system. Simplistically, unpacking means that `dpkg` will extract the files from the archive file that were meant to be installed on your file system, and put them in place. If those packages depend on the existence of some other packages on your system, `dpkg` will refuse to complete the installation (by executing its "configure" action) until the other packages are installed.

However, for some packages, `dpkg` will refuse even to unpack them until certain dependencies are resolved. Such packages are said to "Pre-depend" on the presence of some other packages. The Debian project provided this mechanism to support the safe upgrading of systems from `a.out` format to `ELF` format, where the order in which packages were unpacked was critical. There are other large upgrade situations where this method is useful, e.g. the packages with the required priority and their LibC dependency.

As before, more detailed information about this can be found in the Policy manual.

## 6.11 What is meant by unknown, install, remove purge and hold in the package status?

These "want" flags tell what the user wanted to do with a package (as indicated either by the user's actions in the "Select" section of `dselect`, or by the user's direct invocations of `dpkg`).

Their meanings are:

- unknown - the user has never indicated whether he wants the package

- install - the user wants the package installed or upgraded

- remove - the user wants the package removed, but does not want to remove any existing configuration files.

- purge - the user wants the package to be removed completely, including its configuration files.

- hold - the user wants this package not to be processed, i.e., he wants to keep the current version with the current status whatever that is.

## 6.12 How do I put a package on hold?

There are three ways of holding back packages, with dpkg, aptitude or with dselect.

With dpkg, you just have to export the list of package selections, with:

         dpkg --get-selections \* > selections.txt

Then edit the resulting file `selections.txt`, change the line containing the package you wish to hold, e.g. `libc6`, from this:

         libc6                                           install

to this:

         libc6                                           hold

Save the file, and reload it into dpkg database with:

         dpkg --set-selections < selections.txt

With aptitude, you can hold a package using

         aptitude hold package_name

and remove the hold with

         aptitude unhold package_name

With dselect, you just have to enter the [S]elect screen, find the package you wish to hold in its present state, and press the `=' key (or `H'). The changes will go live immediately after you exit the [S]elect screen.

## 6.13 How do I install a source package?

Debian source packages can't actually be "installed", they are just unpacked in whatever directory you want to build the binary packages they produce.

Source packages are distributed on most of the same mirrors where you can obtain the binary packages. If you set up your APT's `sources.list(5)` to include the appropriate "deb-src" lines, you'll be able to easily download any source packages by running

         apt-get source foo

To help you in actually building the source package, Debian source package provide the so-called build-dependencies mechanism. This means that the source package maintainer keeps a list of other packages that are required to build their package. To see how this is useful, run

         apt-get build-dep foo

before building the source.

## 6.14 How do I build binary packages from a source package?

You will need all of foo_*.dsc, foo_*.tar.gz and foo_*.diff.gz to compile the source (note: there is no .diff.gz for some packages that are native to Debian).

Once you have them (How do I install a source package?, Section 6.13), if you have the `dpkg-dev` package installed, the following command:

         dpkg-source -x foo_version-revision.dsc

will extract the package into a directory called `foo-version`.

If you want just to compile the package, you may cd into `foo-version` directory and issue the command

         dpkg-buildpackage -rfakeroot -b

to build the package (note that this also requires the `fakeroot` package), and then

         dpkg -i ../foo_version-revision_arch.deb

to install the newly-built package(s).

## 6.15 How do I create Debian packages myself?

For more detailed description on this, read the New Maintainers' Guide, available in the `maint-guide` package, or at `http://www.debian.org/doc/devel-manuals#maint-guide`.

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The Debian GNU/Linux FAQ