| Chapter 7. Basics of the Debian package management system | ||
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| Chapter 7. Basics of the Debian package management system | ||
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Table of Contents
This chapter touches on some lower level internals of Debian package management. If you're interested mainly in usage of the relevant tools, skip to chapters Chapter 8, The Debian package management tools and/or Chapter 9, Keeping your Debian system up-to-date.
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 Section 7.2, “What is the format of a Debian binary package?”); they are usually characterized by having a '.deb' file
extension. Binary packages can be unpacked using the Debian utility
dpkg (possibly via a frontend like
aptitude); 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. (The
program apt-get can be used as a frontend for
dpkg-source.)
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 Section 7.9, “What is meant by saying that a package Depends, Recommends, Suggests, Conflicts, Replaces, Breaks or Provides another package?” below.
Debian's packaging tools can be used to:
manipulate and manage packages or parts of packages,
administer local overrides of files in a package,
aid developers in the construction of package archives, and
aid users in the installation of packages which reside on a remote archive site.
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)
if you need to do lowlevel manipulations on .deb files.
The Debian binary package file names conform to the following convention: <foo>_<VersionNumber>-<DebianRevisionNumber>_<DebianArchitecture>.deb
Note that foo is supposed to be the package name. Checking
the package name associated with a particular Debian archive file (.deb file)
can be done in one of these ways:
inspect the "Packages" file in the directory where it was stored at a Debian 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_AAA.deb (where VVV,
RRR and AAA are the version, revision and architecture 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 a user who chooses to rebuild
the package locally). 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.
The AAA component identifies the processor for which the
package was built. This is commonly amd64, which refers to
AMD64, Intel 64 or VIA Nano chips. For other possibilities review Debian's archive
directory structure at Section 6.7, “What are all those directories at the Debian archives?”. For details, see the
description of "Debian architecture" in the manual page
dpkg-architecture(1).
Specifics regarding the contents of a Debian control file are provided in the Debian Policy Manual, section 5, see Section 12.1, “What other documentation exists on and for a Debian system?”.
Briefly, a sample control file is shown below for the Debian package hello:
Package: hello Version: 2.9-2+deb8u1 Architecture: amd64 Maintainer: Santiago Vila <sanvila@debian.org> Installed-Size: 145 Depends: libc6 (>= 2.14) Conflicts: hello-traditional Breaks: hello-debhelper (<< 2.9) Replaces: hello-debhelper (<< 2.9), hello-traditional Section: devel Priority: optional Homepage: https://www.gnu.org/software/hello/ Description: example package based on GNU hello The GNU hello program produces a familiar, friendly greeting. It allows non-programmers 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 Section 7.3, “Why are Debian package file names so long?”.
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 archive sites.
The Priority indicates how important is this package for installation, so that semi-intelligent software like apt or aptitude can sort the package into a category of e.g. packages optionally installed. See Section 7.7, “What is an Essential, Required, Important, Standard, Optional, or Extra package?”.
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", see Section 12.1, “What other documentation exists on and for a Debian system?”.
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:".
These files are executable scripts which are automatically run before or after
a package is installed or removed. 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:
This script is executed before the package it belongs to is 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).
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 users for input,
and/or warn them that if they accept default values, they should remember to go
back and re-configure that package as needed. Many 'postinst' scripts then
execute any commands necessary to start or restart a service once a new package
has been installed or upgraded.
This script typically stops any daemons which are associated with a package. It is executed before the removal of files associated with the package.
This script typically modifies links or other files associated with
foo, and/or removes files created by the package. (Also see
Section 7.8, “What is a Virtual Package?”.)
Currently all of the control files can be found in the 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.)
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 X or TeX or any other large application. 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 send e-mail (with mutt) and download files from archive servers.
This is what will be installed 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) and Exim (for mail delivery, although it can be configured for local delivery only). 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 do not know what they are, or that do not have specialized requirements.
This includes X, 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.
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 konqueror and firefox-esr programs
are web browsers, and should therefore satisfy any dependency of a program that
requires a web browser on a system, in order to work or to be useful. They are
therefore both said to provide the "virtual package" called
www-browser.
Similarly, exim4 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 presence 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 Section 11.11, “Some users like mawk, others like gawk; some like vim, others like elvis; some like trn, others like tin; how does Debian support diversity?”.
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 breaks Package B when both packages cannot be simultaneously configured in a system. The package management system will refuse to install one if the other one is already installed and configured in the system.
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 of these terms can be found in the Debian Policy manual, section 7.2, "Binary Dependencies", see Section 12.1, “What other documentation exists on and for a Debian system?”.
"Pre-Depends" is a special dependency. In the case of most packages,
dpkg will unpack the archive file of a package (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.
These "want" flags tell what the user wanted to do with a package (as indicated
by the user's direct invocations of
dpkg/apt/ aptitude).
Their meanings are:
unknown - the user has never indicated whether the package is wanted.
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 file.
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. wants to keep the current version with the current status whatever that is.
There are three ways of holding back packages, with dpkg, apt or aptitude.
With dpkg, you 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 apt, you can set a package to hold using
apt-mark hold package_name
and remove the hold with
apt-mark unhold package_name
With aptitude, you can hold a package using
aptitude hold package_name
and remove the hold with
aptitude unhold package_name
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 package by running
apt-get source foo
To help you in actually building the source package, Debian source packages 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.
The preferred way to do this is by using various wrapper tools. We'll show how
it's done using the devscripts tools. Install this package
if you haven't done so already.
Now, first get the source package:
apt-get source foo
and change to the source tree:
cd foo-*
Then install needed build-dependencies (if any):
sudo apt-get build-dep foo
Then create a dedicated version of your own build (so that you won't get confused later when Debian itself releases a new version):
dch -l local 'Blah blah blah'
And finally build your package:
debuild -us -uc
If everything worked out fine, you should now be able to install your package by running
sudo dpkg -i ../*.deb
If you prefer to do things manually, and don't want to use
devscripts, follow this procedure:
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 (Section 7.13, “How do I install a source package?”) and 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 just want to compile the package, you may cd into the
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).
For a more detailed description on this, read the New Maintainers' Guide,
available in the maint-guide package or
at https://www.debian.org/doc/devel-manuals#maint-guide,
or the Guide for Debian Maintainers, available in the debmake-doc package or at https://www.debian.org/doc/devel-manuals#debmake-doc.
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| Chapter 6. The Debian archives |  | Chapter 8. The Debian package management tools |