methods: read config in most to least specific order

[apt.git] / doc / guide.dbk

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<book lang="en">

<title>APT User's Guide</title>

<bookinfo>

<authorgroup>
  <author>
    <personname>Jason Gunthorpe</personname><email>jgg@debian.org</email>
  </author>
</authorgroup>

<releaseinfo>Version &apt-product-version;</releaseinfo>

<abstract>
<para>
This document provides an overview of how to use the the APT package manager.
</para>
</abstract>

<copyright><year>1998</year><holder>Jason Gunthorpe</holder></copyright>

<legalnotice>
<title>License Notice</title>
<para>
"APT" and this document are free software; you can redistribute them and/or
modify them under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
</para>
</legalnotice>

<legalnotice>
<para>
For more details, on Debian systems, see the file
/usr/share/common-licenses/GPL for the full license.
</para>
</legalnotice>

</bookinfo>

<chapter id="ch1"><title>General</title>
<para>
The APT package currently contains two sections, the APT
<command>dselect</command> method and the <command>apt-get</command> command
line user interface. Both provide a way to install and remove packages as well
as download new packages from the Internet.
</para>

<section id="s1.1"><title>Anatomy of the Package System</title>
<para>
The Debian packaging system has a large amount of information associated with
each package to help assure that it integrates cleanly and easily into the
system. The most prominent of its features is the dependency system.
</para>
<para>
The dependency system allows individual programs to make use of shared elements
in the system such as libraries. It simplifies placing infrequently used
portions of a program in separate packages to reduce the number of things the
average user is required to install. Also, it allows for choices in mail
transport agents, X servers and so on.
</para>
<para>
The first step to understanding the dependency system is to grasp the concept
of a simple dependency. The meaning of a simple dependency is that a package
requires another package to be installed at the same time to work properly.
</para>
<para>
For instance, mailcrypt is an emacs extension that aids in encrypting email
with GPG. Without GPGP installed mailcrypt is useless, so mailcrypt has a
simple dependency on GPG. Also, because it is an emacs extension it has a
simple dependency on emacs, without emacs it is completely useless.
</para>
<para>
The other important dependency to understand is a conflicting dependency. It
means that a package, when installed with another package, will not work and
may possibly be extremely harmful to the system. As an example consider a mail
transport agent such as sendmail, exim or qmail. It is not possible to have
two mail transport agents installed because both need to listen to the network
to receive mail. Attempting to install two will seriously damage the system so
all mail transport agents have a conflicting dependency with all other mail
transport agents.
</para>
<para>
As an added complication there is the possibility for a package to pretend to
be another package. Consider that exim and sendmail for many intents are
identical, they both deliver mail and understand a common interface. Hence,
the package system has a way for them to declare that they are both
mail-transport-agents. So, exim and sendmail both declare that they provide a
mail-transport-agent and other packages that need a mail transport agent depend
on mail-transport-agent. This can add a great deal of confusion when trying to
manually fix packages.
</para>
<para>
At any given time a single dependency may be met by packages that are already
installed or it may not be. APT attempts to help resolve dependency issues by
providing a number of automatic algorithms that help in selecting packages for
installation.
</para>
</section>

</chapter>

<chapter id="ch2"><title>apt-get</title>
<para>
<command>apt-get</command> provides a simple way to install packages from the
command line. Unlike <command>dpkg</command>, <command>apt-get</command> does
not understand .deb files, it works with the package's proper name and can only
install .deb archives from a <emphasis>Source</emphasis>.
</para>
<para>
The first <footnote><para> If you are using an http proxy server you must set
the http_proxy environment variable first, see sources.list(5) </para>
</footnote> thing that should be done before using <command>apt-get</command>
is to fetch the package lists from the <emphasis>Sources</emphasis> so that it
knows what packages are available. This is done with <literal>apt-get
update</literal>. For instance,
</para>
<screen>
# apt-get update
Get http://ftp.de.debian.org/debian-non-US/ stable/binary-i386/ Packages
Get http://llug.sep.bnl.gov/debian/ testing/contrib Packages
Reading Package Lists... Done
Building Dependency Tree... Done
</screen>
<para>
Once updated there are several commands that can be used:
</para>
<variablelist>
<varlistentry>
<term>upgrade</term>
<listitem>
<para>
Upgrade will attempt to gently upgrade the whole system. Upgrade will never
install a new package or remove an existing package, nor will it ever upgrade a
package that might cause some other package to break. This can be used daily
to relatively safely upgrade the system. Upgrade will list all of the packages
that it could not upgrade, this usually means that they depend on new packages
or conflict with some other package. <command>dselect</command> or
<literal>apt-get install</literal> can be used to force these packages to
install.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>install</term>
<listitem>
<para>
Install is used to install packages by name. The package is automatically
fetched and installed. This can be useful if you already know the name of the
package to install and do not want to go into a GUI to select it. Any number
of packages may be passed to install, they will all be fetched. Install
automatically attempts to resolve dependency problems with the listed packages
and will print a summary and ask for confirmation if anything other than its
arguments are changed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>dist-upgrade</term>
<listitem>
<para>
Dist-upgrade is a complete upgrader designed to simplify upgrading between
releases of Debian. It uses a sophisticated algorithm to determine the best
set of packages to install, upgrade and remove to get as much of the system to
the newest release. In some situations it may be desired to use dist-upgrade
rather than spend the time manually resolving dependencies in
<command>dselect</command>. Once dist-upgrade has completed then
<command>dselect</command> can be used to install any packages that may have
been left out.
</para>
<para>
It is important to closely look at what dist-upgrade is going to do, its
decisions may sometimes be quite surprising.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
<command>apt-get</command> has several command line options that are detailed
in its man page,
<citerefentry><refentrytitle>apt-get</refentrytitle><manvolnum>8</manvolnum></citerefentry>. The
most useful option is <literal>-d</literal> which does not install the
fetched files. If the system has to download a large number of package it
would be undesired to start installing them in case something goes wrong. When
<literal>-d</literal> is used the downloaded archives can be installed by
simply running the command that caused them to be downloaded again without
<literal>-d</literal>.
</para>
</chapter>

<chapter id="ch3"><title>DSelect</title>
<para>
The APT <command>dselect</command> method provides the complete
APT system with the <command>dselect</command> package selection
GUI. <command>dselect</command> is used to select the packages to be
installed or removed and APT actually installs them.
</para>
<para>
To enable the APT method you need to select [A]ccess in
<command>dselect</command> and then choose the APT method. You will be
prompted for a set of <emphasis>Sources</emphasis> which are places to fetch
archives from. These can be remote Internet sites, local Debian mirrors or
CD-ROMs. Each source can provide a fragment of the total Debian archive, APT
will automatically combine them to form a complete set of packages. If you
have a CD-ROM then it is a good idea to specify it first and then specify a
mirror so that you have access to the latest bug fixes. APT will automatically
use packages on your CD-ROM before downloading from the Internet.
</para>
<screen>
   Set up a list of distribution source locations

 Please give the base URL of the debian distribution.
 The access schemes I know about are: http file

 For example:
      file:/mnt/debian,
      ftp://ftp.debian.org/debian,
      http://ftp.de.debian.org/debian,


 URL [http://llug.sep.bnl.gov/debian]:
</screen>
<para>
The <emphasis>Sources</emphasis> setup starts by asking for the base of the
Debian archive, defaulting to a HTTP mirror. Next it asks for the distribution
to get.
</para>
<screen>
 Please give the distribution tag to get or a path to the
 package file ending in a /. The distribution
 tags are typically something like: stable unstable testing non-US

 Distribution [stable]:
</screen>
<para>
The distribution refers to the Debian version in the archive,
<emphasis>stable</emphasis> refers to the latest released version
and <emphasis>unstable</emphasis> refers to the developmental
version. <emphasis>non-US</emphasis> is only available on some mirrors
and refers to packages that contain encryption technology or other
things that cannot be exported from the United States. Importing these
packages into the US is legal however.
</para>
<screen>
 Please give the components to get
 The components are typically something like: main contrib non-free

 Components [main contrib non-free]:
</screen>
<para>
The components list refers to the list of sub distributions to fetch. The
distribution is split up based on software licenses, main being DFSG free
packages while contrib and non-free contain things that have various
restrictions placed on their use and distribution.
</para>
<para>
Any number of sources can be added, the setup script will continue to prompt
until you have specified all that you want.
</para>
<para>
Before starting to use <command>dselect</command> it is necessary to update
the available list by selecting [U]pdate from the menu. This is a superset of
<literal>apt-get update</literal> that makes the fetched information available
to <command>dselect</command>. [U]pdate must be performed even if
<literal>apt-get update</literal> has been run before.
</para>
<para>
You can then go on and make your selections using [S]elect and then perform
the installation using [I]nstall. When using the APT method the [C]onfig and
[R]emove commands have no meaning, the [I]nstall command performs both of
them together.
</para>
<para>
By default APT will automatically remove the package (.deb) files once they
have been successfully installed. To change this behavior place
<literal>Dselect::clean "prompt";</literal> in /etc/apt/apt.conf.
</para>
</chapter>

<chapter id="ch4"><title>The Interface</title>
<para>
Both that APT <command>dselect</command> method and <command>apt-get</command>
share the same interface. It is a simple system that generally tells you what
it will do and then goes and does it. <footnote><para> The
<command>dselect</command> method actually is a set of wrapper scripts to
<command>apt-get</command>. The method actually provides more functionality
than is present in <command>apt-get</command> alone. </para> </footnote> After
printing out a summary of what will happen APT then will print out some
informative status messages so that you can estimate how far along it is and
how much is left to do.
</para>

<section id="s4.1"><title>Startup</title>
<para>
Before all operations except update, APT performs a number of actions
to prepare its internal state. It also does some checks of the system's
state. At any time these operations can be performed by running
<literal>apt-get check</literal>.
</para>
<screen>
# apt-get check
Reading Package Lists... Done
Building Dependency Tree... Done
</screen>
<para>
The first thing it does is read all the package files into memory. APT uses a
caching scheme so this operation will be faster the second time it is run. If
some of the package files are not found then they will be ignored and a
warning will be printed when apt-get exits.
</para>
<para>
The final operation performs a detailed analysis of the system's
dependencies. It checks every dependency of every installed or unpacked
package and considers if it is OK. Should this find a problem then a report
will be printed out and <command>apt-get</command> will refuse to run.
</para>
<screen>
# apt-get check
Reading Package Lists... Done
Building Dependency Tree... Done
You might want to run apt-get -f install' to correct these.
Sorry, but the following packages have unmet dependencies:
  9fonts: Depends: xlib6g but it is not installed
  uucp: Depends: mailx but it is not installed
  blast: Depends: xlib6g (&gt;= 3.3-5) but it is not installed
  adduser: Depends: perl-base but it is not installed
  aumix: Depends: libgpmg1 but it is not installed
  debiandoc-sgml: Depends: sgml-base but it is not installed
  bash-builtins: Depends: bash (&gt;= 2.01) but 2.0-3 is installed
  cthugha: Depends: svgalibg1 but it is not installed
           Depends: xlib6g (&gt;= 3.3-5) but it is not installed
  libreadlineg2: Conflicts:libreadline2 (&lt;&lt; 2.1-2.1)
</screen>
<para>
In this example the system has many problems, including a serious problem with
libreadlineg2. For each package that has unmet dependencies a line is printed
out indicating the package with the problem and the dependencies that are
unmet. A short explanation of why the package has a dependency problem is also
included.
</para>
<para>
There are two ways a system can get into a broken state like this. The
first is caused by <command>dpkg</command> missing some subtle relationships
between packages when performing upgrades. <footnote><para> APT however
considers all known dependencies and attempts to prevent broken
packages </para> </footnote>. The second is if a package installation
fails during an operation. In this situation a package may have been
unpacked without its dependents being installed.
</para>
<para>
The second situation is much less serious than the first because APT places
certain constraints on the order that packages are installed. In both cases
supplying the <literal>-f</literal> option to <command>apt-get</command>
will cause APT to deduce a possible solution to the problem and then
continue on. The APT <command>dselect</command> method always supplies
the <literal>-f</literal> option to allow for easy continuation of failed
maintainer scripts.
</para>
<para>
However, if the <literal>-f</literal> option is used to correct a seriously
broken system caused by the first case then it is possible that it will either
fail immediately or the installation sequence will fail. In either case it is
necessary to manually use dpkg (possibly with forcing options) to correct the
situation enough to allow APT to proceed.
</para>
</section>

<section id="s4.2"><title>The Status Report</title>
<para>
Before proceeding <command>apt-get</command> will present a report on what will
happen. Generally the report reflects the type of operation being performed
but there are several common elements. In all cases the lists reflect the
final state of things, taking into account the <literal>-f</literal> option
and any other relevant activities to the command being executed.
</para>

<section id="s4.2.1"><title>The Extra Package list</title>
<screen>
The following extra packages will be installed:
  libdbd-mysql-perl xlib6 zlib1 xzx libreadline2 libdbd-msql-perl
  mailpgp xdpkg fileutils pinepgp zlib1g xlib6g perl-base
  bin86 libgdbm1 libgdbmg1 quake-lib gmp2 bcc xbuffy
  squake pgp-i python-base debmake ldso perl libreadlineg2
  ssh
</screen>
<para>
The Extra Package list shows all of the packages that will be installed or
upgraded in excess of the ones mentioned on the command line. It is only
generated for an <literal>install</literal> command. The listed packages are
often the result of an Auto Install.
</para>
</section>

<section id="s4.2.2"><title>The Packages to Remove</title>
<screen>
The following packages will be REMOVED:
  xlib6-dev xpat2 tk40-dev xkeycaps xbattle xonix
  xdaliclock tk40 tk41 xforms0.86 ghostview xloadimage xcolorsel
  xadmin xboard perl-debug tkined xtetris libreadline2-dev perl-suid
  nas xpilot xfig
</screen>
<para>
The Packages to Remove list shows all of the packages that will be removed
from the system. It can be shown for any of the operations and should be given
a careful inspection to ensure nothing important is to be taken off. The
<literal>-f</literal> option is especially good at generating packages to
remove so extreme care should be used in that case. The list may contain
packages that are going to be removed because they are only partially
installed, possibly due to an aborted installation.
</para>
</section>

<section id="s4.2.3"><title>The New Packages list</title>
<screen>
The following NEW packages will installed:
  zlib1g xlib6g perl-base libgdbmg1 quake-lib gmp2 pgp-i python-base
</screen>
<para>
The New Packages list is simply a reminder of what will happen. The packages
listed are not presently installed in the system but will be when APT is done.
</para>
</section>

<section id="s4.2.4"><title>The Kept Back list</title>
<screen>
The following packages have been kept back
  compface man-db tetex-base msql libpaper svgalib1
  gs snmp arena lynx xpat2 groff xscreensaver
</screen>
<para>
Whenever the whole system is being upgraded there is the possibility that new
versions of packages cannot be installed because they require new things or
conflict with already installed things. In this case the package will appear
in the Kept Back list. The best way to convince packages listed there to
install is with <literal>apt-get install</literal> or by using
<command>dselect</command> to resolve their problems.
</para>
</section>

<section id="s4.2.5"><title>Held Packages warning</title>
<screen>
The following held packages will be changed:
  cvs
</screen>
<para>
Sometimes you can ask APT to install a package that is on hold, in such a case
it prints out a warning that the held package is going to be changed. This
should only happen during dist-upgrade or install.
</para>
</section>

<section id="s4.2.6"><title>Final summary</title>
<para>
Finally, APT will print out a summary of all the changes that will occur.
</para>
<screen>
206 packages upgraded, 8 newly installed, 23 to remove and 51 not upgraded.
12 packages not fully installed or removed.
Need to get 65.7M/66.7M of archives. After unpacking 26.5M will be used.
</screen>
<para>
The first line of the summary simply is a reduced version of all of the lists
and includes the number of upgrades - that is packages already installed that
have new versions available. The second line indicates the number of poorly
configured packages, possibly the result of an aborted installation. The final
line shows the space requirements that the installation needs. The first pair
of numbers refer to the size of the archive files. The first number indicates
the number of bytes that must be fetched from remote locations and the second
indicates the total size of all the archives required. The next number
indicates the size difference between the presently installed packages and the
newly installed packages. It is roughly equivalent to the space required in
/usr after everything is done. If a large number of packages are being removed
then the value may indicate the amount of space that will be freed.
</para>
<para>
Some other reports can be generated by using the -u option to show packages to
upgrade, they are similar to the previous examples.
</para>
</section>

</section>

<section id="s4.3"><title>The Status Display</title>
<para>
During the download of archives and package files APT prints out a series of
status messages.
</para>
<screen>
# apt-get update
Get:1 http://ftp.de.debian.org/debian-non-US/ stable/non-US/ Packages
Get:2 http://llug.sep.bnl.gov/debian/ testing/contrib Packages
Hit http://llug.sep.bnl.gov/debian/ testing/main Packages
Get:4 http://ftp.de.debian.org/debian-non-US/ unstable/binary-i386/ Packages
Get:5 http://llug.sep.bnl.gov/debian/ testing/non-free Packages
11% [5 testing/non-free `Waiting for file' 0/32.1k 0%] 2203b/s 1m52s
</screen>
<para>
The lines starting with <emphasis>Get</emphasis> are printed out when APT
begins to fetch a file while the last line indicates the progress of the
download. The first percent value on the progress line indicates the total
percent done of all files. Unfortunately since the size of the Package files
is unknown <literal>apt-get update</literal> estimates the percent done which
causes some inaccuracies.
</para>
<para>
The next section of the status line is repeated once for each download
thread and indicates the operation being performed and some useful
information about what is happening. Sometimes this section will simply
read <emphasis>Forking</emphasis> which means the OS is loading the download
module. The first word after the [ is the fetch number as shown on the
history lines. The next word is the short form name of the object being
downloaded. For archives it will contain the name of the package that is
being fetched.
</para>
<para>
Inside of the single quote is an informative string indicating the progress of
the negotiation phase of the download. Typically it progresses from
<emphasis>Connecting</emphasis> to <emphasis>Waiting for file</emphasis> to
<emphasis>Downloading</emphasis> or <emphasis>Resuming</emphasis>. The final
value is the number of bytes downloaded from the remote site. Once the
download begins this is represented as <literal>102/10.2k</literal> indicating
that 102 bytes have been fetched and 10.2 kilobytes is expected. The total
size is always shown in 4 figure notation to preserve space. After the size
display is a percent meter for the file itself. The second last element is the
instantaneous average speed. This values is updated every 5 seconds and
reflects the rate of data transfer for that period. Finally is shown the
estimated transfer time. This is updated regularly and reflects the time to
complete everything at the shown transfer rate.
</para>
<para>
The status display updates every half second to provide a constant feedback on
the download progress while the Get lines scroll back whenever a new file is
started. Since the status display is constantly updated it is unsuitable for
logging to a file, use the <literal>-q</literal> option to remove the status
display.
</para>
</section>

<section id="s4.4"><title>Dpkg</title>
<para>
APT uses <command>dpkg</command> for installing the archives and will
switch over to the <command>dpkg</command> interface once downloading is
completed. <command>dpkg</command> will also ask a number of questions as
it processes the packages and the packages themselves may also ask several
questions. Before each question there is usually a description of what it
is asking and the questions are too varied to discuss completely here.
</para>
</section>

</chapter>

</book>