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 <refmeta>
   <refentrytitle>apt_preferences</>
   <manvolnum>5</>
 </refmeta>

 <!-- Man page title -->
 <refnamediv>
    <refname>apt_preferences</>
    <refpurpose>Preference control file for APT</>
 </refnamediv>

<RefSect1>
<Title>Description</Title>
<para>
The APT preferences file <filename>/etc/apt/preferences</>
can be used to control which versions of packages will be selected
for installation.
</para>

<para>
Several versions of a package may be available for installation when
the &sources-list; file contains references to more than one distribution
(for example, <literal>stable</literal> and <literal>testing</literal>).
APT assigns a priority to each version that is available.
Subject to dependency constraints, <command>apt-get</command> selects the
version with the highest priority for installation.
The APT preferences file overrides the priorities that APT assigns to
package versions by default, thus giving the user control over which
one is selected for installation.
</para>
<para>
Several instances of the same version of a package may be available when
the &sources-list; file contains references to more than one source.
In this case <command>apt-get</command> downloads the instance listed
earliest in the &sources-list; file.
The APT preferences file does not affect the choice of instance, only
the choice of version.
</para>

<RefSect2><Title>APT's Default Priority Assignments</>

<para>
If there is no preferences file or if there is no entry in the file
that applies to a particular version then the priority assigned to that
version is the priority of the distribution to which that version
belongs.  It is possible to single out a distribution, "the target release",
which receives a higher priority than other distributions do by default.
The target release can be set on the <command>apt-get</command> command
line or in the APT configuration file <filename>/etc/apt/apt.conf</filename>.
For example,
<programlisting>
<command>apt-get install -t testing <replaceable>some-package</replaceable></command>
</programlisting>
<programlisting>
APT::Default-Release "stable";
</programlisting>
</para>

<para>
If the target release has been specified then APT uses the following
algorithm to set the priorities of the versions of a package.  Assign:
<variablelist>
<varlistentry>
<term>priority 100</term>
<listitem><simpara>to the version that is already installed (if any).</simpara></listitem>
</varlistentry>
<varlistentry>
<term>priority 500</term>
<listitem><simpara>to the versions that are not installed and do not belong to the target release.</simpara></listitem>
</varlistentry>
<varlistentry>
<term>priority 990</term>
<listitem><simpara>to the versions that are not installed and belong to the target release.</simpara></listitem>
</varlistentry>
</variablelist>
</para>

<para>
If the target release has not been specified then APT simply assigns
priority 100 to all installed package versions and priority 500 to all
uninstalled package versions.
</para>

<para>
APT then applies the following rules, listed in order of precedence,
to determine which version of a package to install.
<itemizedlist>
<listitem><simpara>Never downgrade unless the priority of an available
version exceeds 1000.  ("Downgrading" is installing a less recent version
of a package in place of a more recent version.  Note that none of APT's
default priorities exceeds 1000; such high priorities can only be set in
the preferences file.  Note also that downgrading a package
can be risky.)</simpara></listitem>
<listitem><simpara>Install the highest priority version.</simpara></listitem>
<listitem><simpara>If two or more versions have the same priority,
install the most recent one (that is, the one with the higher version
number).</simpara></listitem>
<listitem><simpara>If two or more versions have the same priority and
version number but either the packages differ in some of their metadata or the
<literal/--reinstall/ option is given, install the uninstalled one.</simpara></listitem>
</itemizedlist>
</para>

<para>
In a typical situation, the installed version of a package (priority 100)
is not as recent as one of the versions available from the sources listed in
the &sources-list; file (priority 500 or 990).  Then the package will be upgraded
when <command>apt-get install <replaceable>some-package</replaceable></command>
or <command>apt-get upgrade</command> is executed.
</para>

<para>
More rarely, the installed version of a package is <emphasis/more/ recent
than any of the other available versions.  The package will not be downgraded
when <command>apt-get install <replaceable>some-package</replaceable></command>
or <command>apt-get upgrade</command> is executed.
</para>

<para>
Sometimes the installed version of a package is more recent than the
version belonging to the target release, but not as recent as a version
belonging to some other distribution.  Such a package will indeed be upgraded
when <command>apt-get install <replaceable>some-package</replaceable></command>
or <command>apt-get upgrade</command> is executed,
because at least <emphasis/one/ of the available versions has a higher
priority than the installed version.
</para>

</RefSect2>

<RefSect2><Title>The Effect of APT Preferences</>

<para>
The APT preferences file allows the system administrator to control the
assignment of priorities.  The file consists of one or more multi-line records
separated by blank lines.  Records can have one of two forms, a specific form
and a general form.
<itemizedlist>
<listitem>
<simpara>
The specific form assigns a priority (a "Pin-Priority") to a
specified package and specified version or version range.  For example,
the following record assigns a high priority to all versions of
the <filename/perl/ package whose version number begins with "<literal/5.8/".
</simpara>

<programlisting>
Package: perl
Pin: version 5.8*
Pin-Priority: 1001
</programlisting>
</listitem>

<listitem><simpara>
The general form assigns a priority to all of the package versions in a
given distribution (that is, to all the versions of packages that are
listed in a certain <filename/Release/ file) or to all of the package
versions coming from a particular Internet site, as identified by the
site's fully qualified domain name.
</simpara>

<simpara>
This general-form entry in the APT preferences file applies only
to groups of packages.  For example, the following record assigns a high
priority to all package versions available from the local site.
</simpara>

<programlisting>
Package: *
Pin: origin ""
Pin-Priority: 999
</programlisting>

<simpara>
A note of caution: the keyword used here is "<literal/origin/".
This should not be confused with the Origin of a distribution as
specified in a <filename/Release/ file.  What follows the "Origin:" tag
in a <filename/Release/ file is not an Internet address
but an author or vendor name, such as "Debian" or "Ximian".
</simpara>

<simpara>
The following record assigns a low priority to all package versions
belonging to any distribution whose Archive name is "<literal/unstable/".
</simpara>

<programlisting>
Package: *
Pin: release a=unstable
Pin-Priority: 50
</programlisting>

<simpara>
The following record assigns a high priority to all package versions
belonging to any release whose Archive name is "<literal/stable/"
and whose release Version number is "<literal/3.0/".
</simpara>

<programlisting>
Package: *
Pin: release a=unstable, v=3.0
Pin-Priority: 50
</programlisting>
</listitem>
</itemizedlist>
</para>

</RefSect2>

<RefSect2>
<Title>How APT Interprets Priorities</Title>

<para>
Priorities (P) assigned in the APT preferences file must be positive
or negative integers.  They are interpreted as follows (roughly speaking):

<variablelist>
<varlistentry>
<term>P &gt; 1000</term>
<listitem><simpara>causes a version to be installed even if this
constitutes a downgrade of the package</simpara></listitem>
</varlistentry>
<varlistentry>
<term>990 &lt; P &lt;=1000</term>
<listitem><simpara>causes a version to be installed
even if it does not come from the target release,
unless the installed version is more recent</simpara></listitem>
</varlistentry>
<varlistentry>
<term>500 &lt; P &lt;=990</term>
<listitem><simpara>causes a version to be installed
unless there is a version available belonging to the target release
or the installed version is more recent</simpara></listitem>
</varlistentry>
<varlistentry>
<term>100 &lt; P &lt;=500</term>
<listitem><simpara>causes a version to be installed
unless there is a version available belonging to some other
distribution or the installed version is more recent</simpara></listitem>
<varlistentry>
<term>0 &lt; P &lt;=100</term>
<listitem><simpara>causes a version to be installed
only if there is no installed version of the package</simpara></listitem>
</varlistentry>
<varlistentry>
<term>P &lt; 0</term>
<listitem><simpara>prevents the version from being installed</simpara></listitem>
</varlistentry>
</variablelist>
</para>

<para>
If any specific-form records match an available package version then the
first such record determines the priority of the package version.  
Failing that,
if any general-form records match an available package version then the
first such record determines the priority of the package version.  
</para>

<para>
For example, suppose the APT preferences file contains the three
records presented earlier:

<programlisting>
Package: perl
Pin: version 5.8*
Pin-Priority: 1001

Package: *
Pin: origin ""
Pin-Priority: 999

Package: *
Pin: release unstable
Pin-Priority: 50
</programlisting>

Then:

<itemizedlist>
<listitem><simpara>The most recent available version of the <literal/perl/
package will be installed, so long as that version's version number begins
with "<literal/5.8/".  If <emphasis/any/ 5.8* version of <literal/perl/ is
available and the installed version is 5.9*, then <literal/perl/ will be
downgraded.</simpara></listitem>
<listitem><simpara>A version of any package other than <literal/perl/
that is available from the local system has priority over other versions,
even versions belonging to the target release.
</simpara></listitem>
<listitem><simpara>A version of a package whose origin is not the local
system but some other site listed in &sources-list; and which belongs to
an <literal/unstable/ distribution is only installed if it is selected
for installation and no version of the package is already installed.
</simpara></listitem>
</itemizedlist>
</para>
</RefSect2>

<RefSect2>
<Title>Determination of Package Version and Distribution Properties</Title>

<para>
The locations listed in the &sources-list; file should provide
<filename>Packages</filename> and <filename>Release</filename> files
to describe the packages available at that location.
</para>

<para>
The <filename>Packages</filename> file is normally found in the directory
<filename>.../dists/<replaceable>dist-name</replaceable>/<replaceable>component</replaceable>/<replaceable>arch</replaceable></filename>:
for example, <filename>.../dists/stable/main/binary-i386/Packages</filename>.
It consists of a series of multi-line records, one for each package available
in that directory.  Only two lines in each record are relevant for setting
APT priorities:
<variablelist>
<varlistentry>
<term>the <literal/Package:/ line</term>
<listitem><simpara>gives the package name</simpara></listitem>
</varlistentry>
<varlistentry>
<term>the <literal/Version:/ line</term>
<listitem><simpara>gives the version number for the named package</simpara></listitem>
</varlistentry>
</variablelist>
</para>

<para>
The <filename>Release</filename> file is normally found in the directory
<filename>.../dists/<replaceable>dist-name</replaceable></filename>:
for example, <filename>.../dists/stable/Release</filename>,
or <filename>.../dists/woody/Release</filename>.
It consists of a single multi-line record which applies to <emphasis/all/ of
the packages in the directory tree below its parent.  Unlike the
<filename/Packages/ file, nearly all of the lines in a <filename/Release/
file are relevant for setting APT priorities:

<variablelist>
<varlistentry>
<term>the <literal/Archive:/ line</term>
<listitem><simpara>names the archive to which all the packages
in the directory tree belong.  For example, the line
"Archive: stable"
specifies that all of the packages in the directory
tree below the parent of the <filename/Release/ file are in a
<literal/stable/ archive.  Specifying this value in the APT preferences file
would require the line:
</simpara>
<programlisting>
Pin: release a=stable
</programlisting>
</listitem>
</varlistentry>

<varlistentry>
<term>the <literal/Version:/ line</term>
<listitem><simpara>names the release version.  For example, the
packages in the tree might belong to Debian GNU/Linux release
version 3.0.  Note that there is normally no version number for the
<literal/testing/ and <literal/unstable/ distributions because they
have not been released yet.  Specifying this in the APT preferences
file would require one of the following lines.
</simpara>

<programlisting>
Pin: release v=3.0
Pin: release a=stable v=3.0
Pin: release 3.0
</programlisting>

</listitem>
</varlistentry>

<varlistentry>
<term>the <literal/Component:/ line</term>
<listitem><simpara>names the licensing component associated with the
packages in the directory tree of the <filename/Release/ file.
For example, the line "Component: main" specifies that
all the packages in the directory tree are from the <literal/main/
component, which entails that they are licensed under terms listed
in the Debian Free Software Guidelines.  Specifying this component
in the APT preferences file would require the line:
</simpara>
<programlisting>
Pin: release c=main
</programlisting>
</listitem>
</varlistentry>

<varlistentry>
<term>the <literal/Origin:/ line</term>
<listitem><simpara>names the originator of the packages in the
directory tree of the <filename/Release/ file.  Most commonly, this is
<literal/Debian/.  Specifying this origin in the APT preferences file
would require the line:
</simpara>
<programlisting>
Pin: release o=Debian
</programlisting>
</listitem>
</varlistentry>

<varlistentry>
<term>the <literal/Label:/ line</term>
<listitem><simpara>names the label of the packages in the directory tree
of the <filename/Release/ file.  Most commonly, this is
<literal/Debian/.  Specifying this label in the APT preferences file
would require the line:
</simpara>
<programlisting>
Pin: release l=Debian
</programlisting>
</listitem>
</varlistentry>
</variablelist>
</para>

<para>
All of the <filename>Packages</filename> and <filename>Release</filename>
files retrieved from locations listed in the &sources-list; file are stored
in the directory <filename>/var/lib/apt/lists</filename>, or in the file named
by the variable <literal/Dir::State::Lists/ in the <filename/apt.conf/ file.
For example, the file
<filename>debian.lcs.mit.edu_debian_dists_unstable_contrib_binary-i386_Release</filename>
contains the <filename>Release</filename> file retrieved from the site
<literal/debian.lcs.mit.edu/ for <literal/binary-i386/ architecture
files from the <literal/contrib/ component of the <literal/unstable/
distribution.
</para>

</RefSect2>

<RefSect2>
<Title>Optional Lines in an APT Preferences Record</Title>

<para>
Each record in the APT preferences file can optionally begin with
one or more lines beginning with the word <literal/Explanation:/.
This provides a place for comments.
</para>

<para>
The <literal/Pin-Priority:/ line in each APT preferences record is
optional.  If omitted, APT assigs a priority of 1 less than the last value
specified on a line beginning with <literal/Pin-Priority: release .../.
</para>
</RefSect2>
</RefSect1>

<RefSect1>
<Title>Examples</Title>
<RefSect2>
<Title>Tracking Stable</Title>

<para>
The following APT preferences file will cause APT to assign a
priority higher than the default (500) to all package versions belonging
to a <literal/stable/ distribution and a prohibitively low priority to
package versions belonging to other <literal/Debian/ distributions.

<programlisting>
Explanation: Uninstall or do not install any Debian-originated
Explanation: package versions other than those in the stable distro
Package: *
Pin: release a=stable
Pin-Priority: 900

Package: *
Pin: release o=Debian
Pin-Priority: -10
</programlisting>
</para>

<para>
With a suitable &sources-list; file and the above preferences file,
any of the following commands will cause APT to upgrade to the
latest <literal/stable/ version(s).

<programlisting>
apt-get install <replaceable>package-name</replaceable>
apt-get upgrade
apt-get dist-upgrade
</programlisting>
</para>

<para>
The following command will cause APT to upgrade the specified
package to the latest version from the <literal/testing/ distribution;
the package will not be upgraded again unless this command is given
again.

<programlisting>
apt-get install <replaceable>package</replaceable>/testing
</programlisting>
</RefSect2>

 <RefSect2>
 <Title>Tracking Testing or Unstable</Title>

<para>
The following APT preferences file will cause APT to assign
a high priority to package versions from the <literal/testing/
distribution, a lower priority to package versions from the
<literal/unstable/ distribution, and a prohibitively low priority
to package versions from other <literal/Debian/ distributions.

<programlisting>
Package: *
Pin: release a=testing
Pin-Priority: 900

Package: *
Pin: release a=unstable
Pin-Priority: 800

Package: *
Pin: release o=Debian
Pin-Priority: -10
</programlisting>
</para>

<para>
With a suitable &sources-list; file and the above preferences file,
any of the following commands will cause APT to upgrade to the latest
<literal/testing/ version(s).

<programlisting>
apt-get install <replaceable>package-name</replaceable>
apt-get upgrade
apt-get dist-upgrade
</programlisting>
</para>

<para>The following command will cause APT to upgrade the specified
package to the latest version from the <literal/unstable/ distribution.
Thereafter, <command>apt-get upgrade</command> will upgrade
the package to the most recent <literal/testing/ version if that is
more recent than the installed version, otherwise, to the most recent
<literal/unstable/ version if that is more recent than the installed
version.

<programlisting>
apt-get install <replaceable>package</replaceable>/unstable
</programlisting>
</para>

</RefSect2>
</RefSect1>

<RefSect1>
<Title>See Also</Title>
<para>
&apt-get; &apt-cache; &apt-conf; &sources-list;
</para>
</RefSect1>

 &manbugs;
 &manauthor;

</refentry>