doc/apt_preferences.5.sgml

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   9 <refentry>
  10  &apt-docinfo;
  11
  12  <refmeta>
  13    <refentrytitle>apt_preferences</>
  14    <manvolnum>5</>
  15  </refmeta>
  16
  17  <!-- Man page title -->
  18  <refnamediv>
  19     <refname>apt_preferences</>
  20     <refpurpose>Preference control file for APT</>
  21  </refnamediv>
  22
  23  <RefSect1><Title>Description</>
  24    <para>
  25    The APT preferences file controls various aspects of the APT system.
  26    It is ment to be user editable and manipulatable from software. The file
  27    consists of a number of records formed like the dpkg status file, space
  28    seperated sections of text with at the start of each line tags seperated
  29    by a colon. It is stored in <filename>/etc/apt/preferences</>.
  30  </RefSect1>
  31
  32  <RefSect1><Title>Versioning</>
  33    <para>
  34    One purpose of the preferences file is to let the user select which version
  35    of a package will be installed. This selection can be made in a number of
  36    ways that fall into three categories, version, release and origin.
  37    <para>
  38    Selection by version can be done by exact match or prefix match. The format
  39    is <literal/2.1.2/ or <literal/2.2*/ for a prefix match. Matching by prefix
  40    can be used to ignore the <literal/r/ in the Debian release versioning, like
  41    <literal/2.1r*/ or to ignore Debian specific revisions, <literal/1.1-*/.
  42    When matching versions with a prefix the highest matching version will
  43    always be picked.
  44    <para>
  45    Selection by release is more complicated and has three forms. The primary
  46    purpose of release selections is to identify a set of packages that match
  47    a specific vendor, or release (ie Debian 2.1). The first two forms are
  48    shortcuts intended for quick command line use. If the first character of the
  49    specification is a digit then it is considered to be a release version match,
  50    otherwise a release label match. Specifications which contain equals are
  51    full release data matches and are a comma seperated list of one letter keys
  52    followed by an equals then by the string. Examples:
  53 <informalexample><programlisting>
  54 v=2.1*,o=Debian,c=main
  55 l=Debian
  56 a=stable
  57 </programlisting></informalexample>
  58    <para>
  59    The data for these matches are taken from the <filename/Release/ files
  60    that APT downloads during an <literal/update/. The available keys are:
  61    <VariableList>
  62      <VarListEntry><term>a= Archive</term>
  63      <ListItem><Para>
  64      This is the common name we give our archives, such as <literal/stable/ or
  65      <literal/unstable/. The special name <literal/now/ is used to designate
  66      the set of packages that are currently installed.
  67      </VarListEntry>
  68
  69      <VarListEntry><term>c= Component</term>
  70      <ListItem><Para>
  71      Referes to the sub-component of the archive, <literal/main/,
  72      <literal/contrib/ etc. Component may be omitted if there are no
  73      components for this archive.
  74      </VarListEntry>
  75
  76      <VarListEntry><term>v= Version</term>
  77      <ListItem><Para>
  78      This is a version string with the same properties as in the Packages file.
  79      It represents the release level of the archive. Typical Debian release
  80      numbers look like <literal/2.1r2/ with the r designating the release of
  81      2.1. New releases are limited to security updates.
  82      </VarListEntry>
  83
  84      <VarListEntry><term>o= Origin</term>
  85      <ListItem><Para>
  86      This specifies who is providing this archive. In the case of Debian the
  87      string will read <literal/Debian/. Other providers may use their own
  88      string.
  89      </VarListEntry>
  90
  91      <VarListEntry><term><term>l= Label</term>
  92      <ListItem><Para>
  93      This carries the encompassing name of the distribution. For Debian proper
  94      this field reads <literal/Debian/. For derived distributions it should
  95      contain their proper name.
  96      </VarListEntry>
  97    </VariableList>
  98    <para>
  99    The final selection method is by origin. This is simply the site name
 100    of the originating package files. The empty string is used for file URIs.
 101    <para>
 102    Version selection, particularly the latter two methods, are used in may
 103    different part of APT, not just the preferences file.
 104  </RefSect1>
 105
 106  <RefSect1><Title>Candidate Version Policy</>
 107    <para>
 108    Interaly APT maintains a list of all available versions for all packages.
 109    If you place multiple releases or vendors in your &sources-list; file then
 110    these features are available. By default APT selects the highest version
 111    from all automatic sources. Some sources, such as
 112    <filename>project/experimental</> are marked Not Automatic - these fall
 113    to the bottom of the selection pile.
 114    <para>
 115    When deciding what version to use APT assigns a priority to each available
 116    version of the package. It then does two things, first it selects
 117    the highest priorty version that is newer than the installed version of the
 118    package, then it selects the highest priority version that is older than
 119    the installed version. Next, if the older versions have a priority greater
 120    than 1000 they are compared with the priority of the upgrade set, the larger
 121    becomes the selected result. Otherwise the downgrade versions are ignored
 122    and the highest priority of the ugprade set is selected.
 123    <para>
 124    It is possible to think of the priorities in strata:
 125    <VariableList>
 126      <VarListEntry><term>1000 and up</term>
 127      <ListItem><Para>
 128      Downgradable priorities
 129      </VarListEntry>
 130
 131      <VarListEntry><term>1000</term>
 132      <ListItem><Para>
 133      The downgrade prevention barrier
 134      </VarListEntry>
 135
 136      <VarListEntry><term>100 to 1000</term>
 137      <ListItem><Para>
 138      Standard priorities. 990 is the priority set by the
 139      <option/--target-release / &apt-get; option. 989 is the start for auto
 140      priorities and 500 are all the default package files.
 141      </VarListEntry>
 142
 143      <VarListEntry><term>100</term>
 144      <ListItem><Para>
 145      The currently installed version
 146      </VarListEntry>
 147
 148      <VarListEntry><term>0 to 100</term>
 149      <ListItem><Para>
 150      Non automatic priorities. These are only used if the package
 151      is not installed and there is no other version available.
 152      </VarListEntry>
 153
 154      <VarListEntry><term>less than 0</term>
 155      <ListItem><Para>
 156      The version is never selected.
 157      </VarListEntry>
 158    </VariableList>
 159    <para>
 160    Giving a pin a priority greater than 1000 will allow APT to downgrade
 161    in order to get to that version.
 162    <para>
 163    Each package may be pinned to a specific version and each Package file
 164    has a priority for every package inside. The highest priority assigned
 165    to a package is the one that is used.
 166    <para>
 167    A package pin looks like this:
 168 <informalexample><programlisting>
 169 Package: apt
 170 Pin: version 0.4.0
 171 Pin-Priority: 1001
 172 </programlisting></informalexample>
 173    The first line specifies the package, the second gives the Pin specification
 174    and the last gives the priority of this pin. The first word of the pin
 175    specification may be version, release or origin, the remainder of the field
 176    is described in the Versioning sectin above.
 177    <para>
 178    A default pin is how the priorities of package files are set. Any number
 179    of default pins may be specified, the first matching default will select
 180    the priority of the package file. Only release or origin may be used in
 181    the Pin specification since they match Package files.
 182 <informalexample><programlisting>
 183 Package: *
 184 Pin: release v=2.1*
 185 Pin-Priority: 998
 186 </programlisting></informalexample>
 187    <para>
 188    If the Pin-Priorty field is omitted then the priority defaults to 989 for
 189    both cases.
 190
 191    <RefSect2><title>Interesting Effects</>
 192    <para>
 193    Due to the downgrade prevention barrier at priority 1000 it is possible
 194    that a lower priority version  will be selected if the higher priority
 195    would casue a downgrade. For instance, if package foo has versions
 196    <literal/1.2/, <literal/1.1/ and <literal/1.0/ installed, with
 197    <literal/1.1/ being the currently installed version and the priorities of
 198    each version being 900, 100 and 950 repectively the winning version will be
 199    <literal/1.2/.
 200    <para>
 201    In practice this is often desired. A user may use a default pin to
 202    make the stable distribution the default and then use the
 203    <option/--target-dist/ option with &apt-get; to select newer versions
 204    from unstable. The packages that have been upgraded to unstable will
 205    continue to follow the versions that are available in unstable since
 206    the stable versions now fall below the downgrade prevention barrier.
 207    <para>
 208    If this is not desired then a default pin should be used to make unstable
 209    have a priority less than 100.
 210    <para>
 211    Users of 3rd party add ons such as Helix GNOME can use this mechanism to
 212    force the usage of Helix packages, or force the usage of Debian packages
 213    by setting the priority of that source sufficiently high. It is even
 214    possible to mass downgrade from one set of packages to another by
 215    using a priority larger than 1000.
 216    </RefSect2>
 217  </RefSect1>
 218
 219  <RefSect1><Title>See Also</>
 220    <para>
 221    &apt-cache; &apt-conf;
 222  </RefSect1>
 223
 224  &manbugs;
 225  &manauthor;
 226
 227 </refentry>