support setting empty values (sanely) & removing support for

[apt.git] / doc / files.dbk

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % aptent SYSTEM "apt.ent"> %aptent;
<!ENTITY % aptverbatiment SYSTEM "apt-verbatim.ent"> %aptverbatiment;
<!ENTITY % aptvendor SYSTEM "apt-vendor.ent"> %aptvendor;
]>

<book lang="en">

<title>APT Files</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 describes the complete implementation and format of the installed
APT directory structure. It also serves as guide to how APT views the Debian
archive.
</para>
</abstract>

<copyright><year>1998-1999</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>
<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>Introduction</title>

<section id="s1.1"><title>General</title>
<para>
This document serves two purposes. The first is to document the installed
directory structure and the format and purpose of each file. The second
purpose is to document how APT views the Debian archive and deals with multiple
package files.
</para>
<para>
The var directory structure is as follows:
</para>
<screen>
  /var/lib/apt/
                lists/
                       partial/
                periodic/
                extended_states
                cdroms.list
  /var/cache/apt/
                  archives/
                          partial/
                  pkgcache.bin
                  srcpkgcache.bin
  /etc/apt/
            sources.list.d/
            apt.conf.d/
            preferences.d/
            trusted.gpg.d/
            sources.list
            apt.conf
            apt_preferences
            trusted.gpg
  /usr/lib/apt/
                methods/
                         bzip2
                         cdrom
                         copy
                         file
                         ftp
                         gpgv
                         gzip
                         http
                         https
                         lzma
                         rred
                         rsh
                         ssh
</screen>
<para>
As is specified in the FHS 2.1 /var/lib/apt is used for application data that
is not expected to be user modified. /var/cache/apt is used for regeneratable
data and is where the package cache and downloaded .debs go. /etc/apt is the
place where configuration should happen and /usr/lib/apt is the place where the
apt and other packages can place binaries which can be used by the acquire
system of APT.
</para>
</section>

</chapter>

<chapter id="ch2"><title>Files</title>

<section id="s2.1"><title>Files and fragment directories in /etc/apt</title>
<para>
All files in /etc/apt are used to modify specific aspects of APT. To enable
other packages to ship needed configuration herself all these files have a
fragment directory packages can place their files in instead of mangling with
the main files. The main files are therefore considered to be only used by the
user and not by a package. The documentation omits this directories most of
the time to be easier readable, so every time the documentation includes a
reference to a main file it really means the file or the fragment directories.
</para>
</section>

<section id="s2.2"><title>Distribution Source list (sources.list)</title>
<para>
The distribution source list is used to locate archives of the debian
distribution. It is designed to support any number of active sources and to
support a mix of source media. The file lists one source per line, with the
fastest source listed first. The format of each line is:
</para>
<para>
<replaceable>type uri args</replaceable>
</para>
<para>
The first item, <replaceable>type</replaceable>, indicates the format for the
remainder of the line. It is designed to indicate the structure of the
distribution the line is talking about. Currently the only defined values are
<emphasis>deb</emphasis> and <emphasis>deb-src</emphasis> which indicate a
standard debian (source) archive with a dists directory. More about these
types and the URI specification can be found in the sources.list manpage.
</para>

<section id="s2.2.1"><title>Hashing the URI</title>
<para>
All permanent information acquired from any of the sources is stored in the
lists directory. Thus, there must be a way to relate the filename in the lists
directory to a line in the sourcelist. To simplify things this is done by
quoting the URI and treating _'s as quoteable characters and converting /
to _. The URI spec says this is done by converting a sensitive character
into %xx where xx is the hexadecimal representation from the ASCII character
set. Examples:
</para>
<screen>
http://www.debian.org/archive/dists/stable/binary-i386/Packages
/var/lib/apt/lists/www.debian.org_archive_dists_stable_binary-i386_Packages

cdrom:Debian 1.3/debian/Packages
/var/lib/apt/info/Debian%201.3_debian_Packages
</screen>
<para>
The other alternative that was considered was to use a deep directory structure
but this poses two problems, it makes it very difficult to prune directories
back when sources are no longer used and complicates the handling of the
partial directory. This gives a very simple way to deal with all of the
situations that can arise. Also note that the same rules described in the
<emphasis>Archive Directory</emphasis> section regarding the partial sub dir
apply here as well.
</para>
</section>

</section>

<section id="s2.3"><title>Extended States File (extended_states)</title>
<para>
The extended_states file serves the same purpose as the normal dpkg status
file (/var/lib/dpkg/status) except that it stores information unique to
apt. This includes currently only the autoflag but is open to store more
unique data that come up over time. It duplicates nothing from the normal
dpkg status file. Please see other APT documentation for a discussion of
the exact internal behavior of these fields. The Package and the Architecture
field are placed directly before the new fields to indicate which package
they apply to. The new fields are as follows:
</para>
<variablelist>
<varlistentry>
<term>Auto-Installed</term>
<listitem>
<para>
The Auto flag can be 1 (Yes) or 0 (No) and controls whether the package was
automatical installed to satisfy a dependency or if the user requested the
installation
</para>
</listitem>
</varlistentry>
</variablelist>
</section>

<section id="s2.4"><title>Binary Package Cache (srcpkgcache.bin and pkgcache.bin)</title>
<para>
Please see cache.sgml for a complete description of what this file
is. The cache file is updated whenever the Packages or Release files of the lists
directory or the dpkg status file changes. If the cache is erased, corrupted or of a non-matching
version it will be automatically rebuilt by all of the tools that need
it. <emphasis>srcpkgcache.bin</emphasis> contains a cache of all of the
package, release files in the source list. In comparison to <emphasis>pkgcache.bin</emphasis>, it does not include the /var/lib/dpkg/status file. This allows regeneration of the cache
when the status files change to use a prebuilt version for greater speed.
</para>
</section>

<section id="s2.5"><title>Downloads Directory (archives)</title>
<para>
The archives directory is where all downloaded .deb archives go. When the file
transfer is initiated the deb is placed in partial. Once the file is fully
downloaded and its MD5 hash and size are verified it is moved from partial
into archives/. Any files found in archives/ can be assumed to be verified.
</para>
<para>
No directory structure is transferred from the receiving site and all .deb file
names conform to debian conventions. No short (msdos) filename should be
placed in archives. If the need arises .debs should be unpacked, scanned and
renamed to their correct internal names. This is mostly to prevent file name
conflicts but other programs may depend on this if convenient. A conforming
.deb is one of the form, name_version_arch.deb. Our archive scripts do not
handle epochs, but they are necessary and should be re-inserted. If necessary
_'s and :'s in the fields should be quoted using the % convention. It must be
possible to extract all 3 fields by examining the file name. Downloaded .debs
must be found in one of the package lists with an exact name + version match..
</para>
</section>

<section id="s2.6"><title>The Methods Directory (/usr/lib/apt/methods)</title>
<para>
The Methods directory is more fully described in the APT Methods interface
document.
</para>
</section>

<section id="s2.7"><title>The Configuration File (/etc/apt/apt.conf)</title>
<para>
The configuration file (and the associated fragments directory
/etc/apt/apt.conf.d/) is described in the apt.conf manpage.
</para>
</section>

<section id="s2.8"><title>The trusted.gpg File (/etc/apt/trusted.gpg)</title>
<para>
The trusted.gpg file (and the files in the associated fragments directory
/etc/apt/trusted.gpg.d/) is a binary file including the keyring used by apt to
validate that the information (e.g. the Release file) it downloads are really
from the distributor it clams to be and is unmodified and is therefore the last
step in the chain of trust between the archive and the end user. This security
system is described in the apt-secure manpage.
</para>
</section>

<section id="s2.9"><title>The Release File</title>
<para>
This file plays an important role in how APT presents the archive to the
user. Its main purpose is to present a descriptive name for the source of
each version of each package. It also is used to detect when new versions
of debian are released. It augments the package file it is associated with
by providing meta information about the entire archive which the Packages
file describes.
</para>
<para>
The full name of the distribution for presentation to the user is formed as
'label version archive', with a possible extended name being 'label version
archive component'.
</para>
<para>
The file is formed as the package file (RFC-822) with the following tags
defined:
</para>
<variablelist>
<varlistentry>
<term>Archive</term>
<listitem>
<para>
This is the common name we give our archives, such as
<emphasis>stable</emphasis> or <emphasis>unstable</emphasis>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Component</term>
<listitem>
<para>
Refers to the sub-component of the archive, <emphasis>main</emphasis>,
<emphasis>contrib</emphasis> etc. Component may be omitted if there are no
components for this archive.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Version</term>
<listitem>
<para>
This is a version string with the same properties as in the Packages file. It
represents the release level of the archive.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Origin</term>
<listitem>
<para>
This specifies who is providing this archive. In the case of Debian the string
will read 'Debian'. Other providers may use their own string
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Label</term>
<listitem>
<para>
This carries the encompassing name of the distribution. For Debian proper this
field reads 'Debian'. For derived distributions it should contain their proper
name.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Architecture</term>
<listitem>
<para>
When the archive has packages for a single architecture then the Architecture
is listed here. If a mixed set of systems are represented then this should
contain the keyword <emphasis>mixed</emphasis>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>NotAutomatic</term>
<listitem>
<para>
A Yes/No flag indicating that the archive is extremely unstable and its
version's should never be automatically selected. This is to be used by
experimental.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Description</term>
<listitem>
<para>
Description is used to describe the release. For instance experimental would
contain a warning that the packages have problems.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The location of the Release file in the archive is very important, it must be
located in the same location as the packages file so that it can be located in
all situations. The following is an example for the current stable release,
1.3.1r6
</para>
<screen>
Archive: stable
Component: main
Version: 1.3.1r6
Origin: Debian
Label: Debian
Architecture: i386
</screen>
<para>
This is an example of experimental,
</para>
<screen>
Archive: experimental
Version: 0
Origin: Debian
Label: Debian
Architecture: mixed
NotAutomatic: Yes
</screen>
<para>
And unstable,
</para>
<screen>
Archive: unstable
Component: main
Version: 2.1
Origin: Debian
Label: Debian
Architecture: i386
</screen>
</section>

</chapter>


</book>