"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.
For more details, on Debian GNU/Linux systems, see the file /usr/share/common-licenses/GPL for the full license.
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.
The var directory structure is as follows:
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.
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.
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:
type uri args
The first item, type, 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 value is deb
which indicates a standard debian archive with a dists dir.
The deb type is to be a typical two level debian distributions,
dist/distribution/component. Typically distribution
is one of stable, unstable or testing while component is one of main,
contrib, non-free or non-us. The format for the deb line is as follows:
deb uri distribution component
[component ...]
uri for the deb type must specify the base of the
debian distribution. APT will automatically generate the proper longer
URIs to get the information it needs. distribution can specify
an exact path, in this case the components must be omitted and
distribution must end in a slash.
Since only one distribution can be specified per deb line it may be
necessary to list a number of deb lines for the same URI. APT will
sort the URI list after it has generated a complete set to allow
connection reuse. It is important to order things in the sourcelist
from most preferred to least preferred (fastest to slowest).
URIs in the source list support a large number of access schemes which
are listed in the sources.list manpage and can be further extended by
transport binaries placed in /usr/lib/apt/methods. The most important
builtin schemes are:
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:
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 Archive Directory section regarding the partial sub dir apply
here as well.
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 behaviour 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:
Please see cache.sgml for a complete description of what this file is. The cache file is updated whenever the contents of the lists directory 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. srcpkgcache.bin contains a cache of all of the package files in the source list. This allows regeneration of the cache when the status files change to use a prebuilt version for greater speed.
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.
No directory structure is transfered 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..
The Methods directory is more fully described in the APT Methods interface document.
The configuration file (and the associated fragments directory /etc/apt/apt.conf.d/) is described in the apt.conf manpage.
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.
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.
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'.
The file is formed as the package file (RFC-822) with the following tags
defined:
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