# APT User's Guide

## General

The APT package currently contains two sections, the APT `dselect` method and the `apt-get` command line user interface. Both provide a way to install and remove packages as well as download new packages from the Internet.

### Anatomy of the Package System

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.

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.

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.

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.

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.

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.

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.

## apt-get

`apt-get` provides a simple way to install packages from the command line. Unlike `dpkg`, `apt-get` does not understand .deb files, it works with the package's proper name and can only install .deb archives from a Source.

The first

If you are using an http proxy server you must set the http_proxy environment variable first, see sources.list(5)

thing that should be done before using `apt-get` is to fetch the package lists from the Sources so that it knows what packages are available. This is done with `apt-get update`. For instance,

    # apt-get update
    Get http://ftp.de.debian.org/debian/ stable/binary-i386/ Packages
    Get http://llug.sep.bnl.gov/debian/ testing/contrib Packages
    Reading Package Lists... Done
    Building Dependency Tree... Done

Once updated there are several commands that can be used:

upgrade

- 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. `dselect` or `apt-get install` can be used to force these packages to install.

install

- 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.

dist-upgrade

- 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 `dselect`. Once dist-upgrade has completed then `dselect` can be used to install any packages that may have been left out.

  It is important to closely look at what dist-upgrade is going to do, its decisions may sometimes be quite surprising.

`apt-get` has several command line options that are detailed in its man page,

apt-get

8. The most useful option is `-d` 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 `-d` is used the downloaded archives can be installed by simply running the command that caused them to be downloaded again without `-d`.

## DSelect

The APT `dselect` method provides the complete APT system with the `dselect` package selection GUI. `dselect` is used to select the packages to be installed or removed and APT actually installs them.

To enable the APT method you need to select [A]ccess in `dselect` and then choose the APT method. You will be prompted for a set of Sources 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.

       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,
          https://deb.debian.org/debian,
          http://deb.debian.org/debian,

     URL [http://llug.sep.bnl.gov/debian]:

The Sources setup starts by asking for the base of the Debian archive, defaulting to an HTTP mirror. Next it asks for the distribution to get.

     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

     Distribution [stable]:

The distribution refers to the Debian version in the archive, stable refers to the latest released version and unstable refers to the developmental version.

     Please give the components to get
     The components are typically something like: main contrib non-free non-free-firmware

     Components [main contrib non-free non-free-firmware]:

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.

Any number of sources can be added, the setup script will continue to prompt until you have specified all that you want.

Before starting to use `dselect` it is necessary to update the available list by selecting [U]pdate from the menu. This is a superset of `apt-get update` that makes the fetched information available to `dselect`. [U]pdate must be performed even if `apt-get update` has been run before.

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.

By default APT will automatically remove the package (.deb) files once they have been successfully installed. To change this behavior place `Dselect::clean "prompt";` in /etc/apt/apt.conf.

## The Interface

Both that APT `dselect` method and `apt-get` share the same interface. It is a simple system that generally tells you what it will do and then goes and does it.

The `dselect` method actually is a set of wrapper scripts to `apt-get`. The method actually provides more functionality than is present in `apt-get` alone.

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.

### Startup

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 `apt-get check`.

    # apt-get check
    Reading Package Lists... Done
    Building Dependency Tree... Done

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.

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 `apt-get` will refuse to run.

    # apt-get check
    Reading Package Lists... Done
    Building Dependency Tree... Done
    You might want to run 'apt --fix-broken 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 (>= 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 (>= 2.01) but 2.0-3 is installed
      cthugha: Depends: svgalibg1 but it is not installed
               Depends: xlib6g (>= 3.3-5) but it is not installed
      libreadlineg2: Conflicts:libreadline2 (<< 2.1-2.1)

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.

There are two ways a system can get into a broken state like this. The first is caused by `dpkg` missing some subtle relationships between packages when performing upgrades.

APT however considers all known dependencies and attempts to prevent broken packages

. 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.

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 `-f` option to `apt-get` will cause APT to deduce a possible solution to the problem and then continue on. The APT `dselect` method always supplies the `-f` option to allow for easy continuation of failed maintainer scripts.

However, if the `-f` 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.

### The Status Report

Before proceeding `apt-get` 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 `-f` option and any other relevant activities to the command being executed.

#### The Extra Package list

    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

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 `install` command. The listed packages are often the result of an Auto Install.

#### The Packages to Remove

    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

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 `-f` 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.

#### The New Packages list

    The following NEW packages will installed:
      zlib1g xlib6g perl-base libgdbmg1 quake-lib gmp2 pgp-i python-base

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.

#### The Kept Back list

    The following packages have been kept back
      compface man-db tetex-base msql libpaper svgalib1
      gs snmp arena lynx xpat2 groff xscreensaver

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 `apt-get install` or by using `dselect` to resolve their problems.

#### Held Packages warning

    The following held packages will be changed:
      cvs

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.

#### Final summary

Finally, APT will print out a summary of all the changes that will occur.

    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.

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.

Some other reports can be generated by using the -u option to show packages to upgrade, they are similar to the previous examples.

### The Status Display

During the download of archives and package files APT prints out a series of status messages.

    # apt-get update
    Get:1 http://ftp.de.debian.org/debian/ stable/main 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/ 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

The lines starting with Get 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 `apt-get update` estimates the percent done which causes some inaccuracies.

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 Forking 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.

Inside of the single quote is an informative string indicating the progress of the negotiation phase of the download. Typically it progresses from Connecting to Waiting for file to Downloading or Resuming. The final value is the number of bytes downloaded from the remote site. Once the download begins this is represented as `102/10.2k` 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.

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 `-q` option to remove the status display.

### Dpkg

APT uses `dpkg` for installing the archives and will switch over to the `dpkg` interface once downloading is completed. `dpkg` 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.