Chapter 4. The Interface
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# Chapter 4. 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. [2] 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.

## 4.1. 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. [3]. 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.

## 4.2. 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.

### 4.2.1. 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.

### 4.2.2. 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.

### 4.2.3. 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.

### 4.2.4. 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.

### 4.2.5. 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.

### 4.2.6. 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.

## 4.3. 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.

## 4.4. 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.

[2] 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.

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

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