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<th colspan="3" align="center">Chapter 4. The Interface</th>
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<div class="chapter">
<div class="titlepage">
<div>
<div>
<h1 class="title"><a id="ch4"/>Chapter 4. The Interface</h1>
</div>
</div>
</div>
<div class="toc">
<p>
<strong>Table of Contents</strong>
</p>
<dl class="toc">
<dt>
<span class="section">
<a href="ch4.html#s4.1">4.1. Startup</a>
</span>
</dt>
<dt>
<span class="section">
<a href="ch4.html#s4.2">4.2. The Status Report</a>
</span>
</dt>
<dt>
<span class="section">
<a href="ch4.html#s4.3">4.3. The Status Display</a>
</span>
</dt>
<dt>
<span class="section">
<a href="ch4.html#s4.4">4.4. Dpkg</a>
</span>
</dt>
</dl>
</div>
<p>
Both that APT <span class="command"><strong>dselect</strong></span> method and <span class="command"><strong>apt-get</strong></span>
share the same interface. It is a simple system that generally tells you what
it will do and then goes and does it. <a href="#ftn.idp52543136" class="footnote" id="idp52543136"><sup class="footnote">[2]</sup></a> 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.
</p>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s4.1"/>4.1. Startup</h2>
</div>
</div>
</div>
<p>
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
<code class="literal">apt-get check</code>.
</p>
<pre class="screen">
# apt-get check
Reading Package Lists... Done
Building Dependency Tree... Done
</pre>
<p>
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.
</p>
<p>
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 <span class="command"><strong>apt-get</strong></span> will refuse to run.
</p>
<pre class="screen">
# apt-get check
Reading Package Lists... Done
Building Dependency Tree... Done
You might want to run apt-get -f 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)
</pre>
<p>
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.
</p>
<p>
There are two ways a system can get into a broken state like this. The
first is caused by <span class="command"><strong>dpkg</strong></span> missing some subtle relationships
between packages when performing upgrades. <a href="#ftn.idp52680272" class="footnote" id="idp52680272"><sup class="footnote">[3]</sup></a>. 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.
</p>
<p>
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 <code class="literal">-f</code> option to <span class="command"><strong>apt-get</strong></span>
will cause APT to deduce a possible solution to the problem and then
continue on. The APT <span class="command"><strong>dselect</strong></span> method always supplies
the <code class="literal">-f</code> option to allow for easy continuation of failed
maintainer scripts.
</p>
<p>
However, if the <code class="literal">-f</code> 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.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s4.2"/>4.2. The Status Report</h2>
</div>
</div>
</div>
<p>
Before proceeding <span class="command"><strong>apt-get</strong></span> 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 <code class="literal">-f</code> option
and any other relevant activities to the command being executed.
</p>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.1"/>4.2.1. The Extra Package list</h3>
</div>
</div>
</div>
<pre class="screen">
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
</pre>
<p>
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 <code class="literal">install</code> command. The listed packages are
often the result of an Auto Install.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.2"/>4.2.2. The Packages to Remove</h3>
</div>
</div>
</div>
<pre class="screen">
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
</pre>
<p>
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
<code class="literal">-f</code> 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.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.3"/>4.2.3. The New Packages list</h3>
</div>
</div>
</div>
<pre class="screen">
The following NEW packages will installed:
zlib1g xlib6g perl-base libgdbmg1 quake-lib gmp2 pgp-i python-base
</pre>
<p>
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.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.4"/>4.2.4. The Kept Back list</h3>
</div>
</div>
</div>
<pre class="screen">
The following packages have been kept back
compface man-db tetex-base msql libpaper svgalib1
gs snmp arena lynx xpat2 groff xscreensaver
</pre>
<p>
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 <code class="literal">apt-get install</code> or by using
<span class="command"><strong>dselect</strong></span> to resolve their problems.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.5"/>4.2.5. Held Packages warning</h3>
</div>
</div>
</div>
<pre class="screen">
The following held packages will be changed:
cvs
</pre>
<p>
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.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="s4.2.6"/>4.2.6. Final summary</h3>
</div>
</div>
</div>
<p>
Finally, APT will print out a summary of all the changes that will occur.
</p>
<pre class="screen">
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.
</pre>
<p>
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.
</p>
<p>
Some other reports can be generated by using the -u option to show packages to
upgrade, they are similar to the previous examples.
</p>
</div>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s4.3"/>4.3. The Status Display</h2>
</div>
</div>
</div>
<p>
During the download of archives and package files APT prints out a series of
status messages.
</p>
<pre class="screen">
# apt-get update
Get:1 http://ftp.de.debian.org/debian-non-US/ stable/non-US/ 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-non-US/ 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
</pre>
<p>
The lines starting with <span class="emphasis"><em>Get</em></span> 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 <code class="literal">apt-get update</code> estimates the percent done which
causes some inaccuracies.
</p>
<p>
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 <span class="emphasis"><em>Forking</em></span> 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.
</p>
<p>
Inside of the single quote is an informative string indicating the progress of
the negotiation phase of the download. Typically it progresses from
<span class="emphasis"><em>Connecting</em></span> to <span class="emphasis"><em>Waiting for file</em></span> to
<span class="emphasis"><em>Downloading</em></span> or <span class="emphasis"><em>Resuming</em></span>. The final
value is the number of bytes downloaded from the remote site. Once the
download begins this is represented as <code class="literal">102/10.2k</code> 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.
</p>
<p>
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 <code class="literal">-q</code> option to remove the status
display.
</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="s4.4"/>4.4. Dpkg</h2>
</div>
</div>
</div>
<p>
APT uses <span class="command"><strong>dpkg</strong></span> for installing the archives and will
switch over to the <span class="command"><strong>dpkg</strong></span> interface once downloading is
completed. <span class="command"><strong>dpkg</strong></span> 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.
</p>
</div>
<div class="footnotes">
<br/>
<hr/>
<div id="ftn.idp52543136" class="footnote">
<p><a href="#idp52543136" class="para"><sup class="para">[2] </sup></a> The
<span class="command"><strong>dselect</strong></span> method actually is a set of wrapper scripts to
<span class="command"><strong>apt-get</strong></span>. The method actually provides more functionality
than is present in <span class="command"><strong>apt-get</strong></span> alone. </p>
</div>
<div id="ftn.idp52680272" class="footnote">
<p><a href="#idp52680272" class="para"><sup class="para">[3] </sup></a> APT however
considers all known dependencies and attempts to prevent broken
packages </p>
</div>
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