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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>Immediate dependency resolution</title><link rel="stylesheet" type="text/css" href="aptitude.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="aptitude user's manual"><link rel="up" href="ch02s03.html" title="Resolving package dependencies"><link rel="prev" href="ch02s03s01.html" title="Dependency resolution in aptitude"><link rel="next" href="ch02s03s03.html" title="Resolving Dependencies Interactively"><link rel="preface" href="pr01.html" title="Introduction"><link rel="chapter" href="ch01.html" title="Chapter 1. Getting started"><link rel="chapter" href="ch02.html" title="Chapter 2. aptitude reference guide"><link rel="chapter" href="ch03.html" title="Chapter 3. aptitude frequently asked questions"><link rel="chapter" href="ch04.html" title="Chapter 4. Credits"><link rel="reference" href="rn01.html" title="Command-line reference"><link rel="refentry" href="rn01re01.html" title="aptitude"><link rel="refentry" href="rn01re02.html" title="aptitude-create-state-bundle"><link rel="refentry" href="rn01re03.html" title="aptitude-run-state-bundle"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Immediate dependency resolution</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02s03s01.html"><img src="images/prev.gif" alt="Prev"></a> </td><th width="60%" align="center">Resolving package dependencies</th><td width="20%" align="right"> <a accesskey="n" href="ch02s03s03.html"><img src="images/next.gif" alt="Next"></a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="secImmediateResolution"></a>Immediate dependency resolution</h3></div></div></div>
	 Whenever you choose to install or upgrade a package in
	 aptitude, aptitude makes an immediate attempt to resolve
	 any of its dependencies that are not fulfilled. For each
	 unsatisfied dependency (either a “Depends”, a
	 “Recommends”, or a “Conflicts”),
	 it performs the following steps:
	<div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
	 If the dependency is a recommendation, aptitude tries
	 to guess whether it is a “new”
	 recommendation or a “previously satisfied”
	 recommendation. aptitude considers a recommendation
	 to be “new” if the package declaring the
	 recommendation is not currently installed, or if its
	 installed version does not recommend a package of the
	 same name. On the other hand, a recommendation is
	 “previously satisfied” if the package
	 declaring the recommendation is installed, the currently
	 installed version recommends a package of the same name,
	 and that recommendation is currently fulfilled.
	 
	 For example: suppose that version <code class="literal">1.0</code>
	 of <code class="literal">prog</code> recommends version
	 <code class="literal">4.0</code> of <code class="literal">libcool1</code>,
	 but version <code class="literal">2.0</code> of
	 <code class="literal">prog</code> recommends version
	 <code class="literal">5.0</code> of <code class="literal">libcool1</code>,
	 and also recommends <code class="literal">apache</code>. If you
	 choose to upgrade <code class="literal">prog</code> from version
	 <code class="literal">1.0</code> to version
	 <code class="literal">2.0</code>, the recommendation of
	 <code class="literal">apache</code> will be considered to be
	 “new” because version
	 <code class="literal">1.0</code> of <code class="literal">prog</code> did
	 not recommend <code class="literal">apache</code>. On the other
	 hand, the recommendation of <code class="literal">libcool1</code>
	 is not “new”, because
	 version <code class="literal">1.0</code> of
	 <code class="literal">prog</code> recommended
	 <code class="literal">libcool1</code>, even though it recommended
	 a different version. However, if
	 <code class="literal">libcool1</code> is installed, then that
	 recommendation will be considered to be
	 “previously satisfied”.
	 
	 If the configuration option <a class="link" href="ch02s05s05.html#configAptInstall-Recommends"><code class="literal">Apt::Install-Recommends</code></a>
	 is <code class="literal">true</code>, aptitude will always
	 attempt to fulfill “new” and
	 “previously satisfied” recommendations; all
	 others will be ignored by immediate resolution. If that
	 option is <code class="literal">false</code>, immediate dependency
	 resolution will ignore all
	 recommendations.
	 </li><li class="listitem">
	 If the dependency is on several packages combined with
	 OR, examine each of the alternatives in the order that
	 they are given. For instance, if a package depends on
	 “<code class="literal">exim |
	 mail-transport-agent</code>”, aptitude will
	 first process <code class="literal">exim</code>, then
	 <code class="literal">mail-transport-agent</code>.
	 </li><li class="listitem">
	 For each alternative, attempt to resolve it. If the
	 dependency is a conflict, remove the current alternative
	 if it is installed (and for an unversioned conflict,
	 also remove any package providing the target of the
	 conflict). Otherwise, install the candidate version of
	 the current alternative if it satisfies the dependency.
	 If not, or if there is no candidate version (for
	 instance, because the current alternative is a virtual
	 package), and if the dependency is unversioned, attempt
	 to install the highest-priority
	 package<a href="#ftn.idp57440688" class="footnote" name="idp57440688">[12]</a> whose
	 candidate version provides the target of the current
	 alternative.
	 
	 For instance, say we are attempting to resolve
	 “<code class="literal">Depends: exim |
	 mail-transport-agent</code>”. aptitude will
	 first attempt to install the package
	 <code class="literal">exim</code>. If <code class="literal">exim</code> is
	 not available, aptitude will then attempt to install
	 the highest-priority package whose candidate version
	 provides <code class="literal">exim</code>. If there is no such
	 package, aptitude will install the highest-priority
	 package whose candidate version provides the virtual
	 package <code class="literal">mail-transport-agent</code>. On the
	 other hand, suppose that the dependency is
	 “<code class="literal">Depends: exim (&gt;= 2.0.0) |
	 mail-transport-agent</code>”, but only version
	 <code class="literal">1.0</code> of <code class="literal">exim</code> is
	 available. In this case, aptitude will not install
	 <code class="literal">exim</code> (because the version does not
	 match), nor will it attempt to install packages
	 providing <code class="literal">exim</code> (because virtual
	 packages cannot match a dependency with a version
	 restriction). Thus, aptitude will fall back to
	 installing the highest-priority package whose candidate
	 version provides
	 <code class="literal">mail-transport-agent</code>.
	 </li><li class="listitem">
	 If a package was installed by the previous step, resolve
	 its dependencies using this algorithm, then stop.
	 </li></ol></div>
	 While this technique very often solves all the outstanding
	 package dependencies, it can fail in a number of common
	 circumstances.
	<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
	 Conflicts are resolved by removing the package that is
	 the target of the conflict. But now other packages that
	 depend on this package have unresolved dependencies; the
	 immediate resolver makes no attempt to fix them.
	 </li><li class="listitem">
	 A dependency might not be satisfiable due to version
	 restrictions and due to the limitation that only
	 candidate versions are considered. For instance, say
	 that versions <code class="literal">1.0</code> and
	 <code class="literal">2.0</code> of <code class="literal">fileutils</code>
	 are available, that the candidate version is
	 <code class="literal">1.0</code>, and that the package
	 <code class="literal">octopus</code> declares a dependency
	 “<code class="literal">Depends: fileutils (&gt;=
	 2.0)</code>”. Immediate resolution is unable
	 to resolve this dependency: it will never consider
	 version <code class="literal">2.0</code> of the package, since
	 that is not the candidate version.
	 </li></ul></div>
	 The interactive dependency resolver can solve these
	 situations and more. When there are broken dependencies
	 left over, or when immediate dependency resolution is
	 disabled, the interactive resolver will automatically start
	 searching for a solution. The next section describes how to
	 use the interactive dependency resolver.
	<div class="footnotes"> <hr style="width:100; text-align:left;margin-left: 0"><div id="ftn.idp57440688" class="footnote"><a href="#idp57440688" class="para">[12] </a>The package with the highest
	 dpkg priority, not the package with
	 the highest <code class="systemitem">apt</code> pin priority.</div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch02s03s01.html"><img src="images/prev.gif" alt="Prev"></a> </td><td width="20%" align="center"><a accesskey="u" href="ch02s03.html"><img src="images/up.gif" alt="Up"></a></td><td width="40%" align="right"> <a accesskey="n" href="ch02s03s03.html"><img src="images/next.gif" alt="Next"></a></td></tr><tr><td width="40%" align="left" valign="top">Dependency resolution in aptitude </td><td width="20%" align="center"><a accesskey="h" href="index.html"><img src="images/home.gif" alt="Home"></a></td><td width="40%" align="right" valign="top"> Resolving Dependencies Interactively</td></tr></table></div></body></html>