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 <section id="declaring-relationships-between-packages">
<h1>7. Declaring relationships between packages<a class="headerlink" href="#declaring-relationships-between-packages" title="Permalink to this headline">¶</a></h1>
<section id="syntax-of-relationship-fields">
<h2>7.1. Syntax of relationship fields<a class="headerlink" href="#syntax-of-relationship-fields" title="Permalink to this headline">¶</a></h2>
These fields all have a uniform syntax. They are a list of package names
separated by commas.
In the <code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Recommends</code>, <code class="docutils literal notranslate">Suggests</code>, <code class="docutils literal notranslate">Pre-Depends</code>,
<code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code> and <code class="docutils literal notranslate">Build-Depends-Arch</code>
control fields of the package, which declare dependencies on other
packages, the package names listed may also include lists of alternative
package names, separated by vertical bar (pipe) symbols <code class="docutils literal notranslate">|</code>. In such a
case, that part of the dependency can be satisfied by any one of the
alternative packages. <a class="footnote-reference brackets" href="#id8" id="id1">1</a>
All of the fields may restrict their applicability to particular versions
of each named package. This is done in parentheses after each individual
package name; the parentheses should contain a relation from the list
below followed by a version number, in the format described in
<a class="reference internal" href="ch-controlfields.html#s-f-version">Version</a>.
The relations allowed are <code class="docutils literal notranslate">&lt;&lt;</code>, <code class="docutils literal notranslate">&lt;=</code>, <code class="docutils literal notranslate">=</code>, <code class="docutils literal notranslate">&gt;=</code> and <code class="docutils literal notranslate">&gt;&gt;</code> for
strictly earlier, earlier or equal, exactly equal, later or equal and
strictly later, respectively. The exception is the Provides field, for
which only <code class="docutils literal notranslate">=</code> is allowed. <a class="footnote-reference brackets" href="#id9" id="id2">2</a>
Whitespace may appear at any point in the version specification subject
to the rules in <a class="reference internal" href="ch-controlfields.html#s-controlsyntax">Syntax of control files</a>, and must appear
where it’s necessary to disambiguate; it is not otherwise significant.
All of the relationship fields can only be folded in source package
control files. For consistency and in case of future changes to <code class="docutils literal notranslate">dpkg</code>
it is recommended that a single space be used after a version
relationship and before a version number; it is also conventional to put
a single space after each comma, on either side of each vertical bar,
and before each open parenthesis. When opening a continuation line in a
relationship field, it is conventional to do so after a comma and before
the space following that comma.
For example, a list of dependencies might appear as:
<div class="highlight-default notranslate"><div class="highlight"><pre>Package: mutt
Version: 1.3.17-1
Depends: libc6 (&gt;= 2.2.1), default-mta | mail-transport-agent
</pre></div>
</div>
Relationships may be restricted to a certain set of architectures. This
is indicated in brackets after each individual package name and the
optional version specification. The brackets enclose a non-empty list of
Debian architecture names in the format described in
<a class="reference internal" href="ch-customized-programs.html#s-arch-spec">Architecture specification strings</a>, separated by whitespace. Exclamation
marks may be prepended to each of the names. (It is not permitted for
some names to be prepended with exclamation marks while others aren’t.)
For build relationship fields (<code class="docutils literal notranslate">Build-Depends</code>,
<code class="docutils literal notranslate">Build-Depends-Indep</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>, <code class="docutils literal notranslate">Build-Conflicts</code>,
<code class="docutils literal notranslate">Build-Conflicts-Indep</code> and <code class="docutils literal notranslate">Build-Conflicts-Arch</code>), if the current
Debian host architecture is not in this list and there are no
exclamation marks in the list, or it is in the list with a prepended
exclamation mark, the package name and the associated version
specification are ignored completely for the purposes of defining the
relationships.
For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>Source: glibc
Build-Depends-Indep: texinfo
Build-Depends: kernel-headers-2.2.10 [!hurd-i386],
 hurd-dev [hurd-i386], gnumach-dev [hurd-i386]
</pre></div>
</div>
requires <code class="docutils literal notranslate">kernel-headers-2.2.10</code> on all architectures other than
hurd-i386 and requires <code class="docutils literal notranslate">hurd-dev</code> and <code class="docutils literal notranslate">gnumach-dev</code> only on
hurd-i386. Here is another example showing multiple architectures
separated by spaces:
<div class="highlight-default notranslate"><div class="highlight"><pre>Build-Depends:
 libluajit5.1-dev [i386 amd64 kfreebsd-i386 armel armhf powerpc mips],
 liblua5.1-dev [hurd-i386 ia64 kfreebsd-amd64 s390x sparc],
</pre></div>
</div>
For binary relationship fields and the <code class="docutils literal notranslate">Built-Using</code> field, the
architecture restriction syntax is only supported in the source package
control file <code class="docutils literal notranslate">debian/control</code>. When the corresponding binary package
control file is generated, the relationship will either be omitted or
included without the architecture restriction based on the architecture
of the binary package. This means that architecture restrictions must
not be used in binary relationship fields for architecture-independent
packages (<code class="docutils literal notranslate">Architecture: all</code>).
For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>Depends: foo [i386], bar [amd64]
</pre></div>
</div>
becomes <code class="docutils literal notranslate">Depends: foo</code> when the package is built on the <code class="docutils literal notranslate">i386</code>
architecture, <code class="docutils literal notranslate">Depends: bar</code> when the package is built on the
<code class="docutils literal notranslate">amd64</code> architecture, and omitted entirely in binary packages built
on all other architectures.
If the architecture-restricted dependency is part of a set of
alternatives using <code class="docutils literal notranslate">|</code>, that alternative is ignored completely on
architectures that do not match the restriction. For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>Build-Depends: foo [!i386] | bar [!amd64]
</pre></div>
</div>
is equivalent to <code class="docutils literal notranslate">bar</code> on the <code class="docutils literal notranslate">i386</code> architecture, to <code class="docutils literal notranslate">foo</code> on
the <code class="docutils literal notranslate">amd64</code> architecture, and to <code class="docutils literal notranslate">foo | bar</code> on all other
architectures.
Relationships may also be restricted to a certain set of architectures
using architecture wildcards in the format described in
<a class="reference internal" href="ch-customized-programs.html#s-arch-wildcard-spec">Architecture wildcards</a>. The syntax for declaring
such restrictions is the same as declaring restrictions using a certain
set of architectures without architecture wildcards. For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>Build-Depends: foo [linux-any], bar [any-i386], baz [!linux-any]
</pre></div>
</div>
is equivalent to <code class="docutils literal notranslate">foo</code> on architectures using the Linux kernel and any
cpu, <code class="docutils literal notranslate">bar</code> on architectures using any kernel and an i386 cpu, and
<code class="docutils literal notranslate">baz</code> on any architecture using a kernel other than Linux.
Note that the binary package relationship fields such as <code class="docutils literal notranslate">Depends</code>
appear in one of the binary package sections of the control file,
whereas the build-time relationships such as <code class="docutils literal notranslate">Build-Depends</code> appear in
the source package section of the control file (which is the first
section).
</section>
<section id="binary-dependencies-depends-recommends-suggests-enhances-pre-depends">
<h2>7.2. Binary Dependencies - <code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Recommends</code>, <code class="docutils literal notranslate">Suggests</code>, <code class="docutils literal notranslate">Enhances</code>, <code class="docutils literal notranslate">Pre-Depends</code><a class="headerlink" href="#binary-dependencies-depends-recommends-suggests-enhances-pre-depends" title="Permalink to this headline">¶</a></h2>
Packages can declare in their control file that they have certain
relationships to other packages - for example, that they cannot be
installed at the same time as certain other packages, and/or that they
depend on the presence of others.
This is done using the <code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Pre-Depends</code>, <code class="docutils literal notranslate">Recommends</code>,
<code class="docutils literal notranslate">Suggests</code>, <code class="docutils literal notranslate">Enhances</code>, <code class="docutils literal notranslate">Breaks</code> and <code class="docutils literal notranslate">Conflicts</code> control fields.
<code class="docutils literal notranslate">Breaks</code> is described in <a class="reference internal" href="#s-breaks">Packages which break other packages - Breaks</a>, and
<code class="docutils literal notranslate">Conflicts</code> is described in <a class="reference internal" href="#s-conflicts">Conflicting binary packages - Conflicts</a>. The
rest are described below.
These seven fields are used to declare a dependency relationship by one
package on another. Except for <code class="docutils literal notranslate">Enhances</code> and <code class="docutils literal notranslate">Breaks</code>, they appear
in the depending (binary) package’s control file. (<code class="docutils literal notranslate">Enhances</code> appears
in the recommending package’s control file, and <code class="docutils literal notranslate">Breaks</code> appears in
the version of depended-on package which causes the named package to
break).
A <code class="docutils literal notranslate">Depends</code> field takes effect only when a package is to be
configured. It does not prevent a package being on the system in an
unconfigured state while its dependencies are unsatisfied, and it is
possible to replace a package whose dependencies are satisfied and which
is properly installed with a different version whose dependencies are
not and cannot be satisfied; when this is done the depending package
will be left unconfigured (since attempts to configure it will give
errors) and will not function properly. If it is necessary, a
<code class="docutils literal notranslate">Pre-Depends</code> field can be used, which has a partial effect even when
a package is being unpacked, as explained in detail below. (The other
three dependency fields, <code class="docutils literal notranslate">Recommends</code>, <code class="docutils literal notranslate">Suggests</code> and <code class="docutils literal notranslate">Enhances</code>,
are only used by the various front-ends to <code class="docutils literal notranslate">dpkg</code> such as <code class="docutils literal notranslate">apt-get</code>,
<code class="docutils literal notranslate">aptitude</code>, and <code class="docutils literal notranslate">dselect</code>.)
Since <code class="docutils literal notranslate">Depends</code> only places requirements on the order in which
packages are configured, packages in an installation run are usually all
unpacked first and all configured later. <a class="footnote-reference brackets" href="#id10" id="id3">3</a>
If there is a circular dependency among packages being installed or
removed, installation or removal order honoring the dependency order is
impossible, requiring the dependency loop be broken at some point and
the dependency requirements violated for at least one package. Packages
involved in circular dependencies may not be able to rely on their
dependencies being configured before they themselves are configured,
depending on which side of the break of the circular dependency loop
they happen to be on. If one of the packages in the loop has no
<code class="docutils literal notranslate">postinst</code> script, then the cycle will be broken at that package; this
ensures that all <code class="docutils literal notranslate">postinst</code> scripts are run with their dependencies
properly configured if this is possible. Otherwise the breaking point is
arbitrary. Packages should therefore avoid circular dependencies where
possible, particularly if they have <code class="docutils literal notranslate">postinst</code> scripts.
The meaning of the five dependency fields is as follows:
<dl>
<dt><code class="docutils literal notranslate">Depends</code></dt><dd>This declares an absolute dependency. A package will not be
configured unless all of the packages listed in its <code class="docutils literal notranslate">Depends</code>
field have been correctly configured (unless there is a circular
dependency as described above).
The <code class="docutils literal notranslate">Depends</code> field should be used if the depended-on package is
required for the depending package to provide a significant amount
of functionality.
The <code class="docutils literal notranslate">Depends</code> field should also be used if the <code class="docutils literal notranslate">postinst</code> or
<code class="docutils literal notranslate">prerm</code> scripts require the depended-on package to be unpacked or
configured in order to run. In the case of <code class="docutils literal notranslate">postinst configure</code>,
the depended-on packages will be unpacked and configured first. (If
both packages are involved in a dependency loop, this might not work
as expected; see the explanation a few paragraphs back.) In the case
of <code class="docutils literal notranslate">prerm</code> or other <code class="docutils literal notranslate">postinst</code> actions, the package dependencies
will normally be at least unpacked, but they may be only
“Half-Installed” if a previous upgrade of the dependency failed.
Finally, the <code class="docutils literal notranslate">Depends</code> field should be used if the depended-on
package is needed by the <code class="docutils literal notranslate">postrm</code> script to fully clean up after
the package removal. There is no guarantee that package dependencies
will be available when <code class="docutils literal notranslate">postrm</code> is run, but the depended-on
package is more likely to be available if the package declares a
dependency (particularly in the case of <code class="docutils literal notranslate">postrm remove</code>). The
<code class="docutils literal notranslate">postrm</code> script must gracefully skip actions that require a
dependency if that dependency isn’t available.
</dd>
<dt><code class="docutils literal notranslate">Recommends</code></dt><dd>This declares a strong, but not absolute, dependency.
The <code class="docutils literal notranslate">Recommends</code> field should list packages that would be found
together with this one in all but unusual installations.
</dd>
<dt><code class="docutils literal notranslate">Suggests</code></dt><dd>This is used to declare that one package may be more useful with one
or more others. Using this field tells the packaging system and the
user that the listed packages are related to this one and can
perhaps enhance its usefulness, but that installing this one without
them is perfectly reasonable.
</dd>
<dt><code class="docutils literal notranslate">Enhances</code></dt><dd>This field is similar to Suggests but works in the opposite
direction. It is used to declare that a package can enhance the
functionality of another package.
</dd>
<dt><code class="docutils literal notranslate">Pre-Depends</code></dt><dd>This field is like <code class="docutils literal notranslate">Depends</code>, except that it also forces <code class="docutils literal notranslate">dpkg</code>
to complete installation of the packages named before even starting
the installation of the package which declares the pre-dependency,
as follows:
When a package declaring a pre-dependency is about to be unpacked
the pre-dependency can be satisfied if the depended-on package is
either fully configured, or even if the depended-on package(s) are
only in the “Unpacked” or the “Half-Configured” state, provided that
they have been configured correctly at some point in the past (and
not removed or partially removed since). In this case, both the
previously-configured and currently “Unpacked” or “Half-Configured”
versions must satisfy any version clause in the <code class="docutils literal notranslate">Pre-Depends</code>
field.
When the package declaring a pre-dependency is about to be
configured, the pre-dependency will be treated as a normal
<code class="docutils literal notranslate">Depends</code>. It will be considered satisfied only if the depended-on
package has been correctly configured. However, unlike with
<code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Pre-Depends</code> does not permit circular dependencies
to be broken. If a circular dependency is encountered while
attempting to honor <code class="docutils literal notranslate">Pre-Depends</code>, the installation will be
aborted.
<code class="docutils literal notranslate">Pre-Depends</code> are also required if the <code class="docutils literal notranslate">preinst</code> script depends
on the named package. It is best to avoid this situation if
possible.
<code class="docutils literal notranslate">Pre-Depends</code> should be used sparingly, preferably only by
packages whose premature upgrade or installation would hamper the
ability of the system to continue with any upgrade that might be in
progress.
You should not specify a <code class="docutils literal notranslate">Pre-Depends</code> entry for a package before
this has been discussed on the <code class="docutils literal notranslate">debian-devel</code> mailing list and a
consensus about doing that has been reached. See
<a class="reference internal" href="ch-binary.html#s-dependencies">Dependencies</a>.
</dd>
</dl>
When selecting which level of dependency to use you should consider how
important the depended-on package is to the functionality of the one
declaring the dependency. Some packages are composed of components of
varying degrees of importance. Such a package should list using
<code class="docutils literal notranslate">Depends</code> the package(s) which are required by the more important
components. The other components’ requirements may be mentioned as
Suggestions or Recommendations, as appropriate to the components’
relative importance.
</section>
<section id="packages-which-break-other-packages-breaks">
<h2>7.3. Packages which break other packages - <code class="docutils literal notranslate">Breaks</code><a class="headerlink" href="#packages-which-break-other-packages-breaks" title="Permalink to this headline">¶</a></h2>
When one binary package declares that it breaks another, <code class="docutils literal notranslate">dpkg</code> will
refuse to allow the package which declares <code class="docutils literal notranslate">Breaks</code> to be unpacked
unless the broken package is deconfigured first, and it will refuse to
allow the broken package to be reconfigured.
A package will not be regarded as causing breakage merely because its
configuration files are still installed; it must be at least
“Half-Installed”.
A special exception is made for packages which declare that they break
their own package name or a virtual package which they provide (see
below): this does not count as a real breakage.
Normally a <code class="docutils literal notranslate">Breaks</code> entry will have an “earlier than” version clause;
such a <code class="docutils literal notranslate">Breaks</code> is introduced in the version of an (implicit or
explicit) dependency which violates an assumption or reveals a bug in
earlier versions of the broken package, or which takes over a file from
earlier versions of the package named in <code class="docutils literal notranslate">Breaks</code>. This use of
<code class="docutils literal notranslate">Breaks</code> will inform higher-level package management tools that the
broken package must be upgraded before the new one.
If the breaking package also overwrites some files from the older
package, it should use <code class="docutils literal notranslate">Replaces</code> to ensure this goes smoothly. See
<a class="reference internal" href="#s-replaces">Overwriting files and replacing packages - Replaces</a> for a full discussion of taking over
files from other packages, including how to use <code class="docutils literal notranslate">Breaks</code> in those
cases.
Many of the cases where <code class="docutils literal notranslate">Breaks</code> should be used were previously
handled with <code class="docutils literal notranslate">Conflicts</code> because <code class="docutils literal notranslate">Breaks</code> did not yet exist. Many
<code class="docutils literal notranslate">Conflicts</code> fields should now be <code class="docutils literal notranslate">Breaks</code>. See
<a class="reference internal" href="#s-conflicts">Conflicting binary packages - Conflicts</a> for more information about the
differences.
</section>
<section id="conflicting-binary-packages-conflicts">
<h2>7.4. Conflicting binary packages - <code class="docutils literal notranslate">Conflicts</code><a class="headerlink" href="#conflicting-binary-packages-conflicts" title="Permalink to this headline">¶</a></h2>
When one binary package declares a conflict with another using a
<code class="docutils literal notranslate">Conflicts</code> field, <code class="docutils literal notranslate">dpkg</code> will refuse to allow them to be unpacked
on the system at the same time. This is a stronger restriction than
<code class="docutils literal notranslate">Breaks</code>, which prevents the broken package from being configured
while the breaking package is in the “Unpacked” state but allows both
packages to be unpacked at the same time.
If one package is to be unpacked, the other must be removed first. If
the package being unpacked is marked as replacing (see
<a class="reference internal" href="#s-replaces">Overwriting files and replacing packages - Replaces</a>, but note that <code class="docutils literal notranslate">Breaks</code> should
normally be used in this case) the one on the system, or the one on the
system is marked as deselected, or both packages are marked
<code class="docutils literal notranslate">Essential</code>, then <code class="docutils literal notranslate">dpkg</code> will automatically remove the package which
is causing the conflict. Otherwise, it will halt the installation of the
new package with an error. This mechanism is specifically designed to
produce an error when the installed package is <code class="docutils literal notranslate">Essential</code>, but the
new package is not.
A package will not cause a conflict merely because its configuration
files are still installed; it must be at least “Half-Installed”.
A special exception is made for packages which declare a conflict with
their own package name, or with a virtual package which they provide
(see below): this does not prevent their installation, and allows a
package to conflict with others providing a replacement for it. You use
this feature when you want the package in question to be the only
package providing some feature.
Normally, <code class="docutils literal notranslate">Breaks</code> should be used instead of <code class="docutils literal notranslate">Conflicts</code> since
<code class="docutils literal notranslate">Conflicts</code> imposes a stronger restriction on the ordering of package
installation or upgrade and can make it more difficult for the package
manager to find a correct solution to an upgrade or installation
problem. <code class="docutils literal notranslate">Breaks</code> should be used
<ul class="simple">
<li>when moving a file from one package to another (see
<a class="reference internal" href="#s-replaces">Overwriting files and replacing packages - Replaces</a>),</li>
<li>when splitting a package (a special case of the previous one), or</li>
<li>when the breaking package exposes a bug in or interacts badly with
particular versions of the broken package.</li>
</ul>
<code class="docutils literal notranslate">Conflicts</code> should be used
<ul class="simple">
<li>when two packages provide the same file and will continue to do so,</li>
<li>in conjunction with <code class="docutils literal notranslate">Provides</code> when only one package providing a
given virtual facility can be unpacked at a time (see
<a class="reference internal" href="#s-virtual">Virtual packages - Provides</a>),</li>
<li>in other cases where one must prevent simultaneous installation of
two packages for reasons that are ongoing (not fixed in a later
version of one of the packages) or that must prevent both packages
from being unpacked at the same time, not just configured.</li>
</ul>
Be aware that adding <code class="docutils literal notranslate">Conflicts</code> is normally not the best solution
when two packages provide the same files. Depending on the reason for
that conflict, using alternatives or renaming the files is often a
better approach. See, for example, <a class="reference internal" href="ch-files.html#s-binaries">Binaries</a>.
Neither <code class="docutils literal notranslate">Breaks</code> nor <code class="docutils literal notranslate">Conflicts</code> should be used unless two packages
cannot be installed at the same time or installing them both causes one
of them to be broken or unusable. Having similar functionality or
performing the same tasks as another package is not sufficient reason to
declare <code class="docutils literal notranslate">Breaks</code> or <code class="docutils literal notranslate">Conflicts</code> with that package.
A <code class="docutils literal notranslate">Conflicts</code> entry may have an “earlier than” version clause if the
reason for the conflict is corrected in a later version of one of the
packages. However, normally the presence of an “earlier than” version
clause is a sign that <code class="docutils literal notranslate">Breaks</code> should have been used instead. An
“earlier than” version clause in <code class="docutils literal notranslate">Conflicts</code> prevents <code class="docutils literal notranslate">dpkg</code> from
upgrading or installing the package which declares such a conflict until
the upgrade or removal of the conflicted-with package has been
completed, which is a strong restriction.
</section>
<section id="virtual-packages-provides">
<h2>7.5. Virtual packages - <code class="docutils literal notranslate">Provides</code><a class="headerlink" href="#virtual-packages-provides" title="Permalink to this headline">¶</a></h2>
As well as the names of actual (“concrete”) packages, the package
relationship fields <code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Recommends</code>, <code class="docutils literal notranslate">Suggests</code>,
<code class="docutils literal notranslate">Enhances</code>, <code class="docutils literal notranslate">Pre-Depends</code>, <code class="docutils literal notranslate">Breaks</code>, <code class="docutils literal notranslate">Conflicts</code>,
<code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>,
<code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Conflicts-Indep</code> and
<code class="docutils literal notranslate">Build-Conflicts-Arch</code> may mention “virtual packages”.
A virtual package is one which appears in the <code class="docutils literal notranslate">Provides</code> control
field of another package. The effect is as if the package(s) which
provide a particular virtual package name had been listed by name
everywhere the virtual package name appears. (See also
<a class="reference internal" href="ch-binary.html#s-virtual-pkg">Virtual packages</a>)
If there are both concrete and virtual packages of the same name, then
the dependency may be satisfied (or the conflict caused) by either the
concrete package with the name in question or any other concrete package
which provides the virtual package with the name in question. This is so
that, for example, supposing we have
<div class="highlight-default notranslate"><div class="highlight"><pre>Package: foo
Depends: bar
</pre></div>
</div>
and someone else releases an enhanced version of the <code class="docutils literal notranslate">bar</code> package
they can say:
<div class="highlight-default notranslate"><div class="highlight"><pre>Package: bar-plus
Provides: bar
</pre></div>
</div>
and the <code class="docutils literal notranslate">bar-plus</code> package will now also satisfy the dependency for
the <code class="docutils literal notranslate">foo</code> package.
A <code class="docutils literal notranslate">Provides</code> field may contain version numbers, and such a version number
will be considered when considering a dependency on or conflict with the
virtual package name. For example, given the following packages:
<div class="highlight-default notranslate"><div class="highlight"><pre>Package: foo
Depends: bar (&gt;= 1.0)

Package: bar
Version: 0.9

Package: bar-plus
Provides: bar (= 1.0)
</pre></div>
</div>
the <code class="docutils literal notranslate">bar-plus</code> package will satisfy the dependency for the <code class="docutils literal notranslate">foo</code>
package with the virtual package name, as above. If the <code class="docutils literal notranslate">Provides</code>
field does not specify a version number, it will not satisfy versioned
dependencies or violate versioned <code class="docutils literal notranslate">Conflicts</code> or <code class="docutils literal notranslate">Breaks</code>. For example,
given the following packages:
<div class="highlight-default notranslate"><div class="highlight"><pre>Package: foo
Depends: bar (&gt;= 1.0)

Package: bar
Version: 0.9

Package: bar-clone
Provides: bar
</pre></div>
</div>
the <code class="docutils literal notranslate">bar-plus</code> package will satisfy the dependency for the <code class="docutils literal notranslate">foo</code>
package, but the <code class="docutils literal notranslate">bar-clone</code> package will not.
To specify which of a set of real packages should be the default to
satisfy a particular dependency on a virtual package, list the real
package as an alternative before the virtual one.
If the virtual package represents a facility that can only be provided
by one real package at a time, such as the mail-transport-agent virtual
package that requires installation of a binary that would conflict with
all other providers of that virtual package (see
<a class="reference internal" href="ch-customized-programs.html#s-mail-transport-agents">Mail transport, delivery and user agents</a>), all packages providing
that virtual package should also declare a conflict with it using
<code class="docutils literal notranslate">Conflicts</code>. This will ensure that at most one provider of that
virtual package is unpacked or installed at a time.
</section>
<section id="overwriting-files-and-replacing-packages-replaces">
<h2>7.6. Overwriting files and replacing packages - <code class="docutils literal notranslate">Replaces</code><a class="headerlink" href="#overwriting-files-and-replacing-packages-replaces" title="Permalink to this headline">¶</a></h2>
Packages can declare in their control file that they should overwrite
files in certain other packages, or completely replace other packages.
The <code class="docutils literal notranslate">Replaces</code> control field has these two distinct purposes.
<section id="overwriting-files-in-other-packages">
<h3>7.6.1. Overwriting files in other packages<a class="headerlink" href="#overwriting-files-in-other-packages" title="Permalink to this headline">¶</a></h3>
It is usually an error for a package to contain files which are on the
system in another package. However, if the overwriting package declares
that it <code class="docutils literal notranslate">Replaces</code> the one containing the file being overwritten, then
<code class="docutils literal notranslate">dpkg</code> will replace the file from the old package with that from the
new. The file will no longer be listed as “owned” by the old package and
will be taken over by the new package. Normally, <code class="docutils literal notranslate">Breaks</code> should be
used in conjunction with <code class="docutils literal notranslate">Replaces</code>. <a class="footnote-reference brackets" href="#id11" id="id4">4</a>
For example, if a package foo is split into foo and foo-data starting at
version 1.2-3, foo-data would have the fields
<div class="highlight-default notranslate"><div class="highlight"><pre>Replaces: foo (&lt;&lt; 1.2-3)
Breaks: foo (&lt;&lt; 1.2-3)
</pre></div>
</div>
in its control file. The new version of the package foo would normally
have the field
<div class="highlight-default notranslate"><div class="highlight"><pre>Depends: foo-data (&gt;= 1.2-3)
</pre></div>
</div>
(or possibly <code class="docutils literal notranslate">Recommends</code> or even <code class="docutils literal notranslate">Suggests</code> if the files moved into
foo-data are not required for normal operation).
If a package is completely replaced in this way, so that <code class="docutils literal notranslate">dpkg</code> does
not know of any files it still contains, it is considered to have
“disappeared”. It will be marked as not wanted on the system (selected
for removal) and “Not-Installed”. Any <code class="docutils literal notranslate">conffile</code>s details noted for
the package will be ignored, as they will have been taken over by the
overwriting package. The package’s <code class="docutils literal notranslate">postrm</code> script will be run with a
special argument to allow the package to do any final cleanup required.
See <a class="reference internal" href="ch-maintainerscripts.html#s-mscriptsinstact">Summary of ways maintainer scripts are called</a>. <a class="footnote-reference brackets" href="#id12" id="id5">5</a>
For this usage of <code class="docutils literal notranslate">Replaces</code>, virtual packages (see
<a class="reference internal" href="#s-virtual">Virtual packages - Provides</a>) are not considered when looking at a
<code class="docutils literal notranslate">Replaces</code> field. The packages declared as being replaced must be
mentioned by their real names.
This usage of <code class="docutils literal notranslate">Replaces</code> only takes effect when both packages are at
least partially on the system at once. It is not relevant if the
packages conflict unless the conflict has been overridden.
</section>
<section id="replacing-whole-packages-forcing-their-removal">
<h3>7.6.2. Replacing whole packages, forcing their removal<a class="headerlink" href="#replacing-whole-packages-forcing-their-removal" title="Permalink to this headline">¶</a></h3>
Second, <code class="docutils literal notranslate">Replaces</code> allows the packaging system to resolve which
package should be removed when there is a conflict (see
<a class="reference internal" href="#s-conflicts">Conflicting binary packages - Conflicts</a>). This usage only takes effect when
the two packages do conflict, so that the two usages of this field do
not interfere with each other.
In this situation, the package declared as being replaced can be a
virtual package, so for example, all mail transport agents (MTAs) would
have the following fields in their control files:
<div class="highlight-default notranslate"><div class="highlight"><pre>Provides: mail-transport-agent
Conflicts: mail-transport-agent
Replaces: mail-transport-agent
</pre></div>
</div>
ensuring that only one MTA can be unpacked at any one time. See
<a class="reference internal" href="#s-virtual">Virtual packages - Provides</a> for more information about this example.
</section>
</section>
<section id="relationships-between-source-and-binary-packages-build-depends-build-depends-indep-build-depends-arch-build-conflicts-build-conflicts-indep-build-conflicts-arch">
<h2>7.7. Relationships between source and binary packages - <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Conflicts-Indep</code>, <code class="docutils literal notranslate">Build-Conflicts-Arch</code><a class="headerlink" href="#relationships-between-source-and-binary-packages-build-depends-build-depends-indep-build-depends-arch-build-conflicts-build-conflicts-indep-build-conflicts-arch" title="Permalink to this headline">¶</a></h2>
Source packages that require certain binary packages to be installed or
absent at the time of building the package may declare relationships to
those binary packages.
This is done using the <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>,
<code class="docutils literal notranslate">Build-Depends-Arch</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Conflicts-Indep</code>
and <code class="docutils literal notranslate">Build-Conflicts-Arch</code> control fields.
Build-dependencies on “build-essential” binary packages can be omitted.
Please see <a class="reference internal" href="ch-source.html#s-pkg-relations">Package relationships</a> for more information.
The dependencies and conflicts they define must be satisfied (as defined
earlier for binary packages) in order to invoke the targets in
<code class="docutils literal notranslate">debian/rules</code>, as follows:
<dl class="simple">
<dt><code class="docutils literal notranslate">clean</code></dt><dd>Only the <code class="docutils literal notranslate">Build-Depends</code> and <code class="docutils literal notranslate">Build-Conflicts</code> fields must be
satisfied when this target is invoked.
</dd>
<dt><code class="docutils literal notranslate">build-arch</code>, and <code class="docutils literal notranslate">binary-arch</code></dt><dd>The <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>,
and <code class="docutils literal notranslate">Build-Conflicts-Arch</code> fields must be satisfied when these
targets are invoked.
</dd>
<dt><code class="docutils literal notranslate">build-indep</code>, and <code class="docutils literal notranslate">binary-indep</code></dt><dd>The <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>,
and <code class="docutils literal notranslate">Build-Conflicts-Indep</code> fields must be satisfied when these
targets are invoked.
</dd>
<dt><code class="docutils literal notranslate">build</code> and <code class="docutils literal notranslate">binary</code></dt><dd>The <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>,
<code class="docutils literal notranslate">Build-Conflicts-Indep</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>, and
<code class="docutils literal notranslate">Build-Conflicts-Arch</code> fields must be satisfied when these targets
are invoked.
</dd>
</dl>
</section>
<section id="additional-source-packages-used-to-build-the-binary-built-using">
<h2>7.8. Additional source packages used to build the binary - <code class="docutils literal notranslate">Built-Using</code><a class="headerlink" href="#additional-source-packages-used-to-build-the-binary-built-using" title="Permalink to this headline">¶</a></h2>
Some binary packages incorporate parts of other packages when built
but do not have to depend on those packages. Examples include linking
with static libraries or incorporating source code from another
package during the build. In this case, the source packages of those
other packages are part of the complete source (the binary package is
not reproducible without them).
When the license of either the incorporated parts or the incorporating
binary package requires that the full source code of the incorporating
binary package be made available, the <code class="docutils literal notranslate">Built-Using</code> field must list
the corresponding source package for any affected binary package
incorporated during the build, <a class="footnote-reference brackets" href="#id13" id="id6">6</a> including an “exactly equal” (“=”)
version relation on the version that was used to build that version of
the incorporating binary package. <a class="footnote-reference brackets" href="#id14" id="id7">7</a>
This causes the Debian archive to retain the versions of the source
packages that were actually incorporated. In particular, if the
versions of the incorporated parts are updated but the incorporating
binary package is not rebuilt, the older versions of the incorporated
parts will remain in the archive in order to satisfy the license.
A package using the source code from the gcc-4.6-source binary package
built from the gcc-4.6 source package would have this field in its
control file:
<div class="highlight-default notranslate"><div class="highlight"><pre>Built-Using: gcc-4.6 (= 4.6.0-11)
</pre></div>
</div>
A package including binaries from grub2 and loadlin would have this
field in its control file:
<div class="highlight-default notranslate"><div class="highlight"><pre>Built-Using: grub2 (= 1.99-9), loadlin (= 1.6e-1)
</pre></div>
</div>
This field should be used only when there are license or DFSG
requirements to retain the referenced source packages. It should not
be added solely as a way to locate packages that need to be rebuilt
against newer versions of their build dependencies.
<dl class="footnote brackets">
<dt class="label" id="id8"><a class="fn-backref" href="#id1">1</a></dt>
<dd>While <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code> and
<code class="docutils literal notranslate">Build-Depends-Arch</code> permit the use of alternative dependencies,
these are not normally used by the Debian autobuilders. To avoid
inconsistency between repeated builds of a package, the
autobuilders will default to selecting the first alternative, after
reducing any architecture-specific restrictions for the build
architecture in question. While this may limit the usefulness of
alternatives in a single release, they can still be used to provide
flexibility in building the same package across multiple
distributions or releases, where a particular dependency is met by
differently named packages.
</dd>
<dt class="label" id="id9"><a class="fn-backref" href="#id2">2</a></dt>
<dd>The relations <code class="docutils literal notranslate">&lt;</code> and <code class="docutils literal notranslate">&gt;</code> were previously allowed, but they were
confusingly defined to mean earlier/later or equal rather than
strictly earlier/later. <code class="docutils literal notranslate">dpkg</code> still supports them with a warning,
but they are no longer allowed by Debian Policy.
</dd>
<dt class="label" id="id10"><a class="fn-backref" href="#id3">3</a></dt>
<dd>This approach makes dependency resolution easier. If two packages A
and B are being upgraded, the installed package A depends on exactly
the installed package B, and the new package A depends on exactly the
new package B (a common situation when upgrading shared libraries and
their corresponding development packages), satisfying the
dependencies at every stage of the upgrade would be impossible. This
relaxed restriction means that both new packages can be unpacked
together and then configured in their dependency order.
</dd>
<dt class="label" id="id11"><a class="fn-backref" href="#id4">4</a></dt>
<dd>To see why <code class="docutils literal notranslate">Breaks</code> is normally needed in addition to <code class="docutils literal notranslate">Replaces</code>,
consider the case of a file in the package foo being taken over by
the package foo-data. <code class="docutils literal notranslate">Replaces</code> will allow foo-data to be
installed and take over that file. However, without <code class="docutils literal notranslate">Breaks</code>,
nothing requires foo to be upgraded to a newer version that knows it
does not include that file and instead depends on foo-data. Nothing
would prevent the new foo-data package from being installed and then
removed, removing the file that it took over from foo. After that
operation, the package manager would think the system was in a
consistent state, but the foo package would be missing one of its
files.
</dd>
<dt class="label" id="id12"><a class="fn-backref" href="#id5">5</a></dt>
<dd>Replaces is a one way relationship. You have to install the replacing
package after the replaced package.
</dd>
<dt class="label" id="id13"><a class="fn-backref" href="#id6">6</a></dt>
<dd><code class="docutils literal notranslate">Build-Depends</code> in the source package is not adequate since it
(rightfully) does not document the exact version used in the build.
</dd>
<dt class="label" id="id14"><a class="fn-backref" href="#id7">7</a></dt>
<dd>The archive software might reject packages that refer to non-existent
sources.
</dd>
</dl>
</section>
</section>

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 <h3><a href="index.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">7. Declaring relationships between packages</a><ul>
<li><a class="reference internal" href="#syntax-of-relationship-fields">7.1. Syntax of relationship fields</a></li>
<li><a class="reference internal" href="#binary-dependencies-depends-recommends-suggests-enhances-pre-depends">7.2. Binary Dependencies - <code class="docutils literal notranslate">Depends</code>, <code class="docutils literal notranslate">Recommends</code>, <code class="docutils literal notranslate">Suggests</code>, <code class="docutils literal notranslate">Enhances</code>, <code class="docutils literal notranslate">Pre-Depends</code></a></li>
<li><a class="reference internal" href="#packages-which-break-other-packages-breaks">7.3. Packages which break other packages - <code class="docutils literal notranslate">Breaks</code></a></li>
<li><a class="reference internal" href="#conflicting-binary-packages-conflicts">7.4. Conflicting binary packages - <code class="docutils literal notranslate">Conflicts</code></a></li>
<li><a class="reference internal" href="#virtual-packages-provides">7.5. Virtual packages - <code class="docutils literal notranslate">Provides</code></a></li>
<li><a class="reference internal" href="#overwriting-files-and-replacing-packages-replaces">7.6. Overwriting files and replacing packages - <code class="docutils literal notranslate">Replaces</code></a><ul>
<li><a class="reference internal" href="#overwriting-files-in-other-packages">7.6.1. Overwriting files in other packages</a></li>
<li><a class="reference internal" href="#replacing-whole-packages-forcing-their-removal">7.6.2. Replacing whole packages, forcing their removal</a></li>
</ul>
</li>
<li><a class="reference internal" href="#relationships-between-source-and-binary-packages-build-depends-build-depends-indep-build-depends-arch-build-conflicts-build-conflicts-indep-build-conflicts-arch">7.7. Relationships between source and binary packages - <code class="docutils literal notranslate">Build-Depends</code>, <code class="docutils literal notranslate">Build-Depends-Indep</code>, <code class="docutils literal notranslate">Build-Depends-Arch</code>, <code class="docutils literal notranslate">Build-Conflicts</code>, <code class="docutils literal notranslate">Build-Conflicts-Indep</code>, <code class="docutils literal notranslate">Build-Conflicts-Arch</code></a></li>
<li><a class="reference internal" href="#additional-source-packages-used-to-build-the-binary-built-using">7.8. Additional source packages used to build the binary - <code class="docutils literal notranslate">Built-Using</code></a></li>
</ul>
</li>
</ul>

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