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<li class="nav-item nav-item-this"><a href=""><span class="section-number">7. </span>Diversions - overriding a package’s version of a file (from old Packaging Manual)</a></li>
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<section id="diversions-overriding-a-package-s-version-of-a-file-from-old-packaging-manual">
<h1><span class="section-number">7. </span>Diversions - overriding a package’s version of a file (from old Packaging Manual)<a class="headerlink" href="#diversions-overriding-a-package-s-version-of-a-file-from-old-packaging-manual" title="Permalink to this headline">¶</a></h1>
<p>It is possible to have <code class="docutils literal notranslate"><span class="pre">dpkg</span></code> not overwrite a file when it reinstalls
the package it belongs to, and to have it put the file from the package
somewhere else instead.</p>
<p>This can be used locally to override a package’s version of a file, or
by one package to override another’s version (or provide a wrapper for
it).</p>
<p>Before deciding to use a diversion, read
<a class="reference internal" href="ap-pkg-alternatives.html"><span class="doc">Alternative versions of an interface - update-alternatives (from old Packaging Manual)</span></a> to see if you really want a
diversion rather than several alternative versions of a program.</p>
<p>There is a diversion list, which is read by <code class="docutils literal notranslate"><span class="pre">dpkg</span></code>, and updated by a
special program <code class="docutils literal notranslate"><span class="pre">dpkg-divert</span></code>. Please see <em class="manpage">dpkg-divert(8)</em> for
full details of its operation.</p>
<p>When a package wishes to divert a file from another, it should call
<code class="docutils literal notranslate"><span class="pre">dpkg-divert</span></code> in its preinst to add the diversion and rename the
existing file. For example, supposing that a <code class="docutils literal notranslate"><span class="pre">smailwrapper</span></code> package
wishes to install a wrapper around <code class="docutils literal notranslate"><span class="pre">/usr/sbin/smail</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dpkg</span><span class="o">-</span><span class="n">divert</span> <span class="o">--</span><span class="n">package</span> <span class="n">smailwrapper</span> <span class="o">--</span><span class="n">add</span> <span class="o">--</span><span class="n">rename</span> \
<span class="o">--</span><span class="n">divert</span> <span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">sbin</span><span class="o">/</span><span class="n">smail</span><span class="o">.</span><span class="n">real</span> <span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">sbin</span><span class="o">/</span><span class="n">smail</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">--package</span> <span class="pre">smailwrapper</span></code> ensures that <code class="docutils literal notranslate"><span class="pre">smailwrapper</span></code>’s copy of
<code class="docutils literal notranslate"><span class="pre">/usr/sbin/smail</span></code> can bypass the diversion and get installed as the
true version. It’s safe to add the diversion unconditionally on upgrades
since it will be left unchanged if it already exists, but
<code class="docutils literal notranslate"><span class="pre">dpkg-divert</span></code> will display a message. To suppress that message, make
the command conditional on the version from which the package is being
upgraded:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>if [ upgrade != "$1 ] || dpkg --compare-versions "$2" lt 1.0-2; then
dpkg-divert --package smailwrapper --add --rename \
--divert /usr/sbin/smail.real /usr/sbin/smail
fi
</pre></div>
</div>
<p>where <code class="docutils literal notranslate"><span class="pre">1.0-2</span></code> is the version at which the diversion was first added to
the package. Running the command during abort-upgrade is pointless but
harmless.</p>
<p>The postrm has to do the reverse:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="p">[</span> <span class="n">remove</span> <span class="o">=</span> <span class="s2">"$1"</span> <span class="o">-</span><span class="n">o</span> <span class="n">abort</span><span class="o">-</span><span class="n">install</span> <span class="o">=</span> <span class="s2">"$1"</span> <span class="o">-</span><span class="n">o</span> <span class="n">disappear</span> <span class="o">=</span> <span class="s2">"$1 ]; then</span>
<span class="n">dpkg</span><span class="o">-</span><span class="n">divert</span> <span class="o">--</span><span class="n">package</span> <span class="n">smailwrapper</span> <span class="o">--</span><span class="n">remove</span> <span class="o">--</span><span class="n">rename</span> \
<span class="o">--</span><span class="n">divert</span> <span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">sbin</span><span class="o">/</span><span class="n">smail</span><span class="o">.</span><span class="n">real</span> <span class="o">/</span><span class="n">usr</span><span class="o">/</span><span class="n">sbin</span><span class="o">/</span><span class="n">smail</span>
<span class="n">fi</span>
</pre></div>
</div>
<p>If the diversion was added at a particular version, the postrm should
also handle the failure case of upgrading from an older version (unless
the older version is so old that direct upgrades are no longer
supported):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>if [ abort-upgrade = "$1 ] && dpkg --compare-versions "$2" lt 1.0-2; then
dpkg-divert --package smailwrapper --remove --rename \
--divert /usr/sbin/smail.real /usr/sbin/smail
fi
</pre></div>
</div>
<p>where <code class="docutils literal notranslate"><span class="pre">1.0-2</span></code> is the version at which the diversion was first added to
the package. The postrm should not remove the diversion on upgrades both
because there’s no reason to remove the diversion only to immediately
re-add it and since the postrm of the old package is run after unpacking
so the removal of the diversion will fail.</p>
<p>Do not attempt to divert a file which is vitally important for the
system’s operation - when using <code class="docutils literal notranslate"><span class="pre">dpkg-divert</span></code> there is a time, after
it has been diverted but before <code class="docutils literal notranslate"><span class="pre">dpkg</span></code> has installed the new version,
when the file does not exist.</p>
<p>Do not attempt to divert a conffile, as <code class="docutils literal notranslate"><span class="pre">dpkg</span></code> does not handle it
well.</p>
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