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<refentry id="systemd.v">

  <refentryinfo>
    <title>systemd.v</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd.v</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd.v</refname>
    <refpurpose>Directory with Versioned Resources</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Description</title>

    <para>In various places systemd components accept paths whose trailing components have the
    <literal>.v/</literal> suffix, pointing to a directory. These components will then automatically look for
    suitable files inside the directory, do a version comparison and open the newest file found (by
    version). Available since version v256. Specifically, two expressions are supported:</para>

    <itemizedlist>

      <listitem><para>When looking for files with a suffix <replaceable>.SUFFIX</replaceable>, and a path
      <filename>…<replaceable>PATH</replaceable>/<replaceable>NAME</replaceable><replaceable>.SUFFIX</replaceable>.v/</filename>
      is specified, then all files
      <filename>…<replaceable>PATH</replaceable>/<replaceable>NAME</replaceable><replaceable>.SUFFIX</replaceable>.v/<replaceable>NAME</replaceable>_*<replaceable>.SUFFIX</replaceable></filename>
      are enumerated, filtered, sorted and the newest file used. The primary sorting key is the
      <emphasis>variable part</emphasis>, here indicated by the wildcard
      <literal>*</literal>.</para></listitem>

      <listitem><para>When a path
      <filename>…<replaceable>PATH</replaceable>.v/<replaceable>NAME</replaceable>___<replaceable>.SUFFIX</replaceable></filename>
      is specified (i.e. the penultimate component of the path ends in <literal>.v</literal> and the final
      component contains a triple underscore), then all files
      <filename>…<replaceable>PATH</replaceable>.v/<replaceable>NAME</replaceable>_*<replaceable>.SUFFIX</replaceable></filename>
      are enumerated, filtered, sorted and the newest file used (again, by the <emphasis>variable
      part</emphasis>, here indicated by the wildcard <literal>*</literal>).</para></listitem>
    </itemizedlist>

    <para>To illustrate this in an example, consider a directory <filename>/var/lib/machines/mymachine.raw.v/</filename>, which is populated with three files:</para>

    <itemizedlist>
      <listitem><para><filename>mymachine_7.5.13.raw</filename></para></listitem>
      <listitem><para><filename>mymachine_7.5.14.raw</filename></para></listitem>
      <listitem><para><filename>mymachine_7.6.0.raw</filename></para></listitem>
    </itemizedlist>

    <para>Invoke a tool such as <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry> with a command line like the following:</para>

    <programlisting># systemd-nspawn --image=/var/lib/machines/mymachine.raw.v --boot</programlisting>

    <para>Then this would automatically be resolved to the equivalent of:</para>

    <programlisting># systemd-nspawn --image=/var/lib/machines/mymachine.raw.v/mymachine_7.6.0.raw --boot</programlisting>

    <para>Much of systemd's functionality that expects a path to a disk image or OS directory hierarchy
    support the <literal>.v/</literal> versioned directory mechanism, for example
    <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry> or
    the <varname>RootDirectory=</varname>/<varname>RootImage=</varname> settings of service files (see
    <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para>

    <para>Use the
    <citerefentry><refentrytitle>systemd-vpick</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool to
    resolve <literal>.v/</literal> paths from the command line, for example for usage in shell
    scripts.</para>
  </refsect1>

  <refsect1>
    <title>Filtering and Sorting</title>

    <para>The variable part of the filenames in the <literal>.v/</literal> directories are filtered and
    compared primarily with a version comparison, implementing <ulink
    url="https://uapi-group.org/specifications/specs/version_format_specification/">Version Format
    Specification</ulink>. However, additional rules apply:</para>

    <itemizedlist>
      <listitem><para>If the variable part is suffixed by one or two integer values ("tries left" and "tries
      done") in the formats <filename>+<replaceable>LEFT</replaceable></filename> or
      <filename>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable></filename>, then these
      indicate usage attempt counters. The idea is that each time before a file is attempted to be used, its
      "tries left" counter is decreased, and the "tries done" counter increased (simply by renaming the
      file). When the file is successfully used (which for example could mean for an OS image: successfully
      booted) the counters are removed from the file name, indicating that the file has been validated to
      work correctly. This mechanism mirrors the boot assessment counters defined by <ulink
      url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT/">Automatic Boot Assessment</ulink>. Any filenames
      with no boot counters or with a non-zero "tries left" counter are sorted before filenames with a zero
      "tries left" counter.</para></listitem>

      <listitem><para>Preceding the use counters (if they are specified), an optional CPU architecture
      identifier may be specified in the filename (separated from the version with an underscore), as defined
      in the architecture vocabulary of the <varname>ConditionArchitecture=</varname> unit file setting, as
      documented in
      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Files
      whose name indicates an architecture not supported locally are filtered and not considered for the
      version comparison.</para></listitem>

      <listitem><para>The rest of the variable part is the version string.</para></listitem>
    </itemizedlist>

    <para>Or in other words, the files in the <literal>.v/</literal> directories should follow one of these
    naming structures:</para>

    <itemizedlist>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>+<replaceable>LEFT</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable>+<replaceable>LEFT</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
      <listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Example</title>

    <para>Here's a more comprehensive example, further extending the one described above. Consider a
    directory <filename>/var/lib/machines/mymachine.raw.v/</filename>, which is populated with the following
    files:</para>

    <itemizedlist>
      <listitem><para><filename>mymachine_7.5.13.raw</filename></para></listitem>
      <listitem><para><filename>mymachine_7.5.14_x86-64.raw</filename></para></listitem>
      <listitem><para><filename>mymachine_7.6.0_arm64.raw</filename></para></listitem>
      <listitem><para><filename>mymachine_7.7.0_x86-64+0-5.raw</filename></para></listitem>
    </itemizedlist>

    <para>Now invoke the following command on an x86-64 machine:</para>

    <programlisting>$ systemd-vpick --suffix=.raw /var/lib/machines/mymachine.raw.v/</programlisting>

    <para>This would resolve the specified path to
    <filename>/var/lib/machines/mymachine.raw.v/mymachine_7.5.14_x86-64.raw</filename>. Explanation: even
    though <filename>mymachine_7.7.0_x86-64+0-5.raw</filename> has the newest version, it is not preferred
    because its tries left counter is zero. And even though <filename>mymachine_7.6.0_arm64.raw</filename>
    has the second newest version it is also not considered, in this case because we operate on an x86_64
    system and the image is intended for arm64 CPUs. Finally, the <filename>mymachine_7.5.13.raw</filename>
    image is not considered because it is older than <filename>mymachine_7.5.14_x86-64.raw</filename>.</para>
  </refsect1>

  <refsect1>
      <title>See Also</title>
      <para><simplelist type="inline">
        <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-vpick</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
        <member><citerefentry><refentrytitle>systemd-sysupdate</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      </simplelist></para>
  </refsect1>

</refentry>
