systemd/man/systemd-poweroff.service.xml

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<refentry id="systemd-poweroff.service">

  <refentryinfo>
    <title>systemd-poweroff.service</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-poweroff.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-poweroff.service</refname>
    <refname>systemd-halt.service</refname>
    <refname>systemd-reboot.service</refname>
    <refname>systemd-kexec.service</refname>
    <refname>systemd-shutdown</refname>
    <refpurpose>System shutdown logic</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-poweroff.service</filename></para>
    <para><filename>systemd-halt.service</filename></para>
    <para><filename>systemd-reboot.service</filename></para>
    <para><filename>systemd-kexec.service</filename></para>
    <para><filename>/usr/lib/systemd/systemd-shutdown</filename></para>
    <para><filename>/usr/lib/systemd/system-shutdown/</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-poweroff.service</filename> is a system service that is pulled in by
    <filename>poweroff.target</filename> and is responsible for the actual system power-off
    operation. Similarly, <filename>systemd-halt.service</filename> is pulled in by
    <filename>halt.target</filename>, <filename>systemd-reboot.service</filename> by
    <filename>reboot.target</filename> and <filename>systemd-kexec.service</filename> by
    <filename>kexec.target</filename> to execute the respective actions.</para>

    <para>When these services are run, they ensure that PID 1 is replaced by the
    <filename>/usr/lib/systemd/systemd-shutdown</filename> tool which is then responsible for the actual
    shutdown. Before shutting down, this binary will try to unmount all remaining file systems (or at least
    remount them read-only), disable all remaining swap devices, detach all remaining storage devices and
    kill all remaining processes.</para>

    <para>It is necessary to have this code in a separate binary because otherwise rebooting after an upgrade
    might be broken — the running PID 1 could still depend on libraries which are not available any more,
    thus keeping the file system busy, which then cannot be re-mounted read-only.</para>

    <para>Shortly before executing the actual system power-off/halt/reboot/kexec
    <filename>systemd-shutdown</filename> will run all executables in
    <filename>/usr/lib/systemd/system-shutdown/</filename> and pass one arguments to them: either
    <literal>poweroff</literal>, <literal>halt</literal>, <literal>reboot</literal>, or
    <literal>kexec</literal>, depending on the chosen action. All executables in this directory are executed
    in parallel, and execution of the action is not continued before all executables finished. Note that
    these executables are run <emphasis>after</emphasis> all services have been shut down, and after most
    mounts have been detached (the root file system as well as <filename>/run/</filename> and various API
    file systems are still around though). This means any programs dropped into this directory must be
    prepared to run in such a limited execution environment and not rely on external services or hierarchies
    such as <filename>/var/</filename> to be around (or writable).</para>

    <para>Note that <filename>systemd-poweroff.service</filename> (and the related units) should never be
    executed directly. Instead, trigger system shutdown with a command such as <literal>systemctl
    poweroff</literal>.</para>

    <para>Another form of shutdown is provided by the
    <citerefentry><refentrytitle>systemd-soft-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    functionality. It reboots only the OS userspace, leaving the kernel, firmware, and hardware as it is.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>reboot</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-suspend.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-soft-reboot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>