man/systemd-halt.service.xml

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

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

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

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

        <refnamediv>
                <refname>systemd-halt.service</refname>
                <refname>systemd-poweroff.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-halt.service</filename></para>
                <para><filename>systemd-poweroff.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>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para><filename>systemd-halt.service</filename> is a
                system service that is pulled in by
                <filename>halt.target</filename> and is responsible
                for the actual system halt. Similarly,
                <filename>systemd-poweroff.service</filename> is
                pulled in by <filename>poweroff.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, 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>Immediately before executing the actual system
                halt/poweroff/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>halt</literal>,
                <literal>poweroff</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.</para>

                <para>Note that
                <filename>systemd-halt.service</filename> (and the
                related units) should never be executed
                directly. Instead, trigger system shutdown with a
                command such as <literal>systemctl halt</literal> or
                suchlike.</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>
                </para>
        </refsect1>

</refentry>