path: root/man/systemd.xml

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  SPDX-License-Identifier: LGPL-2.1+

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-->

<refentry id="systemd"
    xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd</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</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd</refname>
    <refname>init</refname>
    <refpurpose>systemd system and service manager</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <cmdsynopsis>
      <command>/usr/lib/systemd/systemd</command>
      <arg choice="opt" rep="repeat">OPTIONS</arg>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>init</command>
      <arg choice="opt" rep="repeat">OPTIONS</arg>
      <arg choice="req">COMMAND</arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>systemd is a system and service manager for Linux operating
    systems. When run as first process on boot (as PID 1), it acts as
    init system that brings up and maintains userspace
    services.</para>

    <para>For compatibility with SysV, if systemd is called as
    <command>init</command> and a PID that is not 1, it will execute
    <command>telinit</command> and pass all command line arguments
    unmodified. That means <command>init</command> and
    <command>telinit</command> are mostly equivalent when invoked from
    normal login sessions. See
    <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    for more information.</para>

    <para>When run as a system instance, systemd interprets the
    configuration file <filename>system.conf</filename> and the files
    in <filename>system.conf.d</filename> directories; when run as a
    user instance, systemd interprets the configuration file
    <filename>user.conf</filename> and the files in
    <filename>user.conf.d</filename> directories. See
    <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for more information.</para>
  </refsect1>

  <refsect1>
    <title>Options</title>

    <para>The following options are understood:</para>

    <variablelist>
      <varlistentry>
        <term><option>--test</option></term>

        <listitem><para>Determine startup sequence, dump it and exit.
        This is an option useful for debugging only.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--dump-configuration-items</option></term>

        <listitem><para>Dump understood unit configuration items. This
        outputs a terse but complete list of configuration items
        understood in unit definition files.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--dump-bus-properties</option></term>

        <listitem><para>Dump exposed bus properties. This outputs
        a terse but complete list of properties exposed to dbus.
        </para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--unit=</option></term>

        <listitem><para>Set default unit to activate on startup. If
        not specified, defaults to
        <filename>default.target</filename>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--system</option></term>
        <term><option>--user</option></term>

        <listitem><para>For <option>--system</option>, tell systemd to
        run a system instance, even if the process ID is not 1, i.e.
        systemd is not run as init process. <option>--user</option>
        does the opposite, running a user instance even if the process
        ID is 1. Normally, it should not be necessary to pass these
        options, as systemd automatically detects the mode it is
        started in. These options are hence of little use except for
        debugging. Note that it is not supported booting and
        maintaining a full system with systemd running in
        <option>--system</option> mode, but PID not 1. In practice,
        passing <option>--system</option> explicitly is only useful in
        conjunction with <option>--test</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--dump-core</option></term>

        <listitem><para>Enable core dumping on crash. This switch has
        no effect when running as user instance. This setting may also
        be enabled during boot on the kernel command line via the
        <varname>systemd.dump_core=</varname> option, see
        below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>

        <listitem><para>Switch to a specific virtual console (VT) on
        crash. Takes a positive integer in the range 1–63, or a
        boolean argument. If an integer is passed, selects which VT to
        switch to. If <constant>yes</constant>, the VT kernel messages
        are written to is selected. If <constant>no</constant>, no VT
        switch is attempted. This switch has no effect when running as
        user instance. This setting may also be enabled during boot,
        on the kernel command line via the
        <varname>systemd.crash_vt=</varname> option, see
        <!-- FIXME: there is no crash_vt command line option? -->
        below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--crash-shell</option></term>

        <listitem><para>Run a shell on crash. This switch has no
        effect when running as user instance. This setting may also be
        enabled during boot, on the kernel command line via the
        <varname>systemd.crash_shell=</varname> option, see
        below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--crash-reboot</option></term>

        <listitem><para>Automatically reboot the system on crash. This
        switch has no effect when running as user instance. This
        setting may also be enabled during boot, on the kernel command
        line via the <varname>systemd.crash_reboot=</varname> option,
        see below.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--confirm-spawn</option></term>

        <listitem><para>Ask for confirmation when spawning processes.
        This switch has no effect when run as user
        instance.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--show-status=</option></term>

        <listitem><para>Takes a boolean argument or the special value <constant>auto</constant>. If on, terse unit
        status information is shown on the console during boot-up and shutdown. If off, no such status information is
        shown. If set to <constant>auto</constant> behavior is similar to off, except that it is automatically switched
        to on, as soon as the first unit failure or significant boot delay is encountered. This switch has no effect
        when invoked as user instance. If specified, overrides both the kernel command line setting
        <varname>systemd.show_status=</varname> (see below) and the configuration file option
        <option>ShowStatus=</option>, see
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--log-target=</option></term>

        <listitem><para>Set log target. Argument must be one of
        <option>console</option>,
        <option>journal</option>,
        <option>kmsg</option>,
        <option>journal-or-kmsg</option>,
        <option>null</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--log-level=</option></term>

        <listitem><para>Set log level. As
        argument this accepts a numerical log
        level or the well-known <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        symbolic names (lowercase):
        <option>emerg</option>,
        <option>alert</option>,
        <option>crit</option>,
        <option>err</option>,
        <option>warning</option>,
        <option>notice</option>,
        <option>info</option>,
        <option>debug</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--log-color=</option></term>

        <listitem><para>Highlight important log messages. Argument is
        a boolean value. If the argument is omitted, it defaults to
        <option>true</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--log-location=</option></term>

        <listitem><para>Include code location in log messages. This is
        mostly relevant for debugging purposes. Argument is a boolean
        value. If the argument is omitted it defaults to
        <option>true</option>.</para></listitem>
      </varlistentry>
      <varlistentry>
        <term><option>--default-standard-output=</option></term>
        <term><option>--default-standard-error=</option></term>

        <listitem><para>Sets the default output or error output for
        all services and sockets, respectively. That is, controls the
        default for <option>StandardOutput=</option> and
        <option>StandardError=</option> (see
        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        for details). Takes one of
        <option>inherit</option>,
        <option>null</option>,
        <option>tty</option>,
        <option>journal</option>,
        <option>journal+console</option>,
        <option>syslog</option>,
        <option>syslog+console</option>,
        <option>kmsg</option>,
        <option>kmsg+console</option>.  If the
        argument is omitted
        <option>--default-standard-output=</option> defaults to
        <option>journal</option> and
        <option>--default-standard-error=</option> to
        <option>inherit</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--machine-id=</option></term>

        <listitem><para>Override the machine-id set on the hard drive,
        useful for network booting or for containers. May not be set
        to all zeros.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><option>--service-watchdogs=</option></term>

        <listitem><para>Globally enable/disable all service watchdog timeouts and emergency
        actions. This setting may also be specified during boot, on the kernel
        command line via the <varname>systemd.service_watchdogs=</varname>
        option, see below. Defaults to enabled.</para></listitem>
      </varlistentry>

      <xi:include href="standard-options.xml" xpointer="help" />
      <xi:include href="standard-options.xml" xpointer="version" />
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Concepts</title>

    <para>systemd provides a dependency system between various
    entities called "units" of 11 different types. Units encapsulate
    various objects that are relevant for system boot-up and
    maintenance. The majority of units are configured in unit
    configuration files, whose syntax and basic set of options is
    described in
    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
    however some are created automatically from other configuration,
    dynamically from system state or programmatically at runtime.
    Units may be "active" (meaning started, bound, plugged in, …,
    depending on the unit type, see below), or "inactive" (meaning
    stopped, unbound, unplugged, …), as well as in the process of
    being activated or deactivated, i.e. between the two states (these
    states are called "activating", "deactivating"). A special
    "failed" state is available as well, which is very similar to
    "inactive" and is entered when the service failed in some way
    (process returned error code on exit, or crashed, an operation
    timed out, or after too many restarts). If this state is entered,
    the cause will be logged, for later reference. Note that the
    various unit types may have a number of additional substates,
    which are mapped to the five generalized unit states described
    here.</para>

    <para>The following unit types are available:</para>

    <orderedlist>
      <listitem><para>Service units, which start and control daemons
      and the processes they consist of. For details, see
      <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Socket units, which encapsulate local IPC or
      network sockets in the system, useful for socket-based
      activation. For details about socket units, see
      <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      for details on socket-based activation and other forms of
      activation, see
      <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Target units are useful to group units, or
      provide well-known synchronization points during boot-up, see
      <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Device units expose kernel devices in systemd
      and may be used to implement device-based activation. For
      details, see
      <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Mount units control mount points in the file
      system, for details see
      <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Automount units provide automount capabilities,
      for on-demand mounting of file systems as well as parallelized
      boot-up. See
      <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Timer units are useful for triggering activation
      of other units based on timers. You may find details in
      <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Swap units are very similar to mount units and
      encapsulate memory swap partitions or files of the operating
      system. They are described in
      <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Path units may be used to activate other
      services when file system objects change or are modified. See
      <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Slice units may be used to group units which
      manage system processes (such as service and scope units) in a
      hierarchical tree for resource management purposes. See
      <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

      <listitem><para>Scope units are similar to service units, but
      manage foreign processes instead of starting them as well. See
      <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>

    </orderedlist>

    <para>Units are named as their configuration files. Some units
    have special semantics. A detailed list is available in
    <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

    <para>systemd knows various kinds of dependencies, including
    positive and negative requirement dependencies (i.e.
    <varname>Requires=</varname> and <varname>Conflicts=</varname>) as
    well as ordering dependencies (<varname>After=</varname> and
    <varname>Before=</varname>). NB: ordering and requirement
    dependencies are orthogonal. If only a requirement dependency
    exists between two units (e.g. <filename>foo.service</filename>
    requires <filename>bar.service</filename>), but no ordering
    dependency (e.g. <filename>foo.service</filename> after
    <filename>bar.service</filename>) and both are requested to start,
    they will be started in parallel. It is a common pattern that both
    requirement and ordering dependencies are placed between two
    units. Also note that the majority of dependencies are implicitly
    created and maintained by systemd. In most cases, it should be
    unnecessary to declare additional dependencies manually, however
    it is possible to do this.</para>

    <para>Application programs and units (via dependencies) may
    request state changes of units. In systemd, these requests are
    encapsulated as 'jobs' and maintained in a job queue. Jobs may
    succeed or can fail, their execution is ordered based on the
    ordering dependencies of the units they have been scheduled
    for.</para>

    <para>On boot systemd activates the target unit
    <filename>default.target</filename> whose job is to activate
    on-boot services and other on-boot units by pulling them in via
    dependencies. Usually, the unit name is just an alias (symlink) for
    either <filename>graphical.target</filename> (for fully-featured
    boots into the UI) or <filename>multi-user.target</filename> (for
    limited console-only boots for use in embedded or server
    environments, or similar; a subset of graphical.target). However,
    it is at the discretion of the administrator to configure it as an
    alias to any other target unit. See
    <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    for details about these target units.</para>

    <para>Processes systemd spawns are placed in individual Linux
    control groups named after the unit which they belong to in the
    private systemd hierarchy. (see <ulink
    url="https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt">cgroups.txt</ulink>
    for more information about control groups, or short "cgroups").
    systemd uses this to effectively keep track of processes. Control
    group information is maintained in the kernel, and is accessible
    via the file system hierarchy (beneath
    <filename>/sys/fs/cgroup/systemd/</filename>), or in tools such as
    <citerefentry project='man-pages'><refentrytitle>systemd-cgls</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    or
    <citerefentry project='man-pages'><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    (<command>ps xawf -eo pid,user,cgroup,args</command> is
    particularly useful to list all processes and the systemd units
    they belong to.).</para>

    <para>systemd is compatible with the SysV init system to a large
    degree: SysV init scripts are supported and simply read as an
    alternative (though limited) configuration file format. The SysV
    <filename>/dev/initctl</filename> interface is provided, and
    compatibility implementations of the various SysV client tools are
    available. In addition to that, various established Unix
    functionality such as <filename>/etc/fstab</filename> or the
    <filename>utmp</filename> database are supported.</para>

    <para>systemd has a minimal transaction system: if a unit is
    requested to start up or shut down it will add it and all its
    dependencies to a temporary transaction. Then, it will verify if
    the transaction is consistent (i.e. whether the ordering of all
    units is cycle-free). If it is not, systemd will try to fix it up,
    and removes non-essential jobs from the transaction that might
    remove the loop. Also, systemd tries to suppress non-essential
    jobs in the transaction that would stop a running service. Finally
    it is checked whether the jobs of the transaction contradict jobs
    that have already been queued, and optionally the transaction is
    aborted then. If all worked out and the transaction is consistent
    and minimized in its impact it is merged with all already
    outstanding jobs and added to the run queue. Effectively this
    means that before executing a requested operation, systemd will
    verify that it makes sense, fixing it if possible, and only
    failing if it really cannot work.</para>

    <para>systemd contains native implementations of various tasks
    that need to be executed as part of the boot process. For example,
    it sets the hostname or configures the loopback network device. It
    also sets up and mounts various API file systems, such as
    <filename>/sys</filename> or <filename>/proc</filename>.</para>

    <para>For more information about the concepts and
    ideas behind systemd, please refer to the
    <ulink url="http://0pointer.de/blog/projects/systemd.html">Original Design Document</ulink>.</para>

    <para>Note that some but not all interfaces provided
    by systemd are covered by the
    <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
    Stability Promise</ulink>.</para>

    <para>Units may be generated dynamically at boot and system
    manager reload time, for example based on other configuration
    files or parameters passed on the kernel command line. For details, see
    <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

    <para>Systems which invoke systemd in a container or initrd
    environment should implement the
    <ulink url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink> or
    <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
    specifications, respectively.</para>
  </refsect1>

  <refsect1>
    <title>Directories</title>

    <variablelist>
      <varlistentry>
        <term>System unit directories</term>

        <listitem><para>The systemd system manager reads unit
        configuration from various directories. Packages that want to
        install unit files shall place them in the directory returned
        by <command>pkg-config systemd
        --variable=systemdsystemunitdir</command>. Other directories
        checked are <filename>/usr/local/lib/systemd/system</filename>
        and <filename>/usr/lib/systemd/system</filename>. User
        configuration always takes precedence. <command>pkg-config
        systemd --variable=systemdsystemconfdir</command> returns the
        path of the system configuration directory. Packages should
        alter the content of these directories only with the
        <command>enable</command> and <command>disable</command>
        commands of the
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        tool. Full list of directories is provided in
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>
    </variablelist>

    <variablelist>
      <varlistentry>
        <term>User unit directories</term>

        <listitem><para>Similar rules apply for the user unit
        directories. However, here the
        <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
        Base Directory specification</ulink> is followed to find
        units. Applications should place their unit files in the
        directory returned by <command>pkg-config systemd
        --variable=systemduserunitdir</command>. Global configuration
        is done in the directory reported by <command>pkg-config
        systemd --variable=systemduserconfdir</command>. The
        <command>enable</command> and <command>disable</command>
        commands of the
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        tool can handle both global (i.e. for all users) and private
        (for one user) enabling/disabling of units. Full list of
        directories is provided in
        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>
    </variablelist>

    <variablelist>
      <varlistentry>
        <term>SysV init scripts directory</term>

        <listitem><para>The location of the SysV init script directory
        varies between distributions. If systemd cannot find a native
        unit file for a requested service, it will look for a SysV
        init script of the same name (with the
        <filename>.service</filename> suffix
        removed).</para></listitem>
      </varlistentry>
    </variablelist>

    <variablelist>
      <varlistentry>
        <term>SysV runlevel link farm directory</term>

        <listitem><para>The location of the SysV runlevel link farm
        directory varies between distributions. systemd will take the
        link farm into account when figuring out whether a service
        shall be enabled. Note that a service unit with a native unit
        configuration file cannot be started by activating it in the
        SysV runlevel link farm.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Signals</title>

    <variablelist>
      <varlistentry>
        <term><constant>SIGTERM</constant></term>

        <listitem><para>Upon receiving this signal the systemd system
        manager serializes its state, reexecutes itself and
        deserializes the saved state again. This is mostly equivalent
        to <command>systemctl daemon-reexec</command>.</para>

        <para>systemd user managers will start the
        <filename>exit.target</filename> unit when this signal is
        received. This is mostly equivalent to <command>systemctl
        --user start exit.target
        --job-mode=replace-irreversible</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGINT</constant></term>

        <listitem><para>Upon receiving this signal the systemd system
        manager will start the
        <filename>ctrl-alt-del.target</filename> unit. This is mostly
        equivalent to <command>systemctl start ctrl-alt-del.target
        --job-mode=replace-irreversible</command>. If this signal is
        received more than 7 times per 2s, an immediate reboot is
        triggered.  Note that pressing Ctrl-Alt-Del on the console
        will trigger this signal. Hence, if a reboot is hanging,
        pressing Ctrl-Alt-Del more than 7 times in 2s is a relatively
        safe way to trigger an immediate reboot.</para>

        <para>systemd user managers treat this signal the same way as
        <constant>SIGTERM</constant>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGWINCH</constant></term>

        <listitem><para>When this signal is received the systemd
        system manager will start the
        <filename>kbrequest.target</filename> unit. This is mostly
        equivalent to <command>systemctl start
        kbrequest.target</command>.</para>

        <para>This signal is ignored by systemd user
        managers.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGPWR</constant></term>

        <listitem><para>When this signal is received the systemd
        manager will start the <filename>sigpwr.target</filename>
        unit. This is mostly equivalent to <command>systemctl start
        sigpwr.target</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGUSR1</constant></term>

        <listitem><para>When this signal is received the systemd
        manager will try to reconnect to the D-Bus
        bus.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGUSR2</constant></term>

        <listitem><para>When this signal is received the systemd
        manager will log its complete state in human-readable form.
        The data logged is the same as printed by
        <command>systemd-analyze dump</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGHUP</constant></term>

        <listitem><para>Reloads the complete daemon configuration.
        This is mostly equivalent to <command>systemctl
        daemon-reload</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+0</constant></term>

        <listitem><para>Enters default mode, starts the
        <filename>default.target</filename> unit. This is mostly
        equivalent to <command>systemctl isolate
        default.target</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+1</constant></term>

        <listitem><para>Enters rescue mode, starts the
        <filename>rescue.target</filename> unit. This is mostly
        equivalent to <command>systemctl isolate
        rescue.target</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+2</constant></term>

        <listitem><para>Enters emergency mode, starts the
        <filename>emergency.service</filename> unit. This is mostly
        equivalent to <command>systemctl isolate
        emergency.service</command>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+3</constant></term>

        <listitem><para>Halts the machine, starts the
        <filename>halt.target</filename> unit. This is mostly
        equivalent to <command>systemctl start halt.target
        --job-mode=replace-irreversible</command>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+4</constant></term>

        <listitem><para>Powers off the machine, starts the
        <filename>poweroff.target</filename> unit. This is mostly
        equivalent to <command>systemctl start poweroff.target
        --job-mode=replace-irreversible</command>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+5</constant></term>

        <listitem><para>Reboots the machine, starts the
        <filename>reboot.target</filename> unit. This is mostly
        equivalent to <command>systemctl start reboot.target
        --job-mode=replace-irreversible</command>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+6</constant></term>

        <listitem><para>Reboots the machine via kexec, starts the
        <filename>kexec.target</filename> unit. This is mostly
        equivalent to <command>systemctl start kexec.target
        --job-mode=replace-irreversible</command>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+13</constant></term>

        <listitem><para>Immediately halts the machine.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+14</constant></term>

        <listitem><para>Immediately powers off the machine.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+15</constant></term>

        <listitem><para>Immediately reboots the machine.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+16</constant></term>

        <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+20</constant></term>

        <listitem><para>Enables display of status messages on the
        console, as controlled via
        <varname>systemd.show_status=1</varname> on the kernel command
        line.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+21</constant></term>

        <listitem><para>Disables display of
        status messages on the console, as
        controlled via
        <varname>systemd.show_status=0</varname>
        on the kernel command
        line.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+22</constant></term>

        <listitem><para>Sets the service manager's log level to <literal>debug</literal>, in a fashion equivalent to
        <varname>systemd.log_level=debug</varname> on the kernel command line.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+23</constant></term>

        <listitem><para>Restores the log level to its configured value. The configured value is derived from – in order
        of priority – the value specified with <varname>systemd.log-level=</varname> on the kernel command line, or the
        value specified with <option>LogLevel=</option> in the configuration file, or the built-in default of
        <literal>info</literal>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+24</constant></term>

        <listitem><para>Immediately exits the manager (only available
        for --user instances).</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+26</constant></term>

        <listitem><para>Restores the log target to its configured value. The configured value is derived from – in
        order of priority – the value specified with <varname>systemd.log-target=</varname> on the kernel command line,
        or the value specified with <option>LogTarget=</option> in the configuration file, or the built-in
        default.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGRTMIN+27</constant></term>
        <term><constant>SIGRTMIN+28</constant></term>

        <listitem><para>Sets the log target to <literal>console</literal> on <constant>SIGRTMIN+27</constant> (or
        <literal>kmsg</literal> on <constant>SIGRTMIN+28</constant>), in a fashion equivalent to
        <varname>systemd.log_target=console</varname> (or <varname>systemd.log_target=kmsg</varname> on
        <constant>SIGRTMIN+28</constant>) on the kernel command line.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <variablelist class='environment-variables'>
      <varlistentry>
        <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
        <listitem><para>systemd reads the log level from this
        environment variable. This can be overridden with
        <option>--log-level=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
        <listitem><para>systemd reads the log target from this
        environment variable. This can be overridden with
        <option>--log-target=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
        <listitem><para>Controls whether systemd highlights important
        log messages. This can be overridden with
        <option>--log-color=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
        <listitem><para>Controls whether systemd prints the code
        location along with log messages. This can be overridden with
        <option>--log-location=</option>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$XDG_CONFIG_HOME</varname></term>
        <term><varname>$XDG_CONFIG_DIRS</varname></term>
        <term><varname>$XDG_DATA_HOME</varname></term>
        <term><varname>$XDG_DATA_DIRS</varname></term>

        <listitem><para>The systemd user manager uses these variables
        in accordance to the <ulink
        url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
        Base Directory specification</ulink> to find its
        configuration.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_UNIT_PATH</varname></term>

        <listitem><para>Controls where systemd looks for unit
        files.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>

        <listitem><para>Controls where systemd looks for SysV init
        scripts.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>

        <listitem><para>Controls where systemd looks for SysV init
        script runlevel link farms.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_COLORS</varname></term>

        <listitem><para>The value must be a boolean. Controls whether colorized output should be
        generated. This can be specified to override the decision that <command>systemd</command>
        makes based on <varname>$TERM</varname> and what the console is connected to.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$SYSTEMD_URLIFY</varname></term>

        <listitem><para>The value must be a boolean. Controls whether clickable links should be generated in the output
        for terminal emulators supporting this. This can be specified to override the decision that
        <command>systemd</command> makes based on <varname>$TERM</varname> and other conditions.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$LISTEN_PID</varname></term>
        <term><varname>$LISTEN_FDS</varname></term>
        <term><varname>$LISTEN_FDNAMES</varname></term>

        <listitem><para>Set by systemd for supervised processes during
        socket-based activation. See
        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for more information.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>$NOTIFY_SOCKET</varname></term>

        <listitem><para>Set by systemd for supervised processes for
        status and start-up completion notification. See
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
        for more information.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Kernel Command Line</title>

    <para>When run as system instance systemd parses a number of
    kernel command line arguments<footnote><para>If run inside a Linux
    container these arguments may be passed as command line arguments
    to systemd itself, next to any of the command line options listed
    in the Options section above. If run outside of Linux containers,
    these arguments are parsed from <filename>/proc/cmdline</filename>
    instead.</para></footnote>:</para>

    <variablelist class='kernel-commandline-options'>
      <varlistentry>
        <term><varname>systemd.unit=</varname></term>
        <term><varname>rd.systemd.unit=</varname></term>

        <listitem><para>Overrides the unit to activate on boot.
        Defaults to <filename>default.target</filename>. This may be
        used to temporarily boot into a different boot unit, for
        example <filename>rescue.target</filename> or
        <filename>emergency.service</filename>. See
        <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
        for details about these units. The option prefixed with
        <literal>rd.</literal> is honored only in the initial RAM disk
        (initrd), while the one that is not prefixed only in the main
        system.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.dump_core</varname></term>

        <listitem><para>Takes a boolean argument or enables the option if specified
        without an argument. If enabled, the systemd manager (PID 1) dumps core when
        it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.crash_chvt</varname></term>

        <listitem><para>Takes a positive integer, or a boolean argument. Can be also
        specified without an argument, with the same effect as a positive boolean. If
        a positive integer (in the range 1–63) is specified, the system manager (PID
        1) will activate the specified virtual terminal (VT) when it
        crashes. Defaults to disabled, meaning that no such switch is attempted. If
        set to enabled, the VT the kernel messages are written to is selected.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.crash_shell</varname></term>

        <listitem><para>Takes a boolean argument or enables the option if specified
        without an argument. If enabled, the system manager (PID 1) spawns a shell
        when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
        to disabled, for security reasons, as the shell is not protected by password
        authentication.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.crash_reboot</varname></term>

        <listitem><para>Takes a boolean argument or enables the option if specified
        without an argument. If enabled, the system manager (PID 1) will reboot the
        machine automatically when it crashes, after a 10s delay.  Otherwise, the
        system will hang indefinitely. Defaults to disabled, in order to avoid a
        reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
        system is rebooted after the shell exits.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.confirm_spawn</varname></term>

        <listitem><para>Takes a boolean argument or a path to the virtual console
        where the confirmation messages should be emitted. Can be also specified
        without an argument, with the same effect as a positive boolean. If enabled,
        the system manager (PID 1) asks for confirmation when spawning processes
        using <option>/dev/console</option>. If a path or a console name (such as
        <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
        path or described by the give name will be used instead. Defaults to disabled.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.service_watchdogs=</varname></term>

        <listitem><para>Takes a boolean argument. If disabled, all service runtime
        watchdogs (<option>WatchdogSec=</option>) and emergency actions (e.g.
        <option>OnFailure=</option> or <option>StartLimitAction=</option>) are
        ignored by the system manager (PID 1); see
        <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        Defaults to enabled, i.e. watchdogs and failure actions are processed
        normally. The hardware watchdog is not affected by this
        option.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.show_status</varname></term>

        <listitem><para>Takes a boolean argument or the constant
        <constant>auto</constant>. Can be also specified without an argument, with
        the same effect as a positive boolean.  If enabled, the systemd manager (PID
        1) shows terse service status updates on the console during bootup.
        <constant>auto</constant> behaves like <option>false</option> until a unit
        fails or there is a significant delay in boot. Defaults to enabled, unless
        <option>quiet</option> is passed as kernel command line option, in which case
        it defaults to <constant>auto</constant>. If specified overrides the system
        manager configuration file option <option>ShowStatus=</option>, see
        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        However, the process command line option <option>--show-status=</option>
        takes precedence over both this kernel command line option and the
        configuration file option.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.log_target=</varname></term>
        <term><varname>systemd.log_level=</varname></term>
        <term><varname>systemd.log_location=</varname></term>
        <term><varname>systemd.log_color</varname></term>

        <listitem><para>Controls log output, with the same effect as the
        <varname>$SYSTEMD_LOG_TARGET</varname>,
        <varname>$SYSTEMD_LOG_LEVEL</varname>,
        <varname>$SYSTEMD_LOG_LOCATION</varname>,
        <varname>$SYSTEMD_LOG_COLOR</varname> environment variables described above.
        <varname>systemd.log_color</varname> can be specified without an argument,
        with the same effect as a positive boolean.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.default_standard_output=</varname></term>
        <term><varname>systemd.default_standard_error=</varname></term>
        <listitem><para>Controls default standard output and error
        output for services, with the same effect as the
        <option>--default-standard-output=</option> and
        <option>--default-standard-error=</option> command line
        arguments described above, respectively.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.setenv=</varname></term>

        <listitem><para>Takes a string argument in the form
        VARIABLE=VALUE. May be used to set default environment
        variables to add to forked child processes. May be used more
        than once to set multiple variables.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.machine_id=</varname></term>

        <listitem><para>Takes a 32 character hex value to be
        used for setting the machine-id. Intended mostly for
        network booting where the same machine-id is desired
        for every boot.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.unified_cgroup_hierarchy</varname></term>

        <listitem><para>When specified without an argument or with a true argument,
        enables the usage of
        <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">unified cgroup hierarchy</ulink>
        (a.k.a. cgroups-v2). When specified with a false argument, fall back to
        hybrid or full legacy cgroup hierarchy.</para>

        <para>If this option is not specified, the default behaviour is determined
        during compilation (the <option>--with-default-hierarchy=</option>
        option). If the kernel does not support unified cgroup hierarchy, the legacy
        hierarchy will be used even if this option is specified.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>

        <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
        (see previous option). When specified without an argument or with a true
        argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
        tree used for systemd, and
        <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
        cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
        forces a full "legacy" mode. When specified with a false argument, enables
        the use of "hybrid" hierarchy.</para>

        <para>If this option is not specified, the default behaviour is determined
        during compilation (the <option>--with-default-hierarchy=</option>
        option). If the kernel does not support unified cgroup hierarchy, the legacy
        hierarchy will be used  even if this option is specified.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>quiet</varname></term>

        <listitem><para>Turn off status output at boot, much like
        <varname>systemd.show_status=false</varname> would. Note that
        this option is also read by the kernel itself and disables
        kernel log output. Passing this option hence turns off the
        usual output from both the system manager and the kernel.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>debug</varname></term>

        <listitem><para>Turn on debugging output. This is equivalent
        to <varname>systemd.log_level=debug</varname>. Note that this
        option is also read by the kernel itself and enables kernel
        debug output. Passing this option hence turns on the debug
        output from both the system manager and the
        kernel.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>emergency</varname></term>
        <term><varname>rd.emergency</varname></term>
        <term><varname>-b</varname></term>

        <listitem><para>Boot into emergency mode. This is equivalent
        to <varname>systemd.unit=emergency.target</varname> or
        <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
        provided for compatibility reasons and to be easier to type.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>rescue</varname></term>
        <term><varname>rd.rescue</varname></term>
        <term><varname>single</varname></term>
        <term><varname>s</varname></term>
        <term><varname>S</varname></term>
        <term><varname>1</varname></term>

        <listitem><para>Boot into rescue mode. This is equivalent to
        <varname>systemd.unit=rescue.target</varname> or
        <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
        provided for compatibility reasons and to be easier to type.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>2</varname></term>
        <term><varname>3</varname></term>
        <term><varname>4</varname></term>
        <term><varname>5</varname></term>

        <listitem><para>Boot into the specified legacy SysV runlevel.
        These are equivalent to
        <varname>systemd.unit=runlevel2.target</varname>,
        <varname>systemd.unit=runlevel3.target</varname>,
        <varname>systemd.unit=runlevel4.target</varname>, and
        <varname>systemd.unit=runlevel5.target</varname>,
        respectively, and provided for compatibility reasons and to be
        easier to type.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>locale.LANG=</varname></term>
        <term><varname>locale.LANGUAGE=</varname></term>
        <term><varname>locale.LC_CTYPE=</varname></term>
        <term><varname>locale.LC_NUMERIC=</varname></term>
        <term><varname>locale.LC_TIME=</varname></term>
        <term><varname>locale.LC_COLLATE=</varname></term>
        <term><varname>locale.LC_MONETARY=</varname></term>
        <term><varname>locale.LC_MESSAGES=</varname></term>
        <term><varname>locale.LC_PAPER=</varname></term>
        <term><varname>locale.LC_NAME=</varname></term>
        <term><varname>locale.LC_ADDRESS=</varname></term>
        <term><varname>locale.LC_TELEPHONE=</varname></term>
        <term><varname>locale.LC_MEASUREMENT=</varname></term>
        <term><varname>locale.LC_IDENTIFICATION=</varname></term>

        <listitem><para>Set the system locale to use. This overrides
        the settings in <filename>/etc/locale.conf</filename>. For
        more information, see
        <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
        and
        <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>
    </variablelist>

    <para>For other kernel command line parameters understood by
    components of the core OS, please refer to
    <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>Sockets and FIFOs</title>

    <variablelist>
      <varlistentry>
        <term><filename>/run/systemd/notify</filename></term>

        <listitem><para>Daemon status notification socket. This is an
        <constant>AF_UNIX</constant> datagram socket and is used to
        implement the daemon notification logic as implemented by
        <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>

      </varlistentry>

      <varlistentry>
        <term><filename>/run/systemd/private</filename></term>

        <listitem><para>Used internally as communication channel
        between
        <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        and the systemd process. This is an
        <constant>AF_UNIX</constant> stream socket. This interface is
        private to systemd and should not be used in external
        projects.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><filename>/dev/initctl</filename></term>

        <listitem><para>Limited compatibility support for the SysV
        client interface, as implemented by the
        <filename>systemd-initctl.service</filename> unit. This is a
        named pipe in the file system. This interface is obsolete and
        should not be used in new applications.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
      <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>