path: root/man/systemd.xml

<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2010 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<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>systemd <arg choice="opt" rep="repeat">OPTIONS</arg></command>
                </cmdsynopsis>
                <cmdsynopsis>
                        <command>init <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">COMMAND</arg></command>
                </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>--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>Dump core on
                                crash. This switch has no effect when
                                run as user
                                instance.</para></listitem>
                        </varlistentry>
                        <varlistentry>
                                <term><option>--crash-shell</option></term>

                                <listitem><para>Run shell on
                                crash. This switch has no effect when
                                run as user
                                instance.</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>Show terse service
                                status information while booting. This
                                switch has no effect when run as user
                                instance. Takes a boolean argument
                                which may be omitted which is
                                interpreted as
                                <option>true</option>.</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>

                        <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 12 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, or an operation timed out). 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>Snapshot units can be used to
                        temporarily save the state of the set of
                        systemd units, which later may be restored by
                        activating the saved snapshot unit. For more
                        information see
                        <citerefentry><refentrytitle>systemd.snapshot</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/cgroups/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>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="http://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 the <ulink
                url="http://www.freedesktop.org/wiki/Software/systemd/Generators">Generators
                Specification</ulink>.</para>

                <para>Systems which invoke systemd in a container
                or initrd environment should implement the
                <ulink
                url="http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container
                Interface</ulink> or <ulink
                url="http://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</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
                                ctl-alt-del.target</command>. If this
                                signal is received more often 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 start
                                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</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</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</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</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>
                                <term><constant>SIGRTMIN+23</constant></term>

                                <listitem><para>Sets the log level to
                                <literal>debug</literal>
                                (or <literal>info</literal> on
                                <constant>SIGRTMIN+23</constant>), as
                                controlled via
                                <varname>systemd.log_level=debug</varname>
                                (or <varname>systemd.log_level=info</varname>
                                on <constant>SIGRTMIN+23</constant>) on
                                the kernel command
                                line.</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>
                                <term><constant>SIGRTMIN+27</constant></term>
                                <term><constant>SIGRTMIN+28</constant></term>

                                <listitem><para>Sets the log level to
                                <literal>journal-or-kmsg</literal> (or
                                <literal>console</literal> on
                                <constant>SIGRTMIN+27</constant>,
                                <literal>kmsg</literal> on
                                <constant>SIGRTMIN+28</constant>), as
                                controlled via
                                <varname>systemd.log_target=journal-or-kmsg</varname>
                                (or
                                <varname>systemd.log_target=console</varname>
                                on <constant>SIGRTMIN+27</constant> 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>$LISTEN_PID</varname></term>
                                <term><varname>$LISTEN_FDS</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. If <option>true</option>,
                                systemd dumps core when it
                                crashes. Otherwise, no core dump is
                                created. Defaults to
                                <option>true</option>.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>,
                                systemd spawns a shell when it
                                crashes. Otherwise, no shell is
                                spawned. Defaults to
                                <option>false</option>, for security
                                reasons, as the shell is not protected
                                by any password
                                authentication.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Takes an integer
                                argument. If positive systemd
                                activates the specified virtual
                                terminal when it crashes. Defaults to
                                <constant>-1</constant>.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Takes a boolean
                                argument. If <option>true</option>,
                                asks for confirmation when spawning
                                processes. Defaults to
                                <option>false</option>.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Takes a boolean
                                argument or the constant
                                <constant>auto</constant>. If
                                <option>true</option>, shows terse
                                service status updates on the console
                                during bootup.
                                <constant>auto</constant> behaves like
                                <option>false</option> until a service
                                fails or there is a significant delay
                                in boot. Defaults to
                                <option>true</option>, unless
                                <option>quiet</option> is passed as
                                kernel command line option in which
                                case it defaults to
                                <constant>auto</constant>.</para></listitem>
                        </varlistentry>

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

                                <listitem><para>Controls log output,
                                with the same effect as the
                                <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
                                environment variables described above.</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>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>-b</varname></term>

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

                        <varlistentry>
                                <term><varname>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>
                                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><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
                                and
                                <citerefentry><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/shutdownd</filename></term>

                                <listitem><para>Used internally by the
                                <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                                tool to implement delayed
                                shutdowns. This is an <constant>AF_UNIX</constant> datagram
                                socket.</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="http://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
                        <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
                        <citerefentry><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>