man/systemd.xml

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   8   Copyright 2010 Lennart Poettering
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  23
  24 <refentry id="systemd"
  25     xmlns:xi="http://www.w3.org/2001/XInclude">
  26
  27   <refentryinfo>
  28     <title>systemd</title>
  29     <productname>systemd</productname>
  30
  31     <authorgroup>
  32       <author>
  33         <contrib>Developer</contrib>
  34         <firstname>Lennart</firstname>
  35         <surname>Poettering</surname>
  36         <email>lennart@poettering.net</email>
  37       </author>
  38     </authorgroup>
  39   </refentryinfo>
  40
  41   <refmeta>
  42     <refentrytitle>systemd</refentrytitle>
  43     <manvolnum>1</manvolnum>
  44   </refmeta>
  45
  46   <refnamediv>
  47     <refname>systemd</refname>
  48     <refname>init</refname>
  49     <refpurpose>systemd system and service manager</refpurpose>
  50   </refnamediv>
  51
  52   <refsynopsisdiv>
  53     <cmdsynopsis>
  54       <command>systemd</command>
  55       <arg choice="opt" rep="repeat">OPTIONS</arg>
  56     </cmdsynopsis>
  57     <cmdsynopsis>
  58       <command>init</command>
  59       <arg choice="opt" rep="repeat">OPTIONS</arg>
  60       <arg choice="req">COMMAND</arg>
  61     </cmdsynopsis>
  62   </refsynopsisdiv>
  63
  64   <refsect1>
  65     <title>Description</title>
  66
  67     <para>systemd is a system and service manager for Linux operating
  68     systems. When run as first process on boot (as PID 1), it acts as
  69     init system that brings up and maintains userspace
  70     services.</para>
  71
  72     <para>For compatibility with SysV, if systemd is called as
  73     <command>init</command> and a PID that is not 1, it will execute
  74     <command>telinit</command> and pass all command line arguments
  75     unmodified. That means <command>init</command> and
  76     <command>telinit</command> are mostly equivalent when invoked from
  77     normal login sessions. See
  78     <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  79     for more information.</para>
  80
  81     <para>When run as a system instance, systemd interprets the
  82     configuration file <filename>system.conf</filename> and the files
  83     in <filename>system.conf.d</filename> directories; when run as a
  84     user instance, systemd interprets the configuration file
  85     <filename>user.conf</filename> and the files in
  86     <filename>user.conf.d</filename> directories. See
  87     <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  88     for more information.</para>
  89   </refsect1>
  90
  91   <refsect1>
  92     <title>Options</title>
  93
  94     <para>The following options are understood:</para>
  95
  96     <variablelist>
  97       <varlistentry>
  98         <term><option>--test</option></term>
  99
 100         <listitem><para>Determine startup sequence, dump it and exit.
 101         This is an option useful for debugging only.</para></listitem>
 102       </varlistentry>
 103       <varlistentry>
 104         <term><option>--dump-configuration-items</option></term>
 105
 106         <listitem><para>Dump understood unit configuration items. This
 107         outputs a terse but complete list of configuration items
 108         understood in unit definition files.</para></listitem>
 109       </varlistentry>
 110       <varlistentry>
 111         <term><option>--unit=</option></term>
 112
 113         <listitem><para>Set default unit to activate on startup. If
 114         not specified, defaults to
 115         <filename>default.target</filename>.</para></listitem>
 116       </varlistentry>
 117       <varlistentry>
 118         <term><option>--system</option></term>
 119         <term><option>--user</option></term>
 120
 121         <listitem><para>For <option>--system</option>, tell systemd to
 122         run a system instance, even if the process ID is not 1, i.e.
 123         systemd is not run as init process. <option>--user</option>
 124         does the opposite, running a user instance even if the process
 125         ID is 1. Normally, it should not be necessary to pass these
 126         options, as systemd automatically detects the mode it is
 127         started in. These options are hence of little use except for
 128         debugging. Note that it is not supported booting and
 129         maintaining a full system with systemd running in
 130         <option>--system</option> mode, but PID not 1. In practice,
 131         passing <option>--system</option> explicitly is only useful in
 132         conjunction with <option>--test</option>.</para></listitem>
 133       </varlistentry>
 134       <varlistentry>
 135         <term><option>--dump-core</option></term>
 136
 137         <listitem><para>Enable core dumping on crash. This switch has
 138         no effect when running as user instance. This setting may also
 139         be enabled during boot on the kernel command line via the
 140         <varname>systemd.dump_core=</varname> option, see
 141         below.</para></listitem>
 142       </varlistentry>
 143
 144       <varlistentry>
 145         <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>
 146
 147         <listitem><para>Switch to a specific virtual console (VT) on
 148         crash. Takes a positive integer in the range 1–63, or a
 149         boolean argument. If an integer is passed, selects which VT to
 150         switch to. If <constant>yes</constant>, the VT kernel messages
 151         are written to is selected. If <constant>no</constant>, no VT
 152         switch is attempted. This switch has no effect when running as
 153         user instance. This setting may also be enabled during boot,
 154         on the kernel command line via the
 155         <varname>systemd.crash_vt=</varname> option, see
 156         <!-- FIXME: there is no crash_vt command line option? -->
 157         below.</para></listitem>
 158       </varlistentry>
 159
 160       <varlistentry>
 161         <term><option>--crash-shell</option></term>
 162
 163         <listitem><para>Run a shell on crash. This switch has no
 164         effect when running as user instance. This setting may also be
 165         enabled during boot, on the kernel command line via the
 166         <varname>systemd.crash_shell=</varname> option, see
 167         below.</para></listitem>
 168       </varlistentry>
 169
 170       <varlistentry>
 171         <term><option>--crash-reboot</option></term>
 172
 173         <listitem><para>Automatically reboot the system on crash. This
 174         switch has no effect when running as user instance. This
 175         setting may also be enabled during boot, on the kernel command
 176         line via the <varname>systemd.crash_reboot=</varname> option,
 177         see below.</para></listitem>
 178       </varlistentry>
 179
 180       <varlistentry>
 181         <term><option>--confirm-spawn</option></term>
 182
 183         <listitem><para>Ask for confirmation when spawning processes.
 184         This switch has no effect when run as user
 185         instance.</para></listitem>
 186       </varlistentry>
 187       <varlistentry>
 188         <term><option>--show-status=</option></term>
 189
 190         <listitem><para>Takes a boolean argument or the special value <constant>auto</constant>. If on, terse unit
 191         status information is shown on the console during boot-up and shutdown. If off, no such status information is
 192         shown. If set to <constant>auto</constant> behavior is similar to off, except that it is automatically switched
 193         to on, as soon as the first unit failure or significant boot delay is encountered. This switch has no effect
 194         when invoked as user instance. If specified, overrides both the kernel command line setting
 195         <varname>systemd.show_status=</varname> (see below) and the configuration file option
 196         <option>ShowStatus=</option>, see
 197         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 198       </varlistentry>
 199       <varlistentry>
 200         <term><option>--log-target=</option></term>
 201
 202         <listitem><para>Set log target. Argument must be one of
 203         <option>console</option>,
 204         <option>journal</option>,
 205         <option>kmsg</option>,
 206         <option>journal-or-kmsg</option>,
 207         <option>null</option>.</para></listitem>
 208       </varlistentry>
 209       <varlistentry>
 210         <term><option>--log-level=</option></term>
 211
 212         <listitem><para>Set log level. As
 213         argument this accepts a numerical log
 214         level or the well-known <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 215         symbolic names (lowercase):
 216         <option>emerg</option>,
 217         <option>alert</option>,
 218         <option>crit</option>,
 219         <option>err</option>,
 220         <option>warning</option>,
 221         <option>notice</option>,
 222         <option>info</option>,
 223         <option>debug</option>.</para></listitem>
 224       </varlistentry>
 225       <varlistentry>
 226         <term><option>--log-color=</option></term>
 227
 228         <listitem><para>Highlight important log messages. Argument is
 229         a boolean value. If the argument is omitted, it defaults to
 230         <option>true</option>.</para></listitem>
 231       </varlistentry>
 232       <varlistentry>
 233         <term><option>--log-location=</option></term>
 234
 235         <listitem><para>Include code location in log messages. This is
 236         mostly relevant for debugging purposes. Argument is a boolean
 237         value. If the argument is omitted it defaults to
 238         <option>true</option>.</para></listitem>
 239       </varlistentry>
 240       <varlistentry>
 241         <term><option>--default-standard-output=</option></term>
 242         <term><option>--default-standard-error=</option></term>
 243
 244         <listitem><para>Sets the default output or error output for
 245         all services and sockets, respectively. That is, controls the
 246         default for <option>StandardOutput=</option> and
 247         <option>StandardError=</option> (see
 248         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 249         for details). Takes one of
 250         <option>inherit</option>,
 251         <option>null</option>,
 252         <option>tty</option>,
 253         <option>journal</option>,
 254         <option>journal+console</option>,
 255         <option>syslog</option>,
 256         <option>syslog+console</option>,
 257         <option>kmsg</option>,
 258         <option>kmsg+console</option>.  If the
 259         argument is omitted
 260         <option>--default-standard-output=</option> defaults to
 261         <option>journal</option> and
 262         <option>--default-standard-error=</option> to
 263         <option>inherit</option>.</para></listitem>
 264       </varlistentry>
 265
 266       <varlistentry>
 267         <term><option>--machine-id=</option></term>
 268
 269         <listitem><para>Override the machine-id set on the hard drive,
 270         useful for network booting or for containers. May not be set
 271         to all zeros.</para></listitem>
 272       </varlistentry>
 273
 274       <xi:include href="standard-options.xml" xpointer="help" />
 275       <xi:include href="standard-options.xml" xpointer="version" />
 276     </variablelist>
 277   </refsect1>
 278
 279   <refsect1>
 280     <title>Concepts</title>
 281
 282     <para>systemd provides a dependency system between various
 283     entities called "units" of 11 different types. Units encapsulate
 284     various objects that are relevant for system boot-up and
 285     maintenance. The majority of units are configured in unit
 286     configuration files, whose syntax and basic set of options is
 287     described in
 288     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 289     however some are created automatically from other configuration,
 290     dynamically from system state or programmatically at runtime.
 291     Units may be "active" (meaning started, bound, plugged in, …,
 292     depending on the unit type, see below), or "inactive" (meaning
 293     stopped, unbound, unplugged, …), as well as in the process of
 294     being activated or deactivated, i.e. between the two states (these
 295     states are called "activating", "deactivating"). A special
 296     "failed" state is available as well, which is very similar to
 297     "inactive" and is entered when the service failed in some way
 298     (process returned error code on exit, or crashed, or an operation
 299     timed out). If this state is entered, the cause will be logged,
 300     for later reference. Note that the various unit types may have a
 301     number of additional substates, which are mapped to the five
 302     generalized unit states described here.</para>
 303
 304     <para>The following unit types are available:</para>
 305
 306     <orderedlist>
 307       <listitem><para>Service units, which start and control daemons
 308       and the processes they consist of. For details, see
 309       <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 310
 311       <listitem><para>Socket units, which encapsulate local IPC or
 312       network sockets in the system, useful for socket-based
 313       activation. For details about socket units, see
 314       <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 315       for details on socket-based activation and other forms of
 316       activation, see
 317       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 318
 319       <listitem><para>Target units are useful to group units, or
 320       provide well-known synchronization points during boot-up, see
 321       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 322
 323       <listitem><para>Device units expose kernel devices in systemd
 324       and may be used to implement device-based activation. For
 325       details, see
 326       <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 327
 328       <listitem><para>Mount units control mount points in the file
 329       system, for details see
 330       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 331
 332       <listitem><para>Automount units provide automount capabilities,
 333       for on-demand mounting of file systems as well as parallelized
 334       boot-up. See
 335       <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 336
 337       <listitem><para>Timer units are useful for triggering activation
 338       of other units based on timers. You may find details in
 339       <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 340
 341       <listitem><para>Swap units are very similar to mount units and
 342       encapsulate memory swap partitions or files of the operating
 343       system. They are described in
 344       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 345
 346       <listitem><para>Path units may be used to activate other
 347       services when file system objects change or are modified. See
 348       <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 349
 350       <listitem><para>Slice units may be used to group units which
 351       manage system processes (such as service and scope units) in a
 352       hierarchical tree for resource management purposes. See
 353       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 354
 355       <listitem><para>Scope units are similar to service units, but
 356       manage foreign processes instead of starting them as well. See
 357       <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 358
 359     </orderedlist>
 360
 361     <para>Units are named as their configuration files. Some units
 362     have special semantics. A detailed list is available in
 363     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 364
 365     <para>systemd knows various kinds of dependencies, including
 366     positive and negative requirement dependencies (i.e.
 367     <varname>Requires=</varname> and <varname>Conflicts=</varname>) as
 368     well as ordering dependencies (<varname>After=</varname> and
 369     <varname>Before=</varname>). NB: ordering and requirement
 370     dependencies are orthogonal. If only a requirement dependency
 371     exists between two units (e.g. <filename>foo.service</filename>
 372     requires <filename>bar.service</filename>), but no ordering
 373     dependency (e.g. <filename>foo.service</filename> after
 374     <filename>bar.service</filename>) and both are requested to start,
 375     they will be started in parallel. It is a common pattern that both
 376     requirement and ordering dependencies are placed between two
 377     units. Also note that the majority of dependencies are implicitly
 378     created and maintained by systemd. In most cases, it should be
 379     unnecessary to declare additional dependencies manually, however
 380     it is possible to do this.</para>
 381
 382     <para>Application programs and units (via dependencies) may
 383     request state changes of units. In systemd, these requests are
 384     encapsulated as 'jobs' and maintained in a job queue. Jobs may
 385     succeed or can fail, their execution is ordered based on the
 386     ordering dependencies of the units they have been scheduled
 387     for.</para>
 388
 389     <para>On boot systemd activates the target unit
 390     <filename>default.target</filename> whose job is to activate
 391     on-boot services and other on-boot units by pulling them in via
 392     dependencies. Usually, the unit name is just an alias (symlink) for
 393     either <filename>graphical.target</filename> (for fully-featured
 394     boots into the UI) or <filename>multi-user.target</filename> (for
 395     limited console-only boots for use in embedded or server
 396     environments, or similar; a subset of graphical.target). However,
 397     it is at the discretion of the administrator to configure it as an
 398     alias to any other target unit. See
 399     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 400     for details about these target units.</para>
 401
 402     <para>Processes systemd spawns are placed in individual Linux
 403     control groups named after the unit which they belong to in the
 404     private systemd hierarchy. (see <ulink
 405     url="https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
 406     for more information about control groups, or short "cgroups").
 407     systemd uses this to effectively keep track of processes. Control
 408     group information is maintained in the kernel, and is accessible
 409     via the file system hierarchy (beneath
 410     <filename>/sys/fs/cgroup/systemd/</filename>), or in tools such as
 411     <citerefentry project='man-pages'><refentrytitle>systemd-cgls</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 412     or
 413     <citerefentry project='man-pages'><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 414     (<command>ps xawf -eo pid,user,cgroup,args</command> is
 415     particularly useful to list all processes and the systemd units
 416     they belong to.).</para>
 417
 418     <para>systemd is compatible with the SysV init system to a large
 419     degree: SysV init scripts are supported and simply read as an
 420     alternative (though limited) configuration file format. The SysV
 421     <filename>/dev/initctl</filename> interface is provided, and
 422     compatibility implementations of the various SysV client tools are
 423     available. In addition to that, various established Unix
 424     functionality such as <filename>/etc/fstab</filename> or the
 425     <filename>utmp</filename> database are supported.</para>
 426
 427     <para>systemd has a minimal transaction system: if a unit is
 428     requested to start up or shut down it will add it and all its
 429     dependencies to a temporary transaction. Then, it will verify if
 430     the transaction is consistent (i.e. whether the ordering of all
 431     units is cycle-free). If it is not, systemd will try to fix it up,
 432     and removes non-essential jobs from the transaction that might
 433     remove the loop. Also, systemd tries to suppress non-essential
 434     jobs in the transaction that would stop a running service. Finally
 435     it is checked whether the jobs of the transaction contradict jobs
 436     that have already been queued, and optionally the transaction is
 437     aborted then. If all worked out and the transaction is consistent
 438     and minimized in its impact it is merged with all already
 439     outstanding jobs and added to the run queue. Effectively this
 440     means that before executing a requested operation, systemd will
 441     verify that it makes sense, fixing it if possible, and only
 442     failing if it really cannot work.</para>
 443
 444     <para>Systemd contains native implementations of various tasks
 445     that need to be executed as part of the boot process. For example,
 446     it sets the hostname or configures the loopback network device. It
 447     also sets up and mounts various API file systems, such as
 448     <filename>/sys</filename> or <filename>/proc</filename>.</para>
 449
 450     <para>For more information about the concepts and
 451     ideas behind systemd, please refer to the
 452     <ulink url="http://0pointer.de/blog/projects/systemd.html">Original Design Document</ulink>.</para>
 453
 454     <para>Note that some but not all interfaces provided
 455     by systemd are covered by the
 456     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
 457     Stability Promise</ulink>.</para>
 458
 459     <para>Units may be generated dynamically at boot and system
 460     manager reload time, for example based on other configuration
 461     files or parameters passed on the kernel command line. For details, see
 462     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 463
 464     <para>Systems which invoke systemd in a container or initrd
 465     environment should implement the
 466     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink> or
 467     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
 468     specifications, respectively.</para>
 469   </refsect1>
 470
 471   <refsect1>
 472     <title>Directories</title>
 473
 474     <variablelist>
 475       <varlistentry>
 476         <term>System unit directories</term>
 477
 478         <listitem><para>The systemd system manager reads unit
 479         configuration from various directories. Packages that want to
 480         install unit files shall place them in the directory returned
 481         by <command>pkg-config systemd
 482         --variable=systemdsystemunitdir</command>. Other directories
 483         checked are <filename>/usr/local/lib/systemd/system</filename>
 484         and <filename>/usr/lib/systemd/system</filename>. User
 485         configuration always takes precedence. <command>pkg-config
 486         systemd --variable=systemdsystemconfdir</command> returns the
 487         path of the system configuration directory. Packages should
 488         alter the content of these directories only with the
 489         <command>enable</command> and <command>disable</command>
 490         commands of the
 491         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 492         tool. Full list of directories is provided in
 493         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 494         </para></listitem>
 495       </varlistentry>
 496     </variablelist>
 497
 498     <variablelist>
 499       <varlistentry>
 500         <term>User unit directories</term>
 501
 502         <listitem><para>Similar rules apply for the user unit
 503         directories. However, here the
 504         <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 505         Base Directory specification</ulink> is followed to find
 506         units. Applications should place their unit files in the
 507         directory returned by <command>pkg-config systemd
 508         --variable=systemduserunitdir</command>. Global configuration
 509         is done in the directory reported by <command>pkg-config
 510         systemd --variable=systemduserconfdir</command>. The
 511         <command>enable</command> and <command>disable</command>
 512         commands of the
 513         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 514         tool can handle both global (i.e. for all users) and private
 515         (for one user) enabling/disabling of units. Full list of
 516         directories is provided in
 517         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 518         </para></listitem>
 519       </varlistentry>
 520     </variablelist>
 521
 522     <variablelist>
 523       <varlistentry>
 524         <term>SysV init scripts directory</term>
 525
 526         <listitem><para>The location of the SysV init script directory
 527         varies between distributions. If systemd cannot find a native
 528         unit file for a requested service, it will look for a SysV
 529         init script of the same name (with the
 530         <filename>.service</filename> suffix
 531         removed).</para></listitem>
 532       </varlistentry>
 533     </variablelist>
 534
 535     <variablelist>
 536       <varlistentry>
 537         <term>SysV runlevel link farm directory</term>
 538
 539         <listitem><para>The location of the SysV runlevel link farm
 540         directory varies between distributions. systemd will take the
 541         link farm into account when figuring out whether a service
 542         shall be enabled. Note that a service unit with a native unit
 543         configuration file cannot be started by activating it in the
 544         SysV runlevel link farm.</para></listitem>
 545       </varlistentry>
 546     </variablelist>
 547   </refsect1>
 548
 549   <refsect1>
 550     <title>Signals</title>
 551
 552     <variablelist>
 553       <varlistentry>
 554         <term><constant>SIGTERM</constant></term>
 555
 556         <listitem><para>Upon receiving this signal the systemd system
 557         manager serializes its state, reexecutes itself and
 558         deserializes the saved state again. This is mostly equivalent
 559         to <command>systemctl daemon-reexec</command>.</para>
 560
 561         <para>systemd user managers will start the
 562         <filename>exit.target</filename> unit when this signal is
 563         received. This is mostly equivalent to <command>systemctl
 564         --user start exit.target</command>.</para></listitem>
 565       </varlistentry>
 566
 567       <varlistentry>
 568         <term><constant>SIGINT</constant></term>
 569
 570         <listitem><para>Upon receiving this signal the systemd system
 571         manager will start the
 572         <filename>ctrl-alt-del.target</filename> unit. This is mostly
 573         equivalent to <command>systemctl start
 574         ctl-alt-del.target</command>. If this signal is received more
 575         than 7 times per 2s, an immediate reboot is triggered.
 576         Note that pressing Ctrl-Alt-Del on the console will trigger
 577         this signal. Hence, if a reboot is hanging, pressing
 578         Ctrl-Alt-Del more than 7 times in 2s is a relatively safe way
 579         to trigger an immediate reboot.</para>
 580
 581         <para>systemd user managers treat this signal the same way as
 582         <constant>SIGTERM</constant>.</para></listitem>
 583       </varlistentry>
 584
 585       <varlistentry>
 586         <term><constant>SIGWINCH</constant></term>
 587
 588         <listitem><para>When this signal is received the systemd
 589         system manager will start the
 590         <filename>kbrequest.target</filename> unit. This is mostly
 591         equivalent to <command>systemctl start
 592         kbrequest.target</command>.</para>
 593
 594         <para>This signal is ignored by systemd user
 595         managers.</para></listitem>
 596       </varlistentry>
 597
 598       <varlistentry>
 599         <term><constant>SIGPWR</constant></term>
 600
 601         <listitem><para>When this signal is received the systemd
 602         manager will start the <filename>sigpwr.target</filename>
 603         unit. This is mostly equivalent to <command>systemctl start
 604         sigpwr.target</command>.</para></listitem>
 605       </varlistentry>
 606
 607       <varlistentry>
 608         <term><constant>SIGUSR1</constant></term>
 609
 610         <listitem><para>When this signal is received the systemd
 611         manager will try to reconnect to the D-Bus
 612         bus.</para></listitem>
 613       </varlistentry>
 614
 615       <varlistentry>
 616         <term><constant>SIGUSR2</constant></term>
 617
 618         <listitem><para>When this signal is received the systemd
 619         manager will log its complete state in human-readable form.
 620         The data logged is the same as printed by
 621         <command>systemd-analyze dump</command>.</para></listitem>
 622       </varlistentry>
 623
 624       <varlistentry>
 625         <term><constant>SIGHUP</constant></term>
 626
 627         <listitem><para>Reloads the complete daemon configuration.
 628         This is mostly equivalent to <command>systemctl
 629         daemon-reload</command>.</para></listitem>
 630       </varlistentry>
 631
 632       <varlistentry>
 633         <term><constant>SIGRTMIN+0</constant></term>
 634
 635         <listitem><para>Enters default mode, starts the
 636         <filename>default.target</filename> unit. This is mostly
 637         equivalent to <command>systemctl start
 638         default.target</command>.</para></listitem>
 639       </varlistentry>
 640
 641       <varlistentry>
 642         <term><constant>SIGRTMIN+1</constant></term>
 643
 644         <listitem><para>Enters rescue mode, starts the
 645         <filename>rescue.target</filename> unit. This is mostly
 646         equivalent to <command>systemctl isolate
 647         rescue.target</command>.</para></listitem>
 648       </varlistentry>
 649
 650       <varlistentry>
 651         <term><constant>SIGRTMIN+2</constant></term>
 652
 653         <listitem><para>Enters emergency mode, starts the
 654         <filename>emergency.service</filename> unit. This is mostly
 655         equivalent to <command>systemctl isolate
 656         emergency.service</command>.</para></listitem>
 657       </varlistentry>
 658
 659       <varlistentry>
 660         <term><constant>SIGRTMIN+3</constant></term>
 661
 662         <listitem><para>Halts the machine, starts the
 663         <filename>halt.target</filename> unit. This is mostly
 664         equivalent to <command>systemctl start
 665         halt.target</command>.</para></listitem>
 666       </varlistentry>
 667
 668       <varlistentry>
 669         <term><constant>SIGRTMIN+4</constant></term>
 670
 671         <listitem><para>Powers off the machine, starts the
 672         <filename>poweroff.target</filename> unit. This is mostly
 673         equivalent to <command>systemctl start
 674         poweroff.target</command>.</para></listitem>
 675       </varlistentry>
 676
 677       <varlistentry>
 678         <term><constant>SIGRTMIN+5</constant></term>
 679
 680         <listitem><para>Reboots the machine, starts the
 681         <filename>reboot.target</filename> unit. This is mostly
 682         equivalent to <command>systemctl start
 683         reboot.target</command>.</para></listitem>
 684       </varlistentry>
 685
 686       <varlistentry>
 687         <term><constant>SIGRTMIN+6</constant></term>
 688
 689         <listitem><para>Reboots the machine via kexec, starts the
 690         <filename>kexec.target</filename> unit. This is mostly
 691         equivalent to <command>systemctl start
 692         kexec.target</command>.</para></listitem>
 693       </varlistentry>
 694
 695       <varlistentry>
 696         <term><constant>SIGRTMIN+13</constant></term>
 697
 698         <listitem><para>Immediately halts the machine.</para></listitem>
 699       </varlistentry>
 700
 701       <varlistentry>
 702         <term><constant>SIGRTMIN+14</constant></term>
 703
 704         <listitem><para>Immediately powers off the machine.</para></listitem>
 705       </varlistentry>
 706
 707       <varlistentry>
 708         <term><constant>SIGRTMIN+15</constant></term>
 709
 710         <listitem><para>Immediately reboots the machine.</para></listitem>
 711       </varlistentry>
 712
 713       <varlistentry>
 714         <term><constant>SIGRTMIN+16</constant></term>
 715
 716         <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 717       </varlistentry>
 718
 719       <varlistentry>
 720         <term><constant>SIGRTMIN+20</constant></term>
 721
 722         <listitem><para>Enables display of status messages on the
 723         console, as controlled via
 724         <varname>systemd.show_status=1</varname> on the kernel command
 725         line.</para></listitem>
 726       </varlistentry>
 727
 728       <varlistentry>
 729         <term><constant>SIGRTMIN+21</constant></term>
 730
 731         <listitem><para>Disables display of
 732         status messages on the console, as
 733         controlled via
 734         <varname>systemd.show_status=0</varname>
 735         on the kernel command
 736         line.</para></listitem>
 737       </varlistentry>
 738
 739       <varlistentry>
 740         <term><constant>SIGRTMIN+22</constant></term>
 741         <term><constant>SIGRTMIN+23</constant></term>
 742
 743         <listitem><para>Sets the log level to <literal>debug</literal>
 744         (or <literal>info</literal> on
 745         <constant>SIGRTMIN+23</constant>), as controlled via
 746         <varname>systemd.log_level=debug</varname> (or
 747         <varname>systemd.log_level=info</varname> on
 748         <constant>SIGRTMIN+23</constant>) on the kernel command
 749         line.</para></listitem>
 750       </varlistentry>
 751
 752       <varlistentry>
 753         <term><constant>SIGRTMIN+24</constant></term>
 754
 755         <listitem><para>Immediately exits the manager (only available
 756         for --user instances).</para></listitem>
 757       </varlistentry>
 758
 759       <varlistentry>
 760         <term><constant>SIGRTMIN+26</constant></term>
 761         <term><constant>SIGRTMIN+27</constant></term>
 762         <term><constant>SIGRTMIN+28</constant></term>
 763
 764         <listitem><para>Sets the log target to
 765         <literal>journal-or-kmsg</literal> (or
 766         <literal>console</literal> on
 767         <constant>SIGRTMIN+27</constant>, <literal>kmsg</literal> on
 768         <constant>SIGRTMIN+28</constant>), as controlled via
 769         <varname>systemd.log_target=journal-or-kmsg</varname> (or
 770         <varname>systemd.log_target=console</varname> on
 771         <constant>SIGRTMIN+27</constant> or
 772         <varname>systemd.log_target=kmsg</varname> on
 773         <constant>SIGRTMIN+28</constant>) on the kernel command
 774         line.</para></listitem>
 775       </varlistentry>
 776     </variablelist>
 777   </refsect1>
 778
 779   <refsect1>
 780     <title>Environment</title>
 781
 782     <variablelist class='environment-variables'>
 783       <varlistentry>
 784         <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 785         <listitem><para>systemd reads the log level from this
 786         environment variable. This can be overridden with
 787         <option>--log-level=</option>.</para></listitem>
 788       </varlistentry>
 789
 790       <varlistentry>
 791         <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 792         <listitem><para>systemd reads the log target from this
 793         environment variable. This can be overridden with
 794         <option>--log-target=</option>.</para></listitem>
 795       </varlistentry>
 796
 797       <varlistentry>
 798         <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 799         <listitem><para>Controls whether systemd highlights important
 800         log messages. This can be overridden with
 801         <option>--log-color=</option>.</para></listitem>
 802       </varlistentry>
 803
 804       <varlistentry>
 805         <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 806         <listitem><para>Controls whether systemd prints the code
 807         location along with log messages. This can be overridden with
 808         <option>--log-location=</option>.</para></listitem>
 809       </varlistentry>
 810
 811       <varlistentry>
 812         <term><varname>$XDG_CONFIG_HOME</varname></term>
 813         <term><varname>$XDG_CONFIG_DIRS</varname></term>
 814         <term><varname>$XDG_DATA_HOME</varname></term>
 815         <term><varname>$XDG_DATA_DIRS</varname></term>
 816
 817         <listitem><para>The systemd user manager uses these variables
 818         in accordance to the <ulink
 819         url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 820         Base Directory specification</ulink> to find its
 821         configuration.</para></listitem>
 822       </varlistentry>
 823
 824       <varlistentry>
 825         <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 826
 827         <listitem><para>Controls where systemd looks for unit
 828         files.</para></listitem>
 829       </varlistentry>
 830
 831       <varlistentry>
 832         <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 833
 834         <listitem><para>Controls where systemd looks for SysV init
 835         scripts.</para></listitem>
 836       </varlistentry>
 837
 838       <varlistentry>
 839         <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 840
 841         <listitem><para>Controls where systemd looks for SysV init
 842         script runlevel link farms.</para></listitem>
 843       </varlistentry>
 844
 845       <varlistentry>
 846         <term><varname>$SYSTEMD_COLORS</varname></term>
 847
 848         <listitem><para>The value must be a boolean. Controls whether colorized output should be
 849         generated. This can be specified to override the decision that <command>systemd</command>
 850         makes based on <varname>$TERM</varname> and what the console is connected to.</para>
 851         </listitem>
 852       </varlistentry>
 853
 854       <varlistentry>
 855         <term><varname>$LISTEN_PID</varname></term>
 856         <term><varname>$LISTEN_FDS</varname></term>
 857         <term><varname>$LISTEN_FDNAMES</varname></term>
 858
 859         <listitem><para>Set by systemd for supervised processes during
 860         socket-based activation. See
 861         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 862         for more information.</para></listitem>
 863       </varlistentry>
 864
 865       <varlistentry>
 866         <term><varname>$NOTIFY_SOCKET</varname></term>
 867
 868         <listitem><para>Set by systemd for supervised processes for
 869         status and start-up completion notification. See
 870         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 871         for more information.</para></listitem>
 872       </varlistentry>
 873     </variablelist>
 874   </refsect1>
 875
 876   <refsect1>
 877     <title>Kernel Command Line</title>
 878
 879     <para>When run as system instance systemd parses a number of
 880     kernel command line arguments<footnote><para>If run inside a Linux
 881     container these arguments may be passed as command line arguments
 882     to systemd itself, next to any of the command line options listed
 883     in the Options section above. If run outside of Linux containers,
 884     these arguments are parsed from <filename>/proc/cmdline</filename>
 885     instead.</para></footnote>:</para>
 886
 887     <variablelist class='kernel-commandline-options'>
 888       <varlistentry>
 889         <term><varname>systemd.unit=</varname></term>
 890         <term><varname>rd.systemd.unit=</varname></term>
 891
 892         <listitem><para>Overrides the unit to activate on boot.
 893         Defaults to <filename>default.target</filename>. This may be
 894         used to temporarily boot into a different boot unit, for
 895         example <filename>rescue.target</filename> or
 896         <filename>emergency.service</filename>. See
 897         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 898         for details about these units. The option prefixed with
 899         <literal>rd.</literal> is honored only in the initial RAM disk
 900         (initrd), while the one that is not prefixed only in the main
 901         system.</para></listitem>
 902       </varlistentry>
 903
 904       <varlistentry>
 905         <term><varname>systemd.dump_core</varname></term>
 906
 907         <listitem><para>Takes a boolean argument or enables the option if specified
 908         without an argument. If enabled, the systemd manager (PID 1) dumps core when
 909         it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
 910         </listitem>
 911       </varlistentry>
 912
 913       <varlistentry>
 914         <term><varname>systemd.crash_chvt</varname></term>
 915
 916         <listitem><para>Takes a positive integer, or a boolean argument. Can be also
 917         specified without an argument, with the same effect as a positive boolean. If
 918         a positive integer (in the range 1–63) is specified, the system manager (PID
 919         1) will activate the specified virtual terminal (VT) when it
 920         crashes. Defaults to disabled, meaning that no such switch is attempted. If
 921         set to enabled, the VT the kernel messages are written to is selected.
 922         </para></listitem>
 923       </varlistentry>
 924
 925       <varlistentry>
 926         <term><varname>systemd.crash_shell</varname></term>
 927
 928         <listitem><para>Takes a boolean argument or enables the option if specified
 929         without an argument. If enabled, the system manager (PID 1) spawns a shell
 930         when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
 931         to disabled, for security reasons, as the shell is not protected by password
 932         authentication.</para></listitem>
 933       </varlistentry>
 934
 935       <varlistentry>
 936         <term><varname>systemd.crash_reboot</varname></term>
 937
 938         <listitem><para>Takes a boolean argument or enables the option if specified
 939         without an argument. If enabled, the system manager (PID 1) will reboot the
 940         machine automatically when it crashes, after a 10s delay.  Otherwise, the
 941         system will hang indefinitely. Defaults to disabled, in order to avoid a
 942         reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
 943         system is rebooted after the shell exits.</para></listitem>
 944       </varlistentry>
 945
 946       <varlistentry>
 947         <term><varname>systemd.confirm_spawn</varname></term>
 948
 949         <listitem><para>Takes a boolean argument or a path to the virtual console
 950         where the confirmation messages should be emitted. Can be also specified
 951         without an argument, with the same effect as a positive boolean. If enabled,
 952         the system manager (PID 1) asks for confirmation when spawning processes
 953         using <option>/dev/console</option>. If a path or a console name (such as
 954         <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
 955         path or described by the give name will be used instead. Defaults to disabled.
 956         </para></listitem>
 957       </varlistentry>
 958
 959       <varlistentry>
 960         <term><varname>systemd.show_status</varname></term>
 961
 962         <listitem><para>Takes a boolean argument or the constant
 963         <constant>auto</constant>. Can be also specified without an argument, with
 964         the same effect as a positive boolean.  If enabled, the systemd manager (PID
 965         1) shows terse service status updates on the console during bootup.
 966         <constant>auto</constant> behaves like <option>false</option> until a unit
 967         fails or there is a significant delay in boot. Defaults to enabled, unless
 968         <option>quiet</option> is passed as kernel command line option, in which case
 969         it defaults to <constant>auto</constant>. If specified overrides the system
 970         manager configuration file option <option>ShowStatus=</option>, see
 971         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 972         However, the process command line option <option>--show-status=</option>
 973         takes precedence over both this kernel command line option and the
 974         configuration file option.</para></listitem>
 975       </varlistentry>
 976
 977       <varlistentry>
 978         <term><varname>systemd.log_target=</varname></term>
 979         <term><varname>systemd.log_level=</varname></term>
 980         <term><varname>systemd.log_location=</varname></term>
 981         <term><varname>systemd.log_color</varname></term>
 982
 983         <listitem><para>Controls log output, with the same effect as the
 984         <varname>$SYSTEMD_LOG_TARGET</varname>,
 985         <varname>$SYSTEMD_LOG_LEVEL</varname>,
 986         <varname>$SYSTEMD_LOG_LOCATION</varname>,
 987         <varname>$SYSTEMD_LOG_COLOR</varname> environment variables described above.
 988         <varname>systemd.log_color</varname> can be specified without an argument,
 989         with the same effect as a positive boolean.</para></listitem>
 990       </varlistentry>
 991
 992       <varlistentry>
 993         <term><varname>systemd.default_standard_output=</varname></term>
 994         <term><varname>systemd.default_standard_error=</varname></term>
 995         <listitem><para>Controls default standard output and error
 996         output for services, with the same effect as the
 997         <option>--default-standard-output=</option> and
 998         <option>--default-standard-error=</option> command line
 999         arguments described above, respectively.</para></listitem>
1000       </varlistentry>
1001
1002       <varlistentry>
1003         <term><varname>systemd.setenv=</varname></term>
1004
1005         <listitem><para>Takes a string argument in the form
1006         VARIABLE=VALUE. May be used to set default environment
1007         variables to add to forked child processes. May be used more
1008         than once to set multiple variables.</para></listitem>
1009       </varlistentry>
1010
1011       <varlistentry>
1012         <term><varname>systemd.machine_id=</varname></term>
1013
1014         <listitem><para>Takes a 32 character hex value to be
1015         used for setting the machine-id. Intended mostly for
1016         network booting where the same machine-id is desired
1017         for every boot.</para></listitem>
1018       </varlistentry>
1019
1020       <varlistentry>
1021         <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
1022
1023         <listitem><para>When specified without an argument or with a true argument,
1024         enables the usage of
1025         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">unified cgroup hierarchy</ulink>
1026         (a.k.a. cgroups-v2). When specified with a false argument, fall back to
1027         hybrid or full legacy cgroup hierarchy.</para>
1028
1029         <para>If this option is not specified, the default behaviour is determined
1030         during compilation (the <option>--with-default-hierarchy=</option>
1031         option). If the kernel does not support unified cgroup hierarchy, the legacy
1032         hierarchy will be used even if this option is specified.</para>
1033         </listitem>
1034       </varlistentry>
1035
1036       <varlistentry>
1037         <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
1038
1039         <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
1040         (see previous option). When specified without an argument or with a true
1041         argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
1042         tree used for systemd, and
1043         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
1044         cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
1045         forces a full "legacy" mode. When specified with a false argument, enables
1046         the use of "hybrid" hierarchy.</para>
1047
1048         <para>If this option is not specified, the default behaviour is determined
1049         during compilation (the <option>--with-default-hierarchy=</option>
1050         option). If the kernel does not support unified cgroup hierarchy, the legacy
1051         hierarchy will be used  even if this option is specified.</para>
1052         </listitem>
1053       </varlistentry>
1054
1055       <varlistentry>
1056         <term><varname>quiet</varname></term>
1057
1058         <listitem><para>Turn off status output at boot, much like
1059         <varname>systemd.show_status=false</varname> would. Note that
1060         this option is also read by the kernel itself and disables
1061         kernel log output. Passing this option hence turns off the
1062         usual output from both the system manager and the kernel.
1063         </para></listitem>
1064       </varlistentry>
1065
1066       <varlistentry>
1067         <term><varname>debug</varname></term>
1068
1069         <listitem><para>Turn on debugging output. This is equivalent
1070         to <varname>systemd.log_level=debug</varname>. Note that this
1071         option is also read by the kernel itself and enables kernel
1072         debug output. Passing this option hence turns on the debug
1073         output from both the system manager and the
1074         kernel.</para></listitem>
1075       </varlistentry>
1076
1077       <varlistentry>
1078         <term><varname>emergency</varname></term>
1079         <term><varname>rd.emergency</varname></term>
1080         <term><varname>-b</varname></term>
1081
1082         <listitem><para>Boot into emergency mode. This is equivalent
1083         to <varname>systemd.unit=emergency.target</varname> or
1084         <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
1085         provided for compatibility reasons and to be easier to type.</para></listitem>
1086       </varlistentry>
1087
1088       <varlistentry>
1089         <term><varname>rescue</varname></term>
1090         <term><varname>rd.rescue</varname></term>
1091         <term><varname>single</varname></term>
1092         <term><varname>s</varname></term>
1093         <term><varname>S</varname></term>
1094         <term><varname>1</varname></term>
1095
1096         <listitem><para>Boot into rescue mode. This is equivalent to
1097         <varname>systemd.unit=rescue.target</varname> or
1098         <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
1099         provided for compatibility reasons and to be easier to type.</para></listitem>
1100       </varlistentry>
1101
1102       <varlistentry>
1103         <term><varname>2</varname></term>
1104         <term><varname>3</varname></term>
1105         <term><varname>4</varname></term>
1106         <term><varname>5</varname></term>
1107
1108         <listitem><para>Boot into the specified legacy SysV runlevel.
1109         These are equivalent to
1110         <varname>systemd.unit=runlevel2.target</varname>,
1111         <varname>systemd.unit=runlevel3.target</varname>,
1112         <varname>systemd.unit=runlevel4.target</varname>, and
1113         <varname>systemd.unit=runlevel5.target</varname>,
1114         respectively, and provided for compatibility reasons and to be
1115         easier to type.</para></listitem>
1116       </varlistentry>
1117
1118       <varlistentry>
1119         <term><varname>locale.LANG=</varname></term>
1120         <term><varname>locale.LANGUAGE=</varname></term>
1121         <term><varname>locale.LC_CTYPE=</varname></term>
1122         <term><varname>locale.LC_NUMERIC=</varname></term>
1123         <term><varname>locale.LC_TIME=</varname></term>
1124         <term><varname>locale.LC_COLLATE=</varname></term>
1125         <term><varname>locale.LC_MONETARY=</varname></term>
1126         <term><varname>locale.LC_MESSAGES=</varname></term>
1127         <term><varname>locale.LC_PAPER=</varname></term>
1128         <term><varname>locale.LC_NAME=</varname></term>
1129         <term><varname>locale.LC_ADDRESS=</varname></term>
1130         <term><varname>locale.LC_TELEPHONE=</varname></term>
1131         <term><varname>locale.LC_MEASUREMENT=</varname></term>
1132         <term><varname>locale.LC_IDENTIFICATION=</varname></term>
1133
1134         <listitem><para>Set the system locale to use. This overrides
1135         the settings in <filename>/etc/locale.conf</filename>. For
1136         more information, see
1137         <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1138         and
1139         <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
1140         </para></listitem>
1141       </varlistentry>
1142     </variablelist>
1143
1144     <para>For other kernel command line parameters understood by
1145     components of the core OS, please refer to
1146     <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
1147   </refsect1>
1148
1149   <refsect1>
1150     <title>Sockets and FIFOs</title>
1151
1152     <variablelist>
1153       <varlistentry>
1154         <term><filename>/run/systemd/notify</filename></term>
1155
1156         <listitem><para>Daemon status notification socket. This is an
1157         <constant>AF_UNIX</constant> datagram socket and is used to
1158         implement the daemon notification logic as implemented by
1159         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1160
1161       </varlistentry>
1162
1163       <varlistentry>
1164         <term><filename>/run/systemd/private</filename></term>
1165
1166         <listitem><para>Used internally as communication channel
1167         between
1168         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1169         and the systemd process. This is an
1170         <constant>AF_UNIX</constant> stream socket. This interface is
1171         private to systemd and should not be used in external
1172         projects.</para></listitem>
1173       </varlistentry>
1174
1175       <varlistentry>
1176         <term><filename>/dev/initctl</filename></term>
1177
1178         <listitem><para>Limited compatibility support for the SysV
1179         client interface, as implemented by the
1180         <filename>systemd-initctl.service</filename> unit. This is a
1181         named pipe in the file system. This interface is obsolete and
1182         should not be used in new applications.</para></listitem>
1183       </varlistentry>
1184     </variablelist>
1185   </refsect1>
1186
1187   <refsect1>
1188     <title>See Also</title>
1189     <para>
1190       The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
1191       <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1192       <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1193       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1194       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1195       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1196       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1197       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1198       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1199       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1200       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1201       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1202       <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1203       <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1204     </para>
1205   </refsect1>
1206
1207 </refentry>