man/systemd.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="systemd"
   7     xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>systemd</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>systemd</refentrytitle>
  16     <manvolnum>1</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>systemd</refname>
  21     <refname>init</refname>
  22     <refpurpose>systemd system and service manager</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <cmdsynopsis>
  27       <command>/usr/lib/systemd/systemd</command>
  28       <arg choice="opt" rep="repeat">OPTIONS</arg>
  29     </cmdsynopsis>
  30     <cmdsynopsis>
  31       <command>init</command>
  32       <arg choice="opt" rep="repeat">OPTIONS</arg>
  33       <arg choice="req">COMMAND</arg>
  34     </cmdsynopsis>
  35   </refsynopsisdiv>
  36
  37   <refsect1>
  38     <title>Description</title>
  39
  40     <para>systemd is a system and service manager for Linux operating systems. When run as first process on
  41     boot (as PID 1), it acts as init system that brings up and maintains userspace services. Separate
  42     instances are started for logged-in users to start their services.</para>
  43
  44     <para><command>systemd</command> is usually not invoked directly by the user, but is installed as the
  45     <filename>/sbin/init</filename> symlink and started during early boot. The user manager instances are
  46     started automatically through the
  47     <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  48     service.</para>
  49
  50     <para>For compatibility with SysV, if the binary is called as <command>init</command> and is not the
  51     first process on the machine (PID is not 1), it will execute <command>telinit</command> and pass all
  52     command line arguments unmodified. That means <command>init</command> and <command>telinit</command> are
  53     mostly equivalent when invoked from normal login sessions. See
  54     <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry> for more
  55     information.</para>
  56
  57     <para>When run as a system instance, systemd interprets the
  58     configuration file <filename>system.conf</filename> and the files
  59     in <filename>system.conf.d</filename> directories; when run as a
  60     user instance, systemd interprets the configuration file
  61     <filename>user.conf</filename> and the files in
  62     <filename>user.conf.d</filename> directories. See
  63     <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  64     for more information.</para>
  65   </refsect1>
  66
  67   <refsect1>
  68     <title>Concepts</title>
  69
  70     <para>systemd provides a dependency system between various
  71     entities called "units" of 11 different types. Units encapsulate
  72     various objects that are relevant for system boot-up and
  73     maintenance. The majority of units are configured in unit
  74     configuration files, whose syntax and basic set of options is
  75     described in
  76     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
  77     however some are created automatically from other configuration,
  78     dynamically from system state or programmatically at runtime.
  79     Units may be "active" (meaning started, bound, plugged in, …,
  80     depending on the unit type, see below), or "inactive" (meaning
  81     stopped, unbound, unplugged, …), as well as in the process of
  82     being activated or deactivated, i.e. between the two states (these
  83     states are called "activating", "deactivating"). A special
  84     "failed" state is available as well, which is very similar to
  85     "inactive" and is entered when the service failed in some way
  86     (process returned error code on exit, or crashed, an operation
  87     timed out, or after too many restarts). If this state is entered,
  88     the cause will be logged, for later reference. Note that the
  89     various unit types may have a number of additional substates,
  90     which are mapped to the five generalized unit states described
  91     here.</para>
  92
  93     <para>The following unit types are available:</para>
  94
  95     <orderedlist>
  96       <listitem><para>Service units, which start and control daemons
  97       and the processes they consist of. For details, see
  98       <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
  99
 100       <listitem><para>Socket units, which encapsulate local IPC or
 101       network sockets in the system, useful for socket-based
 102       activation. For details about socket units, see
 103       <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 104       for details on socket-based activation and other forms of
 105       activation, see
 106       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 107
 108       <listitem><para>Target units are useful to group units, or
 109       provide well-known synchronization points during boot-up, see
 110       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 111
 112       <listitem><para>Device units expose kernel devices in systemd
 113       and may be used to implement device-based activation. For
 114       details, see
 115       <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 116
 117       <listitem><para>Mount units control mount points in the file
 118       system, for details see
 119       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 120
 121       <listitem><para>Automount units provide automount capabilities,
 122       for on-demand mounting of file systems as well as parallelized
 123       boot-up. See
 124       <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 125
 126       <listitem><para>Timer units are useful for triggering activation
 127       of other units based on timers. You may find details in
 128       <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 129
 130       <listitem><para>Swap units are very similar to mount units and
 131       encapsulate memory swap partitions or files of the operating
 132       system. They are described in
 133       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 134
 135       <listitem><para>Path units may be used to activate other
 136       services when file system objects change or are modified. See
 137       <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 138
 139       <listitem><para>Slice units may be used to group units which
 140       manage system processes (such as service and scope units) in a
 141       hierarchical tree for resource management purposes. See
 142       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 143
 144       <listitem><para>Scope units are similar to service units, but
 145       manage foreign processes instead of starting them as well. See
 146       <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 147
 148     </orderedlist>
 149
 150     <para>Units are named as their configuration files. Some units
 151     have special semantics. A detailed list is available in
 152     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 153
 154     <para>systemd knows various kinds of dependencies, including
 155     positive and negative requirement dependencies (i.e.
 156     <varname>Requires=</varname> and <varname>Conflicts=</varname>) as
 157     well as ordering dependencies (<varname>After=</varname> and
 158     <varname>Before=</varname>). NB: ordering and requirement
 159     dependencies are orthogonal. If only a requirement dependency
 160     exists between two units (e.g. <filename>foo.service</filename>
 161     requires <filename>bar.service</filename>), but no ordering
 162     dependency (e.g. <filename>foo.service</filename> after
 163     <filename>bar.service</filename>) and both are requested to start,
 164     they will be started in parallel. It is a common pattern that both
 165     requirement and ordering dependencies are placed between two
 166     units. Also note that the majority of dependencies are implicitly
 167     created and maintained by systemd. In most cases, it should be
 168     unnecessary to declare additional dependencies manually, however
 169     it is possible to do this.</para>
 170
 171     <para>Application programs and units (via dependencies) may
 172     request state changes of units. In systemd, these requests are
 173     encapsulated as 'jobs' and maintained in a job queue. Jobs may
 174     succeed or can fail, their execution is ordered based on the
 175     ordering dependencies of the units they have been scheduled
 176     for.</para>
 177
 178     <para>On boot systemd activates the target unit
 179     <filename>default.target</filename> whose job is to activate
 180     on-boot services and other on-boot units by pulling them in via
 181     dependencies. Usually, the unit name is just an alias (symlink) for
 182     either <filename>graphical.target</filename> (for fully-featured
 183     boots into the UI) or <filename>multi-user.target</filename> (for
 184     limited console-only boots for use in embedded or server
 185     environments, or similar; a subset of graphical.target). However,
 186     it is at the discretion of the administrator to configure it as an
 187     alias to any other target unit. See
 188     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 189     for details about these target units.</para>
 190
 191     <para>systemd only keeps a minimal set of units loaded into memory. Specifically, the only units that are kept
 192     loaded into memory are those for which at least one of the following conditions is true:</para>
 193
 194     <orderedlist>
 195       <listitem><para>It is in an active, activating, deactivating or failed state (i.e. in any unit state except for <literal>inactive</literal>)</para></listitem>
 196       <listitem><para>It has a job queued for it</para></listitem>
 197       <listitem><para>It is a dependency of some sort of at least one other unit that is loaded into memory</para></listitem>
 198       <listitem><para>It has some form of resource still allocated (e.g. a service unit that is inactive but for which
 199       a process is still lingering that ignored the request to be terminated)</para></listitem>
 200       <listitem><para>It has been pinned into memory programmatically by a D-Bus call</para></listitem>
 201     </orderedlist>
 202
 203     <para>systemd will automatically and implicitly load units from disk — if they are not loaded yet — as soon as
 204     operations are requested for them. Thus, in many respects, the fact whether a unit is loaded or not is invisible to
 205     clients. Use <command>systemctl list-units --all</command> to comprehensively list all units currently loaded. Any
 206     unit for which none of the conditions above applies is promptly unloaded. Note that when a unit is unloaded from
 207     memory its accounting data is flushed out too. However, this data is generally not lost, as a journal log record
 208     is generated declaring the consumed resources whenever a unit shuts down.</para>
 209
 210     <para>Processes systemd spawns are placed in individual Linux
 211     control groups named after the unit which they belong to in the
 212     private systemd hierarchy. (see <ulink
 213     url="https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt">cgroups.txt</ulink>
 214     for more information about control groups, or short "cgroups").
 215     systemd uses this to effectively keep track of processes. Control
 216     group information is maintained in the kernel, and is accessible
 217     via the file system hierarchy (beneath
 218     <filename>/sys/fs/cgroup/systemd/</filename>), or in tools such as
 219     <citerefentry project='man-pages'><refentrytitle>systemd-cgls</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 220     or
 221     <citerefentry project='man-pages'><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 222     (<command>ps xawf -eo pid,user,cgroup,args</command> is
 223     particularly useful to list all processes and the systemd units
 224     they belong to.).</para>
 225
 226     <para>systemd is compatible with the SysV init system to a large
 227     degree: SysV init scripts are supported and simply read as an
 228     alternative (though limited) configuration file format. The SysV
 229     <filename>/dev/initctl</filename> interface is provided, and
 230     compatibility implementations of the various SysV client tools are
 231     available. In addition to that, various established Unix
 232     functionality such as <filename>/etc/fstab</filename> or the
 233     <filename>utmp</filename> database are supported.</para>
 234
 235     <para>systemd has a minimal transaction system: if a unit is
 236     requested to start up or shut down it will add it and all its
 237     dependencies to a temporary transaction. Then, it will verify if
 238     the transaction is consistent (i.e. whether the ordering of all
 239     units is cycle-free). If it is not, systemd will try to fix it up,
 240     and removes non-essential jobs from the transaction that might
 241     remove the loop. Also, systemd tries to suppress non-essential
 242     jobs in the transaction that would stop a running service. Finally
 243     it is checked whether the jobs of the transaction contradict jobs
 244     that have already been queued, and optionally the transaction is
 245     aborted then. If all worked out and the transaction is consistent
 246     and minimized in its impact it is merged with all already
 247     outstanding jobs and added to the run queue. Effectively this
 248     means that before executing a requested operation, systemd will
 249     verify that it makes sense, fixing it if possible, and only
 250     failing if it really cannot work.</para>
 251
 252     <para>Note that transactions are generated independently of a unit's
 253     state at runtime, hence, for example, if a start job is requested on an
 254     already started unit, it will still generate a transaction and wake up any
 255     inactive dependencies (and cause propagation of other jobs as per the
 256     defined relationships). This is because the enqueued job is at the time of
 257     execution compared to the target unit's state and is marked successful and
 258     complete when both satisfy. However, this job also pulls in other
 259     dependencies due to the defined relationships and thus leads to, in our
 260     our example, start jobs for any of those inactive units getting queued as
 261     well.</para>
 262
 263     <para>systemd contains native implementations of various tasks
 264     that need to be executed as part of the boot process. For example,
 265     it sets the hostname or configures the loopback network device. It
 266     also sets up and mounts various API file systems, such as
 267     <filename>/sys</filename> or <filename>/proc</filename>.</para>
 268
 269     <para>For more information about the concepts and
 270     ideas behind systemd, please refer to the
 271     <ulink url="http://0pointer.de/blog/projects/systemd.html">Original Design Document</ulink>.</para>
 272
 273     <para>Note that some but not all interfaces provided
 274     by systemd are covered by the
 275     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
 276     Stability Promise</ulink>.</para>
 277
 278     <para>Units may be generated dynamically at boot and system
 279     manager reload time, for example based on other configuration
 280     files or parameters passed on the kernel command line. For details, see
 281     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 282
 283     <para>Systems which invoke systemd in a container or initrd environment should implement the <ulink
 284     url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> or <ulink
 285     url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
 286     specifications, respectively.</para>
 287   </refsect1>
 288
 289   <refsect1>
 290     <title>Directories</title>
 291
 292     <variablelist>
 293       <varlistentry>
 294         <term>System unit directories</term>
 295
 296         <listitem><para>The systemd system manager reads unit
 297         configuration from various directories. Packages that want to
 298         install unit files shall place them in the directory returned
 299         by <command>pkg-config systemd
 300         --variable=systemdsystemunitdir</command>. Other directories
 301         checked are <filename>/usr/local/lib/systemd/system</filename>
 302         and <filename>/usr/lib/systemd/system</filename>. User
 303         configuration always takes precedence. <command>pkg-config
 304         systemd --variable=systemdsystemconfdir</command> returns the
 305         path of the system configuration directory. Packages should
 306         alter the content of these directories only with the
 307         <command>enable</command> and <command>disable</command>
 308         commands of the
 309         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 310         tool. Full list of directories is provided in
 311         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 312         </para></listitem>
 313       </varlistentry>
 314     </variablelist>
 315
 316     <variablelist>
 317       <varlistentry>
 318         <term>User unit directories</term>
 319
 320         <listitem><para>Similar rules apply for the user unit
 321         directories. However, here the
 322         <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 323         Base Directory specification</ulink> is followed to find
 324         units. Applications should place their unit files in the
 325         directory returned by <command>pkg-config systemd
 326         --variable=systemduserunitdir</command>. Global configuration
 327         is done in the directory reported by <command>pkg-config
 328         systemd --variable=systemduserconfdir</command>. The
 329         <command>enable</command> and <command>disable</command>
 330         commands of the
 331         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 332         tool can handle both global (i.e. for all users) and private
 333         (for one user) enabling/disabling of units. Full list of
 334         directories is provided in
 335         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 336         </para></listitem>
 337       </varlistentry>
 338     </variablelist>
 339
 340     <variablelist>
 341       <varlistentry>
 342         <term>SysV init scripts directory</term>
 343
 344         <listitem><para>The location of the SysV init script directory
 345         varies between distributions. If systemd cannot find a native
 346         unit file for a requested service, it will look for a SysV
 347         init script of the same name (with the
 348         <filename>.service</filename> suffix
 349         removed).</para></listitem>
 350       </varlistentry>
 351     </variablelist>
 352
 353     <variablelist>
 354       <varlistentry>
 355         <term>SysV runlevel link farm directory</term>
 356
 357         <listitem><para>The location of the SysV runlevel link farm
 358         directory varies between distributions. systemd will take the
 359         link farm into account when figuring out whether a service
 360         shall be enabled. Note that a service unit with a native unit
 361         configuration file cannot be started by activating it in the
 362         SysV runlevel link farm.</para></listitem>
 363       </varlistentry>
 364     </variablelist>
 365   </refsect1>
 366
 367   <refsect1>
 368     <title>Signals</title>
 369
 370     <variablelist>
 371       <varlistentry>
 372         <term><constant>SIGTERM</constant></term>
 373
 374         <listitem><para>Upon receiving this signal the systemd system
 375         manager serializes its state, reexecutes itself and
 376         deserializes the saved state again. This is mostly equivalent
 377         to <command>systemctl daemon-reexec</command>.</para>
 378
 379         <para>systemd user managers will start the
 380         <filename>exit.target</filename> unit when this signal is
 381         received. This is mostly equivalent to <command>systemctl
 382         --user start exit.target
 383         --job-mode=replace-irreversibly</command>.</para></listitem>
 384       </varlistentry>
 385
 386       <varlistentry>
 387         <term><constant>SIGINT</constant></term>
 388
 389         <listitem><para>Upon receiving this signal the systemd system manager will start the
 390         <filename>ctrl-alt-del.target</filename> unit. This is mostly equivalent to
 391         <command>systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly</command>. If
 392         this signal is received more than 7 times per 2s, an immediate reboot is triggered. Note
 393         that pressing
 394         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> on the
 395         console will trigger this signal. Hence, if a reboot is hanging, pressing
 396         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> more than
 397         7 times in 2 seconds is a relatively safe way to trigger an immediate reboot.</para>
 398
 399         <para>systemd user managers treat this signal the same way as
 400         <constant>SIGTERM</constant>.</para></listitem>
 401       </varlistentry>
 402
 403       <varlistentry>
 404         <term><constant>SIGWINCH</constant></term>
 405
 406         <listitem><para>When this signal is received the systemd
 407         system manager will start the
 408         <filename>kbrequest.target</filename> unit. This is mostly
 409         equivalent to <command>systemctl start
 410         kbrequest.target</command>.</para>
 411
 412         <para>This signal is ignored by systemd user
 413         managers.</para></listitem>
 414       </varlistentry>
 415
 416       <varlistentry>
 417         <term><constant>SIGPWR</constant></term>
 418
 419         <listitem><para>When this signal is received the systemd
 420         manager will start the <filename>sigpwr.target</filename>
 421         unit. This is mostly equivalent to <command>systemctl start
 422         sigpwr.target</command>.</para></listitem>
 423       </varlistentry>
 424
 425       <varlistentry>
 426         <term><constant>SIGUSR1</constant></term>
 427
 428         <listitem><para>When this signal is received the systemd
 429         manager will try to reconnect to the D-Bus
 430         bus.</para></listitem>
 431       </varlistentry>
 432
 433       <varlistentry>
 434         <term><constant>SIGUSR2</constant></term>
 435
 436         <listitem><para>When this signal is received the systemd
 437         manager will log its complete state in human-readable form.
 438         The data logged is the same as printed by
 439         <command>systemd-analyze dump</command>.</para></listitem>
 440       </varlistentry>
 441
 442       <varlistentry>
 443         <term><constant>SIGHUP</constant></term>
 444
 445         <listitem><para>Reloads the complete daemon configuration.
 446         This is mostly equivalent to <command>systemctl
 447         daemon-reload</command>.</para></listitem>
 448       </varlistentry>
 449
 450       <varlistentry>
 451         <term><constant>SIGRTMIN+0</constant></term>
 452
 453         <listitem><para>Enters default mode, starts the
 454         <filename>default.target</filename> unit. This is mostly
 455         equivalent to <command>systemctl isolate
 456         default.target</command>.</para></listitem>
 457       </varlistentry>
 458
 459       <varlistentry>
 460         <term><constant>SIGRTMIN+1</constant></term>
 461
 462         <listitem><para>Enters rescue mode, starts the
 463         <filename>rescue.target</filename> unit. This is mostly
 464         equivalent to <command>systemctl isolate
 465         rescue.target</command>.</para></listitem>
 466       </varlistentry>
 467
 468       <varlistentry>
 469         <term><constant>SIGRTMIN+2</constant></term>
 470
 471         <listitem><para>Enters emergency mode, starts the
 472         <filename>emergency.service</filename> unit. This is mostly
 473         equivalent to <command>systemctl isolate
 474         emergency.service</command>.</para></listitem>
 475       </varlistentry>
 476
 477       <varlistentry>
 478         <term><constant>SIGRTMIN+3</constant></term>
 479
 480         <listitem><para>Halts the machine, starts the
 481         <filename>halt.target</filename> unit. This is mostly
 482         equivalent to <command>systemctl start halt.target
 483         --job-mode=replace-irreversibly</command>.</para>
 484         </listitem>
 485       </varlistentry>
 486
 487       <varlistentry>
 488         <term><constant>SIGRTMIN+4</constant></term>
 489
 490         <listitem><para>Powers off the machine, starts the
 491         <filename>poweroff.target</filename> unit. This is mostly
 492         equivalent to <command>systemctl start poweroff.target
 493         --job-mode=replace-irreversibly</command>.</para>
 494         </listitem>
 495       </varlistentry>
 496
 497       <varlistentry>
 498         <term><constant>SIGRTMIN+5</constant></term>
 499
 500         <listitem><para>Reboots the machine, starts the
 501         <filename>reboot.target</filename> unit. This is mostly
 502         equivalent to <command>systemctl start reboot.target
 503         --job-mode=replace-irreversibly</command>.</para>
 504         </listitem>
 505       </varlistentry>
 506
 507       <varlistentry>
 508         <term><constant>SIGRTMIN+6</constant></term>
 509
 510         <listitem><para>Reboots the machine via kexec, starts the
 511         <filename>kexec.target</filename> unit. This is mostly
 512         equivalent to <command>systemctl start kexec.target
 513         --job-mode=replace-irreversibly</command>.</para>
 514         </listitem>
 515       </varlistentry>
 516
 517       <varlistentry>
 518         <term><constant>SIGRTMIN+13</constant></term>
 519
 520         <listitem><para>Immediately halts the machine.</para></listitem>
 521       </varlistentry>
 522
 523       <varlistentry>
 524         <term><constant>SIGRTMIN+14</constant></term>
 525
 526         <listitem><para>Immediately powers off the machine.</para></listitem>
 527       </varlistentry>
 528
 529       <varlistentry>
 530         <term><constant>SIGRTMIN+15</constant></term>
 531
 532         <listitem><para>Immediately reboots the machine.</para></listitem>
 533       </varlistentry>
 534
 535       <varlistentry>
 536         <term><constant>SIGRTMIN+16</constant></term>
 537
 538         <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 539       </varlistentry>
 540
 541       <varlistentry>
 542         <term><constant>SIGRTMIN+20</constant></term>
 543
 544         <listitem><para>Enables display of status messages on the
 545         console, as controlled via
 546         <varname>systemd.show_status=1</varname> on the kernel command
 547         line.</para></listitem>
 548       </varlistentry>
 549
 550       <varlistentry>
 551         <term><constant>SIGRTMIN+21</constant></term>
 552
 553         <listitem><para>Disables display of
 554         status messages on the console, as
 555         controlled via
 556         <varname>systemd.show_status=0</varname>
 557         on the kernel command
 558         line.</para></listitem>
 559       </varlistentry>
 560
 561       <varlistentry>
 562         <term><constant>SIGRTMIN+22</constant></term>
 563
 564         <listitem><para>Sets the service manager's log level to <literal>debug</literal>, in a fashion equivalent to
 565         <varname>systemd.log_level=debug</varname> on the kernel command line.</para></listitem>
 566       </varlistentry>
 567
 568       <varlistentry>
 569         <term><constant>SIGRTMIN+23</constant></term>
 570
 571         <listitem><para>Restores the log level to its configured value. The configured value is derived from – in order
 572         of priority – the value specified with <varname>systemd.log-level=</varname> on the kernel command line, or the
 573         value specified with <option>LogLevel=</option> in the configuration file, or the built-in default of
 574         <literal>info</literal>.</para></listitem>
 575       </varlistentry>
 576
 577       <varlistentry>
 578         <term><constant>SIGRTMIN+24</constant></term>
 579
 580         <listitem><para>Immediately exits the manager (only available
 581         for --user instances).</para></listitem>
 582       </varlistentry>
 583
 584       <varlistentry>
 585         <term><constant>SIGRTMIN+26</constant></term>
 586
 587         <listitem><para>Restores the log target to its configured value. The configured value is derived from – in
 588         order of priority – the value specified with <varname>systemd.log-target=</varname> on the kernel command line,
 589         or the value specified with <option>LogTarget=</option> in the configuration file, or the built-in
 590         default.</para></listitem>
 591       </varlistentry>
 592
 593       <varlistentry>
 594         <term><constant>SIGRTMIN+27</constant></term>
 595         <term><constant>SIGRTMIN+28</constant></term>
 596
 597         <listitem><para>Sets the log target to <literal>console</literal> on <constant>SIGRTMIN+27</constant> (or
 598         <literal>kmsg</literal> on <constant>SIGRTMIN+28</constant>), in a fashion equivalent to
 599         <varname>systemd.log_target=console</varname> (or <varname>systemd.log_target=kmsg</varname> on
 600         <constant>SIGRTMIN+28</constant>) on the kernel command line.</para></listitem>
 601       </varlistentry>
 602     </variablelist>
 603   </refsect1>
 604
 605   <refsect1>
 606     <title>Environment</title>
 607
 608     <variablelist class='environment-variables'>
 609       <varlistentry>
 610         <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 611         <listitem><para>systemd reads the log level from this
 612         environment variable. This can be overridden with
 613         <option>--log-level=</option>.</para></listitem>
 614       </varlistentry>
 615
 616       <varlistentry>
 617         <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 618         <listitem><para>systemd reads the log target from this
 619         environment variable. This can be overridden with
 620         <option>--log-target=</option>.</para></listitem>
 621       </varlistentry>
 622
 623       <varlistentry>
 624         <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 625         <listitem><para>Controls whether systemd highlights important
 626         log messages. This can be overridden with
 627         <option>--log-color=</option>.</para></listitem>
 628       </varlistentry>
 629
 630       <varlistentry>
 631         <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 632         <listitem><para>Controls whether systemd prints the code
 633         location along with log messages. This can be overridden with
 634         <option>--log-location=</option>.</para></listitem>
 635       </varlistentry>
 636
 637       <varlistentry>
 638         <term><varname>$XDG_CONFIG_HOME</varname></term>
 639         <term><varname>$XDG_CONFIG_DIRS</varname></term>
 640         <term><varname>$XDG_DATA_HOME</varname></term>
 641         <term><varname>$XDG_DATA_DIRS</varname></term>
 642
 643         <listitem><para>The systemd user manager uses these variables
 644         in accordance to the <ulink
 645         url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 646         Base Directory specification</ulink> to find its
 647         configuration.</para></listitem>
 648       </varlistentry>
 649
 650       <varlistentry>
 651         <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 652         <term><varname>$SYSTEMD_GENERATOR_PATH</varname></term>
 653         <term><varname>$SYSTEMD_ENVIRONMENT_GENERATOR_PATH</varname></term>
 654
 655         <listitem><para>Controls where systemd looks for unit files and
 656         generators.</para>
 657         <para>These variables may contain a list of paths, separated by colons
 658         (<literal>:</literal>). When set, if the list ends with an empty
 659         component (<literal>...:</literal>), this list is prepended to the
 660         usual set of of paths. Otherwise, the specified list replaces the usual
 661         set of paths.
 662         </para></listitem>
 663       </varlistentry>
 664
 665       <varlistentry>
 666         <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 667
 668         <listitem><para>Controls where systemd looks for SysV init
 669         scripts.</para></listitem>
 670       </varlistentry>
 671
 672       <varlistentry>
 673         <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 674
 675         <listitem><para>Controls where systemd looks for SysV init
 676         script runlevel link farms.</para></listitem>
 677       </varlistentry>
 678
 679       <xi:include href="less-variables.xml" xpointer="pager"/>
 680       <xi:include href="less-variables.xml" xpointer="less"/>
 681       <xi:include href="less-variables.xml" xpointer="lesscharset"/>
 682       <xi:include href="less-variables.xml" xpointer="colors"/>
 683       <xi:include href="less-variables.xml" xpointer="urlify"/>
 684
 685       <varlistentry>
 686         <term><varname>$LISTEN_PID</varname></term>
 687         <term><varname>$LISTEN_FDS</varname></term>
 688         <term><varname>$LISTEN_FDNAMES</varname></term>
 689
 690         <listitem><para>Set by systemd for supervised processes during
 691         socket-based activation. See
 692         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 693         for more information.</para></listitem>
 694       </varlistentry>
 695
 696       <varlistentry>
 697         <term><varname>$NOTIFY_SOCKET</varname></term>
 698
 699         <listitem><para>Set by systemd for supervised processes for
 700         status and start-up completion notification. See
 701         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 702         for more information.</para></listitem>
 703       </varlistentry>
 704     </variablelist>
 705
 706     <para>For further environment variables understood by systemd and its various components, see <ulink
 707     url="https://systemd.io/ENVIRONMENT">Known Environment Variables</ulink>.</para>
 708   </refsect1>
 709
 710   <refsect1>
 711     <title>Kernel Command Line</title>
 712
 713     <para>When run as the system instance systemd parses a number of options listed below. They can be
 714     specified as kernel command line arguments<footnote><para>If run inside a Linux container these arguments
 715     may be passed as command line arguments to systemd itself, next to any of the command line options listed
 716     in the Options section above. If run outside of Linux containers, these arguments are parsed from
 717     <filename>/proc/cmdline</filename> instead.</para></footnote>, or through the
 718     <literal>SystemdOptions</literal> EFI variable (on EFI systems). The kernel command line has higher
 719     priority. Following variables are understood:</para>
 720
 721     <variablelist class='kernel-commandline-options'>
 722       <varlistentry>
 723         <term><varname>systemd.unit=</varname></term>
 724         <term><varname>rd.systemd.unit=</varname></term>
 725
 726         <listitem><para>Overrides the unit to activate on boot.
 727         Defaults to <filename>default.target</filename>. This may be
 728         used to temporarily boot into a different boot unit, for
 729         example <filename>rescue.target</filename> or
 730         <filename>emergency.service</filename>. See
 731         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 732         for details about these units. The option prefixed with
 733         <literal>rd.</literal> is honored only in the initial RAM disk
 734         (initrd), while the one that is not prefixed only in the main
 735         system.</para></listitem>
 736       </varlistentry>
 737
 738       <varlistentry>
 739         <term><varname>systemd.dump_core</varname></term>
 740
 741         <listitem><para>Takes a boolean argument or enables the option if specified
 742         without an argument. If enabled, the systemd manager (PID 1) dumps core when
 743         it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
 744         </listitem>
 745       </varlistentry>
 746
 747       <varlistentry>
 748         <term><varname>systemd.crash_chvt</varname></term>
 749
 750         <listitem><para>Takes a positive integer, or a boolean argument. Can be also
 751         specified without an argument, with the same effect as a positive boolean. If
 752         a positive integer (in the range 1–63) is specified, the system manager (PID
 753         1) will activate the specified virtual terminal (VT) when it
 754         crashes. Defaults to disabled, meaning that no such switch is attempted. If
 755         set to enabled, the VT the kernel messages are written to is selected.
 756         </para></listitem>
 757       </varlistentry>
 758
 759       <varlistentry>
 760         <term><varname>systemd.crash_shell</varname></term>
 761
 762         <listitem><para>Takes a boolean argument or enables the option if specified
 763         without an argument. If enabled, the system manager (PID 1) spawns a shell
 764         when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
 765         to disabled, for security reasons, as the shell is not protected by password
 766         authentication.</para></listitem>
 767       </varlistentry>
 768
 769       <varlistentry>
 770         <term><varname>systemd.crash_reboot</varname></term>
 771
 772         <listitem><para>Takes a boolean argument or enables the option if specified
 773         without an argument. If enabled, the system manager (PID 1) will reboot the
 774         machine automatically when it crashes, after a 10s delay.  Otherwise, the
 775         system will hang indefinitely. Defaults to disabled, in order to avoid a
 776         reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
 777         system is rebooted after the shell exits.</para></listitem>
 778       </varlistentry>
 779
 780       <varlistentry>
 781         <term><varname>systemd.confirm_spawn</varname></term>
 782
 783         <listitem><para>Takes a boolean argument or a path to the virtual console
 784         where the confirmation messages should be emitted. Can be also specified
 785         without an argument, with the same effect as a positive boolean. If enabled,
 786         the system manager (PID 1) asks for confirmation when spawning processes
 787         using <option>/dev/console</option>. If a path or a console name (such as
 788         <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
 789         path or described by the give name will be used instead. Defaults to disabled.
 790         </para></listitem>
 791       </varlistentry>
 792
 793       <varlistentry>
 794         <term><varname>systemd.service_watchdogs=</varname></term>
 795
 796         <listitem><para>Takes a boolean argument. If disabled, all service runtime
 797         watchdogs (<option>WatchdogSec=</option>) and emergency actions (e.g.
 798         <option>OnFailure=</option> or <option>StartLimitAction=</option>) are
 799         ignored by the system manager (PID 1); see
 800         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 801         Defaults to enabled, i.e. watchdogs and failure actions are processed
 802         normally. The hardware watchdog is not affected by this
 803         option.</para></listitem>
 804       </varlistentry>
 805
 806       <varlistentry>
 807         <term><varname>systemd.show_status</varname></term>
 808
 809         <listitem><para>Takes a boolean argument or the constants <constant>error</constant> and
 810         <constant>auto</constant>. Can be also specified without an argument, with the same effect as a
 811         positive boolean. If enabled, the systemd manager (PID 1) shows terse service status updates on the
 812         console during bootup. With <constant>error</constant>, only messages about failures are shown, but
 813         boot is otherwise quiet. <constant>auto</constant> behaves like <option>false</option> until there is
 814         a significant delay in boot. Defaults to enabled, unless <option>quiet</option> is passed as kernel
 815         command line option, in which case it defaults to <constant>error</constant>. If specified overrides
 816         the system manager configuration file option <option>ShowStatus=</option>, see
 817         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 818         </para></listitem>
 819       </varlistentry>
 820
 821       <varlistentry>
 822         <term><varname>systemd.status_unit_format=</varname></term>
 823
 824         <listitem><para>Takes either <option>name</option> or <option>description</option> as the value. If
 825         <option>name</option>, the system manager will use unit names in status messages. If specified,
 826         overrides the system manager configuration file option <option>StatusUnitFormat=</option>, see
 827         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 828         </para></listitem>
 829       </varlistentry>
 830
 831       <varlistentry>
 832         <term><varname>systemd.log_target=</varname></term>
 833         <term><varname>systemd.log_level=</varname></term>
 834         <term><varname>systemd.log_location=</varname></term>
 835         <term><varname>systemd.log_color</varname></term>
 836
 837         <listitem><para>Controls log output, with the same effect as the
 838         <varname>$SYSTEMD_LOG_TARGET</varname>,
 839         <varname>$SYSTEMD_LOG_LEVEL</varname>,
 840         <varname>$SYSTEMD_LOG_LOCATION</varname>,
 841         <varname>$SYSTEMD_LOG_COLOR</varname> environment variables described above.
 842         <varname>systemd.log_color</varname> can be specified without an argument,
 843         with the same effect as a positive boolean.</para></listitem>
 844       </varlistentry>
 845
 846       <varlistentry>
 847         <term><varname>systemd.default_standard_output=</varname></term>
 848         <term><varname>systemd.default_standard_error=</varname></term>
 849
 850         <listitem><para>Controls default standard output and error output for services and sockets. That is,
 851         controls the default for <option>StandardOutput=</option> and <option>StandardError=</option> (see
 852         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 853         details). Takes one of <option>inherit</option>, <option>null</option>, <option>tty</option>,
 854         <option>journal</option>, <option>journal+console</option>, <option>kmsg</option>,
 855         <option>kmsg+console</option>. If the argument is omitted
 856         <varname>systemd.default-standard-output=</varname> defaults to <option>journal</option> and
 857         <varname>systemd.default-standard-error=</varname> to <option>inherit</option>.</para></listitem>
 858       </varlistentry>
 859
 860       <varlistentry>
 861         <term><varname>systemd.setenv=</varname></term>
 862
 863         <listitem><para>Takes a string argument in the form
 864         VARIABLE=VALUE. May be used to set default environment
 865         variables to add to forked child processes. May be used more
 866         than once to set multiple variables.</para></listitem>
 867       </varlistentry>
 868
 869       <varlistentry>
 870         <term><varname>systemd.machine_id=</varname></term>
 871
 872         <listitem><para>Takes a 32 character hex value to be
 873         used for setting the machine-id. Intended mostly for
 874         network booting where the same machine-id is desired
 875         for every boot.</para></listitem>
 876       </varlistentry>
 877
 878       <varlistentry>
 879         <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
 880
 881         <listitem><para>When specified without an argument or with a true argument,
 882         enables the usage of
 883         <ulink url="https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html">unified cgroup hierarchy</ulink>
 884         (a.k.a. cgroups-v2). When specified with a false argument, fall back to
 885         hybrid or full legacy cgroup hierarchy.</para>
 886
 887         <para>If this option is not specified, the default behaviour is determined
 888         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 889         option). If the kernel does not support unified cgroup hierarchy, the legacy
 890         hierarchy will be used even if this option is specified.</para>
 891         </listitem>
 892       </varlistentry>
 893
 894       <varlistentry>
 895         <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
 896
 897         <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
 898         (see previous option). When specified without an argument or with a true
 899         argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
 900         tree used for systemd, and
 901         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
 902         cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
 903         forces a full "legacy" mode. When specified with a false argument, enables
 904         the use of "hybrid" hierarchy.</para>
 905
 906         <para>If this option is not specified, the default behaviour is determined
 907         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 908         option). If the kernel does not support unified cgroup hierarchy, the legacy
 909         hierarchy will be used  even if this option is specified.</para>
 910         </listitem>
 911       </varlistentry>
 912
 913       <varlistentry>
 914         <term><varname>quiet</varname></term>
 915
 916         <listitem><para>Turn off status output at boot, much like
 917         <varname>systemd.show_status=no</varname> would. Note that
 918         this option is also read by the kernel itself and disables
 919         kernel log output. Passing this option hence turns off the
 920         usual output from both the system manager and the kernel.
 921         </para></listitem>
 922       </varlistentry>
 923
 924       <varlistentry>
 925         <term><varname>debug</varname></term>
 926
 927         <listitem><para>Turn on debugging output. This is equivalent
 928         to <varname>systemd.log_level=debug</varname>. Note that this
 929         option is also read by the kernel itself and enables kernel
 930         debug output. Passing this option hence turns on the debug
 931         output from both the system manager and the
 932         kernel.</para></listitem>
 933       </varlistentry>
 934
 935       <varlistentry>
 936         <term><varname>emergency</varname></term>
 937         <term><varname>rd.emergency</varname></term>
 938         <term><varname>-b</varname></term>
 939
 940         <listitem><para>Boot into emergency mode. This is equivalent
 941         to <varname>systemd.unit=emergency.target</varname> or
 942         <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
 943         provided for compatibility reasons and to be easier to type.</para></listitem>
 944       </varlistentry>
 945
 946       <varlistentry>
 947         <term><varname>rescue</varname></term>
 948         <term><varname>rd.rescue</varname></term>
 949         <term><varname>single</varname></term>
 950         <term><varname>s</varname></term>
 951         <term><varname>S</varname></term>
 952         <term><varname>1</varname></term>
 953
 954         <listitem><para>Boot into rescue mode. This is equivalent to
 955         <varname>systemd.unit=rescue.target</varname> or
 956         <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
 957         provided for compatibility reasons and to be easier to type.</para></listitem>
 958       </varlistentry>
 959
 960       <varlistentry>
 961         <term><varname>2</varname></term>
 962         <term><varname>3</varname></term>
 963         <term><varname>4</varname></term>
 964         <term><varname>5</varname></term>
 965
 966         <listitem><para>Boot into the specified legacy SysV runlevel.
 967         These are equivalent to
 968         <varname>systemd.unit=runlevel2.target</varname>,
 969         <varname>systemd.unit=runlevel3.target</varname>,
 970         <varname>systemd.unit=runlevel4.target</varname>, and
 971         <varname>systemd.unit=runlevel5.target</varname>,
 972         respectively, and provided for compatibility reasons and to be
 973         easier to type.</para></listitem>
 974       </varlistentry>
 975
 976       <varlistentry>
 977         <term><varname>locale.LANG=</varname></term>
 978         <term><varname>locale.LANGUAGE=</varname></term>
 979         <term><varname>locale.LC_CTYPE=</varname></term>
 980         <term><varname>locale.LC_NUMERIC=</varname></term>
 981         <term><varname>locale.LC_TIME=</varname></term>
 982         <term><varname>locale.LC_COLLATE=</varname></term>
 983         <term><varname>locale.LC_MONETARY=</varname></term>
 984         <term><varname>locale.LC_MESSAGES=</varname></term>
 985         <term><varname>locale.LC_PAPER=</varname></term>
 986         <term><varname>locale.LC_NAME=</varname></term>
 987         <term><varname>locale.LC_ADDRESS=</varname></term>
 988         <term><varname>locale.LC_TELEPHONE=</varname></term>
 989         <term><varname>locale.LC_MEASUREMENT=</varname></term>
 990         <term><varname>locale.LC_IDENTIFICATION=</varname></term>
 991
 992         <listitem><para>Set the system locale to use. This overrides
 993         the settings in <filename>/etc/locale.conf</filename>. For
 994         more information, see
 995         <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 996         and
 997         <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 998         </para></listitem>
 999       </varlistentry>
1000     </variablelist>
1001
1002     <para>For other kernel command line parameters understood by
1003     components of the core OS, please refer to
1004     <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
1005   </refsect1>
1006
1007   <refsect1>
1008     <title>Options</title>
1009
1010     <para><command>systemd</command> is only very rarely invoked directly, since it is started early and is
1011     already running by the time users may interact with it. Normally, tools like
1012     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> are used to
1013     give commands to the manager. Since <command>systemd</command> is usually not invoked directly, the
1014     options listed below are mostly useful for debugging and special purposes.</para>
1015
1016     <refsect2>
1017       <title>Introspection and debugging options</title>
1018
1019       <para>Those options are used for testing and introspection, and <command>systemd</command> may
1020       be invoked with them at any time:</para>
1021
1022       <variablelist>
1023         <varlistentry>
1024           <term><option>--dump-configuration-items</option></term>
1025
1026           <listitem><para>Dump understood unit configuration items. This outputs a terse but complete list of
1027           configuration items understood in unit definition files.</para></listitem>
1028         </varlistentry>
1029
1030         <varlistentry>
1031           <term><option>--dump-bus-properties</option></term>
1032
1033           <listitem><para>Dump exposed bus properties. This outputs a terse but complete list of properties
1034           exposed on D-Bus.</para></listitem>
1035         </varlistentry>
1036
1037         <varlistentry>
1038           <term><option>--test</option></term>
1039
1040           <listitem><para>Determine the initial start-up transaction (i.e. the list of jobs enqueued at
1041           start-up), dump it and exit — without actually executing any of the determined jobs. This option is
1042           useful for debugging only. Note that during regular service manager start-up additional units not
1043           shown by this operation may be started, because hardware, socket, bus or other kinds of activation
1044           might add additional jobs as the transaction is executed. Use <option>--system</option> to request
1045           the initial transaction of the system service manager (this is also the implied default), combine
1046           with <option>--user</option> to request the initial transaction of the per-user service manager
1047           instead.</para></listitem>
1048         </varlistentry>
1049
1050         <varlistentry>
1051           <term><option>--system</option></term>
1052           <term><option>--user</option></term>
1053
1054           <listitem><para>When used in conjunction with <option>--test</option>, selects whether to calculate
1055           the initial transaction for the system instance or for a per-user instance. These options have no
1056           effect when invoked without <option>--test</option>, as during regular
1057           (i.e. non-<option>--test</option>) invocations the service manager will automatically detect
1058           whether it shall operate in system or per-user mode, by checking whether the PID it is run as is 1
1059           or not. Note that it is not supported booting and maintaining a system with the service manager
1060           running in <option>--system</option> mode but with a PID other than 1.</para></listitem>
1061         </varlistentry>
1062
1063         <xi:include href="standard-options.xml" xpointer="help" />
1064         <xi:include href="standard-options.xml" xpointer="version" />
1065       </variablelist>
1066     </refsect2>
1067
1068     <refsect2>
1069       <title>Options that duplicate kernel command line settings</title>
1070
1071       <para>Those options correspond directly to options listed above in "Kernel Command Line". Both forms
1072       may be used equivalently for the system manager, but it is recommended to use the forms listed above in
1073       this context, because they are properly namespaced. When an option is specified both on the kernel
1074       command line, and as a normal command line argument, the latter has higher precedence.</para>
1075
1076       <para>When <command>systemd</command> is used a user manager, the kernel command line is ignored and
1077       the options described are understood. Nevertheless, <command>systemd</command> is usually started in
1078       this mode through the
1079       <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1080       service, which is shared between all users, and it may be more convenient to use configuration files to
1081       modify settings, see
1082       <citerefentry><refentrytitle>systemd-user.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1083       or a drop-in that specifies one of the environment variables listed above in "Environment, see
1084       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
1085
1086       <variablelist>
1087         <varlistentry>
1088           <term><option>--unit=</option></term>
1089
1090           <listitem><para>Set default unit to activate on startup. If not specified, defaults to
1091           <filename>default.target</filename>. See <varname>systemd.unit=</varname> above.</para></listitem>
1092         </varlistentry>
1093
1094         <varlistentry>
1095           <term><option>--dump-core</option></term>
1096
1097           <listitem><para>Enable core dumping on crash. This switch has no effect when running as user
1098           instance. Same as <varname>systemd.dump_core=</varname> above.</para></listitem>
1099         </varlistentry>
1100
1101         <varlistentry>
1102           <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>
1103
1104           <listitem><para>Switch to a specific virtual console (VT) on crash. This switch has no effect when
1105           running as user instance. Same as <varname>systemd.crash_chvt=</varname> above (but not the
1106           different spelling!).</para></listitem>
1107         </varlistentry>
1108
1109         <varlistentry>
1110           <term><option>--crash-shell</option></term>
1111
1112           <listitem><para>Run a shell on crash. This switch has no effect when running as user instance. See
1113           <varname>systemd.crash_shell=</varname> above.</para></listitem>
1114         </varlistentry>
1115
1116         <varlistentry>
1117           <term><option>--crash-reboot</option></term>
1118
1119           <listitem><para>Automatically reboot the system on crash. This switch has no effect when running as
1120           user instance. See <varname>systemd.crash_reboot</varname> above.</para></listitem>
1121         </varlistentry>
1122
1123         <varlistentry>
1124           <term><option>--confirm-spawn</option></term>
1125
1126           <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as
1127           user instance. See <varname>systemd.confirm_spawn</varname> above.</para></listitem>
1128         </varlistentry>
1129
1130         <varlistentry>
1131           <term><option>--show-status</option></term>
1132
1133           <listitem><para>Show terse unit status information is shown on the console during boot-up and
1134           shutdown. See <varname>systemd.show_status</varname> above.</para></listitem>
1135         </varlistentry>
1136
1137         <varlistentry>
1138           <term><option>--log-target=</option></term>
1139
1140           <listitem><para>Set log target. See <varname>systemd.log_target</varname> above.</para></listitem>
1141         </varlistentry>
1142
1143         <varlistentry>
1144           <term><option>--log-level=</option></term>
1145
1146           <listitem><para>Set log level. See <varname>systemd.log_level</varname> above.</para></listitem>
1147         </varlistentry>
1148
1149         <varlistentry>
1150           <term><option>--log-color</option></term>
1151
1152           <listitem><para>Highlight important log messages. See <varname>systemd.log_color</varname> above.
1153           </para></listitem>
1154         </varlistentry>
1155
1156         <varlistentry>
1157           <term><option>--log-location</option></term>
1158
1159           <listitem><para>Include code location in log messages. See <varname>systemd.log_location</varname>
1160           above.</para></listitem>
1161         </varlistentry>
1162
1163         <varlistentry>
1164           <term><option>--machine-id=</option></term>
1165
1166           <listitem><para>Override the machine-id set on the hard drive. See
1167           <varname>systemd.machine_id=</varname> above.</para></listitem>
1168         </varlistentry>
1169
1170         <varlistentry>
1171           <term><option>--service-watchdogs</option></term>
1172
1173           <listitem><para>Globally enable/disable all service watchdog timeouts and emergency actions. See
1174           <varname>systemd.service_watchdogs</varname> above.</para></listitem>
1175         </varlistentry>
1176
1177         <varlistentry>
1178           <term><option>--default-standard-output=</option></term>
1179           <term><option>--default-standard-error=</option></term>
1180
1181           <listitem><para>Sets the default output or error output for all services and sockets,
1182           respectively. See <varname>systemd.default_standard_output=</varname> and
1183           <varname>systemd.default_standard_error=</varname> above.</para></listitem>
1184         </varlistentry>
1185       </variablelist>
1186     </refsect2>
1187   </refsect1>
1188
1189   <refsect1>
1190     <title>Sockets and FIFOs</title>
1191
1192     <variablelist>
1193       <varlistentry>
1194         <term><filename>/run/systemd/notify</filename></term>
1195
1196         <listitem><para>Daemon status notification socket. This is an
1197         <constant>AF_UNIX</constant> datagram socket and is used to
1198         implement the daemon notification logic as implemented by
1199         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1200
1201       </varlistentry>
1202
1203       <varlistentry>
1204         <term><filename>/run/systemd/private</filename></term>
1205
1206         <listitem><para>Used internally as communication channel
1207         between
1208         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1209         and the systemd process. This is an
1210         <constant>AF_UNIX</constant> stream socket. This interface is
1211         private to systemd and should not be used in external
1212         projects.</para></listitem>
1213       </varlistentry>
1214
1215       <varlistentry>
1216         <term><filename>/dev/initctl</filename></term>
1217
1218         <listitem><para>Limited compatibility support for the SysV
1219         client interface, as implemented by the
1220         <filename>systemd-initctl.service</filename> unit. This is a
1221         named pipe in the file system. This interface is obsolete and
1222         should not be used in new applications.</para></listitem>
1223       </varlistentry>
1224     </variablelist>
1225   </refsect1>
1226
1227   <refsect1>
1228     <title>See Also</title>
1229     <para>
1230       The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
1231       <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1232       <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1233       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1234       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1235       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1236       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1237       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1238       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1239       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1240       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1241       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1242       <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1243       <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1244     </para>
1245   </refsect1>
1246
1247 </refentry>