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. Seperate
  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
 284     environment should implement the
 285     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink> or
 286     <ulink url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
 287     specifications, respectively.</para>
 288   </refsect1>
 289
 290   <refsect1>
 291     <title>Directories</title>
 292
 293     <variablelist>
 294       <varlistentry>
 295         <term>System unit directories</term>
 296
 297         <listitem><para>The systemd system manager reads unit
 298         configuration from various directories. Packages that want to
 299         install unit files shall place them in the directory returned
 300         by <command>pkg-config systemd
 301         --variable=systemdsystemunitdir</command>. Other directories
 302         checked are <filename>/usr/local/lib/systemd/system</filename>
 303         and <filename>/usr/lib/systemd/system</filename>. User
 304         configuration always takes precedence. <command>pkg-config
 305         systemd --variable=systemdsystemconfdir</command> returns the
 306         path of the system configuration directory. Packages should
 307         alter the content of these directories only with the
 308         <command>enable</command> and <command>disable</command>
 309         commands of the
 310         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 311         tool. Full list of directories is provided in
 312         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 313         </para></listitem>
 314       </varlistentry>
 315     </variablelist>
 316
 317     <variablelist>
 318       <varlistentry>
 319         <term>User unit directories</term>
 320
 321         <listitem><para>Similar rules apply for the user unit
 322         directories. However, here the
 323         <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 324         Base Directory specification</ulink> is followed to find
 325         units. Applications should place their unit files in the
 326         directory returned by <command>pkg-config systemd
 327         --variable=systemduserunitdir</command>. Global configuration
 328         is done in the directory reported by <command>pkg-config
 329         systemd --variable=systemduserconfdir</command>. The
 330         <command>enable</command> and <command>disable</command>
 331         commands of the
 332         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 333         tool can handle both global (i.e. for all users) and private
 334         (for one user) enabling/disabling of units. Full list of
 335         directories is provided in
 336         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 337         </para></listitem>
 338       </varlistentry>
 339     </variablelist>
 340
 341     <variablelist>
 342       <varlistentry>
 343         <term>SysV init scripts directory</term>
 344
 345         <listitem><para>The location of the SysV init script directory
 346         varies between distributions. If systemd cannot find a native
 347         unit file for a requested service, it will look for a SysV
 348         init script of the same name (with the
 349         <filename>.service</filename> suffix
 350         removed).</para></listitem>
 351       </varlistentry>
 352     </variablelist>
 353
 354     <variablelist>
 355       <varlistentry>
 356         <term>SysV runlevel link farm directory</term>
 357
 358         <listitem><para>The location of the SysV runlevel link farm
 359         directory varies between distributions. systemd will take the
 360         link farm into account when figuring out whether a service
 361         shall be enabled. Note that a service unit with a native unit
 362         configuration file cannot be started by activating it in the
 363         SysV runlevel link farm.</para></listitem>
 364       </varlistentry>
 365     </variablelist>
 366   </refsect1>
 367
 368   <refsect1>
 369     <title>Signals</title>
 370
 371     <variablelist>
 372       <varlistentry>
 373         <term><constant>SIGTERM</constant></term>
 374
 375         <listitem><para>Upon receiving this signal the systemd system
 376         manager serializes its state, reexecutes itself and
 377         deserializes the saved state again. This is mostly equivalent
 378         to <command>systemctl daemon-reexec</command>.</para>
 379
 380         <para>systemd user managers will start the
 381         <filename>exit.target</filename> unit when this signal is
 382         received. This is mostly equivalent to <command>systemctl
 383         --user start exit.target
 384         --job-mode=replace-irreversibly</command>.</para></listitem>
 385       </varlistentry>
 386
 387       <varlistentry>
 388         <term><constant>SIGINT</constant></term>
 389
 390         <listitem><para>Upon receiving this signal the systemd system manager will start the
 391         <filename>ctrl-alt-del.target</filename> unit. This is mostly equivalent to
 392         <command>systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly</command>. If
 393         this signal is received more than 7 times per 2s, an immediate reboot is triggered. Note
 394         that pressing
 395         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> on the
 396         console will trigger this signal. Hence, if a reboot is hanging, pressing
 397         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> more than
 398         7 times in 2 seconds is a relatively safe way to trigger an immediate reboot.</para>
 399
 400         <para>systemd user managers treat this signal the same way as
 401         <constant>SIGTERM</constant>.</para></listitem>
 402       </varlistentry>
 403
 404       <varlistentry>
 405         <term><constant>SIGWINCH</constant></term>
 406
 407         <listitem><para>When this signal is received the systemd
 408         system manager will start the
 409         <filename>kbrequest.target</filename> unit. This is mostly
 410         equivalent to <command>systemctl start
 411         kbrequest.target</command>.</para>
 412
 413         <para>This signal is ignored by systemd user
 414         managers.</para></listitem>
 415       </varlistentry>
 416
 417       <varlistentry>
 418         <term><constant>SIGPWR</constant></term>
 419
 420         <listitem><para>When this signal is received the systemd
 421         manager will start the <filename>sigpwr.target</filename>
 422         unit. This is mostly equivalent to <command>systemctl start
 423         sigpwr.target</command>.</para></listitem>
 424       </varlistentry>
 425
 426       <varlistentry>
 427         <term><constant>SIGUSR1</constant></term>
 428
 429         <listitem><para>When this signal is received the systemd
 430         manager will try to reconnect to the D-Bus
 431         bus.</para></listitem>
 432       </varlistentry>
 433
 434       <varlistentry>
 435         <term><constant>SIGUSR2</constant></term>
 436
 437         <listitem><para>When this signal is received the systemd
 438         manager will log its complete state in human-readable form.
 439         The data logged is the same as printed by
 440         <command>systemd-analyze dump</command>.</para></listitem>
 441       </varlistentry>
 442
 443       <varlistentry>
 444         <term><constant>SIGHUP</constant></term>
 445
 446         <listitem><para>Reloads the complete daemon configuration.
 447         This is mostly equivalent to <command>systemctl
 448         daemon-reload</command>.</para></listitem>
 449       </varlistentry>
 450
 451       <varlistentry>
 452         <term><constant>SIGRTMIN+0</constant></term>
 453
 454         <listitem><para>Enters default mode, starts the
 455         <filename>default.target</filename> unit. This is mostly
 456         equivalent to <command>systemctl isolate
 457         default.target</command>.</para></listitem>
 458       </varlistentry>
 459
 460       <varlistentry>
 461         <term><constant>SIGRTMIN+1</constant></term>
 462
 463         <listitem><para>Enters rescue mode, starts the
 464         <filename>rescue.target</filename> unit. This is mostly
 465         equivalent to <command>systemctl isolate
 466         rescue.target</command>.</para></listitem>
 467       </varlistentry>
 468
 469       <varlistentry>
 470         <term><constant>SIGRTMIN+2</constant></term>
 471
 472         <listitem><para>Enters emergency mode, starts the
 473         <filename>emergency.service</filename> unit. This is mostly
 474         equivalent to <command>systemctl isolate
 475         emergency.service</command>.</para></listitem>
 476       </varlistentry>
 477
 478       <varlistentry>
 479         <term><constant>SIGRTMIN+3</constant></term>
 480
 481         <listitem><para>Halts the machine, starts the
 482         <filename>halt.target</filename> unit. This is mostly
 483         equivalent to <command>systemctl start halt.target
 484         --job-mode=replace-irreversibly</command>.</para>
 485         </listitem>
 486       </varlistentry>
 487
 488       <varlistentry>
 489         <term><constant>SIGRTMIN+4</constant></term>
 490
 491         <listitem><para>Powers off the machine, starts the
 492         <filename>poweroff.target</filename> unit. This is mostly
 493         equivalent to <command>systemctl start poweroff.target
 494         --job-mode=replace-irreversibly</command>.</para>
 495         </listitem>
 496       </varlistentry>
 497
 498       <varlistentry>
 499         <term><constant>SIGRTMIN+5</constant></term>
 500
 501         <listitem><para>Reboots the machine, starts the
 502         <filename>reboot.target</filename> unit. This is mostly
 503         equivalent to <command>systemctl start reboot.target
 504         --job-mode=replace-irreversibly</command>.</para>
 505         </listitem>
 506       </varlistentry>
 507
 508       <varlistentry>
 509         <term><constant>SIGRTMIN+6</constant></term>
 510
 511         <listitem><para>Reboots the machine via kexec, starts the
 512         <filename>kexec.target</filename> unit. This is mostly
 513         equivalent to <command>systemctl start kexec.target
 514         --job-mode=replace-irreversibly</command>.</para>
 515         </listitem>
 516       </varlistentry>
 517
 518       <varlistentry>
 519         <term><constant>SIGRTMIN+13</constant></term>
 520
 521         <listitem><para>Immediately halts the machine.</para></listitem>
 522       </varlistentry>
 523
 524       <varlistentry>
 525         <term><constant>SIGRTMIN+14</constant></term>
 526
 527         <listitem><para>Immediately powers off the machine.</para></listitem>
 528       </varlistentry>
 529
 530       <varlistentry>
 531         <term><constant>SIGRTMIN+15</constant></term>
 532
 533         <listitem><para>Immediately reboots the machine.</para></listitem>
 534       </varlistentry>
 535
 536       <varlistentry>
 537         <term><constant>SIGRTMIN+16</constant></term>
 538
 539         <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 540       </varlistentry>
 541
 542       <varlistentry>
 543         <term><constant>SIGRTMIN+20</constant></term>
 544
 545         <listitem><para>Enables display of status messages on the
 546         console, as controlled via
 547         <varname>systemd.show_status=1</varname> on the kernel command
 548         line.</para></listitem>
 549       </varlistentry>
 550
 551       <varlistentry>
 552         <term><constant>SIGRTMIN+21</constant></term>
 553
 554         <listitem><para>Disables display of
 555         status messages on the console, as
 556         controlled via
 557         <varname>systemd.show_status=0</varname>
 558         on the kernel command
 559         line.</para></listitem>
 560       </varlistentry>
 561
 562       <varlistentry>
 563         <term><constant>SIGRTMIN+22</constant></term>
 564
 565         <listitem><para>Sets the service manager's log level to <literal>debug</literal>, in a fashion equivalent to
 566         <varname>systemd.log_level=debug</varname> on the kernel command line.</para></listitem>
 567       </varlistentry>
 568
 569       <varlistentry>
 570         <term><constant>SIGRTMIN+23</constant></term>
 571
 572         <listitem><para>Restores the log level to its configured value. The configured value is derived from – in order
 573         of priority – the value specified with <varname>systemd.log-level=</varname> on the kernel command line, or the
 574         value specified with <option>LogLevel=</option> in the configuration file, or the built-in default of
 575         <literal>info</literal>.</para></listitem>
 576       </varlistentry>
 577
 578       <varlistentry>
 579         <term><constant>SIGRTMIN+24</constant></term>
 580
 581         <listitem><para>Immediately exits the manager (only available
 582         for --user instances).</para></listitem>
 583       </varlistentry>
 584
 585       <varlistentry>
 586         <term><constant>SIGRTMIN+26</constant></term>
 587
 588         <listitem><para>Restores the log target to its configured value. The configured value is derived from – in
 589         order of priority – the value specified with <varname>systemd.log-target=</varname> on the kernel command line,
 590         or the value specified with <option>LogTarget=</option> in the configuration file, or the built-in
 591         default.</para></listitem>
 592       </varlistentry>
 593
 594       <varlistentry>
 595         <term><constant>SIGRTMIN+27</constant></term>
 596         <term><constant>SIGRTMIN+28</constant></term>
 597
 598         <listitem><para>Sets the log target to <literal>console</literal> on <constant>SIGRTMIN+27</constant> (or
 599         <literal>kmsg</literal> on <constant>SIGRTMIN+28</constant>), in a fashion equivalent to
 600         <varname>systemd.log_target=console</varname> (or <varname>systemd.log_target=kmsg</varname> on
 601         <constant>SIGRTMIN+28</constant>) on the kernel command line.</para></listitem>
 602       </varlistentry>
 603     </variablelist>
 604   </refsect1>
 605
 606   <refsect1>
 607     <title>Environment</title>
 608
 609     <variablelist class='environment-variables'>
 610       <varlistentry>
 611         <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 612         <listitem><para>systemd reads the log level from this
 613         environment variable. This can be overridden with
 614         <option>--log-level=</option>.</para></listitem>
 615       </varlistentry>
 616
 617       <varlistentry>
 618         <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 619         <listitem><para>systemd reads the log target from this
 620         environment variable. This can be overridden with
 621         <option>--log-target=</option>.</para></listitem>
 622       </varlistentry>
 623
 624       <varlistentry>
 625         <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 626         <listitem><para>Controls whether systemd highlights important
 627         log messages. This can be overridden with
 628         <option>--log-color=</option>.</para></listitem>
 629       </varlistentry>
 630
 631       <varlistentry>
 632         <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 633         <listitem><para>Controls whether systemd prints the code
 634         location along with log messages. This can be overridden with
 635         <option>--log-location=</option>.</para></listitem>
 636       </varlistentry>
 637
 638       <varlistentry>
 639         <term><varname>$XDG_CONFIG_HOME</varname></term>
 640         <term><varname>$XDG_CONFIG_DIRS</varname></term>
 641         <term><varname>$XDG_DATA_HOME</varname></term>
 642         <term><varname>$XDG_DATA_DIRS</varname></term>
 643
 644         <listitem><para>The systemd user manager uses these variables
 645         in accordance to the <ulink
 646         url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 647         Base Directory specification</ulink> to find its
 648         configuration.</para></listitem>
 649       </varlistentry>
 650
 651       <varlistentry>
 652         <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 653
 654         <listitem><para>Controls where systemd looks for unit
 655         files.</para></listitem>
 656       </varlistentry>
 657
 658       <varlistentry>
 659         <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 660
 661         <listitem><para>Controls where systemd looks for SysV init
 662         scripts.</para></listitem>
 663       </varlistentry>
 664
 665       <varlistentry>
 666         <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 667
 668         <listitem><para>Controls where systemd looks for SysV init
 669         script runlevel link farms.</para></listitem>
 670       </varlistentry>
 671
 672       <xi:include href="less-variables.xml" xpointer="colors" />
 673       <xi:include href="less-variables.xml" xpointer="urlify" />
 674
 675       <varlistentry>
 676         <term><varname>$LISTEN_PID</varname></term>
 677         <term><varname>$LISTEN_FDS</varname></term>
 678         <term><varname>$LISTEN_FDNAMES</varname></term>
 679
 680         <listitem><para>Set by systemd for supervised processes during
 681         socket-based activation. See
 682         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 683         for more information.</para></listitem>
 684       </varlistentry>
 685
 686       <varlistentry>
 687         <term><varname>$NOTIFY_SOCKET</varname></term>
 688
 689         <listitem><para>Set by systemd for supervised processes for
 690         status and start-up completion notification. See
 691         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 692         for more information.</para></listitem>
 693       </varlistentry>
 694     </variablelist>
 695
 696     <para>For further environment variables understood by systemd and its various components, see <ulink
 697     url="https://systemd.io/ENVIRONMENT">Known Environment Variables</ulink>.</para>
 698   </refsect1>
 699
 700   <refsect1>
 701     <title>Kernel Command Line</title>
 702
 703     <para>When run as the system instance systemd parses a number of options listed below. They can be
 704     specified as kernel command line arguments<footnote><para>If run inside a Linux container these arguments
 705     may be passed as command line arguments to systemd itself, next to any of the command line options listed
 706     in the Options section above. If run outside of Linux containers, these arguments are parsed from
 707     <filename>/proc/cmdline</filename> instead.</para></footnote>, or through the
 708     <literal>SystemdOptions</literal> EFI variable (on EFI systems). The kernel command line has higher
 709     priority. Following variables are understood:</para>
 710
 711     <variablelist class='kernel-commandline-options'>
 712       <varlistentry>
 713         <term><varname>systemd.unit=</varname></term>
 714         <term><varname>rd.systemd.unit=</varname></term>
 715
 716         <listitem><para>Overrides the unit to activate on boot.
 717         Defaults to <filename>default.target</filename>. This may be
 718         used to temporarily boot into a different boot unit, for
 719         example <filename>rescue.target</filename> or
 720         <filename>emergency.service</filename>. See
 721         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 722         for details about these units. The option prefixed with
 723         <literal>rd.</literal> is honored only in the initial RAM disk
 724         (initrd), while the one that is not prefixed only in the main
 725         system.</para></listitem>
 726       </varlistentry>
 727
 728       <varlistentry>
 729         <term><varname>systemd.dump_core</varname></term>
 730
 731         <listitem><para>Takes a boolean argument or enables the option if specified
 732         without an argument. If enabled, the systemd manager (PID 1) dumps core when
 733         it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
 734         </listitem>
 735       </varlistentry>
 736
 737       <varlistentry>
 738         <term><varname>systemd.crash_chvt</varname></term>
 739
 740         <listitem><para>Takes a positive integer, or a boolean argument. Can be also
 741         specified without an argument, with the same effect as a positive boolean. If
 742         a positive integer (in the range 1–63) is specified, the system manager (PID
 743         1) will activate the specified virtual terminal (VT) when it
 744         crashes. Defaults to disabled, meaning that no such switch is attempted. If
 745         set to enabled, the VT the kernel messages are written to is selected.
 746         </para></listitem>
 747       </varlistentry>
 748
 749       <varlistentry>
 750         <term><varname>systemd.crash_shell</varname></term>
 751
 752         <listitem><para>Takes a boolean argument or enables the option if specified
 753         without an argument. If enabled, the system manager (PID 1) spawns a shell
 754         when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
 755         to disabled, for security reasons, as the shell is not protected by password
 756         authentication.</para></listitem>
 757       </varlistentry>
 758
 759       <varlistentry>
 760         <term><varname>systemd.crash_reboot</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) will reboot the
 764         machine automatically when it crashes, after a 10s delay.  Otherwise, the
 765         system will hang indefinitely. Defaults to disabled, in order to avoid a
 766         reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
 767         system is rebooted after the shell exits.</para></listitem>
 768       </varlistentry>
 769
 770       <varlistentry>
 771         <term><varname>systemd.confirm_spawn</varname></term>
 772
 773         <listitem><para>Takes a boolean argument or a path to the virtual console
 774         where the confirmation messages should be emitted. Can be also specified
 775         without an argument, with the same effect as a positive boolean. If enabled,
 776         the system manager (PID 1) asks for confirmation when spawning processes
 777         using <option>/dev/console</option>. If a path or a console name (such as
 778         <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
 779         path or described by the give name will be used instead. Defaults to disabled.
 780         </para></listitem>
 781       </varlistentry>
 782
 783       <varlistentry>
 784         <term><varname>systemd.service_watchdogs=</varname></term>
 785
 786         <listitem><para>Takes a boolean argument. If disabled, all service runtime
 787         watchdogs (<option>WatchdogSec=</option>) and emergency actions (e.g.
 788         <option>OnFailure=</option> or <option>StartLimitAction=</option>) are
 789         ignored by the system manager (PID 1); see
 790         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 791         Defaults to enabled, i.e. watchdogs and failure actions are processed
 792         normally. The hardware watchdog is not affected by this
 793         option.</para></listitem>
 794       </varlistentry>
 795
 796       <varlistentry>
 797         <term><varname>systemd.show_status</varname></term>
 798
 799         <listitem><para>Takes a boolean argument or the constant
 800         <constant>auto</constant>. Can be also specified without an argument, with
 801         the same effect as a positive boolean.  If enabled, the systemd manager (PID
 802         1) shows terse service status updates on the console during bootup.
 803         <constant>auto</constant> behaves like <option>false</option> until a unit
 804         fails or there is a significant delay in boot. Defaults to enabled, unless
 805         <option>quiet</option> is passed as kernel command line option, in which case
 806         it defaults to <constant>auto</constant>. If specified overrides the system
 807         manager configuration file option <option>ShowStatus=</option>, see
 808         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 809         </para></listitem>
 810       </varlistentry>
 811
 812       <varlistentry>
 813         <term><varname>systemd.status_unit_format=</varname></term>
 814
 815         <listitem><para>Takes either <option>name</option> or <option>description</option> as the value. If
 816         <option>name</option>, the system manager will use unit names in status messages. If specified,
 817         overrides the system manager configuration file option <option>StatusUnitFormat=</option>, see
 818         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 819         </para></listitem>
 820       </varlistentry>
 821
 822       <varlistentry>
 823         <term><varname>systemd.log_target=</varname></term>
 824         <term><varname>systemd.log_level=</varname></term>
 825         <term><varname>systemd.log_location=</varname></term>
 826         <term><varname>systemd.log_color</varname></term>
 827
 828         <listitem><para>Controls log output, with the same effect as the
 829         <varname>$SYSTEMD_LOG_TARGET</varname>,
 830         <varname>$SYSTEMD_LOG_LEVEL</varname>,
 831         <varname>$SYSTEMD_LOG_LOCATION</varname>,
 832         <varname>$SYSTEMD_LOG_COLOR</varname> environment variables described above.
 833         <varname>systemd.log_color</varname> can be specified without an argument,
 834         with the same effect as a positive boolean.</para></listitem>
 835       </varlistentry>
 836
 837       <varlistentry>
 838         <term><varname>systemd.default_standard_output=</varname></term>
 839         <term><varname>systemd.default_standard_error=</varname></term>
 840
 841         <listitem><para>Controls default standard output and error output for services and sockets. That is,
 842         controls the default for <option>StandardOutput=</option> and <option>StandardError=</option> (see
 843         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 844         details). Takes one of <option>inherit</option>, <option>null</option>, <option>tty</option>,
 845         <option>journal</option>, <option>journal+console</option>, <option>kmsg</option>,
 846         <option>kmsg+console</option>. If the argument is omitted
 847         <varname>systemd.default-standard-output=</varname> defaults to <option>journal</option> and
 848         <varname>systemd.default-standard-error=</varname> to <option>inherit</option>.</para></listitem>
 849       </varlistentry>
 850
 851       <varlistentry>
 852         <term><varname>systemd.setenv=</varname></term>
 853
 854         <listitem><para>Takes a string argument in the form
 855         VARIABLE=VALUE. May be used to set default environment
 856         variables to add to forked child processes. May be used more
 857         than once to set multiple variables.</para></listitem>
 858       </varlistentry>
 859
 860       <varlistentry>
 861         <term><varname>systemd.machine_id=</varname></term>
 862
 863         <listitem><para>Takes a 32 character hex value to be
 864         used for setting the machine-id. Intended mostly for
 865         network booting where the same machine-id is desired
 866         for every boot.</para></listitem>
 867       </varlistentry>
 868
 869       <varlistentry>
 870         <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
 871
 872         <listitem><para>When specified without an argument or with a true argument,
 873         enables the usage of
 874         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v2.txt">unified cgroup hierarchy</ulink>
 875         (a.k.a. cgroups-v2). When specified with a false argument, fall back to
 876         hybrid or full legacy cgroup hierarchy.</para>
 877
 878         <para>If this option is not specified, the default behaviour is determined
 879         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 880         option). If the kernel does not support unified cgroup hierarchy, the legacy
 881         hierarchy will be used even if this option is specified.</para>
 882         </listitem>
 883       </varlistentry>
 884
 885       <varlistentry>
 886         <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
 887
 888         <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
 889         (see previous option). When specified without an argument or with a true
 890         argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
 891         tree used for systemd, and
 892         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
 893         cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
 894         forces a full "legacy" mode. When specified with a false argument, enables
 895         the use of "hybrid" hierarchy.</para>
 896
 897         <para>If this option is not specified, the default behaviour is determined
 898         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 899         option). If the kernel does not support unified cgroup hierarchy, the legacy
 900         hierarchy will be used  even if this option is specified.</para>
 901         </listitem>
 902       </varlistentry>
 903
 904       <varlistentry>
 905         <term><varname>quiet</varname></term>
 906
 907         <listitem><para>Turn off status output at boot, much like
 908         <varname>systemd.show_status=no</varname> would. Note that
 909         this option is also read by the kernel itself and disables
 910         kernel log output. Passing this option hence turns off the
 911         usual output from both the system manager and the kernel.
 912         </para></listitem>
 913       </varlistentry>
 914
 915       <varlistentry>
 916         <term><varname>debug</varname></term>
 917
 918         <listitem><para>Turn on debugging output. This is equivalent
 919         to <varname>systemd.log_level=debug</varname>. Note that this
 920         option is also read by the kernel itself and enables kernel
 921         debug output. Passing this option hence turns on the debug
 922         output from both the system manager and the
 923         kernel.</para></listitem>
 924       </varlistentry>
 925
 926       <varlistentry>
 927         <term><varname>emergency</varname></term>
 928         <term><varname>rd.emergency</varname></term>
 929         <term><varname>-b</varname></term>
 930
 931         <listitem><para>Boot into emergency mode. This is equivalent
 932         to <varname>systemd.unit=emergency.target</varname> or
 933         <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
 934         provided for compatibility reasons and to be easier to type.</para></listitem>
 935       </varlistentry>
 936
 937       <varlistentry>
 938         <term><varname>rescue</varname></term>
 939         <term><varname>rd.rescue</varname></term>
 940         <term><varname>single</varname></term>
 941         <term><varname>s</varname></term>
 942         <term><varname>S</varname></term>
 943         <term><varname>1</varname></term>
 944
 945         <listitem><para>Boot into rescue mode. This is equivalent to
 946         <varname>systemd.unit=rescue.target</varname> or
 947         <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
 948         provided for compatibility reasons and to be easier to type.</para></listitem>
 949       </varlistentry>
 950
 951       <varlistentry>
 952         <term><varname>2</varname></term>
 953         <term><varname>3</varname></term>
 954         <term><varname>4</varname></term>
 955         <term><varname>5</varname></term>
 956
 957         <listitem><para>Boot into the specified legacy SysV runlevel.
 958         These are equivalent to
 959         <varname>systemd.unit=runlevel2.target</varname>,
 960         <varname>systemd.unit=runlevel3.target</varname>,
 961         <varname>systemd.unit=runlevel4.target</varname>, and
 962         <varname>systemd.unit=runlevel5.target</varname>,
 963         respectively, and provided for compatibility reasons and to be
 964         easier to type.</para></listitem>
 965       </varlistentry>
 966
 967       <varlistentry>
 968         <term><varname>locale.LANG=</varname></term>
 969         <term><varname>locale.LANGUAGE=</varname></term>
 970         <term><varname>locale.LC_CTYPE=</varname></term>
 971         <term><varname>locale.LC_NUMERIC=</varname></term>
 972         <term><varname>locale.LC_TIME=</varname></term>
 973         <term><varname>locale.LC_COLLATE=</varname></term>
 974         <term><varname>locale.LC_MONETARY=</varname></term>
 975         <term><varname>locale.LC_MESSAGES=</varname></term>
 976         <term><varname>locale.LC_PAPER=</varname></term>
 977         <term><varname>locale.LC_NAME=</varname></term>
 978         <term><varname>locale.LC_ADDRESS=</varname></term>
 979         <term><varname>locale.LC_TELEPHONE=</varname></term>
 980         <term><varname>locale.LC_MEASUREMENT=</varname></term>
 981         <term><varname>locale.LC_IDENTIFICATION=</varname></term>
 982
 983         <listitem><para>Set the system locale to use. This overrides
 984         the settings in <filename>/etc/locale.conf</filename>. For
 985         more information, see
 986         <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 987         and
 988         <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 989         </para></listitem>
 990       </varlistentry>
 991     </variablelist>
 992
 993     <para>For other kernel command line parameters understood by
 994     components of the core OS, please refer to
 995     <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 996   </refsect1>
 997
 998   <refsect1>
 999     <title>Options</title>
1000
1001     <para><command>systemd</command> is only very rarely invoked directly, since it is started early and is
1002     already running by the time users may interact with it. Normally, tools like
1003     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> are used to
1004     give commands to the manager. Since <command>systemd</command> is usually not invoked directly, the
1005     options listed below are mostly useful for debugging and special purposes.</para>
1006
1007     <refsect2>
1008       <title>Introspection and debugging options</title>
1009
1010       <para>Those options are used for testing and introspection, and <command>systemd</command> may
1011       be invoked with them at any time:</para>
1012
1013       <variablelist>
1014         <varlistentry>
1015           <term><option>--dump-configuration-items</option></term>
1016
1017           <listitem><para>Dump understood unit configuration items. This outputs a terse but complete list of
1018           configuration items understood in unit definition files.</para></listitem>
1019         </varlistentry>
1020
1021         <varlistentry>
1022           <term><option>--dump-bus-properties</option></term>
1023
1024           <listitem><para>Dump exposed bus properties. This outputs a terse but complete list of properties
1025           exposed on D-Bus.</para></listitem>
1026         </varlistentry>
1027
1028         <varlistentry>
1029           <term><option>--test</option></term>
1030
1031           <listitem><para>Determine the initial start-up transaction (i.e. the list of jobs enqueued at
1032           start-up), dump it and exit — without actually executing any of the determined jobs. This option is
1033           useful for debugging only. Note that during regular service manager start-up additional units not
1034           shown by this operation may be started, because hardware, socket, bus or other kinds of activation
1035           might add additional jobs as the transaction is executed. Use <option>--system</option> to request
1036           the initial transaction of the system service manager (this is also the implied default), combine
1037           with <option>--user</option> to request the initial transaction of the per-user service manager
1038           instead.</para></listitem>
1039         </varlistentry>
1040
1041         <varlistentry>
1042           <term><option>--system</option></term>
1043           <term><option>--user</option></term>
1044
1045           <listitem><para>When used in conjunction with <option>--test</option>, selects whether to calculate
1046           the initial transaction for the system instance or for a per-user instance. These options have no
1047           effect when invoked without <option>--test</option>, as during regular
1048           (i.e. non-<option>--test</option>) invocations the service manager will automatically detect
1049           whether it shall operate in system or per-user mode, by checking whether the PID it is run as is 1
1050           or not. Note that it is not supported booting and maintaining a system with the service manager
1051           running in <option>--system</option> mode but with a PID other than 1.</para></listitem>
1052         </varlistentry>
1053
1054         <xi:include href="standard-options.xml" xpointer="help" />
1055         <xi:include href="standard-options.xml" xpointer="version" />
1056       </variablelist>
1057     </refsect2>
1058
1059     <refsect2>
1060       <title>Options that duplicate kernel command line settings</title>
1061
1062       <para>Those options correspond directly to options listed above in "Kernel Command Line". Both forms
1063       may be used equivalently for the system manager, but it is recommended to use the forms listed above in
1064       this context, because they are properly namespaced. When an option is specified both on the kernel
1065       command line, and as a normal command line argument, the latter has higher precedence.</para>
1066
1067       <para>When <command>systemd</command> is used a user manager, the kernel command line is ignored and
1068       the options described are understood. Nevertheless, <command>systemd</command> is usually started in
1069       this mode through the
1070       <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1071       service, which is shared between all users, and it may be more convenient to use configuration files to
1072       modify settings, see
1073       <citerefentry><refentrytitle>systemd-user.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1074       or a drop-in that specifies one of the environment variables listed above in "Environment, see
1075       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
1076
1077       <variablelist>
1078         <varlistentry>
1079           <term><option>--unit=</option></term>
1080
1081           <listitem><para>Set default unit to activate on startup. If not specified, defaults to
1082           <filename>default.target</filename>. See <varname>systemd.unit=</varname> above.</para></listitem>
1083         </varlistentry>
1084
1085         <varlistentry>
1086           <term><option>--dump-core</option></term>
1087
1088           <listitem><para>Enable core dumping on crash. This switch has no effect when running as user
1089           instance. Same as <varname>systemd.dump_core=</varname> above.</para></listitem>
1090         </varlistentry>
1091
1092         <varlistentry>
1093           <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>
1094
1095           <listitem><para>Switch to a specific virtual console (VT) on crash. This switch has no effect when
1096           running as user instance. Same as <varname>systemd.crash_chvt=</varname> above (but not the
1097           different spelling!).</para></listitem>
1098         </varlistentry>
1099
1100         <varlistentry>
1101           <term><option>--crash-shell</option></term>
1102
1103           <listitem><para>Run a shell on crash. This switch has no effect when running as user instance. See
1104           <varname>systemd.crash_shell=</varname> above.</para></listitem>
1105         </varlistentry>
1106
1107         <varlistentry>
1108           <term><option>--crash-reboot</option></term>
1109
1110           <listitem><para>Automatically reboot the system on crash. This switch has no effect when running as
1111           user instance. See <varname>systemd.crash_reboot</varname> above.</para></listitem>
1112         </varlistentry>
1113
1114         <varlistentry>
1115           <term><option>--confirm-spawn</option></term>
1116
1117           <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as
1118           user instance. See <varname>systemd.confirm_spawn</varname> above.</para></listitem>
1119         </varlistentry>
1120
1121         <varlistentry>
1122           <term><option>--show-status</option></term>
1123
1124           <listitem><para>Show terse unit status information is shown on the console during boot-up and
1125           shutdown. See <varname>systemd.show_status</varname> above.</para></listitem>
1126         </varlistentry>
1127
1128         <varlistentry>
1129           <term><option>--log-target=</option></term>
1130
1131           <listitem><para>Set log target. See <varname>systemd.log_target</varname> above.</para></listitem>
1132         </varlistentry>
1133
1134         <varlistentry>
1135           <term><option>--log-level=</option></term>
1136
1137           <listitem><para>Set log level. See <varname>systemd.log_level</varname> above.</para></listitem>
1138         </varlistentry>
1139
1140         <varlistentry>
1141           <term><option>--log-color</option></term>
1142
1143           <listitem><para>Highlight important log messages. See <varname>systemd.log_color</varname> above.
1144           </para></listitem>
1145         </varlistentry>
1146
1147         <varlistentry>
1148           <term><option>--log-location</option></term>
1149
1150           <listitem><para>Include code location in log messages. See <varname>systemd.log_location</varname>
1151           above.</para></listitem>
1152         </varlistentry>
1153
1154         <varlistentry>
1155           <term><option>--machine-id=</option></term>
1156
1157           <listitem><para>Override the machine-id set on the hard drive. See
1158           <varname>systemd.machine_id=</varname> above.</para></listitem>
1159         </varlistentry>
1160
1161         <varlistentry>
1162           <term><option>--service-watchdogs</option></term>
1163
1164           <listitem><para>Globally enable/disable all service watchdog timeouts and emergency actions. See
1165           <varname>systemd.service_watchdogs</varname> above.</para></listitem>
1166         </varlistentry>
1167
1168         <varlistentry>
1169           <term><option>--default-standard-output=</option></term>
1170           <term><option>--default-standard-error=</option></term>
1171
1172           <listitem><para>Sets the default output or error output for all services and sockets,
1173           respectively. See <varname>systemd.default_standard_output=</varname> and
1174           <varname>systemd.default_standard_error=</varname> above.</para></listitem>
1175         </varlistentry>
1176       </variablelist>
1177     </refsect2>
1178   </refsect1>
1179
1180   <refsect1>
1181     <title>Sockets and FIFOs</title>
1182
1183     <variablelist>
1184       <varlistentry>
1185         <term><filename>/run/systemd/notify</filename></term>
1186
1187         <listitem><para>Daemon status notification socket. This is an
1188         <constant>AF_UNIX</constant> datagram socket and is used to
1189         implement the daemon notification logic as implemented by
1190         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1191
1192       </varlistentry>
1193
1194       <varlistentry>
1195         <term><filename>/run/systemd/private</filename></term>
1196
1197         <listitem><para>Used internally as communication channel
1198         between
1199         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1200         and the systemd process. This is an
1201         <constant>AF_UNIX</constant> stream socket. This interface is
1202         private to systemd and should not be used in external
1203         projects.</para></listitem>
1204       </varlistentry>
1205
1206       <varlistentry>
1207         <term><filename>/dev/initctl</filename></term>
1208
1209         <listitem><para>Limited compatibility support for the SysV
1210         client interface, as implemented by the
1211         <filename>systemd-initctl.service</filename> unit. This is a
1212         named pipe in the file system. This interface is obsolete and
1213         should not be used in new applications.</para></listitem>
1214       </varlistentry>
1215     </variablelist>
1216   </refsect1>
1217
1218   <refsect1>
1219     <title>See Also</title>
1220     <para>
1221       The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
1222       <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1223       <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1224       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1225       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1226       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1227       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1228       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1229       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1230       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1231       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1232       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1233       <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1234       <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1235     </para>
1236   </refsect1>
1237
1238 </refentry>