man/systemd.xml

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   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 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     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 by systemd are covered by the
 274     <ulink url="https://systemd.io/PORTABILITY_AND_STABILITY/">Interface Portability and Stability Promise</ulink>.</para>
 275
 276     <para>Units may be generated dynamically at boot and system
 277     manager reload time, for example based on other configuration
 278     files or parameters passed on the kernel command line. For details, see
 279     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 280
 281     <para>The D-Bus API of <command>systemd</command> is described in
 282     <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 283     and
 284     <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 285     </para>
 286
 287     <para>Systems which invoke systemd in a container or initrd environment should implement the <ulink
 288     url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> or
 289     <ulink url="https://systemd.io/INITRD_INTERFACE/">initrd Interface</ulink>
 290     specifications, respectively.</para>
 291   </refsect1>
 292
 293   <refsect1>
 294     <title>Directories</title>
 295
 296     <variablelist>
 297       <varlistentry>
 298         <term>System unit directories</term>
 299
 300         <listitem><para>The systemd system manager reads unit
 301         configuration from various directories. Packages that want to
 302         install unit files shall place them in the directory returned
 303         by <command>pkg-config systemd
 304         --variable=systemdsystemunitdir</command>. Other directories
 305         checked are <filename>/usr/local/lib/systemd/system</filename>
 306         and <filename>/usr/lib/systemd/system</filename>. User
 307         configuration always takes precedence. <command>pkg-config
 308         systemd --variable=systemdsystemconfdir</command> returns the
 309         path of the system configuration directory. Packages should
 310         alter the content of these directories only with the
 311         <command>enable</command> and <command>disable</command>
 312         commands of the
 313         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 314         tool. Full list of directories is provided in
 315         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 316         </para></listitem>
 317       </varlistentry>
 318     </variablelist>
 319
 320     <variablelist>
 321       <varlistentry>
 322         <term>User unit directories</term>
 323
 324         <listitem><para>Similar rules apply for the user unit
 325         directories. However, here the
 326         <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 327         Base Directory specification</ulink> is followed to find
 328         units. Applications should place their unit files in the
 329         directory returned by <command>pkg-config systemd
 330         --variable=systemduserunitdir</command>. Global configuration
 331         is done in the directory reported by <command>pkg-config
 332         systemd --variable=systemduserconfdir</command>. The
 333         <command>enable</command> and <command>disable</command>
 334         commands of the
 335         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 336         tool can handle both global (i.e. for all users) and private
 337         (for one user) enabling/disabling of units. Full list of
 338         directories is provided in
 339         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 340         </para></listitem>
 341       </varlistentry>
 342     </variablelist>
 343
 344     <variablelist>
 345       <varlistentry>
 346         <term>SysV init scripts directory</term>
 347
 348         <listitem><para>The location of the SysV init script directory
 349         varies between distributions. If systemd cannot find a native
 350         unit file for a requested service, it will look for a SysV
 351         init script of the same name (with the
 352         <filename>.service</filename> suffix
 353         removed).</para></listitem>
 354       </varlistentry>
 355     </variablelist>
 356
 357     <variablelist>
 358       <varlistentry>
 359         <term>SysV runlevel link farm directory</term>
 360
 361         <listitem><para>The location of the SysV runlevel link farm
 362         directory varies between distributions. systemd will take the
 363         link farm into account when figuring out whether a service
 364         shall be enabled. Note that a service unit with a native unit
 365         configuration file cannot be started by activating it in the
 366         SysV runlevel link farm.</para></listitem>
 367       </varlistentry>
 368     </variablelist>
 369   </refsect1>
 370
 371   <refsect1>
 372     <title>Signals</title>
 373
 374     <variablelist>
 375       <varlistentry>
 376         <term><constant>SIGTERM</constant></term>
 377
 378         <listitem><para>Upon receiving this signal the systemd system
 379         manager serializes its state, reexecutes itself and
 380         deserializes the saved state again. This is mostly equivalent
 381         to <command>systemctl daemon-reexec</command>.</para>
 382
 383         <para>systemd user managers will start the
 384         <filename>exit.target</filename> unit when this signal is
 385         received. This is mostly equivalent to <command>systemctl
 386         --user start exit.target
 387         --job-mode=replace-irreversibly</command>.</para></listitem>
 388       </varlistentry>
 389
 390       <varlistentry>
 391         <term><constant>SIGINT</constant></term>
 392
 393         <listitem><para>Upon receiving this signal the systemd system manager will start the
 394         <filename>ctrl-alt-del.target</filename> unit. This is mostly equivalent to
 395         <command>systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly</command>. If
 396         this signal is received more than 7 times per 2s, an immediate reboot is triggered. Note
 397         that pressing
 398         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> on the
 399         console will trigger this signal. Hence, if a reboot is hanging, pressing
 400         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> more than
 401         7 times in 2 seconds is a relatively safe way to trigger an immediate reboot.</para>
 402
 403         <para>systemd user managers treat this signal the same way as
 404         <constant>SIGTERM</constant>.</para></listitem>
 405       </varlistentry>
 406
 407       <varlistentry>
 408         <term><constant>SIGWINCH</constant></term>
 409
 410         <listitem><para>When this signal is received the systemd
 411         system manager will start the
 412         <filename>kbrequest.target</filename> unit. This is mostly
 413         equivalent to <command>systemctl start
 414         kbrequest.target</command>.</para>
 415
 416         <para>This signal is ignored by systemd user
 417         managers.</para></listitem>
 418       </varlistentry>
 419
 420       <varlistentry>
 421         <term><constant>SIGPWR</constant></term>
 422
 423         <listitem><para>When this signal is received the systemd
 424         manager will start the <filename>sigpwr.target</filename>
 425         unit. This is mostly equivalent to <command>systemctl start
 426         sigpwr.target</command>.</para></listitem>
 427       </varlistentry>
 428
 429       <varlistentry>
 430         <term><constant>SIGUSR1</constant></term>
 431
 432         <listitem><para>When this signal is received the systemd
 433         manager will try to reconnect to the D-Bus
 434         bus.</para></listitem>
 435       </varlistentry>
 436
 437       <varlistentry>
 438         <term><constant>SIGUSR2</constant></term>
 439
 440         <listitem><para>When this signal is received the systemd
 441         manager will log its complete state in human-readable form.
 442         The data logged is the same as printed by
 443         <command>systemd-analyze dump</command>.</para></listitem>
 444       </varlistentry>
 445
 446       <varlistentry>
 447         <term><constant>SIGHUP</constant></term>
 448
 449         <listitem><para>Reloads the complete daemon configuration.
 450         This is mostly equivalent to <command>systemctl
 451         daemon-reload</command>.</para></listitem>
 452       </varlistentry>
 453
 454       <varlistentry>
 455         <term><constant>SIGRTMIN+0</constant></term>
 456
 457         <listitem><para>Enters default mode, starts the
 458         <filename>default.target</filename> unit. This is mostly
 459         equivalent to <command>systemctl isolate
 460         default.target</command>.</para></listitem>
 461       </varlistentry>
 462
 463       <varlistentry>
 464         <term><constant>SIGRTMIN+1</constant></term>
 465
 466         <listitem><para>Enters rescue mode, starts the
 467         <filename>rescue.target</filename> unit. This is mostly
 468         equivalent to <command>systemctl isolate
 469         rescue.target</command>.</para></listitem>
 470       </varlistentry>
 471
 472       <varlistentry>
 473         <term><constant>SIGRTMIN+2</constant></term>
 474
 475         <listitem><para>Enters emergency mode, starts the
 476         <filename>emergency.service</filename> unit. This is mostly
 477         equivalent to <command>systemctl isolate
 478         emergency.service</command>.</para></listitem>
 479       </varlistentry>
 480
 481       <varlistentry>
 482         <term><constant>SIGRTMIN+3</constant></term>
 483
 484         <listitem><para>Halts the machine, starts the
 485         <filename>halt.target</filename> unit. This is mostly
 486         equivalent to <command>systemctl start halt.target
 487         --job-mode=replace-irreversibly</command>.</para>
 488         </listitem>
 489       </varlistentry>
 490
 491       <varlistentry>
 492         <term><constant>SIGRTMIN+4</constant></term>
 493
 494         <listitem><para>Powers off the machine, starts the
 495         <filename>poweroff.target</filename> unit. This is mostly
 496         equivalent to <command>systemctl start poweroff.target
 497         --job-mode=replace-irreversibly</command>.</para>
 498         </listitem>
 499       </varlistentry>
 500
 501       <varlistentry>
 502         <term><constant>SIGRTMIN+5</constant></term>
 503
 504         <listitem><para>Reboots the machine, starts the
 505         <filename>reboot.target</filename> unit. This is mostly
 506         equivalent to <command>systemctl start reboot.target
 507         --job-mode=replace-irreversibly</command>.</para>
 508         </listitem>
 509       </varlistentry>
 510
 511       <varlistentry>
 512         <term><constant>SIGRTMIN+6</constant></term>
 513
 514         <listitem><para>Reboots the machine via kexec, starts the
 515         <filename>kexec.target</filename> unit. This is mostly
 516         equivalent to <command>systemctl start kexec.target
 517         --job-mode=replace-irreversibly</command>.</para>
 518         </listitem>
 519       </varlistentry>
 520
 521       <varlistentry>
 522         <term><constant>SIGRTMIN+13</constant></term>
 523
 524         <listitem><para>Immediately halts the machine.</para></listitem>
 525       </varlistentry>
 526
 527       <varlistentry>
 528         <term><constant>SIGRTMIN+14</constant></term>
 529
 530         <listitem><para>Immediately powers off the machine.</para></listitem>
 531       </varlistentry>
 532
 533       <varlistentry>
 534         <term><constant>SIGRTMIN+15</constant></term>
 535
 536         <listitem><para>Immediately reboots the machine.</para></listitem>
 537       </varlistentry>
 538
 539       <varlistentry>
 540         <term><constant>SIGRTMIN+16</constant></term>
 541
 542         <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 543       </varlistentry>
 544
 545       <varlistentry>
 546         <term><constant>SIGRTMIN+20</constant></term>
 547
 548         <listitem><para>Enables display of status messages on the
 549         console, as controlled via
 550         <varname>systemd.show_status=1</varname> on the kernel command
 551         line.</para></listitem>
 552       </varlistentry>
 553
 554       <varlistentry>
 555         <term><constant>SIGRTMIN+21</constant></term>
 556
 557         <listitem><para>Disables display of
 558         status messages on the console, as
 559         controlled via
 560         <varname>systemd.show_status=0</varname>
 561         on the kernel command
 562         line.</para></listitem>
 563       </varlistentry>
 564
 565       <varlistentry>
 566         <term><constant>SIGRTMIN+22</constant></term>
 567
 568         <listitem><para>Sets the service manager's log level to <literal>debug</literal>, in a fashion equivalent to
 569         <varname>systemd.log_level=debug</varname> on the kernel command line.</para></listitem>
 570       </varlistentry>
 571
 572       <varlistentry>
 573         <term><constant>SIGRTMIN+23</constant></term>
 574
 575         <listitem><para>Restores the log level to its configured value. The configured value is derived from – in order
 576         of priority – the value specified with <varname>systemd.log-level=</varname> on the kernel command line, or the
 577         value specified with <option>LogLevel=</option> in the configuration file, or the built-in default of
 578         <literal>info</literal>.</para></listitem>
 579       </varlistentry>
 580
 581       <varlistentry>
 582         <term><constant>SIGRTMIN+24</constant></term>
 583
 584         <listitem><para>Immediately exits the manager (only available
 585         for --user instances).</para></listitem>
 586       </varlistentry>
 587
 588       <varlistentry>
 589         <term><constant>SIGRTMIN+26</constant></term>
 590
 591         <listitem><para>Restores the log target to its configured value. The configured value is derived from – in
 592         order of priority – the value specified with <varname>systemd.log-target=</varname> on the kernel command line,
 593         or the value specified with <option>LogTarget=</option> in the configuration file, or the built-in
 594         default.</para></listitem>
 595       </varlistentry>
 596
 597       <varlistentry>
 598         <term><constant>SIGRTMIN+27</constant></term>
 599         <term><constant>SIGRTMIN+28</constant></term>
 600
 601         <listitem><para>Sets the log target to <literal>console</literal> on <constant>SIGRTMIN+27</constant> (or
 602         <literal>kmsg</literal> on <constant>SIGRTMIN+28</constant>), in a fashion equivalent to
 603         <varname>systemd.log_target=console</varname> (or <varname>systemd.log_target=kmsg</varname> on
 604         <constant>SIGRTMIN+28</constant>) on the kernel command line.</para></listitem>
 605       </varlistentry>
 606     </variablelist>
 607   </refsect1>
 608
 609   <refsect1>
 610     <title>Environment</title>
 611
 612     <variablelist class='environment-variables'>
 613       <varlistentry>
 614         <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 615         <listitem><para>Controls whether systemd highlights important log messages. This can be overridden
 616         with <option>--log-color=</option>.</para></listitem>
 617       </varlistentry>
 618
 619       <varlistentry>
 620         <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 621         <listitem><para>systemd reads the log level from this environment variable. This can be overridden
 622         with <option>--log-level=</option>.</para></listitem>
 623       </varlistentry>
 624
 625       <varlistentry>
 626         <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 627         <listitem><para>Controls whether systemd prints the code location along with log messages. This can
 628         be overridden with <option>--log-location=</option>.</para></listitem>
 629       </varlistentry>
 630
 631       <varlistentry>
 632         <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 633         <listitem><para>systemd reads the log target from this environment variable. This can be overridden
 634         with <option>--log-target=</option>.</para></listitem>
 635       </varlistentry>
 636
 637       <varlistentry>
 638         <term><varname>$SYSTEMD_LOG_TIME</varname></term>
 639         <listitem><para>Controls whether systemd prefixes log messages with the current time. This can be
 640         overridden with <option>--log-time=</option>.</para></listitem>
 641       </varlistentry>
 642
 643       <varlistentry>
 644         <term><varname>$SYSTEMD_LOG_TID</varname></term>
 645         <listitem><para>Controls whether systemd prefixes log messages with the current thread ID
 646         (TID).</para></listitem>
 647       </varlistentry>
 648
 649       <varlistentry>
 650         <term><varname>$XDG_CONFIG_HOME</varname></term>
 651         <term><varname>$XDG_CONFIG_DIRS</varname></term>
 652         <term><varname>$XDG_DATA_HOME</varname></term>
 653         <term><varname>$XDG_DATA_DIRS</varname></term>
 654
 655         <listitem><para>The systemd user manager uses these variables
 656         in accordance to the <ulink
 657         url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 658         Base Directory specification</ulink> to find its
 659         configuration.</para></listitem>
 660       </varlistentry>
 661
 662       <varlistentry>
 663         <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 664         <term><varname>$SYSTEMD_GENERATOR_PATH</varname></term>
 665         <term><varname>$SYSTEMD_ENVIRONMENT_GENERATOR_PATH</varname></term>
 666
 667         <listitem><para>Controls where systemd looks for unit files and
 668         generators.</para>
 669         <para>These variables may contain a list of paths, separated by colons
 670         (<literal>:</literal>). When set, if the list ends with an empty
 671         component (<literal>...:</literal>), this list is prepended to the
 672         usual set of paths. Otherwise, the specified list replaces the usual
 673         set of paths.
 674         </para></listitem>
 675       </varlistentry>
 676
 677       <varlistentry>
 678         <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 679
 680         <listitem><para>Controls where systemd looks for SysV init
 681         scripts.</para></listitem>
 682       </varlistentry>
 683
 684       <varlistentry>
 685         <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 686
 687         <listitem><para>Controls where systemd looks for SysV init
 688         script runlevel link farms.</para></listitem>
 689       </varlistentry>
 690
 691       <xi:include href="less-variables.xml" xpointer="pager"/>
 692       <xi:include href="less-variables.xml" xpointer="less"/>
 693       <xi:include href="less-variables.xml" xpointer="lesscharset"/>
 694       <xi:include href="less-variables.xml" xpointer="colors"/>
 695       <xi:include href="less-variables.xml" xpointer="urlify"/>
 696
 697       <varlistentry>
 698         <term><varname>$LISTEN_PID</varname></term>
 699         <term><varname>$LISTEN_FDS</varname></term>
 700         <term><varname>$LISTEN_FDNAMES</varname></term>
 701
 702         <listitem><para>Set by systemd for supervised processes during
 703         socket-based activation. See
 704         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 705         for more information.</para></listitem>
 706       </varlistentry>
 707
 708       <varlistentry>
 709         <term><varname>$NOTIFY_SOCKET</varname></term>
 710
 711         <listitem><para>Set by systemd for supervised processes for
 712         status and start-up completion notification. See
 713         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 714         for more information.</para></listitem>
 715       </varlistentry>
 716     </variablelist>
 717
 718     <para>For further environment variables understood by systemd and its various components, see <ulink
 719     url="https://systemd.io/ENVIRONMENT">Known Environment Variables</ulink>.</para>
 720   </refsect1>
 721
 722   <refsect1>
 723     <title>Kernel Command Line</title>
 724
 725     <para>When run as the system instance systemd parses a number of options listed below. They can be
 726     specified as kernel command line arguments<footnote><para>If run inside a Linux container these arguments
 727     may be passed as command line arguments to systemd itself, next to any of the command line options listed
 728     in the Options section above. If run outside of Linux containers, these arguments are parsed from
 729     <filename>/proc/cmdline</filename> instead.</para></footnote>, or through the
 730     <literal>SystemdOptions</literal> EFI variable (on EFI systems). The kernel command line has higher
 731     priority. Following variables are understood:</para>
 732
 733     <variablelist class='kernel-commandline-options'>
 734       <varlistentry>
 735         <term><varname>systemd.unit=</varname></term>
 736         <term><varname>rd.systemd.unit=</varname></term>
 737
 738         <listitem><para>Overrides the unit to activate on boot.
 739         Defaults to <filename>default.target</filename>. This may be
 740         used to temporarily boot into a different boot unit, for
 741         example <filename>rescue.target</filename> or
 742         <filename>emergency.service</filename>. See
 743         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 744         for details about these units. The option prefixed with
 745         <literal>rd.</literal> is honored only in the initial RAM disk
 746         (initrd), while the one that is not prefixed only in the main
 747         system.</para></listitem>
 748       </varlistentry>
 749
 750       <varlistentry>
 751         <term><varname>systemd.dump_core</varname></term>
 752
 753         <listitem><para>Takes a boolean argument or enables the option if specified
 754         without an argument. If enabled, the systemd manager (PID 1) dumps core when
 755         it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
 756         </listitem>
 757       </varlistentry>
 758
 759       <varlistentry>
 760         <term><varname>systemd.crash_chvt</varname></term>
 761
 762         <listitem><para>Takes a positive integer, or a boolean argument. Can be also specified without an
 763         argument, with the same effect as a positive boolean. If a positive integer (in the range 1–63) is
 764         specified, the system manager (PID 1) will activate the specified virtual terminal when it crashes.
 765         Defaults to disabled, meaning that no such switch is attempted. If set to enabled, the virtual
 766         terminal the kernel messages are written to is used instead.</para></listitem>
 767       </varlistentry>
 768
 769       <varlistentry>
 770         <term><varname>systemd.crash_shell</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) spawns a shell
 774         when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
 775         to disabled, for security reasons, as the shell is not protected by password
 776         authentication.</para></listitem>
 777       </varlistentry>
 778
 779       <varlistentry>
 780         <term><varname>systemd.crash_reboot</varname></term>
 781
 782         <listitem><para>Takes a boolean argument or enables the option if specified
 783         without an argument. If enabled, the system manager (PID 1) will reboot the
 784         machine automatically when it crashes, after a 10s delay.  Otherwise, the
 785         system will hang indefinitely. Defaults to disabled, in order to avoid a
 786         reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
 787         system is rebooted after the shell exits.</para></listitem>
 788       </varlistentry>
 789
 790       <varlistentry>
 791         <term><varname>systemd.confirm_spawn</varname></term>
 792
 793         <listitem><para>Takes a boolean argument or a path to the virtual console
 794         where the confirmation messages should be emitted. Can be also specified
 795         without an argument, with the same effect as a positive boolean. If enabled,
 796         the system manager (PID 1) asks for confirmation when spawning processes
 797         using <option>/dev/console</option>. If a path or a console name (such as
 798         <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
 799         path or described by the give name will be used instead. Defaults to disabled.
 800         </para></listitem>
 801       </varlistentry>
 802
 803       <varlistentry>
 804         <term><varname>systemd.service_watchdogs=</varname></term>
 805
 806         <listitem><para>Takes a boolean argument. If disabled, all service runtime
 807         watchdogs (<option>WatchdogSec=</option>) and emergency actions (e.g.
 808         <option>OnFailure=</option> or <option>StartLimitAction=</option>) are
 809         ignored by the system manager (PID 1); see
 810         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 811         Defaults to enabled, i.e. watchdogs and failure actions are processed
 812         normally. The hardware watchdog is not affected by this
 813         option.</para></listitem>
 814       </varlistentry>
 815
 816       <varlistentry>
 817         <term><varname>systemd.show_status</varname></term>
 818
 819         <listitem><para>Takes a boolean argument or the constants <constant>error</constant> and
 820         <constant>auto</constant>. Can be also specified without an argument, with the same effect as a
 821         positive boolean. If enabled, the systemd manager (PID 1) shows terse service status updates on the
 822         console during bootup. With <constant>error</constant>, only messages about failures are shown, but
 823         boot is otherwise quiet. <constant>auto</constant> behaves like <option>false</option> until there is
 824         a significant delay in boot. Defaults to enabled, unless <option>quiet</option> is passed as kernel
 825         command line option, in which case it defaults to <constant>error</constant>. If specified overrides
 826         the system manager configuration file option <option>ShowStatus=</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.status_unit_format=</varname></term>
 833
 834         <listitem><para>Takes either <option>name</option> or <option>description</option> as the value. If
 835         <option>name</option>, the system manager will use unit names in status messages. If specified,
 836         overrides the system manager configuration file option <option>StatusUnitFormat=</option>, see
 837         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 838         </para></listitem>
 839       </varlistentry>
 840
 841       <varlistentry>
 842         <term><varname>systemd.log_color</varname></term>
 843         <term><varname>systemd.log_level=</varname></term>
 844         <term><varname>systemd.log_location</varname></term>
 845         <term><varname>systemd.log_target=</varname></term>
 846         <term><varname>systemd.log_time</varname></term>
 847         <term><varname>systemd.log_tid</varname></term>
 848
 849         <listitem><para>Controls log output, with the same effect as the
 850         <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>,
 851         <varname>$SYSTEMD_LOG_LOCATION</varname>, <varname>$SYSTEMD_LOG_TARGET</varname>,
 852         <varname>$SYSTEMD_LOG_TIME</varname>, and <varname>$SYSTEMD_LOG_TID</varname> environment variables
 853         described above. <varname>systemd.log_color</varname>, <varname>systemd.log_location</varname>,
 854         <varname>systemd.log_time</varname>, and <varname>systemd.log_tid=</varname> can be specified without
 855         an argument, with the same effect as a positive boolean.</para></listitem>
 856       </varlistentry>
 857
 858       <varlistentry>
 859         <term><varname>systemd.default_standard_output=</varname></term>
 860         <term><varname>systemd.default_standard_error=</varname></term>
 861
 862         <listitem><para>Controls default standard output and error output for services and sockets. That is,
 863         controls the default for <option>StandardOutput=</option> and <option>StandardError=</option> (see
 864         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 865         details). Takes one of <option>inherit</option>, <option>null</option>, <option>tty</option>,
 866         <option>journal</option>, <option>journal+console</option>, <option>kmsg</option>,
 867         <option>kmsg+console</option>. If the argument is omitted
 868         <varname>systemd.default-standard-output=</varname> defaults to <option>journal</option> and
 869         <varname>systemd.default-standard-error=</varname> to <option>inherit</option>.</para></listitem>
 870       </varlistentry>
 871
 872       <varlistentry>
 873         <term><varname>systemd.setenv=</varname></term>
 874
 875         <listitem><para>Takes a string argument in the form
 876         VARIABLE=VALUE. May be used to set default environment
 877         variables to add to forked child processes. May be used more
 878         than once to set multiple variables.</para></listitem>
 879       </varlistentry>
 880
 881       <varlistentry>
 882         <term><varname>systemd.machine_id=</varname></term>
 883
 884         <listitem><para>Takes a 32 character hex value to be
 885         used for setting the machine-id. Intended mostly for
 886         network booting where the same machine-id is desired
 887         for every boot.</para></listitem>
 888       </varlistentry>
 889
 890       <varlistentry>
 891         <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
 892
 893         <listitem><para>When specified without an argument or with a true argument,
 894         enables the usage of
 895         <ulink url="https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html">unified cgroup hierarchy</ulink>
 896         (a.k.a. cgroups-v2). When specified with a false argument, fall back to
 897         hybrid or full legacy cgroup hierarchy.</para>
 898
 899         <para>If this option is not specified, the default behaviour is determined
 900         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 901         option). If the kernel does not support unified cgroup hierarchy, the legacy
 902         hierarchy will be used even if this option is specified.</para>
 903         </listitem>
 904       </varlistentry>
 905
 906       <varlistentry>
 907         <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
 908
 909         <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
 910         (see previous option). When specified without an argument or with a true
 911         argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
 912         tree used for systemd, and
 913         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
 914         cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
 915         forces a full "legacy" mode. When specified with a false argument, enables
 916         the use of "hybrid" hierarchy.</para>
 917
 918         <para>If this option is not specified, the default behaviour is determined
 919         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 920         option). If the kernel does not support unified cgroup hierarchy, the legacy
 921         hierarchy will be used  even if this option is specified.</para>
 922         </listitem>
 923       </varlistentry>
 924
 925       <varlistentry>
 926         <term><varname>quiet</varname></term>
 927
 928         <listitem><para>Turn off status output at boot, much like
 929         <varname>systemd.show_status=no</varname> would. Note that
 930         this option is also read by the kernel itself and disables
 931         kernel log output. Passing this option hence turns off the
 932         usual output from both the system manager and the kernel.
 933         </para></listitem>
 934       </varlistentry>
 935
 936       <varlistentry>
 937         <term><varname>debug</varname></term>
 938
 939         <listitem><para>Turn on debugging output. This is equivalent
 940         to <varname>systemd.log_level=debug</varname>. Note that this
 941         option is also read by the kernel itself and enables kernel
 942         debug output. Passing this option hence turns on the debug
 943         output from both the system manager and the
 944         kernel.</para></listitem>
 945       </varlistentry>
 946
 947       <varlistentry>
 948         <term><varname>emergency</varname></term>
 949         <term><varname>rd.emergency</varname></term>
 950         <term><varname>-b</varname></term>
 951
 952         <listitem><para>Boot into emergency mode. This is equivalent
 953         to <varname>systemd.unit=emergency.target</varname> or
 954         <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
 955         provided for compatibility reasons and to be easier to type.</para></listitem>
 956       </varlistentry>
 957
 958       <varlistentry>
 959         <term><varname>rescue</varname></term>
 960         <term><varname>rd.rescue</varname></term>
 961         <term><varname>single</varname></term>
 962         <term><varname>s</varname></term>
 963         <term><varname>S</varname></term>
 964         <term><varname>1</varname></term>
 965
 966         <listitem><para>Boot into rescue mode. This is equivalent to
 967         <varname>systemd.unit=rescue.target</varname> or
 968         <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
 969         provided for compatibility reasons and to be easier to type.</para></listitem>
 970       </varlistentry>
 971
 972       <varlistentry>
 973         <term><varname>2</varname></term>
 974         <term><varname>3</varname></term>
 975         <term><varname>4</varname></term>
 976         <term><varname>5</varname></term>
 977
 978         <listitem><para>Boot into the specified legacy SysV runlevel.
 979         These are equivalent to
 980         <varname>systemd.unit=runlevel2.target</varname>,
 981         <varname>systemd.unit=runlevel3.target</varname>,
 982         <varname>systemd.unit=runlevel4.target</varname>, and
 983         <varname>systemd.unit=runlevel5.target</varname>,
 984         respectively, and provided for compatibility reasons and to be
 985         easier to type.</para></listitem>
 986       </varlistentry>
 987
 988       <varlistentry>
 989         <term><varname>locale.LANG=</varname></term>
 990         <term><varname>locale.LANGUAGE=</varname></term>
 991         <term><varname>locale.LC_CTYPE=</varname></term>
 992         <term><varname>locale.LC_NUMERIC=</varname></term>
 993         <term><varname>locale.LC_TIME=</varname></term>
 994         <term><varname>locale.LC_COLLATE=</varname></term>
 995         <term><varname>locale.LC_MONETARY=</varname></term>
 996         <term><varname>locale.LC_MESSAGES=</varname></term>
 997         <term><varname>locale.LC_PAPER=</varname></term>
 998         <term><varname>locale.LC_NAME=</varname></term>
 999         <term><varname>locale.LC_ADDRESS=</varname></term>
1000         <term><varname>locale.LC_TELEPHONE=</varname></term>
1001         <term><varname>locale.LC_MEASUREMENT=</varname></term>
1002         <term><varname>locale.LC_IDENTIFICATION=</varname></term>
1003
1004         <listitem><para>Set the system locale to use. This overrides
1005         the settings in <filename>/etc/locale.conf</filename>. For
1006         more information, see
1007         <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1008         and
1009         <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
1010         </para></listitem>
1011       </varlistentry>
1012     </variablelist>
1013
1014     <para>For other kernel command line parameters understood by
1015     components of the core OS, please refer to
1016     <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
1017   </refsect1>
1018
1019   <refsect1>
1020     <title>Options</title>
1021
1022     <para><command>systemd</command> is only very rarely invoked directly, since it is started early and is
1023     already running by the time users may interact with it. Normally, tools like
1024     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> are used to
1025     give commands to the manager. Since <command>systemd</command> is usually not invoked directly, the
1026     options listed below are mostly useful for debugging and special purposes.</para>
1027
1028     <refsect2>
1029       <title>Introspection and debugging options</title>
1030
1031       <para>Those options are used for testing and introspection, and <command>systemd</command> may
1032       be invoked with them at any time:</para>
1033
1034       <variablelist>
1035         <varlistentry>
1036           <term><option>--dump-configuration-items</option></term>
1037
1038           <listitem><para>Dump understood unit configuration items. This outputs a terse but complete list of
1039           configuration items understood in unit definition files.</para></listitem>
1040         </varlistentry>
1041
1042         <varlistentry>
1043           <term><option>--dump-bus-properties</option></term>
1044
1045           <listitem><para>Dump exposed bus properties. This outputs a terse but complete list of properties
1046           exposed on D-Bus.</para></listitem>
1047         </varlistentry>
1048
1049         <varlistentry>
1050           <term><option>--test</option></term>
1051
1052           <listitem><para>Determine the initial start-up transaction (i.e. the list of jobs enqueued at
1053           start-up), dump it and exit — without actually executing any of the determined jobs. This option is
1054           useful for debugging only. Note that during regular service manager start-up additional units not
1055           shown by this operation may be started, because hardware, socket, bus or other kinds of activation
1056           might add additional jobs as the transaction is executed. Use <option>--system</option> to request
1057           the initial transaction of the system service manager (this is also the implied default), combine
1058           with <option>--user</option> to request the initial transaction of the per-user service manager
1059           instead.</para></listitem>
1060         </varlistentry>
1061
1062         <varlistentry>
1063           <term><option>--system</option></term>
1064           <term><option>--user</option></term>
1065
1066           <listitem><para>When used in conjunction with <option>--test</option>, selects whether to calculate
1067           the initial transaction for the system instance or for a per-user instance. These options have no
1068           effect when invoked without <option>--test</option>, as during regular
1069           (i.e. non-<option>--test</option>) invocations the service manager will automatically detect
1070           whether it shall operate in system or per-user mode, by checking whether the PID it is run as is 1
1071           or not. Note that it is not supported booting and maintaining a system with the service manager
1072           running in <option>--system</option> mode but with a PID other than 1.</para></listitem>
1073         </varlistentry>
1074
1075         <xi:include href="standard-options.xml" xpointer="help" />
1076         <xi:include href="standard-options.xml" xpointer="version" />
1077       </variablelist>
1078     </refsect2>
1079
1080     <refsect2>
1081       <title>Options that duplicate kernel command line settings</title>
1082
1083       <para>Those options correspond directly to options listed above in "Kernel Command Line". Both forms
1084       may be used equivalently for the system manager, but it is recommended to use the forms listed above in
1085       this context, because they are properly namespaced. When an option is specified both on the kernel
1086       command line and as a normal command line argument, the latter has higher precedence.</para>
1087
1088       <para>When <command>systemd</command> is used as a user manager, the kernel command line is ignored and
1089       only the options described below are understood. Nevertheless, <command>systemd</command> is usually
1090       started in this mode through the
1091       <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1092       service, which is shared between all users, and it may be more convenient to use configuration files to
1093       modify settings (see
1094       <citerefentry><refentrytitle>systemd-user.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
1095       or a drop-in that specifies one of the environment variables listed above in the Environment section
1096       (see the discussion of <varname>Environment=</varname> and <varname>EnvironmentFile=</varname> in
1097       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para>
1098
1099       <variablelist>
1100         <varlistentry>
1101           <term><option>--unit=</option></term>
1102
1103           <listitem><para>Set default unit to activate on startup. If not specified, defaults to
1104           <filename>default.target</filename>. See <varname>systemd.unit=</varname> above.</para></listitem>
1105         </varlistentry>
1106
1107         <varlistentry>
1108           <term><option>--dump-core</option></term>
1109
1110           <listitem><para>Enable core dumping on crash. This switch has no effect when running as user
1111           instance. Same as <varname>systemd.dump_core=</varname> above.</para></listitem>
1112         </varlistentry>
1113
1114         <varlistentry>
1115           <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>
1116
1117           <listitem><para>Switch to a specific virtual console (VT) on crash. This switch has no effect when
1118           running as user instance. Same as <varname>systemd.crash_chvt=</varname> above (but not the
1119           different spelling!).</para></listitem>
1120         </varlistentry>
1121
1122         <varlistentry>
1123           <term><option>--crash-shell</option></term>
1124
1125           <listitem><para>Run a shell on crash. This switch has no effect when running as user instance. See
1126           <varname>systemd.crash_shell=</varname> above.</para></listitem>
1127         </varlistentry>
1128
1129         <varlistentry>
1130           <term><option>--crash-reboot</option></term>
1131
1132           <listitem><para>Automatically reboot the system on crash. This switch has no effect when running as
1133           user instance. See <varname>systemd.crash_reboot</varname> above.</para></listitem>
1134         </varlistentry>
1135
1136         <varlistentry>
1137           <term><option>--confirm-spawn</option></term>
1138
1139           <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as
1140           user instance. See <varname>systemd.confirm_spawn</varname> above.</para></listitem>
1141         </varlistentry>
1142
1143         <varlistentry>
1144           <term><option>--show-status</option></term>
1145
1146           <listitem><para>Show terse unit status information on the console during boot-up and shutdown. See
1147           <varname>systemd.show_status</varname> above.</para></listitem>
1148         </varlistentry>
1149
1150         <varlistentry>
1151           <term><option>--log-color</option></term>
1152
1153           <listitem><para>Highlight important log messages. See <varname>systemd.log_color</varname> above.
1154           </para></listitem>
1155         </varlistentry>
1156
1157         <varlistentry>
1158           <term><option>--log-level=</option></term>
1159
1160           <listitem><para>Set log level. See <varname>systemd.log_level</varname> above.</para></listitem>
1161         </varlistentry>
1162
1163         <varlistentry>
1164           <term><option>--log-location</option></term>
1165
1166           <listitem><para>Include code location in log messages. See <varname>systemd.log_location</varname>
1167           above.</para></listitem>
1168         </varlistentry>
1169
1170         <varlistentry>
1171           <term><option>--log-target=</option></term>
1172
1173           <listitem><para>Set log target. See <varname>systemd.log_target</varname> above.</para></listitem>
1174         </varlistentry>
1175
1176         <varlistentry>
1177           <term><option>--log-time=</option></term>
1178
1179           <listitem><para>Prefix messages with timestamp. See <varname>systemd.log_time</varname> above.
1180           </para></listitem>
1181         </varlistentry>
1182
1183         <varlistentry>
1184           <term><option>--machine-id=</option></term>
1185
1186           <listitem><para>Override the machine-id set on the hard drive. See
1187           <varname>systemd.machine_id=</varname> above.</para></listitem>
1188         </varlistentry>
1189
1190         <varlistentry>
1191           <term><option>--service-watchdogs</option></term>
1192
1193           <listitem><para>Globally enable/disable all service watchdog timeouts and emergency actions. See
1194           <varname>systemd.service_watchdogs</varname> above.</para></listitem>
1195         </varlistentry>
1196
1197         <varlistentry>
1198           <term><option>--default-standard-output=</option></term>
1199           <term><option>--default-standard-error=</option></term>
1200
1201           <listitem><para>Sets the default output or error output for all services and sockets,
1202           respectively. See <varname>systemd.default_standard_output=</varname> and
1203           <varname>systemd.default_standard_error=</varname> above.</para></listitem>
1204         </varlistentry>
1205       </variablelist>
1206     </refsect2>
1207   </refsect1>
1208
1209   <refsect1>
1210     <title>Sockets and FIFOs</title>
1211
1212     <variablelist>
1213       <varlistentry>
1214         <term><filename>/run/systemd/notify</filename></term>
1215
1216         <listitem><para>Daemon status notification socket. This is an
1217         <constant>AF_UNIX</constant> datagram socket and is used to
1218         implement the daemon notification logic as implemented by
1219         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1220
1221       </varlistentry>
1222
1223       <varlistentry>
1224         <term><filename>/run/systemd/private</filename></term>
1225
1226         <listitem><para>Used internally as communication channel
1227         between
1228         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1229         and the systemd process. This is an
1230         <constant>AF_UNIX</constant> stream socket. This interface is
1231         private to systemd and should not be used in external
1232         projects.</para></listitem>
1233       </varlistentry>
1234
1235       <varlistentry>
1236         <term><filename>/dev/initctl</filename></term>
1237
1238         <listitem><para>Limited compatibility support for the SysV
1239         client interface, as implemented by the
1240         <filename>systemd-initctl.service</filename> unit. This is a
1241         named pipe in the file system. This interface is obsolete and
1242         should not be used in new applications.</para></listitem>
1243       </varlistentry>
1244     </variablelist>
1245   </refsect1>
1246
1247   <refsect1>
1248     <title>See Also</title>
1249     <para>
1250       The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
1251       <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1252       <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1253       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1254       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1255       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1256       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1257       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1258       <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1259       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1260       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1261       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1262       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1263       <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1264       <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1265     </para>
1266   </refsect1>
1267
1268 </refentry>