man/systemd.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="systemd"
   7     xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>systemd</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>systemd</refentrytitle>
  16     <manvolnum>1</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>systemd</refname>
  21     <refname>init</refname>
  22     <refpurpose>systemd system and service manager</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <cmdsynopsis>
  27       <command>/usr/lib/systemd/systemd</command>
  28       <arg choice="opt" rep="repeat">OPTIONS</arg>
  29     </cmdsynopsis>
  30     <cmdsynopsis>
  31       <command>init</command>
  32       <arg choice="opt" rep="repeat">OPTIONS</arg>
  33       <arg choice="req">COMMAND</arg>
  34     </cmdsynopsis>
  35   </refsynopsisdiv>
  36
  37   <refsect1>
  38     <title>Description</title>
  39
  40     <para>systemd is a system and service manager for Linux operating systems. When run as first process on
  41     boot (as PID 1), it acts as init system that brings up and maintains userspace services. Separate
  42     instances are started for logged-in users to start their services.</para>
  43
  44     <para><command>systemd</command> is usually not invoked directly by the user, but is installed as the
  45     <filename>/sbin/init</filename> symlink and started during early boot. The user manager instances are
  46     started automatically through the
  47     <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  48     service.</para>
  49
  50     <para>For compatibility with SysV, if the binary is called as <command>init</command> and is not the
  51     first process on the machine (PID is not 1), it will execute <command>telinit</command> and pass all
  52     command line arguments unmodified. That means <command>init</command> and <command>telinit</command> are
  53     mostly equivalent when invoked from normal login sessions. See
  54     <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry> for more
  55     information.</para>
  56
  57     <para>When run as a system instance, systemd interprets the
  58     configuration file <filename>system.conf</filename> and the files
  59     in <filename>system.conf.d</filename> directories; when run as a
  60     user instance, systemd interprets the configuration file
  61     <filename>user.conf</filename> and the files in
  62     <filename>user.conf.d</filename> directories. See
  63     <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  64     for more information.</para>
  65   </refsect1>
  66
  67   <refsect1>
  68     <title>Concepts</title>
  69
  70     <para>systemd provides a dependency system between various
  71     entities called "units" of 11 different types. Units encapsulate
  72     various objects that are relevant for system boot-up and
  73     maintenance. The majority of units are configured in unit
  74     configuration files, whose syntax and basic set of options is
  75     described in
  76     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
  77     however some are created automatically from other configuration,
  78     dynamically from system state or programmatically at runtime.
  79     Units may be "active" (meaning started, bound, plugged in, …,
  80     depending on the unit type, see below), or "inactive" (meaning
  81     stopped, unbound, unplugged, …), as well as in the process of
  82     being activated or deactivated, i.e. between the two states (these
  83     states are called "activating", "deactivating"). A special
  84     "failed" state is available as well, which is very similar to
  85     "inactive" and is entered when the service failed in some way
  86     (process returned error code on exit, or crashed, an operation
  87     timed out, or after too many restarts). If this state is entered,
  88     the cause will be logged, for later reference. Note that the
  89     various unit types may have a number of additional substates,
  90     which are mapped to the five generalized unit states described
  91     here.</para>
  92
  93     <para>The following unit types are available:</para>
  94
  95     <orderedlist>
  96       <listitem><para>Service units, which start and control daemons
  97       and the processes they consist of. For details, see
  98       <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
  99
 100       <listitem><para>Socket units, which encapsulate local IPC or
 101       network sockets in the system, useful for socket-based
 102       activation. For details about socket units, see
 103       <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 104       for details on socket-based activation and other forms of
 105       activation, see
 106       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 107
 108       <listitem><para>Target units are useful to group units, or
 109       provide well-known synchronization points during boot-up, see
 110       <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 111
 112       <listitem><para>Device units expose kernel devices in systemd
 113       and may be used to implement device-based activation. For
 114       details, see
 115       <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 116
 117       <listitem><para>Mount units control mount points in the file
 118       system, for details see
 119       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 120
 121       <listitem><para>Automount units provide automount capabilities,
 122       for on-demand mounting of file systems as well as parallelized
 123       boot-up. See
 124       <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 125
 126       <listitem><para>Timer units are useful for triggering activation
 127       of other units based on timers. You may find details in
 128       <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 129
 130       <listitem><para>Swap units are very similar to mount units and
 131       encapsulate memory swap partitions or files of the operating
 132       system. They are described in
 133       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 134
 135       <listitem><para>Path units may be used to activate other
 136       services when file system objects change or are modified. See
 137       <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 138
 139       <listitem><para>Slice units may be used to group units which
 140       manage system processes (such as service and scope units) in a
 141       hierarchical tree for resource management purposes. See
 142       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 143
 144       <listitem><para>Scope units are similar to service units, but
 145       manage foreign processes instead of starting them as well. See
 146       <citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 147
 148     </orderedlist>
 149
 150     <para>Units are named as their configuration files. Some units
 151     have special semantics. A detailed list is available in
 152     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 153
 154     <para>systemd knows various kinds of dependencies, including
 155     positive and negative requirement dependencies (i.e.
 156     <varname>Requires=</varname> and <varname>Conflicts=</varname>) as
 157     well as ordering dependencies (<varname>After=</varname> and
 158     <varname>Before=</varname>). NB: ordering and requirement
 159     dependencies are orthogonal. If only a requirement dependency
 160     exists between two units (e.g. <filename>foo.service</filename>
 161     requires <filename>bar.service</filename>), but no ordering
 162     dependency (e.g. <filename>foo.service</filename> after
 163     <filename>bar.service</filename>) and both are requested to start,
 164     they will be started in parallel. It is a common pattern that both
 165     requirement and ordering dependencies are placed between two
 166     units. Also note that the majority of dependencies are implicitly
 167     created and maintained by systemd. In most cases, it should be
 168     unnecessary to declare additional dependencies manually, however
 169     it is possible to do this.</para>
 170
 171     <para>Application programs and units (via dependencies) may
 172     request state changes of units. In systemd, these requests are
 173     encapsulated as 'jobs' and maintained in a job queue. Jobs may
 174     succeed or can fail, their execution is ordered based on the
 175     ordering dependencies of the units they have been scheduled
 176     for.</para>
 177
 178     <para>On boot systemd activates the target unit
 179     <filename>default.target</filename> whose job is to activate
 180     on-boot services and other on-boot units by pulling them in via
 181     dependencies. Usually, the unit name is just an alias (symlink) for
 182     either <filename>graphical.target</filename> (for fully-featured
 183     boots into the UI) or <filename>multi-user.target</filename> (for
 184     limited console-only boots for use in embedded or server
 185     environments, or similar; a subset of graphical.target). However,
 186     it is at the discretion of the administrator to configure it as an
 187     alias to any other target unit. See
 188     <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 189     for details about these target units.</para>
 190
 191     <para>systemd only keeps a minimal set of units loaded into memory. Specifically, the only units that are kept
 192     loaded into memory are those for which at least one of the following conditions is true:</para>
 193
 194     <orderedlist>
 195       <listitem><para>It is in an active, activating, deactivating or failed state (i.e. in any unit state except for <literal>inactive</literal>)</para></listitem>
 196       <listitem><para>It has a job queued for it</para></listitem>
 197       <listitem><para>It is a dependency of 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
 274     by systemd are covered by the
 275     <ulink url="Portability and">Interface Portability and Stability Promise</ulink>.</para>
 276
 277     <para>Units may be generated dynamically at boot and system
 278     manager reload time, for example based on other configuration
 279     files or parameters passed on the kernel command line. For details, see
 280     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 281
 282     <para>The D-Bus API of <command>systemd</command> is described in
 283     <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 284     and
 285     <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 286     </para>
 287
 288     <para>Systems which invoke systemd in a container or initrd environment should implement the <ulink
 289     url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> or
 290     <ulink url="https://systemd.io/INITRD_INTERFACE/">initrd Interface</ulink>
 291     specifications, respectively.</para>
 292   </refsect1>
 293
 294   <refsect1>
 295     <title>Directories</title>
 296
 297     <variablelist>
 298       <varlistentry>
 299         <term>System unit directories</term>
 300
 301         <listitem><para>The systemd system manager reads unit
 302         configuration from various directories. Packages that want to
 303         install unit files shall place them in the directory returned
 304         by <command>pkg-config systemd
 305         --variable=systemdsystemunitdir</command>. Other directories
 306         checked are <filename>/usr/local/lib/systemd/system</filename>
 307         and <filename>/usr/lib/systemd/system</filename>. User
 308         configuration always takes precedence. <command>pkg-config
 309         systemd --variable=systemdsystemconfdir</command> returns the
 310         path of the system configuration directory. Packages should
 311         alter the content of these directories only with the
 312         <command>enable</command> and <command>disable</command>
 313         commands of the
 314         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 315         tool. Full list of directories is provided in
 316         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 317         </para></listitem>
 318       </varlistentry>
 319     </variablelist>
 320
 321     <variablelist>
 322       <varlistentry>
 323         <term>User unit directories</term>
 324
 325         <listitem><para>Similar rules apply for the user unit
 326         directories. However, here the
 327         <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 328         Base Directory specification</ulink> is followed to find
 329         units. Applications should place their unit files in the
 330         directory returned by <command>pkg-config systemd
 331         --variable=systemduserunitdir</command>. Global configuration
 332         is done in the directory reported by <command>pkg-config
 333         systemd --variable=systemduserconfdir</command>. The
 334         <command>enable</command> and <command>disable</command>
 335         commands of the
 336         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 337         tool can handle both global (i.e. for all users) and private
 338         (for one user) enabling/disabling of units. Full list of
 339         directories is provided in
 340         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 341         </para></listitem>
 342       </varlistentry>
 343     </variablelist>
 344
 345     <variablelist>
 346       <varlistentry>
 347         <term>SysV init scripts directory</term>
 348
 349         <listitem><para>The location of the SysV init script directory
 350         varies between distributions. If systemd cannot find a native
 351         unit file for a requested service, it will look for a SysV
 352         init script of the same name (with the
 353         <filename>.service</filename> suffix
 354         removed).</para></listitem>
 355       </varlistentry>
 356     </variablelist>
 357
 358     <variablelist>
 359       <varlistentry>
 360         <term>SysV runlevel link farm directory</term>
 361
 362         <listitem><para>The location of the SysV runlevel link farm
 363         directory varies between distributions. systemd will take the
 364         link farm into account when figuring out whether a service
 365         shall be enabled. Note that a service unit with a native unit
 366         configuration file cannot be started by activating it in the
 367         SysV runlevel link farm.</para></listitem>
 368       </varlistentry>
 369     </variablelist>
 370   </refsect1>
 371
 372   <refsect1>
 373     <title>Signals</title>
 374
 375     <variablelist>
 376       <varlistentry>
 377         <term><constant>SIGTERM</constant></term>
 378
 379         <listitem><para>Upon receiving this signal the systemd system
 380         manager serializes its state, reexecutes itself and
 381         deserializes the saved state again. This is mostly equivalent
 382         to <command>systemctl daemon-reexec</command>.</para>
 383
 384         <para>systemd user managers will start the
 385         <filename>exit.target</filename> unit when this signal is
 386         received. This is mostly equivalent to <command>systemctl
 387         --user start exit.target
 388         --job-mode=replace-irreversibly</command>.</para></listitem>
 389       </varlistentry>
 390
 391       <varlistentry>
 392         <term><constant>SIGINT</constant></term>
 393
 394         <listitem><para>Upon receiving this signal the systemd system manager will start the
 395         <filename>ctrl-alt-del.target</filename> unit. This is mostly equivalent to
 396         <command>systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly</command>. If
 397         this signal is received more than 7 times per 2s, an immediate reboot is triggered. Note
 398         that pressing
 399         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> on the
 400         console will trigger this signal. Hence, if a reboot is hanging, pressing
 401         <keycombo><keycap>Ctrl</keycap><keycap>Alt</keycap><keycap>Del</keycap></keycombo> more than
 402         7 times in 2 seconds is a relatively safe way to trigger an immediate reboot.</para>
 403
 404         <para>systemd user managers treat this signal the same way as
 405         <constant>SIGTERM</constant>.</para></listitem>
 406       </varlistentry>
 407
 408       <varlistentry>
 409         <term><constant>SIGWINCH</constant></term>
 410
 411         <listitem><para>When this signal is received the systemd
 412         system manager will start the
 413         <filename>kbrequest.target</filename> unit. This is mostly
 414         equivalent to <command>systemctl start
 415         kbrequest.target</command>.</para>
 416
 417         <para>This signal is ignored by systemd user
 418         managers.</para></listitem>
 419       </varlistentry>
 420
 421       <varlistentry>
 422         <term><constant>SIGPWR</constant></term>
 423
 424         <listitem><para>When this signal is received the systemd
 425         manager will start the <filename>sigpwr.target</filename>
 426         unit. This is mostly equivalent to <command>systemctl start
 427         sigpwr.target</command>.</para></listitem>
 428       </varlistentry>
 429
 430       <varlistentry>
 431         <term><constant>SIGUSR1</constant></term>
 432
 433         <listitem><para>When this signal is received the systemd
 434         manager will try to reconnect to the D-Bus
 435         bus.</para></listitem>
 436       </varlistentry>
 437
 438       <varlistentry>
 439         <term><constant>SIGUSR2</constant></term>
 440
 441         <listitem><para>When this signal is received the systemd
 442         manager will log its complete state in human-readable form.
 443         The data logged is the same as printed by
 444         <command>systemd-analyze dump</command>.</para></listitem>
 445       </varlistentry>
 446
 447       <varlistentry>
 448         <term><constant>SIGHUP</constant></term>
 449
 450         <listitem><para>Reloads the complete daemon configuration.
 451         This is mostly equivalent to <command>systemctl
 452         daemon-reload</command>.</para></listitem>
 453       </varlistentry>
 454
 455       <varlistentry>
 456         <term><constant>SIGRTMIN+0</constant></term>
 457
 458         <listitem><para>Enters default mode, starts the
 459         <filename>default.target</filename> unit. This is mostly
 460         equivalent to <command>systemctl isolate
 461         default.target</command>.</para></listitem>
 462       </varlistentry>
 463
 464       <varlistentry>
 465         <term><constant>SIGRTMIN+1</constant></term>
 466
 467         <listitem><para>Enters rescue mode, starts the
 468         <filename>rescue.target</filename> unit. This is mostly
 469         equivalent to <command>systemctl isolate
 470         rescue.target</command>.</para></listitem>
 471       </varlistentry>
 472
 473       <varlistentry>
 474         <term><constant>SIGRTMIN+2</constant></term>
 475
 476         <listitem><para>Enters emergency mode, starts the
 477         <filename>emergency.service</filename> unit. This is mostly
 478         equivalent to <command>systemctl isolate
 479         emergency.service</command>.</para></listitem>
 480       </varlistentry>
 481
 482       <varlistentry>
 483         <term><constant>SIGRTMIN+3</constant></term>
 484
 485         <listitem><para>Halts the machine, starts the
 486         <filename>halt.target</filename> unit. This is mostly
 487         equivalent to <command>systemctl start halt.target
 488         --job-mode=replace-irreversibly</command>.</para>
 489         </listitem>
 490       </varlistentry>
 491
 492       <varlistentry>
 493         <term><constant>SIGRTMIN+4</constant></term>
 494
 495         <listitem><para>Powers off the machine, starts the
 496         <filename>poweroff.target</filename> unit. This is mostly
 497         equivalent to <command>systemctl start poweroff.target
 498         --job-mode=replace-irreversibly</command>.</para>
 499         </listitem>
 500       </varlistentry>
 501
 502       <varlistentry>
 503         <term><constant>SIGRTMIN+5</constant></term>
 504
 505         <listitem><para>Reboots the machine, starts the
 506         <filename>reboot.target</filename> unit. This is mostly
 507         equivalent to <command>systemctl start reboot.target
 508         --job-mode=replace-irreversibly</command>.</para>
 509         </listitem>
 510       </varlistentry>
 511
 512       <varlistentry>
 513         <term><constant>SIGRTMIN+6</constant></term>
 514
 515         <listitem><para>Reboots the machine via kexec, starts the
 516         <filename>kexec.target</filename> unit. This is mostly
 517         equivalent to <command>systemctl start kexec.target
 518         --job-mode=replace-irreversibly</command>.</para>
 519         </listitem>
 520       </varlistentry>
 521
 522       <varlistentry>
 523         <term><constant>SIGRTMIN+13</constant></term>
 524
 525         <listitem><para>Immediately halts the machine.</para></listitem>
 526       </varlistentry>
 527
 528       <varlistentry>
 529         <term><constant>SIGRTMIN+14</constant></term>
 530
 531         <listitem><para>Immediately powers off the machine.</para></listitem>
 532       </varlistentry>
 533
 534       <varlistentry>
 535         <term><constant>SIGRTMIN+15</constant></term>
 536
 537         <listitem><para>Immediately reboots the machine.</para></listitem>
 538       </varlistentry>
 539
 540       <varlistentry>
 541         <term><constant>SIGRTMIN+16</constant></term>
 542
 543         <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 544       </varlistentry>
 545
 546       <varlistentry>
 547         <term><constant>SIGRTMIN+20</constant></term>
 548
 549         <listitem><para>Enables display of status messages on the
 550         console, as controlled via
 551         <varname>systemd.show_status=1</varname> on the kernel command
 552         line.</para></listitem>
 553       </varlistentry>
 554
 555       <varlistentry>
 556         <term><constant>SIGRTMIN+21</constant></term>
 557
 558         <listitem><para>Disables display of
 559         status messages on the console, as
 560         controlled via
 561         <varname>systemd.show_status=0</varname>
 562         on the kernel command
 563         line.</para></listitem>
 564       </varlistentry>
 565
 566       <varlistentry>
 567         <term><constant>SIGRTMIN+22</constant></term>
 568
 569         <listitem><para>Sets the service manager's log level to <literal>debug</literal>, in a fashion equivalent to
 570         <varname>systemd.log_level=debug</varname> on the kernel command line.</para></listitem>
 571       </varlistentry>
 572
 573       <varlistentry>
 574         <term><constant>SIGRTMIN+23</constant></term>
 575
 576         <listitem><para>Restores the log level to its configured value. The configured value is derived from – in order
 577         of priority – the value specified with <varname>systemd.log-level=</varname> on the kernel command line, or the
 578         value specified with <option>LogLevel=</option> in the configuration file, or the built-in default of
 579         <literal>info</literal>.</para></listitem>
 580       </varlistentry>
 581
 582       <varlistentry>
 583         <term><constant>SIGRTMIN+24</constant></term>
 584
 585         <listitem><para>Immediately exits the manager (only available
 586         for --user instances).</para></listitem>
 587       </varlistentry>
 588
 589       <varlistentry>
 590         <term><constant>SIGRTMIN+26</constant></term>
 591
 592         <listitem><para>Restores the log target to its configured value. The configured value is derived from – in
 593         order of priority – the value specified with <varname>systemd.log-target=</varname> on the kernel command line,
 594         or the value specified with <option>LogTarget=</option> in the configuration file, or the built-in
 595         default.</para></listitem>
 596       </varlistentry>
 597
 598       <varlistentry>
 599         <term><constant>SIGRTMIN+27</constant></term>
 600         <term><constant>SIGRTMIN+28</constant></term>
 601
 602         <listitem><para>Sets the log target to <literal>console</literal> on <constant>SIGRTMIN+27</constant> (or
 603         <literal>kmsg</literal> on <constant>SIGRTMIN+28</constant>), in a fashion equivalent to
 604         <varname>systemd.log_target=console</varname> (or <varname>systemd.log_target=kmsg</varname> on
 605         <constant>SIGRTMIN+28</constant>) on the kernel command line.</para></listitem>
 606       </varlistentry>
 607     </variablelist>
 608   </refsect1>
 609
 610   <refsect1>
 611     <title>Environment</title>
 612
 613     <variablelist class='environment-variables'>
 614       <varlistentry>
 615         <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 616         <listitem><para>Controls whether systemd highlights important log messages. This can be overridden
 617         with <option>--log-color=</option>.</para></listitem>
 618       </varlistentry>
 619
 620       <varlistentry>
 621         <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 622         <listitem><para>systemd reads the log level from this environment variable. This can be overridden
 623         with <option>--log-level=</option>.</para></listitem>
 624       </varlistentry>
 625
 626       <varlistentry>
 627         <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 628         <listitem><para>Controls whether systemd prints the code location along with log messages. This can
 629         be overridden with <option>--log-location=</option>.</para></listitem>
 630       </varlistentry>
 631
 632       <varlistentry>
 633         <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 634         <listitem><para>systemd reads the log target from this environment variable. This can be overridden
 635         with <option>--log-target=</option>.</para></listitem>
 636       </varlistentry>
 637
 638       <varlistentry>
 639         <term><varname>$SYSTEMD_LOG_TIME</varname></term>
 640         <listitem><para>Controls whether systemd prefixes log messages with the current time. This can be
 641         overridden with <option>--log-time=</option>.</para></listitem>
 642       </varlistentry>
 643
 644       <varlistentry>
 645         <term><varname>$SYSTEMD_LOG_TID</varname></term>
 646         <listitem><para>Controls whether systemd prefixes log messages with the current thread ID
 647         (TID).</para></listitem>
 648       </varlistentry>
 649
 650       <varlistentry>
 651         <term><varname>$XDG_CONFIG_HOME</varname></term>
 652         <term><varname>$XDG_CONFIG_DIRS</varname></term>
 653         <term><varname>$XDG_DATA_HOME</varname></term>
 654         <term><varname>$XDG_DATA_DIRS</varname></term>
 655
 656         <listitem><para>The systemd user manager uses these variables
 657         in accordance to the <ulink
 658         url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 659         Base Directory specification</ulink> to find its
 660         configuration.</para></listitem>
 661       </varlistentry>
 662
 663       <varlistentry>
 664         <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 665         <term><varname>$SYSTEMD_GENERATOR_PATH</varname></term>
 666         <term><varname>$SYSTEMD_ENVIRONMENT_GENERATOR_PATH</varname></term>
 667
 668         <listitem><para>Controls where systemd looks for unit files and
 669         generators.</para>
 670         <para>These variables may contain a list of paths, separated by colons
 671         (<literal>:</literal>). When set, if the list ends with an empty
 672         component (<literal>...:</literal>), this list is prepended to the
 673         usual set of paths. Otherwise, the specified list replaces the usual
 674         set of paths.
 675         </para></listitem>
 676       </varlistentry>
 677
 678       <varlistentry>
 679         <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 680
 681         <listitem><para>Controls where systemd looks for SysV init
 682         scripts.</para></listitem>
 683       </varlistentry>
 684
 685       <varlistentry>
 686         <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 687
 688         <listitem><para>Controls where systemd looks for SysV init
 689         script runlevel link farms.</para></listitem>
 690       </varlistentry>
 691
 692       <xi:include href="less-variables.xml" xpointer="pager"/>
 693       <xi:include href="less-variables.xml" xpointer="less"/>
 694       <xi:include href="less-variables.xml" xpointer="lesscharset"/>
 695       <xi:include href="less-variables.xml" xpointer="colors"/>
 696       <xi:include href="less-variables.xml" xpointer="urlify"/>
 697
 698       <varlistentry>
 699         <term><varname>$LISTEN_PID</varname></term>
 700         <term><varname>$LISTEN_FDS</varname></term>
 701         <term><varname>$LISTEN_FDNAMES</varname></term>
 702
 703         <listitem><para>Set by systemd for supervised processes during
 704         socket-based activation. See
 705         <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 706         for more information.</para></listitem>
 707       </varlistentry>
 708
 709       <varlistentry>
 710         <term><varname>$NOTIFY_SOCKET</varname></term>
 711
 712         <listitem><para>Set by systemd for supervised processes for
 713         status and start-up completion notification. See
 714         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 715         for more information.</para></listitem>
 716       </varlistentry>
 717     </variablelist>
 718
 719     <para>For further environment variables understood by systemd and its various components, see <ulink
 720     url="https://systemd.io/ENVIRONMENT">Known Environment Variables</ulink>.</para>
 721   </refsect1>
 722
 723   <refsect1>
 724     <title>Kernel Command Line</title>
 725
 726     <para>When run as the system instance systemd parses a number of options listed below. They can be
 727     specified as kernel command line arguments<footnote><para>If run inside a Linux container these arguments
 728     may be passed as command line arguments to systemd itself, next to any of the command line options listed
 729     in the Options section above. If run outside of Linux containers, these arguments are parsed from
 730     <filename>/proc/cmdline</filename> instead.</para></footnote>, or through the
 731     <literal>SystemdOptions</literal> EFI variable (on EFI systems). The kernel command line has higher
 732     priority. Following variables are understood:</para>
 733
 734     <variablelist class='kernel-commandline-options'>
 735       <varlistentry>
 736         <term><varname>systemd.unit=</varname></term>
 737         <term><varname>rd.systemd.unit=</varname></term>
 738
 739         <listitem><para>Overrides the unit to activate on boot.
 740         Defaults to <filename>default.target</filename>. This may be
 741         used to temporarily boot into a different boot unit, for
 742         example <filename>rescue.target</filename> or
 743         <filename>emergency.service</filename>. See
 744         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 745         for details about these units. The option prefixed with
 746         <literal>rd.</literal> is honored only in the initial RAM disk
 747         (initrd), while the one that is not prefixed only in the main
 748         system.</para></listitem>
 749       </varlistentry>
 750
 751       <varlistentry>
 752         <term><varname>systemd.dump_core</varname></term>
 753
 754         <listitem><para>Takes a boolean argument or enables the option if specified
 755         without an argument. If enabled, the systemd manager (PID 1) dumps core when
 756         it crashes. Otherwise, no core dump is created. Defaults to enabled.</para>
 757         </listitem>
 758       </varlistentry>
 759
 760       <varlistentry>
 761         <term><varname>systemd.crash_chvt</varname></term>
 762
 763         <listitem><para>Takes a positive integer, or a boolean argument. Can be also specified without an
 764         argument, with the same effect as a positive boolean. If a positive integer (in the range 1–63) is
 765         specified, the system manager (PID 1) will activate the specified virtual terminal when it crashes.
 766         Defaults to disabled, meaning that no such switch is attempted. If set to enabled, the virtual
 767         terminal the kernel messages are written to is used instead.</para></listitem>
 768       </varlistentry>
 769
 770       <varlistentry>
 771         <term><varname>systemd.crash_shell</varname></term>
 772
 773         <listitem><para>Takes a boolean argument or enables the option if specified
 774         without an argument. If enabled, the system manager (PID 1) spawns a shell
 775         when it crashes, after a 10s delay. Otherwise, no shell is spawned. Defaults
 776         to disabled, for security reasons, as the shell is not protected by password
 777         authentication.</para></listitem>
 778       </varlistentry>
 779
 780       <varlistentry>
 781         <term><varname>systemd.crash_reboot</varname></term>
 782
 783         <listitem><para>Takes a boolean argument or enables the option if specified
 784         without an argument. If enabled, the system manager (PID 1) will reboot the
 785         machine automatically when it crashes, after a 10s delay.  Otherwise, the
 786         system will hang indefinitely. Defaults to disabled, in order to avoid a
 787         reboot loop. If combined with <varname>systemd.crash_shell</varname>, the
 788         system is rebooted after the shell exits.</para></listitem>
 789       </varlistentry>
 790
 791       <varlistentry>
 792         <term><varname>systemd.confirm_spawn</varname></term>
 793
 794         <listitem><para>Takes a boolean argument or a path to the virtual console
 795         where the confirmation messages should be emitted. Can be also specified
 796         without an argument, with the same effect as a positive boolean. If enabled,
 797         the system manager (PID 1) asks for confirmation when spawning processes
 798         using <option>/dev/console</option>. If a path or a console name (such as
 799         <literal>ttyS0</literal>) is provided, the virtual console pointed to by this
 800         path or described by the give name will be used instead. Defaults to disabled.
 801         </para></listitem>
 802       </varlistentry>
 803
 804       <varlistentry>
 805         <term><varname>systemd.service_watchdogs=</varname></term>
 806
 807         <listitem><para>Takes a boolean argument. If disabled, all service runtime
 808         watchdogs (<option>WatchdogSec=</option>) and emergency actions (e.g.
 809         <option>OnFailure=</option> or <option>StartLimitAction=</option>) are
 810         ignored by the system manager (PID 1); see
 811         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 812         Defaults to enabled, i.e. watchdogs and failure actions are processed
 813         normally. The hardware watchdog is not affected by this
 814         option.</para></listitem>
 815       </varlistentry>
 816
 817       <varlistentry>
 818         <term><varname>systemd.show_status</varname></term>
 819
 820         <listitem><para>Takes a boolean argument or the constants <constant>error</constant> and
 821         <constant>auto</constant>. Can be also specified without an argument, with the same effect as a
 822         positive boolean. If enabled, the systemd manager (PID 1) shows terse service status updates on the
 823         console during bootup. With <constant>error</constant>, only messages about failures are shown, but
 824         boot is otherwise quiet. <constant>auto</constant> behaves like <option>false</option> until there is
 825         a significant delay in boot. Defaults to enabled, unless <option>quiet</option> is passed as kernel
 826         command line option, in which case it defaults to <constant>error</constant>. If specified overrides
 827         the system manager configuration file option <option>ShowStatus=</option>, see
 828         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 829         </para></listitem>
 830       </varlistentry>
 831
 832       <varlistentry>
 833         <term><varname>systemd.status_unit_format=</varname></term>
 834
 835         <listitem><para>Takes either <option>name</option> or <option>description</option> as the value. If
 836         <option>name</option>, the system manager will use unit names in status messages. If specified,
 837         overrides the system manager configuration file option <option>StatusUnitFormat=</option>, see
 838         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 839         </para></listitem>
 840       </varlistentry>
 841
 842       <varlistentry>
 843         <term><varname>systemd.log_color</varname></term>
 844         <term><varname>systemd.log_level=</varname></term>
 845         <term><varname>systemd.log_location</varname></term>
 846         <term><varname>systemd.log_target=</varname></term>
 847         <term><varname>systemd.log_time</varname></term>
 848         <term><varname>systemd.log_tid</varname></term>
 849
 850         <listitem><para>Controls log output, with the same effect as the
 851         <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>,
 852         <varname>$SYSTEMD_LOG_LOCATION</varname>, <varname>$SYSTEMD_LOG_TARGET</varname>,
 853         <varname>$SYSTEMD_LOG_TIME</varname>, and <varname>$SYSTEMD_LOG_TID</varname> environment variables
 854         described above. <varname>systemd.log_color</varname>, <varname>systemd.log_location</varname>,
 855         <varname>systemd.log_time</varname>, and <varname>systemd.log_tid=</varname> can be specified without
 856         an argument, with the same effect as a positive boolean.</para></listitem>
 857       </varlistentry>
 858
 859       <varlistentry>
 860         <term><varname>systemd.default_standard_output=</varname></term>
 861         <term><varname>systemd.default_standard_error=</varname></term>
 862
 863         <listitem><para>Controls default standard output and error output for services and sockets. That is,
 864         controls the default for <option>StandardOutput=</option> and <option>StandardError=</option> (see
 865         <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 866         details). Takes one of <option>inherit</option>, <option>null</option>, <option>tty</option>,
 867         <option>journal</option>, <option>journal+console</option>, <option>kmsg</option>,
 868         <option>kmsg+console</option>. If the argument is omitted
 869         <varname>systemd.default-standard-output=</varname> defaults to <option>journal</option> and
 870         <varname>systemd.default-standard-error=</varname> to <option>inherit</option>.</para></listitem>
 871       </varlistentry>
 872
 873       <varlistentry>
 874         <term><varname>systemd.setenv=</varname></term>
 875
 876         <listitem><para>Takes a string argument in the form
 877         VARIABLE=VALUE. May be used to set default environment
 878         variables to add to forked child processes. May be used more
 879         than once to set multiple variables.</para></listitem>
 880       </varlistentry>
 881
 882       <varlistentry>
 883         <term><varname>systemd.machine_id=</varname></term>
 884
 885         <listitem><para>Takes a 32 character hex value to be
 886         used for setting the machine-id. Intended mostly for
 887         network booting where the same machine-id is desired
 888         for every boot.</para></listitem>
 889       </varlistentry>
 890
 891       <varlistentry>
 892         <term><varname>systemd.unified_cgroup_hierarchy</varname></term>
 893
 894         <listitem><para>When specified without an argument or with a true argument,
 895         enables the usage of
 896         <ulink url="https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html">unified cgroup hierarchy</ulink>
 897         (a.k.a. cgroups-v2). When specified with a false argument, fall back to
 898         hybrid or full legacy cgroup hierarchy.</para>
 899
 900         <para>If this option is not specified, the default behaviour is determined
 901         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 902         option). If the kernel does not support unified cgroup hierarchy, the legacy
 903         hierarchy will be used even if this option is specified.</para>
 904         </listitem>
 905       </varlistentry>
 906
 907       <varlistentry>
 908         <term><varname>systemd.legacy_systemd_cgroup_controller</varname></term>
 909
 910         <listitem><para>Takes effect if the full unified cgroup hierarchy is not used
 911         (see previous option). When specified without an argument or with a true
 912         argument, disables the use of "hybrid" cgroup hierarchy (i.e. a cgroups-v2
 913         tree used for systemd, and
 914         <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/">legacy
 915         cgroup hierarchy</ulink>, a.k.a. cgroups-v1, for other controllers), and
 916         forces a full "legacy" mode. When specified with a false argument, enables
 917         the use of "hybrid" hierarchy.</para>
 918
 919         <para>If this option is not specified, the default behaviour is determined
 920         during compilation (the <option>-Ddefault-hierarchy=</option> meson
 921         option). If the kernel does not support unified cgroup hierarchy, the legacy
 922         hierarchy will be used  even if this option is specified.</para>
 923         </listitem>
 924       </varlistentry>
 925
 926       <varlistentry>
 927         <term><varname>quiet</varname></term>
 928
 929         <listitem><para>Turn off status output at boot, much like
 930         <varname>systemd.show_status=no</varname> would. Note that
 931         this option is also read by the kernel itself and disables
 932         kernel log output. Passing this option hence turns off the
 933         usual output from both the system manager and the kernel.
 934         </para></listitem>
 935       </varlistentry>
 936
 937       <varlistentry>
 938         <term><varname>debug</varname></term>
 939
 940         <listitem><para>Turn on debugging output. This is equivalent
 941         to <varname>systemd.log_level=debug</varname>. Note that this
 942         option is also read by the kernel itself and enables kernel
 943         debug output. Passing this option hence turns on the debug
 944         output from both the system manager and the
 945         kernel.</para></listitem>
 946       </varlistentry>
 947
 948       <varlistentry>
 949         <term><varname>emergency</varname></term>
 950         <term><varname>rd.emergency</varname></term>
 951         <term><varname>-b</varname></term>
 952
 953         <listitem><para>Boot into emergency mode. This is equivalent
 954         to <varname>systemd.unit=emergency.target</varname> or
 955         <varname>rd.systemd.unit=emergency.target</varname>, respectively, and
 956         provided for compatibility reasons and to be easier to type.</para></listitem>
 957       </varlistentry>
 958
 959       <varlistentry>
 960         <term><varname>rescue</varname></term>
 961         <term><varname>rd.rescue</varname></term>
 962         <term><varname>single</varname></term>
 963         <term><varname>s</varname></term>
 964         <term><varname>S</varname></term>
 965         <term><varname>1</varname></term>
 966
 967         <listitem><para>Boot into rescue mode. This is equivalent to
 968         <varname>systemd.unit=rescue.target</varname> or
 969         <varname>rd.systemd.unit=rescue.target</varname>, respectively, and
 970         provided for compatibility reasons and to be easier to type.</para></listitem>
 971       </varlistentry>
 972
 973       <varlistentry>
 974         <term><varname>2</varname></term>
 975         <term><varname>3</varname></term>
 976         <term><varname>4</varname></term>
 977         <term><varname>5</varname></term>
 978
 979         <listitem><para>Boot into the specified legacy SysV runlevel.
 980         These are equivalent to
 981         <varname>systemd.unit=runlevel2.target</varname>,
 982         <varname>systemd.unit=runlevel3.target</varname>,
 983         <varname>systemd.unit=runlevel4.target</varname>, and
 984         <varname>systemd.unit=runlevel5.target</varname>,
 985         respectively, and provided for compatibility reasons and to be
 986         easier to type.</para></listitem>
 987       </varlistentry>
 988
 989       <varlistentry>
 990         <term><varname>locale.LANG=</varname></term>
 991         <term><varname>locale.LANGUAGE=</varname></term>
 992         <term><varname>locale.LC_CTYPE=</varname></term>
 993         <term><varname>locale.LC_NUMERIC=</varname></term>
 994         <term><varname>locale.LC_TIME=</varname></term>
 995         <term><varname>locale.LC_COLLATE=</varname></term>
 996         <term><varname>locale.LC_MONETARY=</varname></term>
 997         <term><varname>locale.LC_MESSAGES=</varname></term>
 998         <term><varname>locale.LC_PAPER=</varname></term>
 999         <term><varname>locale.LC_NAME=</varname></term>
1000         <term><varname>locale.LC_ADDRESS=</varname></term>
1001         <term><varname>locale.LC_TELEPHONE=</varname></term>
1002         <term><varname>locale.LC_MEASUREMENT=</varname></term>
1003         <term><varname>locale.LC_IDENTIFICATION=</varname></term>
1004
1005         <listitem><para>Set the system locale to use. This overrides
1006         the settings in <filename>/etc/locale.conf</filename>. For
1007         more information, see
1008         <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1009         and
1010         <citerefentry project='man-pages'><refentrytitle>locale</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
1011         </para></listitem>
1012       </varlistentry>
1013     </variablelist>
1014
1015     <para>For other kernel command line parameters understood by
1016     components of the core OS, please refer to
1017     <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
1018   </refsect1>
1019
1020   <refsect1>
1021     <title>Options</title>
1022
1023     <para><command>systemd</command> is only very rarely invoked directly, since it is started early and is
1024     already running by the time users may interact with it. Normally, tools like
1025     <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> are used to
1026     give commands to the manager. Since <command>systemd</command> is usually not invoked directly, the
1027     options listed below are mostly useful for debugging and special purposes.</para>
1028
1029     <refsect2>
1030       <title>Introspection and debugging options</title>
1031
1032       <para>Those options are used for testing and introspection, and <command>systemd</command> may
1033       be invoked with them at any time:</para>
1034
1035       <variablelist>
1036         <varlistentry>
1037           <term><option>--dump-configuration-items</option></term>
1038
1039           <listitem><para>Dump understood unit configuration items. This outputs a terse but complete list of
1040           configuration items understood in unit definition files.</para></listitem>
1041         </varlistentry>
1042
1043         <varlistentry>
1044           <term><option>--dump-bus-properties</option></term>
1045
1046           <listitem><para>Dump exposed bus properties. This outputs a terse but complete list of properties
1047           exposed on D-Bus.</para></listitem>
1048         </varlistentry>
1049
1050         <varlistentry>
1051           <term><option>--test</option></term>
1052
1053           <listitem><para>Determine the initial start-up transaction (i.e. the list of jobs enqueued at
1054           start-up), dump it and exit — without actually executing any of the determined jobs. This option is
1055           useful for debugging only. Note that during regular service manager start-up additional units not
1056           shown by this operation may be started, because hardware, socket, bus or other kinds of activation
1057           might add additional jobs as the transaction is executed. Use <option>--system</option> to request
1058           the initial transaction of the system service manager (this is also the implied default), combine
1059           with <option>--user</option> to request the initial transaction of the per-user service manager
1060           instead.</para></listitem>
1061         </varlistentry>
1062
1063         <varlistentry>
1064           <term><option>--system</option></term>
1065           <term><option>--user</option></term>
1066
1067           <listitem><para>When used in conjunction with <option>--test</option>, selects whether to calculate
1068           the initial transaction for the system instance or for a per-user instance. These options have no
1069           effect when invoked without <option>--test</option>, as during regular
1070           (i.e. non-<option>--test</option>) invocations the service manager will automatically detect
1071           whether it shall operate in system or per-user mode, by checking whether the PID it is run as is 1
1072           or not. Note that it is not supported booting and maintaining a system with the service manager
1073           running in <option>--system</option> mode but with a PID other than 1.</para></listitem>
1074         </varlistentry>
1075
1076         <xi:include href="standard-options.xml" xpointer="help" />
1077         <xi:include href="standard-options.xml" xpointer="version" />
1078       </variablelist>
1079     </refsect2>
1080
1081     <refsect2>
1082       <title>Options that duplicate kernel command line settings</title>
1083
1084       <para>Those options correspond directly to options listed above in "Kernel Command Line". Both forms
1085       may be used equivalently for the system manager, but it is recommended to use the forms listed above in
1086       this context, because they are properly namespaced. When an option is specified both on the kernel
1087       command line and as a normal command line argument, the latter has higher precedence.</para>
1088
1089       <para>When <command>systemd</command> is used as a user manager, the kernel command line is ignored and
1090       only the options described below are understood. Nevertheless, <command>systemd</command> is usually
1091       started in this mode through the
1092       <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1093       service, which is shared between all users, and it may be more convenient to use configuration files to
1094       modify settings (see
1095       <citerefentry><refentrytitle>systemd-user.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
1096       or a drop-in that specifies one of the environment variables listed above in the Environment section
1097       (see the discussion of <varname>Environment=</varname> and <varname>EnvironmentFile=</varname> in
1098       <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para>
1099
1100       <variablelist>
1101         <varlistentry>
1102           <term><option>--unit=</option></term>
1103
1104           <listitem><para>Set default unit to activate on startup. If not specified, defaults to
1105           <filename>default.target</filename>. See <varname>systemd.unit=</varname> above.</para></listitem>
1106         </varlistentry>
1107
1108         <varlistentry>
1109           <term><option>--dump-core</option></term>
1110
1111           <listitem><para>Enable core dumping on crash. This switch has no effect when running as user
1112           instance. Same as <varname>systemd.dump_core=</varname> above.</para></listitem>
1113         </varlistentry>
1114
1115         <varlistentry>
1116           <term><option>--crash-vt=</option><replaceable>VT</replaceable></term>
1117
1118           <listitem><para>Switch to a specific virtual console (VT) on crash. This switch has no effect when
1119           running as user instance. Same as <varname>systemd.crash_chvt=</varname> above (but not the
1120           different spelling!).</para></listitem>
1121         </varlistentry>
1122
1123         <varlistentry>
1124           <term><option>--crash-shell</option></term>
1125
1126           <listitem><para>Run a shell on crash. This switch has no effect when running as user instance. See
1127           <varname>systemd.crash_shell=</varname> above.</para></listitem>
1128         </varlistentry>
1129
1130         <varlistentry>
1131           <term><option>--crash-reboot</option></term>
1132
1133           <listitem><para>Automatically reboot the system on crash. This switch has no effect when running as
1134           user instance. See <varname>systemd.crash_reboot</varname> above.</para></listitem>
1135         </varlistentry>
1136
1137         <varlistentry>
1138           <term><option>--confirm-spawn</option></term>
1139
1140           <listitem><para>Ask for confirmation when spawning processes. This switch has no effect when run as
1141           user instance. See <varname>systemd.confirm_spawn</varname> above.</para></listitem>
1142         </varlistentry>
1143
1144         <varlistentry>
1145           <term><option>--show-status</option></term>
1146
1147           <listitem><para>Show terse unit status information on the console during boot-up and shutdown. See
1148           <varname>systemd.show_status</varname> above.</para></listitem>
1149         </varlistentry>
1150
1151         <varlistentry>
1152           <term><option>--log-color</option></term>
1153
1154           <listitem><para>Highlight important log messages. See <varname>systemd.log_color</varname> above.
1155           </para></listitem>
1156         </varlistentry>
1157
1158         <varlistentry>
1159           <term><option>--log-level=</option></term>
1160
1161           <listitem><para>Set log level. See <varname>systemd.log_level</varname> above.</para></listitem>
1162         </varlistentry>
1163
1164         <varlistentry>
1165           <term><option>--log-location</option></term>
1166
1167           <listitem><para>Include code location in log messages. See <varname>systemd.log_location</varname>
1168           above.</para></listitem>
1169         </varlistentry>
1170
1171         <varlistentry>
1172           <term><option>--log-target=</option></term>
1173
1174           <listitem><para>Set log target. See <varname>systemd.log_target</varname> above.</para></listitem>
1175         </varlistentry>
1176
1177         <varlistentry>
1178           <term><option>--log-time=</option></term>
1179
1180           <listitem><para>Prefix messages with timestamp. See <varname>systemd.log_time</varname> above.
1181           </para></listitem>
1182         </varlistentry>
1183
1184         <varlistentry>
1185           <term><option>--machine-id=</option></term>
1186
1187           <listitem><para>Override the machine-id set on the hard drive. See
1188           <varname>systemd.machine_id=</varname> above.</para></listitem>
1189         </varlistentry>
1190
1191         <varlistentry>
1192           <term><option>--service-watchdogs</option></term>
1193
1194           <listitem><para>Globally enable/disable all service watchdog timeouts and emergency actions. See
1195           <varname>systemd.service_watchdogs</varname> above.</para></listitem>
1196         </varlistentry>
1197
1198         <varlistentry>
1199           <term><option>--default-standard-output=</option></term>
1200           <term><option>--default-standard-error=</option></term>
1201
1202           <listitem><para>Sets the default output or error output for all services and sockets,
1203           respectively. See <varname>systemd.default_standard_output=</varname> and
1204           <varname>systemd.default_standard_error=</varname> above.</para></listitem>
1205         </varlistentry>
1206       </variablelist>
1207     </refsect2>
1208   </refsect1>
1209
1210   <refsect1>
1211     <title>Sockets and FIFOs</title>
1212
1213     <variablelist>
1214       <varlistentry>
1215         <term><filename>/run/systemd/notify</filename></term>
1216
1217         <listitem><para>Daemon status notification socket. This is an
1218         <constant>AF_UNIX</constant> datagram socket and is used to
1219         implement the daemon notification logic as implemented by
1220         <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1221
1222       </varlistentry>
1223
1224       <varlistentry>
1225         <term><filename>/run/systemd/private</filename></term>
1226
1227         <listitem><para>Used internally as communication channel
1228         between
1229         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1230         and the systemd process. This is an
1231         <constant>AF_UNIX</constant> stream socket. This interface is
1232         private to systemd and should not be used in external
1233         projects.</para></listitem>
1234       </varlistentry>
1235
1236       <varlistentry>
1237         <term><filename>/dev/initctl</filename></term>
1238
1239         <listitem><para>Limited compatibility support for the SysV
1240         client interface, as implemented by the
1241         <filename>systemd-initctl.service</filename> unit. This is a
1242         named pipe in the file system. This interface is obsolete and
1243         should not be used in new applications.</para></listitem>
1244       </varlistentry>
1245     </variablelist>
1246   </refsect1>
1247
1248   <refsect1>
1249     <title>See Also</title>
1250     <para>
1251       The <ulink url="https://www.freedesktop.org/wiki/Software/systemd/">systemd Homepage</ulink>,
1252       <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1253       <citerefentry project='man-pages'><refentrytitle>locale.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1254       <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1255       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1256       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1257       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1258       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
1259       <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1260       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1261       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1262       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1263       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1264       <citerefentry project='man-pages'><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1265       <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1266     </para>
1267   </refsect1>
1268
1269 </refentry>