man/systemd.xml

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   8   Copyright 2010 Lennart Poettering
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  23
  24 <refentry id="systemd">
  25
  26         <refentryinfo>
  27                 <title>systemd</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>systemd</refentrytitle>
  42                 <manvolnum>1</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>systemd</refname>
  47                 <refname>init</refname>
  48                 <refpurpose>systemd System and Service Manager</refpurpose>
  49         </refnamediv>
  50
  51         <refsynopsisdiv>
  52                 <cmdsynopsis>
  53                         <command>systemd <arg choice="opt" rep="repeat">OPTIONS</arg></command>
  54                 </cmdsynopsis>
  55                 <cmdsynopsis>
  56                         <command>init <arg choice="opt" rep="repeat">OPTIONS</arg> <arg choice="req">COMMAND</arg></command>
  57                 </cmdsynopsis>
  58         </refsynopsisdiv>
  59
  60         <refsect1>
  61                 <title>Description</title>
  62
  63                 <para>systemd is a system and service manager for
  64                 Linux operating systems. When run as first process on
  65                 boot (as PID 1), it acts as init system that brings
  66                 up and maintains userspace services.</para>
  67
  68                 <para>For compatibility with SysV, if systemd is called
  69                 as <command>init</command> and a PID that is not
  70                 1, it will execute <command>telinit</command> and pass
  71                 all command line arguments unmodified. That means
  72                 <command>init</command> and <command>telinit</command>
  73                 are mostly equivalent when invoked from normal login sessions. See
  74                 <citerefentry><refentrytitle>telinit</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  75                 for more information.</para>
  76
  77                 <para>When run as system instance, systemd interprets
  78                 the configuration file
  79                 <filename>system.conf</filename>, otherwise
  80                 <filename>user.conf</filename>. See
  81                 <citerefentry><refentrytitle>systemd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  82                 for more information.</para>
  83         </refsect1>
  84
  85         <refsect1>
  86                 <title>Options</title>
  87
  88                 <para>The following options are understood:</para>
  89
  90                 <variablelist>
  91                         <varlistentry>
  92                                 <term><option>-h</option></term>
  93                                 <term><option>--help</option></term>
  94
  95                                 <listitem><para>Prints a short help
  96                                 text and exits.</para></listitem>
  97                         </varlistentry>
  98                         <varlistentry>
  99                                 <term><option>--test</option></term>
 100
 101                                 <listitem><para>Determine startup
 102                                 sequence, dump it and exit. This is an
 103                                 option useful for debugging
 104                                 only.</para></listitem>
 105                         </varlistentry>
 106                         <varlistentry>
 107                                 <term><option>--dump-configuration-items</option></term>
 108
 109                                 <listitem><para>Dump understood unit
 110                                 configuration items. This outputs a
 111                                 terse but complete list of
 112                                 configuration items understood in unit
 113                                 definition files.</para></listitem>
 114                         </varlistentry>
 115                         <varlistentry>
 116                                 <term><option>--introspect=</option></term>
 117
 118                                 <listitem><para>Extract D-Bus
 119                                 interface introspection data. This is
 120                                 mostly useful at install time
 121                                 to generate data suitable for the
 122                                 D-Bus interfaces
 123                                 repository. Optionally the interface
 124                                 name for the introspection data may be
 125                                 specified. If omitted, the
 126                                 introspection data for all interfaces
 127                                 is dumped.</para></listitem>
 128                         </varlistentry>
 129                         <varlistentry>
 130                                 <term><option>--unit=</option></term>
 131
 132                                 <listitem><para>Set default unit to
 133                                 activate on startup. If not specified
 134                                 defaults to
 135                                 <filename>default.target</filename>.</para></listitem>
 136                         </varlistentry>
 137                         <varlistentry>
 138                                 <term><option>--system</option></term>
 139                                 <term><option>--user</option></term>
 140
 141                                 <listitem><para>Tell systemd to run a
 142                                 system instance (resp. user
 143                                 instance), even if the process ID is
 144                                 not 1 (resp. is 1), i.e. systemd is
 145                                 not (resp. is) run as init process.
 146                                 Normally it should not be necessary to
 147                                 pass these options, as systemd
 148                                 automatically detects the mode it is
 149                                 started in. These options are hence of
 150                                 little use except for debugging. Note
 151                                 that it is not supported booting and
 152                                 maintaining a full system with systemd
 153                                 running in <option>--system</option>
 154                                 mode, but PID not 1. In practice,
 155                                 passing <option>--system</option> explicitly is
 156                                 only useful in conjunction with
 157                                 <option>--test</option>.</para></listitem>
 158                         </varlistentry>
 159                         <varlistentry>
 160                                 <term><option>--dump-core</option></term>
 161
 162                                 <listitem><para>Dump core on
 163                                 crash. This switch has no effect when
 164                                 run as user
 165                                 instance.</para></listitem>
 166                         </varlistentry>
 167                         <varlistentry>
 168                                 <term><option>--crash-shell</option></term>
 169
 170                                 <listitem><para>Run shell on
 171                                 crash. This switch has no effect when
 172                                 run as user
 173                                 instance.</para></listitem>
 174                         </varlistentry>
 175                         <varlistentry>
 176                                 <term><option>--confirm-spawn</option></term>
 177
 178                                 <listitem><para>Ask for confirmation
 179                                 when spawning processes. This switch
 180                                 has no effect when run as user
 181                                 instance.</para></listitem>
 182                         </varlistentry>
 183                         <varlistentry>
 184                                 <term><option>--show-status=</option></term>
 185
 186                                 <listitem><para>Show terse service
 187                                 status information while booting. This
 188                                 switch has no effect when run as user
 189                                 instance. Takes a boolean argument
 190                                 which may be omitted which is
 191                                 interpreted as
 192                                 <option>true</option>.</para></listitem>
 193                         </varlistentry>
 194                         <varlistentry>
 195                                 <term><option>--sysv-console=</option></term>
 196
 197                                 <listitem><para>Controls whether
 198                                 output of SysV init scripts will be
 199                                 directed to the console. This switch
 200                                 has no effect when run as user
 201                                 instance. Takes a boolean argument
 202                                 which may be omitted which is
 203                                 interpreted as
 204                                 <option>true</option>.</para></listitem>
 205                         </varlistentry>
 206                         <varlistentry>
 207                                 <term><option>--log-target=</option></term>
 208
 209                                 <listitem><para>Set log
 210                                 target. Argument must be one of
 211                                 <option>console</option>,
 212                                 <option>syslog</option>,
 213                                 <option>kmsg</option>,
 214                                 <option>syslog-or-kmsg</option>,
 215                                 <option>null</option>.</para></listitem>
 216                         </varlistentry>
 217                         <varlistentry>
 218                                 <term><option>--log-level=</option></term>
 219
 220                                 <listitem><para>Set log level. As
 221                                 argument this accepts a numerical log
 222                                 level or the well-known <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 223                                 symbolic names (lowercase):
 224                                 <option>emerg</option>,
 225                                 <option>alert</option>,
 226                                 <option>crit</option>,
 227                                 <option>err</option>,
 228                                 <option>warning</option>,
 229                                 <option>notice</option>,
 230                                 <option>info</option>,
 231                                 <option>debug</option>.</para></listitem>
 232                         </varlistentry>
 233                         <varlistentry>
 234                                 <term><option>--log-color=</option></term>
 235
 236                                 <listitem><para>Highlight important
 237                                 log messages. Argument is a boolean
 238                                 value. If the argument is omitted it
 239                                 defaults to
 240                                 <option>true</option>.</para></listitem>
 241                         </varlistentry>
 242                         <varlistentry>
 243                                 <term><option>--log-location=</option></term>
 244
 245                                 <listitem><para>Include code location
 246                                 in log messages. This is mostly
 247                                 relevant for debugging
 248                                 purposes. Argument is a boolean
 249                                 value. If the argument is omitted
 250                                 it defaults to
 251                                 <option>true</option>.</para></listitem>
 252                         </varlistentry>
 253                         <varlistentry>
 254                                 <term><option>--default-standard-output=</option></term>
 255                                 <term><option>--default-standard-error=</option></term>
 256
 257                                 <listitem><para>Sets the default
 258                                 output resp. error output for all
 259                                 services and sockets, i.e. controls
 260                                 the default for
 261                                 <option>StandardOutput=</option>
 262                                 resp. <option>StandardExecute=</option>
 263                                 (see
 264                                 <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 265                                 for details). Takes one of
 266                                 <option>inherit</option>,
 267                                 <option>null</option>,
 268                                 <option>tty</option>,
 269                                 <option>syslog</option>,
 270                                 <option>syslog+console</option>,
 271                                 <option>kmsg</option>,
 272                                 <option>kmsg-console</option>.  If the
 273                                 argument is omitted it defaults to
 274                                 <option>null</option>
 275                                 resp. <option>inherit</option>.</para></listitem>
 276                         </varlistentry>
 277                 </variablelist>
 278         </refsect1>
 279
 280         <refsect1>
 281                 <title>Concepts</title>
 282
 283                 <para>systemd provides a dependency system between
 284                 various entities called "units". Units encapsulate
 285                 various objects that are relevant for system boot-up
 286                 and maintenance. The majority of units are configured
 287                 in unit configuration files, whose syntax and basic
 288                 set of options is described in
 289                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 290                 however some are created automatically from other
 291                 configuration or dynamically from system state. Units
 292                 may be 'active' (meaning started, bound, plugged in,
 293                 ...  depending on the unit type, see below), or
 294                 'inactive' (meaning stopped, unbound, unplugged, ...),
 295                 as well as in the process of being activated or
 296                 deactivated, i.e. between the two states (these states
 297                 are called 'activating', 'deactivating'). A special
 298                 'failed' state is available as well which is very
 299                 similar to 'inactive' and is entered when the service
 300                 failed in some way (process returned error code on
 301                 exit, or crashed, or an operation timed out). If this
 302                 state is entered the cause will be logged, for later
 303                 reference. Note that the various unit types may have a
 304                 number of additional substates, which are mapped to
 305                 the five generalized unit states described
 306                 here.</para>
 307
 308                 <para>The following unit types are available:</para>
 309
 310                 <orderedlist>
 311                         <listitem><para>Service units, which control
 312                         daemons and the processes they consist of. For
 313                         details see
 314                         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 315
 316                         <listitem><para>Socket units, which
 317                         encapsulate local IPC or network sockets in
 318                         the system, useful for socket-based
 319                         activation. For details about socket units see
 320                         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 321                         for details on socket-based activation and
 322                         other forms of activation, see
 323                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 324
 325                         <listitem><para>Target units are useful to
 326                         group units, or provide well-known
 327                         synchronization points during boot-up, see
 328                         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 329
 330                         <listitem><para>Device units expose kernel
 331                         devices in systemd and may be used to
 332                         implement device-based activation. For details
 333                         see
 334                         <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 335
 336                         <listitem><para>Mount units control mount
 337                         points in the file system, for details see
 338                         <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 339
 340                         <listitem><para>Automount units provide
 341                         automount capabilities, for on-demand mounting
 342                         of file systems as well as parallelized
 343                         boot-up. See
 344                         <citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 345
 346                         <listitem><para>Snapshot units can be used to
 347                         temporarily save the state of the set of
 348                         systemd units, which later may be restored by
 349                         activating the saved snapshot unit. For more
 350                         information see
 351                         <citerefentry><refentrytitle>systemd.snapshot</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 352
 353                         <listitem><para>Timer units are useful for
 354                         triggering activation of other units based on
 355                         timers. You may find details in
 356                         <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 357
 358                         <listitem><para>Swap units are very similar to
 359                         mount units and encapsulate memory swap
 360                         partitions or files of the operating
 361                         system. They are described in <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 362
 363                         <listitem><para>Path units may be used
 364                         to activate other services when file system
 365                         objects change or are modified. See
 366                         <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
 367
 368                 </orderedlist>
 369
 370                 <para>Units are named as their configuration
 371                 files. Some units have special semantics. A detailed
 372                 list is available in
 373                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 374
 375                 <para>systemd knows various kinds of dependencies,
 376                 including positive and negative requirement
 377                 dependencies (i.e. <varname>Requires=</varname> and
 378                 <varname>Conflicts=</varname>) as well as ordering
 379                 dependencies (<varname>After=</varname> and
 380                 <varname>Before=</varname>). NB: ordering and
 381                 requirement dependencies are orthogonal. If only a
 382                 requirement dependency exists between two units
 383                 (e.g. <filename>foo.service</filename> requires
 384                 <filename>bar.service</filename>), but no ordering
 385                 dependency (e.g. <filename>foo.service</filename>
 386                 after <filename>bar.service</filename>) and both are
 387                 requested to start, they will be started in
 388                 parallel. It is a common pattern that both requirement
 389                 and ordering dependencies are placed between two
 390                 units. Also note that the majority of dependencies are
 391                 implicitly created and maintained by systemd. In most
 392                 cases it should be unnecessary to declare additional
 393                 dependencies manually, however it is possible to do
 394                 this.</para>
 395
 396                 <para>Application programs and units (via
 397                 dependencies) may request state changes of units. In
 398                 systemd, these requests are encapsulated as 'jobs' and
 399                 maintained in a job queue. Jobs may succeed or can
 400                 fail, their execution is ordered based on the ordering
 401                 dependencies of the units they have been scheduled
 402                 for.</para>
 403
 404                 <para>On boot systemd activates the target unit
 405                 <filename>default.target</filename> whose job is to
 406                 activate on-boot services and other on-boot units by
 407                 pulling them in via dependencies. Usually the unit
 408                 name is just an alias (symlink) for either
 409                 <filename>graphical.target</filename> (for
 410                 fully-featured boots into the UI) or
 411                 <filename>multi-user.target</filename> (for limited
 412                 console-only boots for use in embedded or server
 413                 environments, or similar; a subset of
 414                 graphical.target). However it is at the discretion of
 415                 the administrator to configure it as an alias to any
 416                 other target unit. See
 417                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 418                 for details about these target units.</para>
 419
 420                 <para>Processes systemd spawns are placed in
 421                 individual Linux control groups named after the unit
 422                 which they belong to in the private systemd
 423                 hierarchy. (see <ulink
 424                 url="http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt">cgroups.txt</ulink>
 425                 for more information about control groups, or short
 426                 "cgroups"). systemd uses this to effectively keep
 427                 track of processes. Control group information is
 428                 maintained in the kernel, and is accessible via the
 429                 file system hierarchy (beneath
 430                 <filename>/sys/fs/cgroup/systemd/</filename>), or in tools
 431                 such as
 432                 <citerefentry><refentrytitle>ps</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 433                 (<command>ps xawf -eo pid,user,cgroup,args</command>
 434                 is particularly useful to list all processes and the
 435                 systemd units they belong to.).</para>
 436
 437                 <para>systemd is compatible with the SysV init system
 438                 to a large degree: SysV init scripts are supported and
 439                 simply read as an alternative (though limited)
 440                 configuration file format. The SysV
 441                 <filename>/dev/initctl</filename> interface is
 442                 provided, and compatibility implementations of the
 443                 various SysV client tools are available. In addition to
 444                 that, various established Unix functionality such as
 445                 <filename>/etc/fstab</filename> or the
 446                 <filename>utmp</filename> database are
 447                 supported.</para>
 448
 449                 <para>systemd has a minimal transaction system: if a
 450                 unit is requested to start up or shut down it will add
 451                 it and all its dependencies to a temporary
 452                 transaction. Then, it will verify if the transaction
 453                 is consistent (i.e. whether the ordering of all units
 454                 is cycle-free). If it is not, systemd will try to fix
 455                 it up, and removes non-essential jobs from the
 456                 transaction that might remove the loop. Also, systemd
 457                 tries to suppress non-essential jobs in the
 458                 transaction that would stop a running service. Finally
 459                 it is checked whether the jobs of the transaction
 460                 contradict jobs that have already been queued, and
 461                 optionally the transaction is aborted then. If all
 462                 worked out and the transaction is consistent and
 463                 minimized in its impact it is merged with all already
 464                 outstanding jobs and added to the run
 465                 queue. Effectively this means that before executing a
 466                 requested operation, systemd will verify that it makes
 467                 sense, fixing it if possible, and only failing if it
 468                 really cannot work.</para>
 469
 470                 <para>Systemd contains native implementations of
 471                 various tasks that need to be executed as part of the
 472                 boot process. For example, it sets the host name or
 473                 configures the loopback network device. It also sets
 474                 up and mounts various API file systems, such as
 475                 <filename>/sys</filename> or
 476                 <filename>/proc</filename>.</para>
 477
 478                 <para>For more information about the concepts and
 479                 ideas behind systemd please refer to the <ulink
 480                 url="http://0pointer.de/blog/projects/systemd.html">Original
 481                 Design Document</ulink>.</para>
 482
 483                 <para>Note that some but not all interfaces provided
 484                 by systemd are covered by the <ulink
 485                 url="http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
 486                 Stability Promise</ulink>.</para>
 487         </refsect1>
 488
 489         <refsect1>
 490                 <title>Directories</title>
 491
 492                 <variablelist>
 493                         <varlistentry>
 494                                 <term>System unit directories</term>
 495
 496                                 <listitem><para>The systemd system
 497                                 manager reads unit configuration from
 498                                 various directories. Packages that
 499                                 want to install unit files shall place
 500                                 them in the directory returned by
 501                                 <command>pkg-config systemd
 502                                 --variable=systemdsystemunitdir</command>. Other
 503                                 directories checked are
 504                                 <filename>/usr/local/share/systemd/system</filename>
 505                                 and
 506                                 <filename>/usr/share/systemd/system</filename>. User
 507                                 configuration always takes
 508                                 precedence. <command>pkg-config
 509                                 systemd
 510                                 --variable=systemdsystemconfdir</command>
 511                                 returns the path of the system
 512                                 configuration directory. Packages
 513                                 should alter the content of these
 514                                 directories only with the
 515                                 <command>enable</command> and
 516                                 <command>disable</command> commands of
 517                                 the
 518                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 519                                 tool.</para></listitem>
 520                         </varlistentry>
 521                 </variablelist>
 522
 523                 <variablelist>
 524                         <varlistentry>
 525                                 <term>User unit directories</term>
 526
 527                                 <listitem><para>Similar rules apply
 528                                 for the user unit
 529                                 directories. However, here the <ulink
 530                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 531                                 Base Directory specification</ulink>
 532                                 is followed to find
 533                                 units. Applications should place their
 534                                 unit files in the directory returned
 535                                 by <command>pkg-config systemd
 536                                 --variable=systemduserunitdir</command>. Global
 537                                 configuration is done in the directory
 538                                 reported by <command>pkg-config
 539                                 systemd
 540                                 --variable=systemduserconfdir</command>. The
 541                                 <command>enable</command> and
 542                                 <command>disable</command> commands of
 543                                 the
 544                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 545                                 tool can handle both global (i.e. for
 546                                 all users) and private (for one user)
 547                                 enabling/disabling of
 548                                 units.</para></listitem>
 549                         </varlistentry>
 550                 </variablelist>
 551
 552                 <variablelist>
 553                         <varlistentry>
 554                                 <term>SysV init scripts directory</term>
 555
 556                                 <listitem><para>The location of the
 557                                 SysV init script directory varies
 558                                 between distributions. If systemd
 559                                 cannot find a native unit file for a
 560                                 requested service, it will look for a
 561                                 SysV init script of the same name
 562                                 (with the
 563                                 <filename>.service</filename> suffix
 564                                 removed).</para></listitem>
 565                         </varlistentry>
 566                 </variablelist>
 567
 568                 <variablelist>
 569                         <varlistentry>
 570                                 <term>SysV runlevel link farm directory</term>
 571
 572                                 <listitem><para>The location of the
 573                                 SysV runlevel link farm directory
 574                                 varies between distributions. systemd
 575                                 will take the link farm into account
 576                                 when figuring out whether a service
 577                                 shall be enabled. Note that a service
 578                                 unit with a native unit configuration
 579                                 file cannot be started by activating it
 580                                 in the SysV runlevel link
 581                                 farm.</para></listitem>
 582                         </varlistentry>
 583                 </variablelist>
 584         </refsect1>
 585
 586         <refsect1>
 587                 <title>Signals</title>
 588
 589                 <variablelist>
 590                         <varlistentry>
 591                                 <term>SIGTERM</term>
 592
 593                                 <listitem><para>Upon receiving this
 594                                 signal the systemd system manager
 595                                 serializes its state, reexecutes
 596                                 itself and deserializes the saved
 597                                 state again. This is mostly equivalent
 598                                 to <command>systemctl
 599                                 daemon-reexec</command>.</para>
 600
 601                                 <para>systemd user managers will
 602                                 start the
 603                                 <filename>exit.target</filename> unit
 604                                 when this signal is received. This is
 605                                 mostly equivalent to
 606                                 <command>systemctl --user start
 607                                 exit.target</command>.</para></listitem>
 608                         </varlistentry>
 609
 610                         <varlistentry>
 611                                 <term>SIGINT</term>
 612
 613                                 <listitem><para>Upon receiving this
 614                                 signal the systemd system manager will
 615                                 start the
 616                                 <filename>ctrl-alt-del.target</filename> unit. This
 617                                 is mostly equivalent to
 618                                 <command>systemctl start
 619                                 ctl-alt-del.target</command>.</para>
 620
 621                                 <para>systemd user managers
 622                                 treat this signal the same way as
 623                                 SIGTERM.</para></listitem>
 624                         </varlistentry>
 625
 626                         <varlistentry>
 627                                 <term>SIGWINCH</term>
 628
 629                                 <listitem><para>When this signal is
 630                                 received the systemd system manager
 631                                 will start the
 632                                 <filename>kbrequest.target</filename>
 633                                 unit. This is mostly equivalent to
 634                                 <command>systemctl start
 635                                 kbrequest.target</command>.</para>
 636
 637                                 <para>This signal is ignored by
 638                                 systemd user
 639                                 managers.</para></listitem>
 640                         </varlistentry>
 641
 642                         <varlistentry>
 643                                 <term>SIGPWR</term>
 644
 645                                 <listitem><para>When this signal is
 646                                 received the systemd manager
 647                                 will start the
 648                                 <filename>sigpwr.target</filename>
 649                                 unit. This is mostly equivalent to
 650                                 <command>systemctl start
 651                                 sigpwr.target</command>.</para></listitem>
 652                         </varlistentry>
 653
 654                         <varlistentry>
 655                                 <term>SIGUSR1</term>
 656
 657                                 <listitem><para>When this signal is
 658                                 received the systemd manager will try
 659                                 to reconnect to the D-Bus
 660                                 bus.</para></listitem>
 661                         </varlistentry>
 662
 663                         <varlistentry>
 664                                 <term>SIGUSR2</term>
 665
 666                                 <listitem><para>When this signal is
 667                                 received the systemd manager will log
 668                                 its complete state in human readable
 669                                 form. The data logged is the same as
 670                                 printed by <command>systemctl
 671                                 dump</command>.</para></listitem>
 672                         </varlistentry>
 673
 674                         <varlistentry>
 675                                 <term>SIGHUP</term>
 676
 677                                 <listitem><para>Reloads the complete
 678                                 daemon configuration. This is mostly
 679                                 equivalent to <command>systemctl
 680                                 daemon-reload</command>.</para></listitem>
 681                         </varlistentry>
 682
 683                         <varlistentry>
 684                                 <term>SIGRTMIN+0</term>
 685
 686                                 <listitem><para>Enters default mode, starts the
 687                                 <filename>default.target</filename>
 688                                 unit. This is mostly equivalent to
 689                                 <command>systemctl start
 690                                 default.target</command>.</para></listitem>
 691                         </varlistentry>
 692
 693                         <varlistentry>
 694                                 <term>SIGRTMIN+1</term>
 695
 696                                 <listitem><para>Enters rescue mode,
 697                                 starts the
 698                                 <filename>rescue.target</filename>
 699                                 unit. This is mostly equivalent to
 700                                 <command>systemctl isolate
 701                                 rescue.target</command>.</para></listitem>
 702                         </varlistentry>
 703
 704                         <varlistentry>
 705                                 <term>SIGRTMIN+2</term>
 706
 707                                 <listitem><para>Enters emergency mode,
 708                                 starts the
 709                                 <filename>emergency.service</filename>
 710                                 unit. This is mostly equivalent to
 711                                 <command>systemctl isolate
 712                                 emergency.service</command>.</para></listitem>
 713                         </varlistentry>
 714
 715                         <varlistentry>
 716                                 <term>SIGRTMIN+3</term>
 717
 718                                 <listitem><para>Halts the machine,
 719                                 starts the
 720                                 <filename>halt.target</filename>
 721                                 unit. This is mostly equivalent to
 722                                 <command>systemctl start
 723                                 halt.target</command>.</para></listitem>
 724                         </varlistentry>
 725
 726                         <varlistentry>
 727                                 <term>SIGRTMIN+4</term>
 728
 729                                 <listitem><para>Powers off the machine,
 730                                 starts the
 731                                 <filename>poweroff.target</filename>
 732                                 unit. This is mostly equivalent to
 733                                 <command>systemctl start
 734                                 poweroff.target</command>.</para></listitem>
 735                         </varlistentry>
 736
 737                         <varlistentry>
 738                                 <term>SIGRTMIN+5</term>
 739
 740                                 <listitem><para>Reboots the machine,
 741                                 starts the
 742                                 <filename>reboot.target</filename>
 743                                 unit. This is mostly equivalent to
 744                                 <command>systemctl start
 745                                 reboot.target</command>.</para></listitem>
 746                         </varlistentry>
 747
 748                         <varlistentry>
 749                                 <term>SIGRTMIN+6</term>
 750
 751                                 <listitem><para>Reboots the machine via kexec,
 752                                 starts the
 753                                 <filename>kexec.target</filename>
 754                                 unit. This is mostly equivalent to
 755                                 <command>systemctl start
 756                                 kexec.target</command>.</para></listitem>
 757                         </varlistentry>
 758
 759                         <varlistentry>
 760                                 <term>SIGRTMIN+13</term>
 761
 762                                 <listitem><para>Immediately halts the machine.</para></listitem>
 763                         </varlistentry>
 764
 765                         <varlistentry>
 766                                 <term>SIGRTMIN+14</term>
 767
 768                                 <listitem><para>Immediately powers off the machine.</para></listitem>
 769                         </varlistentry>
 770
 771                         <varlistentry>
 772                                 <term>SIGRTMIN+15</term>
 773
 774                                 <listitem><para>Immediately reboots the machine.</para></listitem>
 775                         </varlistentry>
 776
 777                         <varlistentry>
 778                                 <term>SIGRTMIN+16</term>
 779
 780                                 <listitem><para>Immediately reboots the machine with kexec.</para></listitem>
 781                         </varlistentry>
 782
 783                         <varlistentry>
 784                                 <term>SIGRTMIN+20</term>
 785
 786                                 <listitem><para>Enables display of
 787                                 status messages on the console, as
 788                                 controlled via
 789                                 <varname>systemd.show_status=1</varname>
 790                                 on the kernel command
 791                                 line.</para></listitem>
 792                         </varlistentry>
 793
 794                         <varlistentry>
 795                                 <term>SIGRTMIN+21</term>
 796
 797                                 <listitem><para>Disables display of
 798                                 status messages on the console, as
 799                                 controlled via
 800                                 <varname>systemd.show_status=0</varname>
 801                                 on the kernel command
 802                                 line.</para></listitem>
 803                         </varlistentry>
 804                 </variablelist>
 805         </refsect1>
 806
 807         <refsect1>
 808                 <title>Environment</title>
 809
 810                 <variablelist>
 811                         <varlistentry>
 812                                 <term><varname>$SYSTEMD_LOG_LEVEL</varname></term>
 813                                 <listitem><para>systemd reads the
 814                                 log level from this environment
 815                                 variable. This can be overridden with
 816                                 <option>--log-level=</option>.</para></listitem>
 817                         </varlistentry>
 818
 819                         <varlistentry>
 820                                 <term><varname>$SYSTEMD_LOG_TARGET</varname></term>
 821                                 <listitem><para>systemd reads the
 822                                 log target from this environment
 823                                 variable. This can be overridden with
 824                                 <option>--log-target=</option>.</para></listitem>
 825                         </varlistentry>
 826
 827                         <varlistentry>
 828                                 <term><varname>$SYSTEMD_LOG_COLOR</varname></term>
 829                                 <listitem><para>Controls whether
 830                                 systemd highlights important log
 831                                 messages. This can be overridden with
 832                                 <option>--log-color=</option>.</para></listitem>
 833                         </varlistentry>
 834
 835                         <varlistentry>
 836                                 <term><varname>$SYSTEMD_LOG_LOCATION</varname></term>
 837                                 <listitem><para>Controls whether
 838                                 systemd prints the code location along
 839                                 with log messages. This can be
 840                                 overridden with
 841                                 <option>--log-location=</option>.</para></listitem>
 842                         </varlistentry>
 843
 844                         <varlistentry>
 845                                 <term><varname>$XDG_CONFIG_HOME</varname></term>
 846                                 <term><varname>$XDG_CONFIG_DIRS</varname></term>
 847                                 <term><varname>$XDG_DATA_HOME</varname></term>
 848                                 <term><varname>$XDG_DATA_DIRS</varname></term>
 849
 850                                 <listitem><para>The systemd user
 851                                 manager uses these variables in
 852                                 accordance to the <ulink
 853                                 url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG
 854                                 Base Directory specification</ulink>
 855                                 to find its configuration.</para></listitem>
 856                         </varlistentry>
 857
 858                         <varlistentry>
 859                                 <term><varname>$SYSTEMD_UNIT_PATH</varname></term>
 860
 861                                 <listitem><para>Controls where systemd
 862                                 looks for unit
 863                                 files.</para></listitem>
 864                         </varlistentry>
 865
 866                         <varlistentry>
 867                                 <term><varname>$SYSTEMD_SYSVINIT_PATH</varname></term>
 868
 869                                 <listitem><para>Controls where systemd
 870                                 looks for SysV init scripts.</para></listitem>
 871                         </varlistentry>
 872
 873                         <varlistentry>
 874                                 <term><varname>$SYSTEMD_SYSVRCND_PATH</varname></term>
 875
 876                                 <listitem><para>Controls where systemd
 877                                 looks for SysV init script runlevel link
 878                                 farms.</para></listitem>
 879                         </varlistentry>
 880
 881                         <varlistentry>
 882                                 <term><varname>$LISTEN_PID</varname></term>
 883                                 <term><varname>$LISTEN_FDS</varname></term>
 884
 885                                 <listitem><para>Set by systemd for
 886                                 supervised processes during
 887                                 socket-based activation. See
 888                                 <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 889                                 for more information.
 890                                 </para></listitem>
 891                         </varlistentry>
 892
 893                         <varlistentry>
 894                                 <term><varname>$NOTIFY_SOCKET</varname></term>
 895
 896                                 <listitem><para>Set by systemd for
 897                                 supervised processes for status and
 898                                 start-up completion notification. See
 899                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 900                                 for more information.
 901                                 </para></listitem>
 902                         </varlistentry>
 903                 </variablelist>
 904         </refsect1>
 905
 906         <refsect1>
 907                 <title>Kernel Command Line</title>
 908
 909                 <para>When run as system instance systemd parses a few kernel command line arguments:</para>
 910
 911                 <variablelist>
 912                         <varlistentry>
 913                                 <term><varname>systemd.unit=</varname></term>
 914
 915                                 <listitem><para>Overrides the unit to
 916                                 activate on boot. Defaults to
 917                                 <filename>default.target</filename>. This
 918                                 may be used to temporarily boot into a
 919                                 different boot unit, for example
 920                                 <filename>rescue.target</filename> or
 921                                 <filename>emergency.service</filename>. See
 922                                 <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 923                                 for details about these
 924                                 units.</para></listitem>
 925                         </varlistentry>
 926
 927                         <varlistentry>
 928                                 <term><varname>systemd.dump_core=</varname></term>
 929
 930                                 <listitem><para>Takes a boolean
 931                                 argument. If <option>true</option>
 932                                 systemd dumps core when it
 933                                 crashes. Otherwise no core dump is
 934                                 created. Defaults to
 935                                 <option>true</option>.</para></listitem>
 936                         </varlistentry>
 937
 938                         <varlistentry>
 939                                 <term><varname>systemd.crash_shell=</varname></term>
 940
 941                                 <listitem><para>Takes a boolean
 942                                 argument. If <option>true</option>
 943                                 systemd spawns a shell when it
 944                                 crashes. Otherwise no shell is
 945                                 spawned. Defaults to
 946                                 <option>false</option>, for security
 947                                 reasons, as the shell is not protected
 948                                 by any password
 949                                 authentication.</para></listitem>
 950                         </varlistentry>
 951
 952                         <varlistentry>
 953                                 <term><varname>systemd.crash_chvt=</varname></term>
 954
 955                                 <listitem><para>Takes an integer
 956                                 argument. If positive systemd
 957                                 activates the specified virtual
 958                                 terminal when it crashes. Defaults to
 959                                 <literal>-1</literal>.</para></listitem>
 960                         </varlistentry>
 961
 962                         <varlistentry>
 963                                 <term><varname>systemd.confirm_spawn=</varname></term>
 964
 965                                 <listitem><para>Takes a boolean
 966                                 argument. If <option>true</option>
 967                                 asks for confirmation when spawning
 968                                 processes. Defaults to
 969                                 <option>false</option>.</para></listitem>
 970                         </varlistentry>
 971
 972                         <varlistentry>
 973                                 <term><varname>systemd.show_status=</varname></term>
 974
 975                                 <listitem><para>Takes a boolean
 976                                 argument. If <option>true</option>
 977                                 shows terse service status updates on
 978                                 the console during bootup. Defaults to
 979                                 <option>true</option>.</para></listitem>
 980                         </varlistentry>
 981
 982                         <varlistentry>
 983                                 <term><varname>systemd.sysv_console=</varname></term>
 984
 985                                 <listitem><para>Takes a boolean
 986                                 argument. If <option>true</option>
 987                                 output of SysV init scripts will be
 988                                 directed to the console. Defaults to
 989                                 <option>true</option>, unless
 990                                 <option>quiet</option> is passed as
 991                                 kernel command line option in which
 992                                 case it defaults to
 993                                 <option>false</option>.</para></listitem>
 994                         </varlistentry>
 995
 996                         <varlistentry>
 997                                 <term><varname>systemd.log_target=</varname></term>
 998                                 <term><varname>systemd.log_level=</varname></term>
 999                                 <term><varname>systemd.log_color=</varname></term>
1000                                 <term><varname>systemd.log_location=</varname></term>
1001
1002                                 <listitem><para>Controls log output,
1003                                 with the same effect as the
1004                                 <varname>$SYSTEMD_LOG_TARGET</varname>, <varname>$SYSTEMD_LOG_LEVEL</varname>, <varname>$SYSTEMD_LOG_COLOR</varname>, <varname>$SYSTEMD_LOG_LOCATION</varname>
1005                                 environment variables described above.</para></listitem>
1006                         </varlistentry>
1007
1008                         <varlistentry>
1009                                 <term><varname>systemd.default_standard_output=</varname></term>
1010                                 <term><varname>systemd.default_standard_error=</varname></term>
1011                                 <listitem><para>Controls default
1012                                 standard output/error output for
1013                                 services, with the same effect as the
1014                                 <option>--default-standard-output=</option>
1015                                 resp. <option>--default-standard-error=</option>
1016                                 command line arguments described
1017                                 above.</para></listitem>
1018                         </varlistentry>
1019
1020                 </variablelist>
1021         </refsect1>
1022
1023         <refsect1>
1024                 <title>Sockets and FIFOs</title>
1025
1026                 <variablelist>
1027                         <varlistentry>
1028                                 <term><filename>@/org/freedesktop/systemd1/notify</filename></term>
1029
1030                                 <listitem><para>Daemon status
1031                                 notification socket. This is an AF_UNIX
1032                                 datagram socket in the Linux abstract
1033                                 namespace, and is used to implement
1034                                 the daemon notification logic as
1035                                 implemented by
1036                                 <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
1037
1038                         </varlistentry>
1039
1040                         <varlistentry>
1041                                 <term><filename>@/org/freedesktop/systemd1/logger</filename></term>
1042
1043                                 <listitem><para>Used internally by the
1044                                 <filename>systemd-logger.service</filename>
1045                                 unit to connect STDOUT and/or STDERR
1046                                 of spawned processes to
1047                                 <citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
1048                                 or the kernel log buffer. This is an
1049                                 AF_UNIX stream socket in the Linux
1050                                 abstract namespace.</para></listitem>
1051                         </varlistentry>
1052
1053                         <varlistentry>
1054                                 <term><filename>@/org/freedesktop/systemd1/shutdown</filename></term>
1055
1056                                 <listitem><para>Used internally by the
1057                                 <citerefentry><refentrytitle>shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1058                                 tool to implement delayed
1059                                 shutdowns. This is an AF_UNIX datagram
1060                                 socket in the Linux abstract
1061                                 namespace.</para></listitem>
1062                         </varlistentry>
1063
1064                         <varlistentry>
1065                                 <term><filename>@/org/freedesktop/systemd1/private</filename></term>
1066
1067                                 <listitem><para>Used internally as
1068                                 communication channel between
1069                                 <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1070                                 and the systemd process. This is an
1071                                 AF_UNIX stream socket in the Linux
1072                                 abstract namespace. This interface is
1073                                 private to systemd and should not be
1074                                 used in external
1075                                 projects.</para></listitem>
1076                         </varlistentry>
1077
1078                         <varlistentry>
1079                                 <term><filename>/dev/initctl</filename></term>
1080
1081                                 <listitem><para>Limited compatibility
1082                                 support for the SysV client interface,
1083                                 as implemented by the
1084                                 <filename>systemd-initctl.service</filename>
1085                                 unit. This is a named pipe in the file
1086                                 system. This interface is obsolete and
1087                                 should not be used in new
1088                                 applications.</para></listitem>
1089                         </varlistentry>
1090                 </variablelist>
1091         </refsect1>
1092
1093         <refsect1>
1094                 <title>See Also</title>
1095                 <para>
1096                         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1097                         <citerefentry><refentrytitle>systemadm</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1098                         <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1099                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1100                         <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1101                         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1102                         <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1103                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
1104                 </para>
1105         </refsect1>
1106
1107 </refentry>