man/systemd.special.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd.special">
   7
   8   <refentryinfo>
   9     <title>systemd.special</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd.special</refentrytitle>
  15     <manvolnum>7</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd.special</refname>
  20     <refpurpose>Special systemd units</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv><para>
  24     <!-- sort alphabetically, targets first --><filename>basic.target</filename>,
  25     <filename>bluetooth.target</filename>,
  26     <filename>cryptsetup-pre.target</filename>,
  27     <filename>cryptsetup.target</filename>,
  28     <filename>veritysetup-pre.target</filename>,
  29     <filename>veritysetup.target</filename>,
  30     <filename>ctrl-alt-del.target</filename>,
  31     <filename>blockdev@.target</filename>,
  32     <filename>boot-complete.target</filename>,
  33     <filename>default.target</filename>,
  34     <filename>emergency.target</filename>,
  35     <filename>exit.target</filename>,
  36     <filename>final.target</filename>,
  37     <filename>first-boot-complete.target</filename>,
  38     <filename>getty.target</filename>,
  39     <filename>getty-pre.target</filename>,
  40     <filename>graphical.target</filename>,
  41     <filename>halt.target</filename>,
  42     <filename>hibernate.target</filename>,
  43     <filename>hybrid-sleep.target</filename>,
  44     <filename>suspend-then-hibernate.target</filename>,
  45     <filename>initrd.target</filename>,
  46     <filename>initrd-fs.target</filename>,
  47     <filename>initrd-root-device.target</filename>,
  48     <filename>initrd-root-fs.target</filename>,
  49     <filename>initrd-usr-fs.target</filename>,
  50     <filename>kbrequest.target</filename>,
  51     <filename>kexec.target</filename>,
  52     <filename>local-fs-pre.target</filename>,
  53     <filename>local-fs.target</filename>,
  54     <filename>machines.target</filename>
  55     <filename>multi-user.target</filename>,
  56     <filename>network-online.target</filename>,
  57     <filename>network-pre.target</filename>,
  58     <filename>network.target</filename>,
  59     <filename>nss-lookup.target</filename>,
  60     <filename>nss-user-lookup.target</filename>,
  61     <filename>paths.target</filename>,
  62     <filename>poweroff.target</filename>,
  63     <filename>printer.target</filename>,
  64     <filename>reboot.target</filename>,
  65     <filename>remote-cryptsetup.target</filename>,
  66     <filename>remote-veritysetup.target</filename>,
  67     <filename>remote-fs-pre.target</filename>,
  68     <filename>remote-fs.target</filename>,
  69     <filename>rescue.target</filename>,
  70     <filename>rpcbind.target</filename>,
  71     <filename>runlevel2.target</filename>,
  72     <filename>runlevel3.target</filename>,
  73     <filename>runlevel4.target</filename>,
  74     <filename>runlevel5.target</filename>,
  75     <filename>shutdown.target</filename>,
  76     <filename>sigpwr.target</filename>,
  77     <filename>sleep.target</filename>,
  78     <filename>slices.target</filename>,
  79     <filename>smartcard.target</filename>,
  80     <filename>sockets.target</filename>,
  81     <filename>sound.target</filename>,
  82     <filename>suspend.target</filename>,
  83     <filename>swap.target</filename>,
  84     <filename>sysinit.target</filename>,
  85     <filename>system-update.target</filename>,
  86     <filename>system-update-pre.target</filename>,
  87     <filename>time-set.target</filename>,
  88     <filename>time-sync.target</filename>,
  89     <filename>timers.target</filename>,
  90     <filename>umount.target</filename>,
  91     <filename>usb-gadget.target</filename>,
  92     <!-- slices --><filename>-.slice</filename>,
  93     <filename>system.slice</filename>,
  94     <filename>user.slice</filename>,
  95     <filename>machine.slice</filename>,
  96     <!-- the rest --><filename>-.mount</filename>,
  97     <filename>dbus.service</filename>,
  98     <filename>dbus.socket</filename>,
  99     <filename>display-manager.service</filename>,
 100     <filename>init.scope</filename>,
 101     <filename>syslog.socket</filename>,
 102     <filename>system-update-cleanup.service</filename>
 103   </para></refsynopsisdiv>
 104
 105   <refsect1>
 106     <title>Description</title>
 107
 108     <para>A few units are treated specially by systemd. Many of them have
 109     special internal semantics and cannot be renamed, while others simply
 110     have a standard meaning and should be present on all systems.</para>
 111   </refsect1>
 112
 113   <refsect1>
 114     <title>Units managed by the system service manager</title>
 115
 116     <refsect2>
 117       <title>Special System Units</title>
 118
 119       <variablelist>
 120         <varlistentry>
 121           <term><filename>-.mount</filename></term>
 122           <listitem>
 123             <para>The root mount point, i.e. the mount unit for the <filename>/</filename>
 124             path. This unit is unconditionally active, during the entire time the system is up, as
 125             this mount point is where the basic userspace is running from.</para>
 126           </listitem>
 127         </varlistentry>
 128
 129         <varlistentry>
 130           <term><filename>basic.target</filename></term>
 131           <listitem>
 132             <para>A special target unit covering basic boot-up.</para>
 133
 134             <para>systemd automatically adds dependency of the type
 135             <varname>After=</varname> for this target unit to all
 136             services (except for those with
 137             <varname>DefaultDependencies=no</varname>).</para>
 138
 139             <para>Usually, this should pull-in all local mount points plus
 140             <filename>/var/</filename>, <filename>/tmp/</filename> and
 141             <filename>/var/tmp/</filename>, swap devices, sockets, timers,
 142             path units and other basic initialization necessary for general
 143             purpose daemons. The mentioned mount points are special cased
 144             to allow them to be remote.
 145             </para>
 146
 147             <para>This target usually does not pull in any non-target units
 148             directly, but rather does so indirectly via other early boot targets.
 149             It is instead meant as a synchronization point for late boot
 150             services. Refer to
 151             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 152             for details on the targets involved.
 153             </para>
 154           </listitem>
 155         </varlistentry>
 156         <varlistentry>
 157           <term><filename>boot-complete.target</filename></term>
 158           <listitem>
 159             <para>This target is intended as generic synchronization point for services that shall determine or act on
 160             whether the boot process completed successfully. Order units that are required to succeed for a boot process
 161             to be considered successful before this unit, and add a <varname>Requires=</varname> dependency from the
 162             target unit to them. Order units that shall only run when the boot process is considered successful after the
 163             target unit and pull in the target from it, also with <varname>Requires=</varname>. Note that by default this
 164             target unit is not part of the initial boot transaction, but is supposed to be pulled in only if required by
 165             units that want to run only on successful boots.</para>
 166
 167             <para>See
 168             <citerefentry><refentrytitle>systemd-boot-check-no-failures.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 169             for a service that implements a generic system health check and orders itself before
 170             <filename>boot-complete.target</filename>.</para>
 171
 172             <para>See
 173             <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 174             for a service that propagates boot success information to the boot loader, and orders itself after
 175             <filename>boot-complete.target</filename>.</para>
 176           </listitem>
 177         </varlistentry>
 178         <varlistentry>
 179           <term><filename>ctrl-alt-del.target</filename></term>
 180           <listitem>
 181             <para>systemd starts this target whenever Control+Alt+Del is
 182             pressed on the console. Usually, this should be aliased
 183             (symlinked) to <filename>reboot.target</filename>.</para>
 184           </listitem>
 185         </varlistentry>
 186         <varlistentry>
 187           <term><filename>cryptsetup.target</filename></term>
 188           <listitem>
 189             <para>A target that pulls in setup services for all
 190             encrypted block devices.</para>
 191           </listitem>
 192         </varlistentry>
 193         <varlistentry>
 194           <term><filename>veritysetup.target</filename></term>
 195           <listitem>
 196             <para>A target that pulls in setup services for all
 197             verity integrity protected block devices.</para>
 198           </listitem>
 199         </varlistentry>
 200         <varlistentry>
 201           <term><filename>dbus.service</filename></term>
 202           <listitem>
 203             <para>A special unit for the D-Bus bus daemon. As soon as
 204             this service is fully started up systemd will connect to it
 205             and register its service.</para>
 206           </listitem>
 207         </varlistentry>
 208         <varlistentry>
 209           <term><filename>dbus.socket</filename></term>
 210           <listitem>
 211             <para>A special unit for the D-Bus system bus socket. All
 212             units with <varname>Type=dbus</varname> automatically gain a
 213             dependency on this unit.</para>
 214           </listitem>
 215         </varlistentry>
 216         <varlistentry>
 217           <term><filename>default.target</filename></term>
 218           <listitem>
 219             <para>The default unit systemd starts at bootup. Usually, this should be aliased (symlinked) to
 220             <filename>multi-user.target</filename> or <filename>graphical.target</filename>. See
 221             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 222             more discussion.</para>
 223
 224             <para>The default unit systemd starts at bootup can be overridden with the
 225             <varname>systemd.unit=</varname> kernel command line option, or more conveniently, with the short
 226             names like <varname>single</varname>, <varname>rescue</varname>, <varname>1</varname>,
 227             <varname>3</varname>, <varname>5</varname>, …; see
 228             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 229           </listitem>
 230         </varlistentry>
 231         <varlistentry>
 232           <term><filename>display-manager.service</filename></term>
 233           <listitem>
 234             <para>The display manager service. Usually, this should be
 235             aliased (symlinked) to <filename>gdm.service</filename> or a
 236             similar display manager service.</para>
 237           </listitem>
 238         </varlistentry>
 239         <varlistentry>
 240           <term><filename>emergency.target</filename></term>
 241           <listitem>
 242             <para>A special target unit that starts an emergency shell on the main console. This
 243             target does not pull in other services or mounts. It is the most minimal version of
 244             starting the system in order to acquire an interactive shell; the only processes running
 245             are usually just the system manager (PID 1) and the shell process. This unit may be used
 246             by specifying <varname>emergency</varname> on the kernel command line; it is
 247             also used when a file system check on a required file system fails and boot-up cannot
 248             continue. Compare with <filename>rescue.target</filename>, which serves a similar
 249             purpose, but also starts the most basic services and mounts all file systems.</para>
 250
 251             <para>In many ways booting into <filename>emergency.target</filename> is similar to the
 252             effect of booting with <literal>init=/bin/sh</literal> on the kernel command line,
 253             except that emergency mode provides you with the full system and service manager, and
 254             allows starting individual units in order to continue the boot process in steps.</para>
 255
 256             <para>Note that depending on how <filename>emergency.target</filename> is reached, the root file
 257             system might be mounted read-only or read-write (no remounting is done specially for this
 258             target). For example, the system may boot with root mounted read-only when <varname>ro</varname>
 259             is used on the kernel command line and remain this way for <filename>emergency.target</filename>,
 260             or the system may transition to <filename>emergency.target</filename> after the system has been
 261             partially booted and disks have already been remounted read-write.</para>
 262           </listitem>
 263         </varlistentry>
 264         <varlistentry>
 265           <term><filename>exit.target</filename></term>
 266           <listitem>
 267             <para>A special service unit for shutting down the system or
 268             user service manager. It is equivalent to
 269             <filename>poweroff.target</filename> on non-container
 270             systems, and also works in containers.</para>
 271
 272             <para>systemd will start this unit when it receives the
 273             <constant>SIGTERM</constant> or <constant>SIGINT</constant>
 274             signal when running as user service daemon.</para>
 275
 276             <para>Normally, this (indirectly) pulls in
 277             <filename>shutdown.target</filename>, which in turn should be
 278             conflicted by all units that want to be scheduled for
 279             shutdown when the service manager starts to exit.</para>
 280           </listitem>
 281         </varlistentry>
 282         <varlistentry>
 283           <term><filename>final.target</filename></term>
 284           <listitem>
 285             <para>A special target unit that is used during the shutdown
 286             logic and may be used to pull in late services after all
 287             normal services are already terminated and all mounts
 288             unmounted.
 289             </para>
 290           </listitem>
 291         </varlistentry>
 292         <varlistentry>
 293           <term><filename>getty.target</filename></term>
 294           <listitem>
 295             <para>A special target unit that pulls in statically
 296             configured local TTY <filename>getty</filename> instances.
 297             </para>
 298           </listitem>
 299         </varlistentry>
 300         <varlistentry>
 301           <term><filename>graphical.target</filename></term>
 302           <listitem>
 303             <para>A special target unit for setting up a graphical login
 304             screen. This pulls in
 305             <filename>multi-user.target</filename>.</para>
 306
 307             <para>Units that are needed for graphical logins shall add
 308             <varname>Wants=</varname> dependencies for their unit to
 309             this unit (or <filename>multi-user.target</filename>) during
 310             installation. This is best configured via
 311             <varname>WantedBy=graphical.target</varname> in the unit's
 312             [Install] section.</para>
 313           </listitem>
 314         </varlistentry>
 315         <varlistentry>
 316           <term><filename>hibernate.target</filename></term>
 317           <listitem>
 318             <para>A special target unit for hibernating the system. This
 319             pulls in <filename>sleep.target</filename>.</para>
 320           </listitem>
 321         </varlistentry>
 322         <varlistentry>
 323           <term><filename>hybrid-sleep.target</filename></term>
 324           <listitem>
 325             <para>A special target unit for hibernating and suspending
 326             the system at the same time. This pulls in
 327             <filename>sleep.target</filename>.</para>
 328           </listitem>
 329         </varlistentry>
 330         <varlistentry>
 331           <term><filename>suspend-then-hibernate.target</filename></term>
 332           <listitem>
 333             <para>A special target unit for suspending the system for a period
 334             of time, waking it and putting it into hibernate. This pulls in
 335             <filename>sleep.target</filename>.</para>
 336           </listitem>
 337         </varlistentry>
 338
 339         <varlistentry>
 340           <term><filename>halt.target</filename></term>
 341           <listitem>
 342             <para>A special target unit for shutting down and halting
 343             the system. Note that this target is distinct from
 344             <filename>poweroff.target</filename> in that it generally
 345             really just halts the system rather than powering it
 346             down.</para>
 347
 348             <para>Applications wanting to halt the system should not start this unit
 349             directly, but should instead execute <command>systemctl halt</command>
 350             (possibly with the <option>--no-block</option> option) or call
 351             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 352             <command>org.freedesktop.systemd1.Manager.Halt</command> D-Bus method
 353             directly.</para>
 354           </listitem>
 355         </varlistentry>
 356         <varlistentry>
 357           <term><filename>init.scope</filename></term>
 358           <listitem>
 359             <para>This scope unit is where the system and service manager (PID 1) itself resides. It
 360             is active as long as the system is running.</para>
 361           </listitem>
 362         </varlistentry>
 363         <varlistentry>
 364           <term><filename>initrd.target</filename></term>
 365           <listitem>
 366             <para>This is the default target in the initramfs, similar to <filename>default.target</filename>
 367             in the main system. It is used to mount the real root and transition to it. See
 368             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 369             more discussion.</para>
 370           </listitem>
 371         </varlistentry>
 372         <varlistentry>
 373           <term><filename>initrd-fs.target</filename></term>
 374           <listitem>
 375             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 376             automatically adds dependencies of type <varname>Before=</varname> to
 377             <filename>sysroot-usr.mount</filename> and all mount points found in
 378             <filename>/etc/fstab</filename> that have the <option>x-initrd.mount</option> mount option set
 379             and do not have the <option>noauto</option> mount option set. It is also indirectly ordered after
 380             <filename>sysroot.mount</filename>. Thus, once this target is reached the
 381             <filename>/sysroot/</filename> hierarchy is fully set up, in preparation for the transition to
 382             the host OS.</para>
 383           </listitem>
 384         </varlistentry>
 385         <varlistentry>
 386           <term><filename>initrd-root-device.target</filename></term>
 387           <listitem>
 388             <para>A special initrd target unit that is reached when the root filesystem device is available, but before
 389             it has been mounted.
 390             <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 391             and
 392             <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 393             automatically setup the appropriate dependencies to make this happen.
 394             </para>
 395           </listitem>
 396         </varlistentry>
 397         <varlistentry>
 398           <term><filename>initrd-root-fs.target</filename></term>
 399           <listitem>
 400             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 401             automatically adds dependencies of type <varname>Before=</varname> to the
 402             <filename>sysroot.mount</filename> unit, which is generated from the kernel command line's
 403             <varname>root=</varname> setting (or equivalent).</para>
 404           </listitem>
 405         </varlistentry>
 406         <varlistentry>
 407           <term><filename>initrd-usr-fs.target</filename></term>
 408           <listitem>
 409             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 410             automatically adds dependencies of type <varname>Before=</varname> to the
 411             <filename>sysusr-usr.mount</filename> unit, which is generated from the kernel command line's
 412             <varname>usr=</varname> switch. Services may order themselves after this target unit in order to
 413             run once the <filename>/sysusr/</filename> hierarchy becomes available, on systems that come up
 414             initially without a root file system, but with an initialized <filename>/usr/</filename> and need
 415             to access that before setting up the root file system to ultimately switch to. On systems where
 416             <varname>usr=</varname> is not used this target is ordered after
 417             <filename>sysroot.mount</filename> and thus mostly equivalent to
 418             <filename>initrd-root-fs.target</filename>. In effect on any system once this target is reached
 419             the file system backing <filename>/usr/</filename> is mounted, though possibly at two different
 420             locations, either below the <filename>/sysusr/</filename> or the <filename>/sysroot/</filename>
 421             hierarchies.</para>
 422           </listitem>
 423         </varlistentry>
 424         <varlistentry>
 425           <term><filename>kbrequest.target</filename></term>
 426           <listitem>
 427             <para>systemd starts this target whenever Alt+ArrowUp is
 428             pressed on the console. Note that any user with physical access
 429             to the machine will be able to do this, without authentication,
 430             so this should be used carefully.</para>
 431           </listitem>
 432         </varlistentry>
 433         <varlistentry>
 434           <term><filename>kexec.target</filename></term>
 435           <listitem>
 436             <para>A special target unit for shutting down and rebooting
 437             the system via kexec.</para>
 438
 439             <para>Applications wanting to reboot the system should not start this unit
 440             directly, but should instead execute <command>systemctl kexec</command>
 441             (possibly with the <option>--no-block</option> option) or call
 442             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 443             <command>org.freedesktop.systemd1.Manager.KExec</command> D-Bus method
 444             directly.</para>
 445           </listitem>
 446         </varlistentry>
 447         <varlistentry>
 448           <term><filename>local-fs.target</filename></term>
 449           <listitem>
 450             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 451             automatically adds dependencies of type
 452             <varname>Before=</varname> to all mount units that refer to
 453             local mount points for this target unit. In addition, it
 454             adds dependencies of type <varname>Wants=</varname> to this
 455             target unit for those mounts listed in
 456             <filename>/etc/fstab</filename> that have the
 457             <option>auto</option> mount option set.</para>
 458           </listitem>
 459         </varlistentry>
 460         <varlistentry>
 461           <term><filename>machines.target</filename></term>
 462           <listitem>
 463             <para>A standard target unit for starting all the containers
 464             and other virtual machines. See <filename>systemd-nspawn@.service</filename>
 465             for an example.</para>
 466           </listitem>
 467         </varlistentry>
 468         <varlistentry>
 469           <term><filename>multi-user.target</filename></term>
 470           <listitem>
 471             <para>A special target unit for setting up a multi-user
 472             system (non-graphical). This is pulled in by
 473             <filename>graphical.target</filename>.</para>
 474
 475             <para>Units that are needed for a multi-user system shall
 476             add <varname>Wants=</varname> dependencies for their unit to
 477             this unit during installation. This is best configured via
 478             <varname>WantedBy=multi-user.target</varname> in the unit's
 479             [Install] section.</para>
 480           </listitem>
 481         </varlistentry>
 482         <varlistentry>
 483           <term><filename>network-online.target</filename></term>
 484           <listitem>
 485             <para>Units that strictly require a configured network
 486             connection should pull in
 487             <filename>network-online.target</filename> (via a
 488             <varname>Wants=</varname> type dependency) and order
 489             themselves after it. This target unit is intended to pull in
 490             a service that delays further execution until the network is
 491             sufficiently set up. What precisely this requires is left to
 492             the implementation of the network managing service.</para>
 493
 494             <para>Note the distinction between this unit and
 495             <filename>network.target</filename>. This unit is an active
 496             unit (i.e. pulled in by the consumer rather than the
 497             provider of this functionality) and pulls in a service which
 498             possibly adds substantial delays to further execution. In
 499             contrast, <filename>network.target</filename> is a passive
 500             unit (i.e. pulled in by the provider of the functionality,
 501             rather than the consumer) that usually does not delay
 502             execution much. Usually, <filename>network.target</filename>
 503             is part of the boot of most systems, while
 504             <filename>network-online.target</filename> is not, except
 505             when at least one unit requires it. Also see <ulink
 506             url="https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running
 507             Services After the Network is up</ulink> for more
 508             information.</para>
 509
 510             <para>All mount units for remote network file systems automatically pull in this unit, and order
 511             themselves after it. Note that networking daemons that simply <emphasis>provide</emphasis>
 512             functionality to other hosts (as opposed to <emphasis>consume</emphasis> functionality of other
 513             hosts) generally do not need to pull this in.</para>
 514
 515             <para>systemd automatically adds dependencies of type <varname>Wants=</varname> and
 516             <varname>After=</varname> for this target unit to all SysV init script service units
 517             with an LSB header referring to the <literal>$network</literal> facility.</para>
 518
 519             <para>Note that this unit is only useful during the original system start-up
 520             logic. After the system has completed booting up, it will not track the online state of
 521             the system anymore. Due to this it cannot be used as a network connection monitor
 522             concept, it is purely a one-time system start-up concept.</para>
 523             </listitem>
 524         </varlistentry>
 525         <varlistentry>
 526           <term><filename>paths.target</filename></term>
 527           <listitem>
 528             <para>A special target unit that sets up all path units (see
 529             <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 530             for details) that shall be active after boot.</para>
 531
 532             <para>It is recommended that path units installed by
 533             applications get pulled in via <varname>Wants=</varname>
 534             dependencies from this unit. This is best configured via a
 535             <varname>WantedBy=paths.target</varname> in the path unit's
 536             [Install] section.</para>
 537           </listitem>
 538         </varlistentry>
 539         <varlistentry>
 540           <term><filename>poweroff.target</filename></term>
 541           <listitem>
 542             <para>A special target unit for shutting down and powering
 543             off the system.</para>
 544
 545             <para>Applications wanting to power off the system should not start this unit
 546             directly, but should instead execute <command>systemctl poweroff</command>
 547             (possibly with the <option>--no-block</option> option) or call
 548             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 549             <command>org.freedesktop.login1.Manager.PowerOff</command> D-Bus method
 550             directly.</para>
 551
 552             <para><filename>runlevel0.target</filename> is an alias for
 553             this target unit, for compatibility with SysV.</para>
 554           </listitem>
 555         </varlistentry>
 556         <varlistentry>
 557           <term><filename>reboot.target</filename></term>
 558           <listitem>
 559             <para>A special target unit for shutting down and rebooting
 560             the system.</para>
 561
 562             <para>Applications wanting to reboot the system should not start this unit
 563             directly, but should instead execute <command>systemctl reboot</command>
 564             (possibly with the <option>--no-block</option> option) or call
 565             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 566             <command>org.freedesktop.login1.Manager.Reboot</command> D-Bus method
 567             directly.</para>
 568
 569             <para><filename>runlevel6.target</filename> is an alias for
 570             this target unit, for compatibility with SysV.</para>
 571           </listitem>
 572         </varlistentry>
 573         <varlistentry>
 574           <term><filename>remote-cryptsetup.target</filename></term>
 575           <listitem>
 576             <para>Similar to <filename>cryptsetup.target</filename>, but for encrypted
 577             devices which are accessed over the network. It is used for
 578             <citerefentry><refentrytitle>crypttab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 579             entries marked with <option>_netdev</option>.</para>
 580           </listitem>
 581         </varlistentry>
 582         <varlistentry>
 583           <term><filename>remote-veritysetup.target</filename></term>
 584           <listitem>
 585             <para>Similar to <filename>veritysetup.target</filename>, but for verity
 586             integrity protected devices which are accessed over the network. It is used for
 587             <citerefentry><refentrytitle>veritytab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 588             entries marked with <option>_netdev</option>.</para>
 589           </listitem>
 590         </varlistentry>
 591         <varlistentry>
 592           <term><filename>remote-fs.target</filename></term>
 593           <listitem>
 594             <para>Similar to <filename>local-fs.target</filename>, but
 595             for remote mount points.</para>
 596
 597             <para>systemd automatically adds dependencies of type
 598             <varname>After=</varname> for this target unit to all SysV
 599             init script service units with an LSB header referring to
 600             the <literal>$remote_fs</literal> facility.</para>
 601           </listitem>
 602         </varlistentry>
 603         <varlistentry>
 604           <term><filename>rescue.target</filename></term>
 605           <listitem>
 606             <para>A special target unit that pulls in the base system (including system mounts) and
 607             spawns a rescue shell. Isolate to this target in order to administer the system in
 608             single-user mode with all file systems mounted but with no services running, except for
 609             the most basic. Compare with <filename>emergency.target</filename>, which is much more
 610             reduced and does not provide the file systems or most basic services. Compare with
 611             <filename>multi-user.target</filename>, this target could be seen as
 612             <filename>single-user.target</filename>.</para>
 613
 614             <para><filename>runlevel1.target</filename> is an alias for this target unit, for
 615             compatibility with SysV.</para>
 616
 617             <para>Use the <literal>systemd.unit=rescue.target</literal> kernel command line option
 618             to boot into this mode. A short alias for this kernel command line option is
 619             <literal>1</literal>, for compatibility with SysV.</para>
 620           </listitem>
 621         </varlistentry>
 622         <varlistentry>
 623           <term><filename>runlevel2.target</filename></term>
 624           <term><filename>runlevel3.target</filename></term>
 625           <term><filename>runlevel4.target</filename></term>
 626           <term><filename>runlevel5.target</filename></term>
 627           <listitem>
 628             <para>These are targets that are called whenever the SysV
 629             compatibility code asks for runlevel 2, 3, 4, 5,
 630             respectively. It is a good idea to make this an alias for
 631             (i.e. symlink to) <filename>graphical.target</filename>
 632             (for runlevel 5) or <filename>multi-user.target</filename>
 633             (the others).</para>
 634           </listitem>
 635         </varlistentry>
 636         <varlistentry>
 637           <term><filename>shutdown.target</filename></term>
 638           <listitem>
 639             <para>A special target unit that terminates the services on
 640             system shutdown.</para>
 641
 642             <para>Services that shall be terminated on system shutdown
 643             shall add <varname>Conflicts=</varname> and
 644             <varname>Before=</varname> dependencies to this unit for
 645             their service unit, which is implicitly done when
 646             <varname>DefaultDependencies=yes</varname> is set (the
 647             default).</para>
 648           </listitem>
 649         </varlistentry>
 650         <varlistentry>
 651           <term><filename>sigpwr.target</filename></term>
 652           <listitem>
 653             <para>A special target that is started when systemd receives
 654             the SIGPWR process signal, which is normally sent by the
 655             kernel or UPS daemons when power fails.</para>
 656           </listitem>
 657         </varlistentry>
 658         <varlistentry>
 659           <term><filename>sleep.target</filename></term>
 660           <listitem>
 661             <para>A special target unit that is pulled in by
 662             <filename>suspend.target</filename>,
 663             <filename>hibernate.target</filename> and
 664             <filename>hybrid-sleep.target</filename> and may be used to
 665             hook units into the sleep state logic.</para>
 666           </listitem>
 667         </varlistentry>
 668         <varlistentry>
 669           <term><filename>slices.target</filename></term>
 670           <listitem>
 671             <para>A special target unit that sets up all slice units (see
 672             <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 673             for details) that shall always be active after boot. By default the generic
 674             <filename>system.slice</filename> slice unit as well as the root slice unit
 675             <filename>-.slice</filename> are pulled in and ordered before this unit (see
 676             below).</para>
 677
 678             <para>Adding slice units to <filename>slices.target</filename> is generally not
 679             necessary. Instead, when some unit that uses <varname>Slice=</varname> is started, the
 680             specified slice will be started automatically. Adding
 681             <varname>WantedBy=slices.target</varname> lines to the [Install]
 682             section should only be done for units that need to be always active. In that case care
 683             needs to be taken to avoid creating a loop through the automatic dependencies on
 684             "parent" slices.</para>
 685           </listitem>
 686         </varlistentry>
 687         <varlistentry>
 688           <term><filename>sockets.target</filename></term>
 689           <listitem>
 690             <para>A special target unit that sets up all socket
 691             units (see
 692             <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 693             for details) that shall be active after boot.</para>
 694
 695             <para>Services that can be socket-activated shall add
 696             <varname>Wants=</varname> dependencies to this unit for
 697             their socket unit during installation. This is best
 698             configured via a <varname>WantedBy=sockets.target</varname>
 699             in the socket unit's [Install]
 700             section.</para>
 701           </listitem>
 702         </varlistentry>
 703         <varlistentry>
 704           <term><filename>suspend.target</filename></term>
 705           <listitem>
 706             <para>A special target unit for suspending the system. This
 707             pulls in <filename>sleep.target</filename>.</para>
 708           </listitem>
 709         </varlistentry>
 710         <varlistentry>
 711           <term><filename>swap.target</filename></term>
 712           <listitem>
 713             <para>Similar to <filename>local-fs.target</filename>, but
 714             for swap partitions and swap files.</para>
 715           </listitem>
 716         </varlistentry>
 717         <varlistentry>
 718           <term><filename>sysinit.target</filename></term>
 719           <listitem>
 720             <para>systemd automatically adds dependencies of the types
 721             <varname>Requires=</varname> and <varname>After=</varname>
 722             for this target unit to all services (except for those with
 723             <varname>DefaultDependencies=no</varname>).</para>
 724
 725             <para>This target pulls in the services required for system
 726             initialization. System services pulled in by this target should
 727             declare <varname>DefaultDependencies=no</varname> and specify
 728             all their dependencies manually, including access to anything
 729             more than a read only root filesystem. For details on the
 730             dependencies of this target, refer to
 731             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 732             </para>
 733           </listitem>
 734         </varlistentry>
 735         <varlistentry>
 736           <term><filename>syslog.socket</filename></term>
 737           <listitem>
 738             <para>The socket unit syslog implementations should listen
 739             on. All userspace log messages will be made available on
 740             this socket. For more information about syslog integration,
 741             please consult the <ulink
 742             url="https://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
 743             Interface</ulink> document.</para>
 744           </listitem>
 745         </varlistentry>
 746         <varlistentry>
 747           <term><filename>system-update.target</filename></term>
 748           <term><filename>system-update-pre.target</filename></term>
 749           <term><filename>system-update-cleanup.service</filename></term>
 750           <listitem>
 751             <para>A special target unit that is used for offline system updates.
 752             <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 753             will redirect the boot process to this target if <filename>/system-update</filename>
 754             exists. For more information see
 755             <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 756             </para>
 757
 758             <para>Updates should happen before the <filename>system-update.target</filename> is
 759             reached, and the services which implement them should cause the machine to reboot. The
 760             main units executing the update should order themselves after
 761             <filename>system-update-pre.target</filename> but not pull it in. Services which want to
 762             run during system updates only, but before the actual system update is executed should
 763             order themselves before this unit and pull it in. As a safety measure, if this does not
 764             happen, and <filename>/system-update</filename> still exists after
 765             <filename>system-update.target</filename> is reached,
 766             <filename>system-update-cleanup.service</filename> will remove this symlink and reboot
 767             the machine.</para>
 768           </listitem>
 769         </varlistentry>
 770         <varlistentry>
 771           <term><filename>timers.target</filename></term>
 772           <listitem>
 773             <para>A special target unit that sets up all timer units
 774             (see
 775             <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 776             for details) that shall be active after boot.</para>
 777
 778             <para>It is recommended that timer units installed by
 779             applications get pulled in via <varname>Wants=</varname>
 780             dependencies from this unit. This is best configured via
 781             <varname>WantedBy=timers.target</varname> in the timer
 782             unit's [Install] section.</para>
 783           </listitem>
 784         </varlistentry>
 785         <varlistentry>
 786           <term><filename>umount.target</filename></term>
 787           <listitem>
 788             <para>A special target unit that unmounts all mount and
 789             automount points on system shutdown.</para>
 790
 791             <para>Mounts that shall be unmounted on system shutdown
 792             shall add Conflicts dependencies to this unit for their
 793             mount unit, which is implicitly done when
 794             <varname>DefaultDependencies=yes</varname> is set (the
 795             default).</para>
 796           </listitem>
 797         </varlistentry>
 798
 799       </variablelist>
 800     </refsect2>
 801
 802     <refsect2>
 803       <title>Special System Units for Devices</title>
 804
 805       <para>Some target units are automatically pulled in as devices of
 806       certain kinds show up in the system. These may be used to
 807       automatically activate various services based on the specific type
 808       of the available hardware.</para>
 809
 810       <variablelist>
 811         <varlistentry>
 812           <term><filename>bluetooth.target</filename></term>
 813           <listitem>
 814             <para>This target is started automatically as soon as a
 815             Bluetooth controller is plugged in or becomes available at
 816             boot.</para>
 817
 818             <para>This may be used to pull in Bluetooth management
 819             daemons dynamically when Bluetooth hardware is found.</para>
 820           </listitem>
 821         </varlistentry>
 822         <varlistentry>
 823           <term><filename>printer.target</filename></term>
 824           <listitem>
 825             <para>This target is started automatically as soon as a
 826             printer is plugged in or becomes available at boot.</para>
 827
 828             <para>This may be used to pull in printer management daemons
 829             dynamically when printer hardware is found.</para>
 830           </listitem>
 831         </varlistentry>
 832         <varlistentry>
 833           <term><filename>smartcard.target</filename></term>
 834           <listitem>
 835             <para>This target is started automatically as soon as a
 836             smartcard controller is plugged in or becomes available at
 837             boot.</para>
 838
 839             <para>This may be used to pull in smartcard management
 840             daemons dynamically when smartcard hardware is found.</para>
 841           </listitem>
 842         </varlistentry>
 843         <varlistentry>
 844           <term><filename>sound.target</filename></term>
 845           <listitem>
 846             <para>This target is started automatically as soon as a
 847             sound card is plugged in or becomes available at
 848             boot.</para>
 849
 850             <para>This may be used to pull in audio management daemons
 851             dynamically when audio hardware is found.</para>
 852           </listitem>
 853         </varlistentry>
 854         <varlistentry>
 855           <term><filename>usb-gadget.target</filename></term>
 856           <listitem>
 857             <para>This target is started automatically as soon as a
 858             USB Device Controller becomes available at boot.</para>
 859
 860             <para>This may be used to pull in usb gadget
 861             dynamically when UDC hardware is found.</para>
 862           </listitem>
 863         </varlistentry>
 864       </variablelist>
 865     </refsect2>
 866
 867     <refsect2>
 868       <title>Special Passive System Units </title>
 869
 870       <para>A number of special system targets are defined that can be
 871       used to properly order boot-up of optional services. These targets
 872       are generally not part of the initial boot transaction, unless
 873       they are explicitly pulled in by one of the implementing services.
 874       Note specifically that these <emphasis>passive</emphasis> target
 875       units are generally not pulled in by the consumer of a service,
 876       but by the provider of the service. This means: a consuming
 877       service should order itself after these targets (as appropriate),
 878       but not pull it in. A providing service should order itself before
 879       these targets (as appropriate) and pull it in (via a
 880       <varname>Wants=</varname> type dependency).</para>
 881
 882       <para>Note that these passive units cannot be started manually,
 883       i.e. <literal>systemctl start time-sync.target</literal> will fail
 884       with an error. They can only be pulled in by dependency. This is
 885       enforced since they exist for ordering purposes only and thus are
 886       not useful as only unit within a transaction.</para>
 887
 888       <variablelist>
 889         <varlistentry>
 890           <term><filename>blockdev@.target</filename></term>
 891           <listitem><para>This template unit is used to order mount units and other consumers of block
 892           devices after services that synthesize these block devices. In particular, this is intended to be
 893           used with storage services (such as
 894           <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>/
 895           <citerefentry><refentrytitle>systemd-veritysetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
 896           that allocate and manage a virtual block device. Storage services are ordered before an instance of
 897           <filename>blockdev@.target</filename>, and the consumer units after it. The ordering is
 898           particularly relevant during shutdown, as it ensures that the mount is deactivated first and the
 899           service backing the mount later. The <filename>blockdev@.target</filename> instance should be
 900           pulled in via a <option>Wants=</option> dependency of the storage daemon and thus generally not be
 901           part of any transaction unless a storage daemon is used. The instance name for instances of this
 902           template unit must be a properly escaped block device node path, e.g.
 903           <filename index="false">blockdev@dev-mapper-foobar.target</filename> for the storage device
 904           <filename index="false">/dev/mapper/foobar</filename>.</para></listitem>
 905         </varlistentry>
 906         <varlistentry>
 907           <term><filename>cryptsetup-pre.target</filename></term>
 908           <listitem>
 909             <para>This passive target unit may be pulled in by services
 910             that want to run before any encrypted block device is set
 911             up. All encrypted block devices are set up after this target
 912             has been reached. Since the shutdown order is implicitly the
 913             reverse start-up order between units, this target is
 914             particularly useful to ensure that a service is shut down
 915             only after all encrypted block devices are fully
 916             stopped.</para>
 917           </listitem>
 918         </varlistentry>
 919         <varlistentry>
 920           <term><filename>veritysetup-pre.target</filename></term>
 921           <listitem>
 922             <para>This passive target unit may be pulled in by services
 923             that want to run before any verity integrity protected block
 924             device is set up. All verity integrity protected block
 925             devices are set up after this target has been reached. Since
 926             the shutdown order is implicitly the reverse start-up order
 927             between units, this target is particularly useful to ensure
 928             that a service is shut down only after all verity integrity
 929             protected block devices are fully stopped.</para>
 930           </listitem>
 931         </varlistentry>
 932         <varlistentry>
 933           <term><filename>first-boot-complete.target</filename></term>
 934           <listitem>
 935             <para>This passive target is intended as a synchronization point for units that need to run once
 936             during the first boot.  Only after all units ordered before this target have finished, will the
 937             <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 938             be committed to disk, marking the first boot as completed.  If the boot is aborted at any time
 939             before that, the next boot will re-run any units with <varname>ConditionFirstBoot=yes</varname>.
 940             </para>
 941           </listitem>
 942         </varlistentry>
 943         <varlistentry>
 944           <term><filename>getty-pre.target</filename></term>
 945           <listitem>
 946             <para>A special passive target unit. Users of this target
 947             are expected to pull it in the boot transaction via
 948             a dependency (e.g. <varname>Wants=</varname>). Order your
 949             unit before this unit if you want to make use of the console
 950             just before <filename>getty</filename> is started.
 951             </para>
 952           </listitem>
 953         </varlistentry>
 954         <varlistentry>
 955           <term><filename>local-fs-pre.target</filename></term>
 956           <listitem>
 957             <para>This target unit is
 958             automatically ordered before
 959             all local mount points marked
 960             with <option>auto</option>
 961             (see above). It can be used to
 962             execute certain units before
 963             all local mounts.</para>
 964           </listitem>
 965         </varlistentry>
 966         <varlistentry>
 967           <term><filename>network.target</filename></term>
 968           <listitem>
 969             <para>This unit is supposed to indicate when network functionality is available, but it is only
 970             very weakly defined what that is supposed to mean. However, the following should apply at
 971             minimum:</para>
 972
 973             <itemizedlist>
 974               <listitem><para>At start-up, any configured synthetic network devices (i.e. not physical ones
 975               that require hardware to show up and be probed, but virtual ones like bridge devices and
 976               similar which are created programmatically) that do not depend on any underlying hardware
 977               should be allocated by the time this target is reached. It is not necessary for these
 978               interfaces to also have completed IP level configuration by the time
 979               <filename>network.target</filename> is reached.</para></listitem>
 980
 981               <listitem><para>At shutdown, a unit that is ordered after <filename>network.target</filename>
 982               will be stopped before the network — to whatever level it might be set up by then — is shut
 983               down. It is hence useful when writing service files that require network access on shutdown,
 984               which should order themselves after this target, but not pull it in. Also see <ulink
 985               url="https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget">Running Services After
 986               the Network is up</ulink> for more information.</para></listitem>
 987             </itemizedlist>
 988
 989             <para>It must emphasized that at start-up there's no guarantee that hardware-based devices have
 990             shown up by the time this target is reached, or even acquired complete IP configuration. For that
 991             purpose use <filename>network-online.target</filename> as described above.</para>
 992           </listitem>
 993         </varlistentry>
 994         <varlistentry>
 995           <term><filename>network-pre.target</filename></term>
 996           <listitem>
 997             <para>This passive target unit may be pulled in by services
 998             that want to run before any network is set up, for example
 999             for the purpose of setting up a firewall. All network
1000             management software orders itself after this target, but
1001             does not pull it in.</para>
1002           </listitem>
1003         </varlistentry>
1004         <varlistentry>
1005           <term><filename>nss-lookup.target</filename></term>
1006           <listitem>
1007             <para>A target that should be used as synchronization point for all host/network name
1008             service lookups. Note that this is independent of UNIX user/group name lookups for which
1009             <filename>nss-user-lookup.target</filename> should be used.  All services for which the
1010             availability of full host/network name resolution is essential should be ordered after
1011             this target, but not pull it in. systemd automatically adds dependencies of type
1012             <varname>After=</varname> for this target unit to all SysV init script service units
1013             with an LSB header referring to the <literal>$named</literal> facility.</para>
1014           </listitem>
1015         </varlistentry>
1016         <varlistentry>
1017           <term><filename>nss-user-lookup.target</filename></term>
1018           <listitem>
1019             <para>A target that should be used as synchronization point for all regular UNIX
1020             user/group name service lookups. Note that this is independent of host/network name
1021             lookups for which <filename>nss-lookup.target</filename> should be used. All services
1022             for which the availability of the full user/group database is essential should be
1023             ordered after this target, but not pull it in. All services which provide parts of the
1024             user/group database should be ordered before this target, and pull it in. Note that this
1025             unit is only relevant for regular users and groups — system users and groups are
1026             required to be resolvable during earliest boot already, and hence do not need any
1027             special ordering against this target.</para>
1028           </listitem>
1029         </varlistentry>
1030         <varlistentry>
1031           <term><filename>remote-fs-pre.target</filename></term>
1032           <listitem>
1033             <para>This target unit is automatically ordered before all
1034             mount point units (see above) and cryptsetup/veritysetup devices
1035             marked with the <option>_netdev</option>. It can be used to run
1036             certain units before remote encrypted devices and mounts are established.
1037             Note that this unit is generally not part of the initial
1038             transaction, unless the unit that wants to be ordered before
1039             all remote mounts pulls it in via a
1040             <varname>Wants=</varname> type dependency. If the unit wants
1041             to be pulled in by the first remote mount showing up, it
1042             should use <filename>network-online.target</filename> (see
1043             above).</para>
1044           </listitem>
1045         </varlistentry>
1046         <varlistentry>
1047           <term><filename>rpcbind.target</filename></term>
1048           <listitem>
1049             <para>The portmapper/rpcbind pulls in this target and orders
1050             itself before it, to indicate its availability. systemd
1051             automatically adds dependencies of type
1052             <varname>After=</varname> for this target unit to all SysV
1053             init script service units with an LSB header referring to
1054             the <literal>$portmap</literal> facility.</para>
1055           </listitem>
1056         </varlistentry>
1057         <varlistentry>
1058           <term><filename>time-set.target</filename></term>
1059           <listitem>
1060             <para>Services responsible for setting the system clock (<constant>CLOCK_REALTIME</constant>)
1061             from a local source (such as a maintained timestamp file or imprecise real-time clock) should
1062             pull in this target and order themselves before it. Services where approximate, roughly monotonic
1063             time is desired should be ordered after this unit, but not pull it in.</para>
1064
1065             <para>This target does not provide the accuracy guarantees of
1066             <filename>time-sync.target</filename> (see below), however does not depend on remote clock
1067             sources to be reachable, i.e. the target is typically not delayed by network problems and
1068             similar. Use of this target is recommended for services where approximate clock accuracy and
1069             rough monotonicity is desired but activation shall not be delayed for possibly unreliable network
1070             communication.</para>
1071
1072             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1073             this target unit to all timer units with at least one <varname>OnCalendar=</varname>
1074             directive.</para>
1075
1076             <para>The
1077             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1078             service is a simple daemon that pulls in this target and orders itself before it. Besides
1079             implementing the SNTP network protocol it maintains a timestamp file on disk whose modification
1080             time is regularlary updated. At service start-up the local system clock is set from that modification time,
1081             ensuring it increases roughly monotonically.</para>
1082
1083             <para>Note that ordering a unit after <filename>time-set.target</filename> only has effect if
1084             there's actually a service ordered before it that delays it until the clock is adjusted for rough
1085             monotonicity. Otherwise, this target might get reached before the clock is adjusted to be roughly
1086             monotonic. Enable
1087             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1088             or an alternative NTP implementation to delay the target.</para>
1089           </listitem>
1090         </varlistentry>
1091         <varlistentry>
1092           <term><filename>time-sync.target</filename></term>
1093           <listitem>
1094             <para>Services indicating completed synchronization of the system clock
1095             (<constant>CLOCK_REALTIME</constant>) to a remote source should pull in this target and order
1096             themselves before it. Services where accurate time is essential should be ordered after this
1097             unit, but not pull it in.</para>
1098
1099             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1100             this target unit to all SysV init script service units with an LSB header referring to the
1101             <literal>$time</literal> facility, as well to all timer units with at least one
1102             <varname>OnCalendar=</varname> directive.</para>
1103
1104             <para>This target provides stricter clock accuracy guarantees than
1105             <filename>time-set.target</filename> (see above), but likely requires
1106             network communication and thus introduces unpredictable delays.
1107             Services that require clock accuracy and where network
1108             communication delays are acceptable should use this target. Services that require a less accurate
1109             clock, and only approximate and roughly monotonic clock behaviour should use
1110             <filename>time-set.target</filename> instead.</para>
1111
1112             <para>Note that ordering a unit after <filename>time-sync.target</filename> only has effect if
1113             there's actually a service ordered before it that delays it until clock synchronization is
1114             reached. Otherwise, this target might get reached before the clock is synchronized to any remote
1115             accurate reference clock. When using
1116             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1117             enable
1118             <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1119             to delay the target; or use an equivalent service for other NTP implementations.</para>
1120
1121             <table>
1122               <title>Comparison</title>
1123               <tgroup cols='2' align='left' colsep='1' rowsep='1'>
1124                 <colspec colname="time-set" />
1125                 <colspec colname="time-sync" />
1126                 <thead>
1127                   <row>
1128                     <entry><filename>time-set.target</filename></entry>
1129                     <entry><filename>time-sync.target</filename></entry>
1130                   </row>
1131                 </thead>
1132                 <tbody>
1133                   <row>
1134                     <entry>"quick" to reach</entry>
1135                     <entry>"slow" to reach</entry>
1136                   </row>
1137                   <row>
1138                     <entry>typically uses local clock sources, boot process not affected by availability of external resources</entry>
1139                     <entry>typically uses remote clock sources, inserts dependencies on remote resources into boot process</entry>
1140                   </row>
1141                   <row>
1142                     <entry>reliable, because local</entry>
1143                     <entry>unreliable, because typically network involved</entry>
1144                   </row>
1145                   <row>
1146                     <entry>typically guarantees an approximate and roughly monotonic clock only</entry>
1147                     <entry>typically guarantees an accurate clock</entry>
1148                   </row>
1149                   <row>
1150                     <entry>implemented by <filename>systemd-timesyncd.service</filename></entry>
1151                     <entry>implemented by <filename>systemd-time-wait-sync.service</filename></entry>
1152                   </row>
1153                 </tbody>
1154               </tgroup>
1155             </table>
1156
1157           </listitem>
1158         </varlistentry>
1159       </variablelist>
1160     </refsect2>
1161
1162     <refsect2>
1163       <title>Special Slice Units</title>
1164
1165       <para>There are four <literal>.slice</literal> units which form the basis of the hierarchy for
1166       assignment of resources for services, users, and virtual machines or containers. See
1167       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1168       for details about slice units.</para>
1169
1170       <variablelist>
1171         <varlistentry>
1172           <term><filename>-.slice</filename></term>
1173           <listitem>
1174             <para>The root slice is the root of the slice hierarchy. It usually does not contain
1175             units directly, but may be used to set defaults for the whole tree.</para>
1176           </listitem>
1177         </varlistentry>
1178
1179         <varlistentry>
1180           <term><filename>system.slice</filename></term>
1181           <listitem>
1182             <para>By default, all system services started by
1183             <command>systemd</command> are found in this slice.</para>
1184           </listitem>
1185         </varlistentry>
1186
1187         <varlistentry>
1188           <term><filename>user.slice</filename></term>
1189           <listitem>
1190             <para>By default, all user processes and services started on
1191             behalf of the user, including the per-user systemd instance
1192             are found in this slice.  This is pulled in by
1193             <filename>systemd-logind.service</filename>.</para>
1194           </listitem>
1195         </varlistentry>
1196
1197         <varlistentry>
1198           <term><filename>machine.slice</filename></term>
1199           <listitem>
1200             <para>By default, all virtual machines and containers
1201             registered with <command>systemd-machined</command> are
1202             found in this slice. This is pulled in by
1203             <filename>systemd-machined.service</filename>.</para>
1204           </listitem>
1205         </varlistentry>
1206       </variablelist>
1207     </refsect2>
1208   </refsect1>
1209
1210   <refsect1>
1211     <title>Units managed by the user service manager</title>
1212
1213     <refsect2>
1214       <title>Special User Units</title>
1215
1216       <para>When systemd runs as a user instance, the following special
1217       units are available:</para>
1218
1219       <variablelist>
1220         <varlistentry>
1221           <term><filename>default.target</filename></term>
1222           <listitem>
1223             <para>This is the main target of the user session, started by default. Various services that
1224             compose the normal user session should be pulled into this target. In this regard,
1225             <filename>default.target</filename> is similar to <filename>multi-user.target</filename> in the
1226             system instance, but it is a real unit, not an alias.</para>
1227           </listitem>
1228         </varlistentry>
1229       </variablelist>
1230
1231       <para>In addition, the following units are available which have definitions similar to their
1232       system counterparts:
1233       <filename>exit.target</filename>,
1234       <filename>shutdown.target</filename>,
1235       <filename>sockets.target</filename>,
1236       <filename>timers.target</filename>,
1237       <filename>paths.target</filename>,
1238       <filename>bluetooth.target</filename>,
1239       <filename>printer.target</filename>,
1240       <filename>smartcard.target</filename>,
1241       <filename>sound.target</filename>.</para>
1242     </refsect2>
1243
1244     <refsect2>
1245       <title>Special Passive User Units</title>
1246
1247       <variablelist>
1248         <varlistentry>
1249           <term><filename>graphical-session.target</filename></term>
1250           <listitem>
1251             <para>This target is active whenever any graphical session is running. It is used to
1252             stop user services which only apply to a graphical (X, Wayland, etc.) session when the
1253             session is terminated. Such services should have
1254             <literal>PartOf=graphical-session.target</literal> in their [Unit]
1255             section. A target for a particular session (e. g.
1256             <filename>gnome-session.target</filename>) starts and stops
1257             <literal>graphical-session.target</literal> with
1258             <literal>BindsTo=graphical-session.target</literal>.</para>
1259
1260             <para>Which services are started by a session target is determined by the
1261             <literal>Wants=</literal> and <literal>Requires=</literal> dependencies.  For services
1262             that can be enabled independently, symlinks in <literal>.wants/</literal> and
1263             <literal>.requires/</literal> should be used, see
1264             <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
1265             Those symlinks should either be shipped in packages, or should be added dynamically
1266             after installation, for example using <literal>systemctl add-wants</literal>, see
1267             <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
1268             </para>
1269
1270             <example>
1271               <title>Nautilus as part of a GNOME session</title>
1272
1273               <para><literal>gnome-session.target</literal> pulls in Nautilus as top-level service:</para>
1274
1275               <programlisting>[Unit]
1276 Description=User systemd services for GNOME graphical session
1277 Wants=nautilus.service
1278 BindsTo=graphical-session.target</programlisting>
1279
1280               <para><literal>nautilus.service</literal> gets stopped when the session stops:</para>
1281
1282               <programlisting>[Unit]
1283 Description=Render the desktop icons with Nautilus
1284 PartOf=graphical-session.target
1285
1286 [Service]
1287 …</programlisting>
1288             </example>
1289           </listitem>
1290         </varlistentry>
1291
1292         <varlistentry>
1293           <term><filename>graphical-session-pre.target</filename></term>
1294           <listitem>
1295             <para>This target contains services which set up the environment or global configuration
1296             of a graphical session, such as SSH/GPG agents (which need to export an environment
1297             variable into all desktop processes) or migration of obsolete d-conf keys after an OS
1298             upgrade (which needs to happen before starting any process that might use them). This
1299             target must be started before starting a graphical session like
1300             <filename>gnome-session.target</filename>.</para>
1301           </listitem>
1302         </varlistentry>
1303
1304         <varlistentry>
1305           <term><filename>xdg-desktop-autostart.target</filename></term>
1306           <listitem>
1307             <para>The XDG specification defines a way to autostart applications using XDG desktop files.
1308             systemd ships
1309             <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1310             for the XDG desktop files in autostart directories. Desktop Environments can opt-in to use this
1311             service by adding a <varname>Wants=</varname> dependency on
1312             <filename>xdg-desktop-autostart.target</filename>.</para>
1313           </listitem>
1314         </varlistentry>
1315       </variablelist>
1316     </refsect2>
1317
1318     <refsect2>
1319       <title>Special User Slice Units</title>
1320
1321       <para>There are four <literal>.slice</literal> units which form the basis of the user hierarchy for
1322       assignment of resources for user applications and services. See
1323       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1324       for details about slice units and the documentation about
1325       <ulink url="https://systemd.io/DESKTOP_ENVIRONMENTS">Desktop Environments</ulink>
1326       for further information.</para>
1327
1328       <variablelist>
1329         <varlistentry>
1330           <term><filename>-.slice</filename></term>
1331           <listitem>
1332             <para>The root slice is the root of the user's slice hierarchy.
1333             It usually does not contain units directly, but may be used to set defaults for the whole tree.</para>
1334           </listitem>
1335         </varlistentry>
1336
1337         <varlistentry>
1338           <term><filename>app.slice</filename></term>
1339           <listitem>
1340             <para>By default, all user services and applications managed by
1341             <command>systemd</command> are found in this slice.
1342             All interactively launched applications like web browsers and text editors
1343             as well as non-critical services should be placed into this slice.</para>
1344           </listitem>
1345         </varlistentry>
1346
1347         <varlistentry>
1348           <term><filename>session.slice</filename></term>
1349           <listitem>
1350             <para>All essential services and applications required for the
1351             session should use this slice.
1352             These are services that either cannot be restarted easily
1353             or where latency issues may affect the interactivity of the system and applications.
1354             This includes the display server, screen readers and other services such as DBus or XDG portals.
1355             Such services should be configured to be part of this slice by
1356             adding <varname>Slice=session.slice</varname> to their unit files.</para>
1357           </listitem>
1358         </varlistentry>
1359
1360         <varlistentry>
1361           <term><filename>background.slice</filename></term>
1362           <listitem>
1363             <para>All services running low-priority background tasks should use this slice.
1364             This permits resources to be preferentially assigned to the other slices.
1365             Examples include non-interactive tasks like file indexing or backup operations
1366             where latency is not important.</para>
1367           </listitem>
1368         </varlistentry>
1369       </variablelist>
1370     </refsect2>
1371   </refsect1>
1372
1373   <refsect1>
1374       <title>See Also</title>
1375       <para>
1376         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1377         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1378         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1379         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1380         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1381         <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1382         <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1383         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1384         <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1385       </para>
1386   </refsect1>
1387
1388 </refentry>