man/systemd.special.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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   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>factory-reset.target</filename>,
  37     <filename>final.target</filename>,
  38     <filename>first-boot-complete.target</filename>,
  39     <filename>getty.target</filename>,
  40     <filename>getty-pre.target</filename>,
  41     <filename>graphical.target</filename>,
  42     <filename>halt.target</filename>,
  43     <filename>hibernate.target</filename>,
  44     <filename>hybrid-sleep.target</filename>,
  45     <filename>suspend-then-hibernate.target</filename>,
  46     <filename>initrd.target</filename>,
  47     <filename>initrd-fs.target</filename>,
  48     <filename>initrd-root-device.target</filename>,
  49     <filename>initrd-root-fs.target</filename>,
  50     <filename>initrd-usr-fs.target</filename>,
  51     <filename>integritysetup-pre.target</filename>,
  52     <filename>integritysetup.target</filename>,
  53     <filename>kbrequest.target</filename>,
  54     <filename>kexec.target</filename>,
  55     <filename>local-fs-pre.target</filename>,
  56     <filename>local-fs.target</filename>,
  57     <filename>machines.target</filename>
  58     <filename>multi-user.target</filename>,
  59     <filename>network-online.target</filename>,
  60     <filename>network-pre.target</filename>,
  61     <filename>network.target</filename>,
  62     <filename>nss-lookup.target</filename>,
  63     <filename>nss-user-lookup.target</filename>,
  64     <filename>paths.target</filename>,
  65     <filename>poweroff.target</filename>,
  66     <filename>printer.target</filename>,
  67     <filename>reboot.target</filename>,
  68     <filename>remote-cryptsetup.target</filename>,
  69     <filename>remote-veritysetup.target</filename>,
  70     <filename>remote-fs-pre.target</filename>,
  71     <filename>remote-fs.target</filename>,
  72     <filename>rescue.target</filename>,
  73     <filename>rpcbind.target</filename>,
  74     <filename>runlevel2.target</filename>,
  75     <filename>runlevel3.target</filename>,
  76     <filename>runlevel4.target</filename>,
  77     <filename>runlevel5.target</filename>,
  78     <filename>shutdown.target</filename>,
  79     <filename>sigpwr.target</filename>,
  80     <filename>sleep.target</filename>,
  81     <filename>slices.target</filename>,
  82     <filename>smartcard.target</filename>,
  83     <filename>sockets.target</filename>,
  84     <filename>sound.target</filename>,
  85     <filename>suspend.target</filename>,
  86     <filename>swap.target</filename>,
  87     <filename>sysinit.target</filename>,
  88     <filename>system-update.target</filename>,
  89     <filename>system-update-pre.target</filename>,
  90     <filename>time-set.target</filename>,
  91     <filename>time-sync.target</filename>,
  92     <filename>timers.target</filename>,
  93     <filename>umount.target</filename>,
  94     <filename>usb-gadget.target</filename>,
  95     <!-- slices --><filename>-.slice</filename>,
  96     <filename>system.slice</filename>,
  97     <filename>user.slice</filename>,
  98     <filename>machine.slice</filename>,
  99     <!-- the rest --><filename>-.mount</filename>,
 100     <filename>dbus.service</filename>,
 101     <filename>dbus.socket</filename>,
 102     <filename>display-manager.service</filename>,
 103     <filename>init.scope</filename>,
 104     <filename>syslog.socket</filename>,
 105     <filename>system-update-cleanup.service</filename>
 106   </para></refsynopsisdiv>
 107
 108   <refsect1>
 109     <title>Description</title>
 110
 111     <para>A few units are treated specially by systemd. Many of them have
 112     special internal semantics and cannot be renamed, while others simply
 113     have a standard meaning and should be present on all systems.</para>
 114   </refsect1>
 115
 116   <refsect1>
 117     <title>Units managed by the system service manager</title>
 118
 119     <refsect2>
 120       <title>Special System Units</title>
 121
 122       <variablelist>
 123         <varlistentry>
 124           <term><filename>-.mount</filename></term>
 125           <listitem>
 126             <para>The root mount point, i.e. the mount unit for the <filename>/</filename>
 127             path. This unit is unconditionally active, during the entire time the system is up, as
 128             this mount point is where the basic userspace is running from.</para>
 129           </listitem>
 130         </varlistentry>
 131
 132         <varlistentry>
 133           <term><filename>basic.target</filename></term>
 134           <listitem>
 135             <para>A special target unit covering basic boot-up.</para>
 136
 137             <para>systemd automatically adds dependency of the type
 138             <varname>After=</varname> for this target unit to all
 139             services (except for those with
 140             <varname>DefaultDependencies=no</varname>).</para>
 141
 142             <para>Usually, this should pull-in all local mount points plus
 143             <filename>/var/</filename>, <filename>/tmp/</filename> and
 144             <filename>/var/tmp/</filename>, swap devices, sockets, timers,
 145             path units and other basic initialization necessary for general
 146             purpose daemons. The mentioned mount points are special cased
 147             to allow them to be remote.
 148             </para>
 149
 150             <para>This target usually does not pull in any non-target units
 151             directly, but rather does so indirectly via other early boot targets.
 152             It is instead meant as a synchronization point for late boot
 153             services. Refer to
 154             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 155             for details on the targets involved.
 156             </para>
 157           </listitem>
 158         </varlistentry>
 159         <varlistentry>
 160           <term><filename>boot-complete.target</filename></term>
 161           <listitem>
 162             <para>This target is intended as generic synchronization point for services that shall determine or act on
 163             whether the boot process completed successfully. Order units that are required to succeed for a boot process
 164             to be considered successful before this unit, and add a <varname>Requires=</varname> dependency from the
 165             target unit to them. Order units that shall only run when the boot process is considered successful after the
 166             target unit and pull in the target from it, also with <varname>Requires=</varname>. Note that by default this
 167             target unit is not part of the initial boot transaction, but is supposed to be pulled in only if required by
 168             units that want to run only on successful boots.</para>
 169
 170             <para>See
 171             <citerefentry><refentrytitle>systemd-boot-check-no-failures.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 172             for a service that implements a generic system health check and orders itself before
 173             <filename>boot-complete.target</filename>.</para>
 174
 175             <para>See
 176             <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 177             for a service that propagates boot success information to the boot loader, and orders itself after
 178             <filename>boot-complete.target</filename>.</para>
 179           </listitem>
 180         </varlistentry>
 181         <varlistentry>
 182           <term><filename>ctrl-alt-del.target</filename></term>
 183           <listitem>
 184             <para>systemd starts this target whenever Control+Alt+Del is
 185             pressed on the console. Usually, this should be aliased
 186             (symlinked) to <filename>reboot.target</filename>.</para>
 187           </listitem>
 188         </varlistentry>
 189         <varlistentry>
 190           <term><filename>cryptsetup.target</filename></term>
 191           <listitem>
 192             <para>A target that pulls in setup services for all
 193             encrypted block devices.</para>
 194           </listitem>
 195         </varlistentry>
 196         <varlistentry>
 197           <term><filename>veritysetup.target</filename></term>
 198           <listitem>
 199             <para>A target that pulls in setup services for all
 200             verity integrity protected block devices.</para>
 201           </listitem>
 202         </varlistentry>
 203         <varlistentry>
 204           <term><filename>dbus.service</filename></term>
 205           <listitem>
 206             <para>A special unit for the D-Bus bus daemon. As soon as
 207             this service is fully started up systemd will connect to it
 208             and register its service.</para>
 209           </listitem>
 210         </varlistentry>
 211         <varlistentry>
 212           <term><filename>dbus.socket</filename></term>
 213           <listitem>
 214             <para>A special unit for the D-Bus system bus socket. All
 215             units with <varname>Type=dbus</varname> automatically gain a
 216             dependency on this unit.</para>
 217           </listitem>
 218         </varlistentry>
 219         <varlistentry>
 220           <term><filename>default.target</filename></term>
 221           <listitem>
 222             <para>The default unit systemd starts at bootup. Usually, this should be aliased (symlinked) to
 223             <filename>multi-user.target</filename> or <filename>graphical.target</filename>. See
 224             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 225             more discussion.</para>
 226
 227             <para>The default unit systemd starts at bootup can be overridden with the
 228             <varname>systemd.unit=</varname> kernel command line option, or more conveniently, with the short
 229             names like <varname>single</varname>, <varname>rescue</varname>, <varname>1</varname>,
 230             <varname>3</varname>, <varname>5</varname>, …; see
 231             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
 232           </listitem>
 233         </varlistentry>
 234         <varlistentry>
 235           <term><filename>display-manager.service</filename></term>
 236           <listitem>
 237             <para>The display manager service. Usually, this should be
 238             aliased (symlinked) to <filename>gdm.service</filename> or a
 239             similar display manager service.</para>
 240           </listitem>
 241         </varlistentry>
 242         <varlistentry>
 243           <term><filename>emergency.target</filename></term>
 244           <listitem>
 245             <para>A special target unit that starts an emergency shell on the main console. This
 246             target does not pull in other services or mounts. It is the most minimal version of
 247             starting the system in order to acquire an interactive shell; the only processes running
 248             are usually just the system manager (PID 1) and the shell process. This unit may be used
 249             by specifying <varname>emergency</varname> on the kernel command line; it is
 250             also used when a file system check on a required file system fails and boot-up cannot
 251             continue. Compare with <filename>rescue.target</filename>, which serves a similar
 252             purpose, but also starts the most basic services and mounts all file systems.</para>
 253
 254             <para>In many ways booting into <filename>emergency.target</filename> is similar to the
 255             effect of booting with <literal>init=/bin/sh</literal> on the kernel command line,
 256             except that emergency mode provides you with the full system and service manager, and
 257             allows starting individual units in order to continue the boot process in steps.</para>
 258
 259             <para>Note that depending on how <filename>emergency.target</filename> is reached, the root file
 260             system might be mounted read-only or read-write (no remounting is done specially for this
 261             target). For example, the system may boot with root mounted read-only when <varname>ro</varname>
 262             is used on the kernel command line and remain this way for <filename>emergency.target</filename>,
 263             or the system may transition to <filename>emergency.target</filename> after the system has been
 264             partially booted and disks have already been remounted read-write.</para>
 265           </listitem>
 266         </varlistentry>
 267         <varlistentry>
 268           <term><filename>exit.target</filename></term>
 269           <listitem>
 270             <para>A special service unit for shutting down the system or
 271             user service manager. It is equivalent to
 272             <filename>poweroff.target</filename> on non-container
 273             systems, and also works in containers.</para>
 274
 275             <para>systemd will start this unit when it receives the
 276             <constant>SIGTERM</constant> or <constant>SIGINT</constant>
 277             signal when running as user service daemon.</para>
 278
 279             <para>Normally, this (indirectly) pulls in
 280             <filename>shutdown.target</filename>, which in turn should be
 281             conflicted by all units that want to be scheduled for
 282             shutdown when the service manager starts to exit.</para>
 283           </listitem>
 284         </varlistentry>
 285         <varlistentry>
 286           <term><filename>factory-reset.target</filename></term>
 287           <listitem>
 288             <para>A special target to trigger a factory reset.</para>
 289           </listitem>
 290         </varlistentry>
 291         <varlistentry>
 292           <term><filename>final.target</filename></term>
 293           <listitem>
 294             <para>A special target unit that is used during the shutdown
 295             logic and may be used to pull in late services after all
 296             normal services are already terminated and all mounts
 297             unmounted.
 298             </para>
 299           </listitem>
 300         </varlistentry>
 301         <varlistentry>
 302           <term><filename>getty.target</filename></term>
 303           <listitem>
 304             <para>A special target unit that pulls in statically
 305             configured local TTY <filename>getty</filename> instances.
 306             </para>
 307           </listitem>
 308         </varlistentry>
 309         <varlistentry>
 310           <term><filename>graphical.target</filename></term>
 311           <listitem>
 312             <para>A special target unit for setting up a graphical login
 313             screen. This pulls in
 314             <filename>multi-user.target</filename>.</para>
 315
 316             <para>Units that are needed for graphical logins shall add
 317             <varname>Wants=</varname> dependencies for their unit to
 318             this unit (or <filename>multi-user.target</filename>) during
 319             installation. This is best configured via
 320             <varname>WantedBy=graphical.target</varname> in the unit's
 321             [Install] section.</para>
 322           </listitem>
 323         </varlistentry>
 324         <varlistentry>
 325           <term><filename>hibernate.target</filename></term>
 326           <listitem>
 327             <para>A special target unit for hibernating the system. This
 328             pulls in <filename>sleep.target</filename>.</para>
 329           </listitem>
 330         </varlistentry>
 331         <varlistentry>
 332           <term><filename>hybrid-sleep.target</filename></term>
 333           <listitem>
 334             <para>A special target unit for hibernating and suspending
 335             the system at the same time. This pulls in
 336             <filename>sleep.target</filename>.</para>
 337           </listitem>
 338         </varlistentry>
 339         <varlistentry>
 340           <term><filename>suspend-then-hibernate.target</filename></term>
 341           <listitem>
 342             <para>A special target unit for suspending the system for a period
 343             of time, waking it and putting it into hibernate. This pulls in
 344             <filename>sleep.target</filename>.</para>
 345           </listitem>
 346         </varlistentry>
 347
 348         <varlistentry>
 349           <term><filename>halt.target</filename></term>
 350           <listitem>
 351             <para>A special target unit for shutting down and halting
 352             the system. Note that this target is distinct from
 353             <filename>poweroff.target</filename> in that it generally
 354             really just halts the system rather than powering it
 355             down.</para>
 356
 357             <para>Applications wanting to halt the system should not start this unit
 358             directly, but should instead execute <command>systemctl halt</command>
 359             (possibly with the <option>--no-block</option> option) or call
 360             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 361             <command>org.freedesktop.systemd1.Manager.Halt</command> D-Bus method
 362             directly.</para>
 363           </listitem>
 364         </varlistentry>
 365         <varlistentry>
 366           <term><filename>init.scope</filename></term>
 367           <listitem>
 368             <para>This scope unit is where the system and service manager (PID 1) itself resides. It
 369             is active as long as the system is running.</para>
 370           </listitem>
 371         </varlistentry>
 372         <varlistentry>
 373           <term><filename>initrd.target</filename></term>
 374           <listitem>
 375             <para>This is the default target in the initramfs, similar to <filename>default.target</filename>
 376             in the main system. It is used to mount the real root and transition to it. See
 377             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
 378             more discussion.</para>
 379           </listitem>
 380         </varlistentry>
 381         <varlistentry>
 382           <term><filename>initrd-fs.target</filename></term>
 383           <listitem>
 384             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 385             automatically adds dependencies of type <varname>Before=</varname> to
 386             <filename>sysroot-usr.mount</filename> and all mount points found in
 387             <filename>/etc/fstab</filename> that have the <option>x-initrd.mount</option> mount option set
 388             and do not have the <option>noauto</option> mount option set. It is also indirectly ordered after
 389             <filename>sysroot.mount</filename>. Thus, once this target is reached the
 390             <filename>/sysroot/</filename> hierarchy is fully set up, in preparation for the transition to
 391             the host OS.</para>
 392           </listitem>
 393         </varlistentry>
 394         <varlistentry>
 395           <term><filename>initrd-root-device.target</filename></term>
 396           <listitem>
 397             <para>A special initrd target unit that is reached when the root filesystem device is available, but before
 398             it has been mounted.
 399             <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 400             and
 401             <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 402             automatically setup the appropriate dependencies to make this happen.
 403             </para>
 404           </listitem>
 405         </varlistentry>
 406         <varlistentry>
 407           <term><filename>initrd-root-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>sysroot.mount</filename> unit, which is generated from the kernel command line's
 412             <varname>root=</varname> setting (or equivalent).</para>
 413           </listitem>
 414         </varlistentry>
 415         <varlistentry>
 416           <term><filename>initrd-usr-fs.target</filename></term>
 417           <listitem>
 418             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 419             automatically adds dependencies of type <varname>Before=</varname> to the
 420             <filename>sysusr-usr.mount</filename> unit, which is generated from the kernel command line's
 421             <varname>usr=</varname> switch. Services may order themselves after this target unit in order to
 422             run once the <filename>/sysusr/</filename> hierarchy becomes available, on systems that come up
 423             initially without a root file system, but with an initialized <filename>/usr/</filename> and need
 424             to access that before setting up the root file system to ultimately switch to. On systems where
 425             <varname>usr=</varname> is not used this target is ordered after
 426             <filename>sysroot.mount</filename> and thus mostly equivalent to
 427             <filename>initrd-root-fs.target</filename>. In effect on any system once this target is reached
 428             the file system backing <filename>/usr/</filename> is mounted, though possibly at two different
 429             locations, either below the <filename>/sysusr/</filename> or the <filename>/sysroot/</filename>
 430             hierarchies.</para>
 431           </listitem>
 432         </varlistentry>
 433         <varlistentry>
 434           <term><filename>kbrequest.target</filename></term>
 435           <listitem>
 436             <para>systemd starts this target whenever Alt+ArrowUp is
 437             pressed on the console. Note that any user with physical access
 438             to the machine will be able to do this, without authentication,
 439             so this should be used carefully.</para>
 440           </listitem>
 441         </varlistentry>
 442         <varlistentry>
 443           <term><filename>kexec.target</filename></term>
 444           <listitem>
 445             <para>A special target unit for shutting down and rebooting
 446             the system via kexec.</para>
 447
 448             <para>Applications wanting to reboot the system should not start this unit
 449             directly, but should instead execute <command>systemctl kexec</command>
 450             (possibly with the <option>--no-block</option> option) or call
 451             <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 452             <command>org.freedesktop.systemd1.Manager.KExec</command> D-Bus method
 453             directly.</para>
 454           </listitem>
 455         </varlistentry>
 456         <varlistentry>
 457           <term><filename>local-fs.target</filename></term>
 458           <listitem>
 459             <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 460             automatically adds dependencies of type
 461             <varname>Before=</varname> to all mount units that refer to
 462             local mount points for this target unit. In addition, it
 463             adds dependencies of type <varname>Wants=</varname> to this
 464             target unit for those mounts listed in
 465             <filename>/etc/fstab</filename> that have the
 466             <option>auto</option> mount option set.</para>
 467           </listitem>
 468         </varlistentry>
 469         <varlistentry>
 470           <term><filename>machines.target</filename></term>
 471           <listitem>
 472             <para>A standard target unit for starting all the containers
 473             and other virtual machines. See <filename>systemd-nspawn@.service</filename>
 474             for an example.</para>
 475           </listitem>
 476         </varlistentry>
 477         <varlistentry>
 478           <term><filename>multi-user.target</filename></term>
 479           <listitem>
 480             <para>A special target unit for setting up a multi-user
 481             system (non-graphical). This is pulled in by
 482             <filename>graphical.target</filename>.</para>
 483
 484             <para>Units that are needed for a multi-user system shall
 485             add <varname>Wants=</varname> dependencies for their unit to
 486             this unit during installation. This is best configured via
 487             <varname>WantedBy=multi-user.target</varname> in the unit's
 488             [Install] section.</para>
 489           </listitem>
 490         </varlistentry>
 491         <varlistentry>
 492           <term><filename>network-online.target</filename></term>
 493           <listitem>
 494             <para>Units that strictly require a configured network
 495             connection should pull in
 496             <filename>network-online.target</filename> (via a
 497             <varname>Wants=</varname> type dependency) and order
 498             themselves after it. This target unit is intended to pull in
 499             a service that delays further execution until the network is
 500             sufficiently set up. What precisely this requires is left to
 501             the implementation of the network managing service.</para>
 502
 503             <para>Note the distinction between this unit and <filename>network.target</filename>. This unit
 504             is an active unit (i.e. pulled in by the consumer rather than the provider of this functionality)
 505             and pulls in a service which possibly adds substantial delays to further execution. In contrast,
 506             <filename>network.target</filename> is a passive unit (i.e. pulled in by the provider of the
 507             functionality, rather than the consumer) that usually does not delay execution much. Usually,
 508             <filename>network.target</filename> is part of the boot of most systems, while
 509             <filename>network-online.target</filename> is not, except when at least one unit requires
 510             it. Also see <ulink url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is
 511             Up</ulink> for more information.</para>
 512
 513             <para>All mount units for remote network file systems automatically pull in this unit, and order
 514             themselves after it. Note that networking daemons that simply <emphasis>provide</emphasis>
 515             functionality to other hosts (as opposed to <emphasis>consume</emphasis> functionality of other
 516             hosts) generally do not need to pull this in.</para>
 517
 518             <para>systemd automatically adds dependencies of type <varname>Wants=</varname> and
 519             <varname>After=</varname> for this target unit to all SysV init script service units
 520             with an LSB header referring to the <literal>$network</literal> facility.</para>
 521
 522             <para>Note that this unit is only useful during the original system start-up
 523             logic. After the system has completed booting up, it will not track the online state of
 524             the system anymore. Due to this it cannot be used as a network connection monitor
 525             concept, it is purely a one-time system start-up concept.</para>
 526             </listitem>
 527         </varlistentry>
 528         <varlistentry>
 529           <term><filename>paths.target</filename></term>
 530           <listitem>
 531             <para>A special target unit that sets up all path units (see
 532             <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 533             for details) that shall be active after boot.</para>
 534
 535             <para>It is recommended that path units installed by
 536             applications get pulled in via <varname>Wants=</varname>
 537             dependencies from this unit. This is best configured via a
 538             <varname>WantedBy=paths.target</varname> in the path unit's
 539             [Install] section.</para>
 540           </listitem>
 541         </varlistentry>
 542         <varlistentry>
 543           <term><filename>poweroff.target</filename></term>
 544           <listitem>
 545             <para>A special target unit for shutting down and powering
 546             off the system.</para>
 547
 548             <para>Applications wanting to power off the system should not start this unit
 549             directly, but should instead execute <command>systemctl poweroff</command>
 550             (possibly with the <option>--no-block</option> option) or call
 551             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 552             <command>org.freedesktop.login1.Manager.PowerOff</command> D-Bus method
 553             directly.</para>
 554
 555             <para><filename>runlevel0.target</filename> is an alias for
 556             this target unit, for compatibility with SysV.</para>
 557           </listitem>
 558         </varlistentry>
 559         <varlistentry>
 560           <term><filename>reboot.target</filename></term>
 561           <listitem>
 562             <para>A special target unit for shutting down and rebooting
 563             the system.</para>
 564
 565             <para>Applications wanting to reboot the system should not start this unit
 566             directly, but should instead execute <command>systemctl reboot</command>
 567             (possibly with the <option>--no-block</option> option) or call
 568             <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>'s
 569             <command>org.freedesktop.login1.Manager.Reboot</command> D-Bus method
 570             directly.</para>
 571
 572             <para><filename>runlevel6.target</filename> is an alias for
 573             this target unit, for compatibility with SysV.</para>
 574           </listitem>
 575         </varlistentry>
 576         <varlistentry>
 577           <term><filename>remote-cryptsetup.target</filename></term>
 578           <listitem>
 579             <para>Similar to <filename>cryptsetup.target</filename>, but for encrypted
 580             devices which are accessed over the network. It is used for
 581             <citerefentry><refentrytitle>crypttab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 582             entries marked with <option>_netdev</option>.</para>
 583           </listitem>
 584         </varlistentry>
 585         <varlistentry>
 586           <term><filename>remote-veritysetup.target</filename></term>
 587           <listitem>
 588             <para>Similar to <filename>veritysetup.target</filename>, but for verity
 589             integrity protected devices which are accessed over the network. It is used for
 590             <citerefentry><refentrytitle>veritytab</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 591             entries marked with <option>_netdev</option>.</para>
 592           </listitem>
 593         </varlistentry>
 594         <varlistentry>
 595           <term><filename>remote-fs.target</filename></term>
 596           <listitem>
 597             <para>Similar to <filename>local-fs.target</filename>, but
 598             for remote mount points.</para>
 599
 600             <para>systemd automatically adds dependencies of type
 601             <varname>After=</varname> for this target unit to all SysV
 602             init script service units with an LSB header referring to
 603             the <literal>$remote_fs</literal> facility.</para>
 604           </listitem>
 605         </varlistentry>
 606         <varlistentry>
 607           <term><filename>rescue.target</filename></term>
 608           <listitem>
 609             <para>A special target unit that pulls in the base system (including system mounts) and
 610             spawns a rescue shell. Isolate to this target in order to administer the system in
 611             single-user mode with all file systems mounted but with no services running, except for
 612             the most basic. Compare with <filename>emergency.target</filename>, which is much more
 613             reduced and does not provide the file systems or most basic services. Compare with
 614             <filename>multi-user.target</filename>, this target could be seen as
 615             <filename>single-user.target</filename>.</para>
 616
 617             <para><filename>runlevel1.target</filename> is an alias for this target unit, for
 618             compatibility with SysV.</para>
 619
 620             <para>Use the <literal>systemd.unit=rescue.target</literal> kernel command line option
 621             to boot into this mode. A short alias for this kernel command line option is
 622             <literal>1</literal>, for compatibility with SysV.</para>
 623           </listitem>
 624         </varlistentry>
 625         <varlistentry>
 626           <term><filename>runlevel2.target</filename></term>
 627           <term><filename>runlevel3.target</filename></term>
 628           <term><filename>runlevel4.target</filename></term>
 629           <term><filename>runlevel5.target</filename></term>
 630           <listitem>
 631             <para>These are targets that are called whenever the SysV
 632             compatibility code asks for runlevel 2, 3, 4, 5,
 633             respectively. It is a good idea to make this an alias for
 634             (i.e. symlink to) <filename>graphical.target</filename>
 635             (for runlevel 5) or <filename>multi-user.target</filename>
 636             (the others).</para>
 637           </listitem>
 638         </varlistentry>
 639         <varlistentry>
 640           <term><filename>shutdown.target</filename></term>
 641           <listitem>
 642             <para>A special target unit that terminates the services on
 643             system shutdown.</para>
 644
 645             <para>Services that shall be terminated on system shutdown
 646             shall add <varname>Conflicts=</varname> and
 647             <varname>Before=</varname> dependencies to this unit for
 648             their service unit, which is implicitly done when
 649             <varname>DefaultDependencies=yes</varname> is set (the
 650             default).</para>
 651           </listitem>
 652         </varlistentry>
 653         <varlistentry>
 654           <term><filename>sigpwr.target</filename></term>
 655           <listitem>
 656             <para>A special target that is started when systemd receives
 657             the SIGPWR process signal, which is normally sent by the
 658             kernel or UPS daemons when power fails.</para>
 659           </listitem>
 660         </varlistentry>
 661         <varlistentry>
 662           <term><filename>sleep.target</filename></term>
 663           <listitem>
 664             <para>A special target unit that is pulled in by
 665             <filename>suspend.target</filename>,
 666             <filename>hibernate.target</filename> and
 667             <filename>hybrid-sleep.target</filename> and may be used to
 668             hook units into the sleep state logic.</para>
 669           </listitem>
 670         </varlistentry>
 671         <varlistentry>
 672           <term><filename>slices.target</filename></term>
 673           <listitem>
 674             <para>A special target unit that sets up all slice units (see
 675             <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 676             for details) that shall always be active after boot. By default the generic
 677             <filename>system.slice</filename> slice unit as well as the root slice unit
 678             <filename>-.slice</filename> are pulled in and ordered before this unit (see
 679             below).</para>
 680
 681             <para>Adding slice units to <filename>slices.target</filename> is generally not
 682             necessary. Instead, when some unit that uses <varname>Slice=</varname> is started, the
 683             specified slice will be started automatically. Adding
 684             <varname>WantedBy=slices.target</varname> lines to the [Install]
 685             section should only be done for units that need to be always active. In that case care
 686             needs to be taken to avoid creating a loop through the automatic dependencies on
 687             "parent" slices.</para>
 688           </listitem>
 689         </varlistentry>
 690         <varlistentry>
 691           <term><filename>sockets.target</filename></term>
 692           <listitem>
 693             <para>A special target unit that sets up all socket
 694             units (see
 695             <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 696             for details) that shall be active after boot.</para>
 697
 698             <para>Services that can be socket-activated shall add
 699             <varname>Wants=</varname> dependencies to this unit for
 700             their socket unit during installation. This is best
 701             configured via a <varname>WantedBy=sockets.target</varname>
 702             in the socket unit's [Install]
 703             section.</para>
 704           </listitem>
 705         </varlistentry>
 706         <varlistentry>
 707           <term><filename>suspend.target</filename></term>
 708           <listitem>
 709             <para>A special target unit for suspending the system. This
 710             pulls in <filename>sleep.target</filename>.</para>
 711           </listitem>
 712         </varlistentry>
 713         <varlistentry>
 714           <term><filename>swap.target</filename></term>
 715           <listitem>
 716             <para>Similar to <filename>local-fs.target</filename>, but
 717             for swap partitions and swap files.</para>
 718           </listitem>
 719         </varlistentry>
 720         <varlistentry>
 721           <term><filename>sysinit.target</filename></term>
 722           <listitem>
 723             <para>systemd automatically adds dependencies of the types
 724             <varname>Requires=</varname> and <varname>After=</varname>
 725             for this target unit to all services (except for those with
 726             <varname>DefaultDependencies=no</varname>).</para>
 727
 728             <para>This target pulls in the services required for system
 729             initialization. System services pulled in by this target should
 730             declare <varname>DefaultDependencies=no</varname> and specify
 731             all their dependencies manually, including access to anything
 732             more than a read only root filesystem. For details on the
 733             dependencies of this target, refer to
 734             <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 735             </para>
 736           </listitem>
 737         </varlistentry>
 738         <varlistentry>
 739           <term><filename>syslog.socket</filename></term>
 740           <listitem>
 741             <para>The socket unit syslog implementations should listen
 742             on. All userspace log messages will be made available on
 743             this socket. For more information about syslog integration,
 744             please consult the <ulink
 745             url="https://www.freedesktop.org/wiki/Software/systemd/syslog">Syslog
 746             Interface</ulink> document.</para>
 747           </listitem>
 748         </varlistentry>
 749         <varlistentry>
 750           <term><filename>system-update.target</filename></term>
 751           <term><filename>system-update-pre.target</filename></term>
 752           <term><filename>system-update-cleanup.service</filename></term>
 753           <listitem>
 754             <para>A special target unit that is used for offline system updates.
 755             <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 756             will redirect the boot process to this target if <filename>/system-update</filename>
 757             exists. For more information see
 758             <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 759             </para>
 760
 761             <para>Updates should happen before the <filename>system-update.target</filename> is
 762             reached, and the services which implement them should cause the machine to reboot. The
 763             main units executing the update should order themselves after
 764             <filename>system-update-pre.target</filename> but not pull it in. Services which want to
 765             run during system updates only, but before the actual system update is executed should
 766             order themselves before this unit and pull it in. As a safety measure, if this does not
 767             happen, and <filename>/system-update</filename> still exists after
 768             <filename>system-update.target</filename> is reached,
 769             <filename>system-update-cleanup.service</filename> will remove this symlink and reboot
 770             the machine.</para>
 771           </listitem>
 772         </varlistentry>
 773         <varlistentry>
 774           <term><filename>timers.target</filename></term>
 775           <listitem>
 776             <para>A special target unit that sets up all timer units
 777             (see
 778             <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 779             for details) that shall be active after boot.</para>
 780
 781             <para>It is recommended that timer units installed by
 782             applications get pulled in via <varname>Wants=</varname>
 783             dependencies from this unit. This is best configured via
 784             <varname>WantedBy=timers.target</varname> in the timer
 785             unit's [Install] section.</para>
 786           </listitem>
 787         </varlistentry>
 788         <varlistentry>
 789           <term><filename>umount.target</filename></term>
 790           <listitem>
 791             <para>A special target unit that unmounts all mount and
 792             automount points on system shutdown.</para>
 793
 794             <para>Mounts that shall be unmounted on system shutdown
 795             shall add Conflicts dependencies to this unit for their
 796             mount unit, which is implicitly done when
 797             <varname>DefaultDependencies=yes</varname> is set (the
 798             default).</para>
 799           </listitem>
 800         </varlistentry>
 801
 802       </variablelist>
 803     </refsect2>
 804
 805     <refsect2>
 806       <title>Special System Units for Devices</title>
 807
 808       <para>Some target units are automatically pulled in as devices of
 809       certain kinds show up in the system. These may be used to
 810       automatically activate various services based on the specific type
 811       of the available hardware.</para>
 812
 813       <variablelist>
 814         <varlistentry>
 815           <term><filename>bluetooth.target</filename></term>
 816           <listitem>
 817             <para>This target is started automatically as soon as a
 818             Bluetooth controller is plugged in or becomes available at
 819             boot.</para>
 820
 821             <para>This may be used to pull in Bluetooth management
 822             daemons dynamically when Bluetooth hardware is found.</para>
 823           </listitem>
 824         </varlistentry>
 825         <varlistentry>
 826           <term><filename>printer.target</filename></term>
 827           <listitem>
 828             <para>This target is started automatically as soon as a
 829             printer is plugged in or becomes available at boot.</para>
 830
 831             <para>This may be used to pull in printer management daemons
 832             dynamically when printer hardware is found.</para>
 833           </listitem>
 834         </varlistentry>
 835         <varlistentry>
 836           <term><filename>smartcard.target</filename></term>
 837           <listitem>
 838             <para>This target is started automatically as soon as a
 839             smartcard controller is plugged in or becomes available at
 840             boot.</para>
 841
 842             <para>This may be used to pull in smartcard management
 843             daemons dynamically when smartcard hardware is found.</para>
 844           </listitem>
 845         </varlistentry>
 846         <varlistentry>
 847           <term><filename>sound.target</filename></term>
 848           <listitem>
 849             <para>This target is started automatically as soon as a
 850             sound card is plugged in or becomes available at
 851             boot.</para>
 852
 853             <para>This may be used to pull in audio management daemons
 854             dynamically when audio hardware is found.</para>
 855           </listitem>
 856         </varlistentry>
 857         <varlistentry>
 858           <term><filename>usb-gadget.target</filename></term>
 859           <listitem>
 860             <para>This target is started automatically as soon as a
 861             USB Device Controller becomes available at boot.</para>
 862
 863             <para>This may be used to pull in usb gadget
 864             dynamically when UDC hardware is found.</para>
 865           </listitem>
 866         </varlistentry>
 867       </variablelist>
 868     </refsect2>
 869
 870     <refsect2>
 871       <title>Special Passive System Units </title>
 872
 873       <para>A number of special system targets are defined that can be
 874       used to properly order boot-up of optional services. These targets
 875       are generally not part of the initial boot transaction, unless
 876       they are explicitly pulled in by one of the implementing services.
 877       Note specifically that these <emphasis>passive</emphasis> target
 878       units are generally not pulled in by the consumer of a service,
 879       but by the provider of the service. This means: a consuming
 880       service should order itself after these targets (as appropriate),
 881       but not pull it in. A providing service should order itself before
 882       these targets (as appropriate) and pull it in (via a
 883       <varname>Wants=</varname> type dependency).</para>
 884
 885       <para>Note that these passive units cannot be started manually,
 886       i.e. <literal>systemctl start time-sync.target</literal> will fail
 887       with an error. They can only be pulled in by dependency. This is
 888       enforced since they exist for ordering purposes only and thus are
 889       not useful as only unit within a transaction.</para>
 890
 891       <variablelist>
 892         <varlistentry>
 893           <term><filename>blockdev@.target</filename></term>
 894           <listitem><para>This template unit is used to order mount units and other consumers of block
 895           devices after services that synthesize these block devices. In particular, this is intended to be
 896           used with storage services (such as
 897           <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>/
 898           <citerefentry><refentrytitle>systemd-veritysetup@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>)
 899           that allocate and manage a virtual block device. Storage services are ordered before an instance of
 900           <filename>blockdev@.target</filename>, and the consumer units after it. The ordering is
 901           particularly relevant during shutdown, as it ensures that the mount is deactivated first and the
 902           service backing the mount later. The <filename>blockdev@.target</filename> instance should be
 903           pulled in via a <option>Wants=</option> dependency of the storage daemon and thus generally not be
 904           part of any transaction unless a storage daemon is used. The instance name for instances of this
 905           template unit must be a properly escaped block device node path, e.g.
 906           <filename index="false">blockdev@dev-mapper-foobar.target</filename> for the storage device
 907           <filename index="false">/dev/mapper/foobar</filename>.</para></listitem>
 908         </varlistentry>
 909         <varlistentry>
 910           <term><filename>cryptsetup-pre.target</filename></term>
 911           <listitem>
 912             <para>This passive target unit may be pulled in by services
 913             that want to run before any encrypted block device is set
 914             up. All encrypted block devices are set up after this target
 915             has been reached. Since the shutdown order is implicitly the
 916             reverse start-up order between units, this target is
 917             particularly useful to ensure that a service is shut down
 918             only after all encrypted block devices are fully
 919             stopped.</para>
 920           </listitem>
 921         </varlistentry>
 922         <varlistentry>
 923           <term><filename>veritysetup-pre.target</filename></term>
 924           <listitem>
 925             <para>This passive target unit may be pulled in by services
 926             that want to run before any verity integrity protected block
 927             device is set up. All verity integrity protected block
 928             devices are set up after this target has been reached. Since
 929             the shutdown order is implicitly the reverse start-up order
 930             between units, this target is particularly useful to ensure
 931             that a service is shut down only after all verity integrity
 932             protected block devices are fully stopped.</para>
 933           </listitem>
 934         </varlistentry>
 935         <varlistentry>
 936           <term><filename>first-boot-complete.target</filename></term>
 937           <listitem>
 938             <para>This passive target is intended as a synchronization point for units that need to run once
 939             during the first boot.  Only after all units ordered before this target have finished, will the
 940             <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 941             be committed to disk, marking the first boot as completed.  If the boot is aborted at any time
 942             before that, the next boot will re-run any units with <varname>ConditionFirstBoot=yes</varname>.
 943             </para>
 944           </listitem>
 945         </varlistentry>
 946         <varlistentry>
 947           <term><filename>getty-pre.target</filename></term>
 948           <listitem>
 949             <para>A special passive target unit. Users of this target
 950             are expected to pull it in the boot transaction via
 951             a dependency (e.g. <varname>Wants=</varname>). Order your
 952             unit before this unit if you want to make use of the console
 953             just before <filename>getty</filename> is started.
 954             </para>
 955           </listitem>
 956         </varlistentry>
 957         <varlistentry>
 958           <term><filename>local-fs-pre.target</filename></term>
 959           <listitem>
 960             <para>This target unit is
 961             automatically ordered before
 962             all local mount points marked
 963             with <option>auto</option>
 964             (see above). It can be used to
 965             execute certain units before
 966             all local mounts.</para>
 967           </listitem>
 968         </varlistentry>
 969         <varlistentry>
 970           <term><filename>network.target</filename></term>
 971           <listitem>
 972             <para>This unit is supposed to indicate when network functionality is available, but it is only
 973             very weakly defined what that is supposed to mean. However, the following should apply at
 974             minimum:</para>
 975
 976             <itemizedlist>
 977               <listitem><para>At start-up, any configured synthetic network devices (i.e. not physical ones
 978               that require hardware to show up and be probed, but virtual ones like bridge devices and
 979               similar which are created programmatically) that do not depend on any underlying hardware
 980               should be allocated by the time this target is reached. It is not necessary for these
 981               interfaces to also have completed IP level configuration by the time
 982               <filename>network.target</filename> is reached.</para></listitem>
 983
 984               <listitem><para>At shutdown, a unit that is ordered after <filename>network.target</filename>
 985               will be stopped before the network — to whatever level it might be set up by then — is shut
 986               down. It is hence useful when writing service files that require network access on shutdown,
 987               which should order themselves after this target, but not pull it in. Also see <ulink
 988               url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for
 989               more information.</para></listitem>
 990             </itemizedlist>
 991
 992             <para>It must emphasized that at start-up there's no guarantee that hardware-based devices have
 993             shown up by the time this target is reached, or even acquired complete IP configuration. For that
 994             purpose use <filename>network-online.target</filename> as described above.</para>
 995           </listitem>
 996         </varlistentry>
 997         <varlistentry>
 998           <term><filename>network-pre.target</filename></term>
 999           <listitem>
1000             <para>This passive target unit may be pulled in by services that want to run before any network
1001             is set up, for example for the purpose of setting up a firewall. All network management software
1002             orders itself after this target, but does not pull it in. Also see <ulink
1003             url="https://systemd.io/NETWORK_ONLINE">Running Services After the Network Is Up</ulink> for more
1004             information.</para>
1005           </listitem>
1006         </varlistentry>
1007         <varlistentry>
1008           <term><filename>nss-lookup.target</filename></term>
1009           <listitem>
1010             <para>A target that should be used as synchronization point for all host/network name
1011             service lookups. Note that this is independent of UNIX user/group name lookups for which
1012             <filename>nss-user-lookup.target</filename> should be used.  All services for which the
1013             availability of full host/network name resolution is essential should be ordered after
1014             this target, but not pull it in. systemd automatically adds dependencies of type
1015             <varname>After=</varname> for this target unit to all SysV init script service units
1016             with an LSB header referring to the <literal>$named</literal> facility.</para>
1017           </listitem>
1018         </varlistentry>
1019         <varlistentry>
1020           <term><filename>nss-user-lookup.target</filename></term>
1021           <listitem>
1022             <para>A target that should be used as synchronization point for all regular UNIX
1023             user/group name service lookups. Note that this is independent of host/network name
1024             lookups for which <filename>nss-lookup.target</filename> should be used. All services
1025             for which the availability of the full user/group database is essential should be
1026             ordered after this target, but not pull it in. All services which provide parts of the
1027             user/group database should be ordered before this target, and pull it in. Note that this
1028             unit is only relevant for regular users and groups — system users and groups are
1029             required to be resolvable during earliest boot already, and hence do not need any
1030             special ordering against this target.</para>
1031           </listitem>
1032         </varlistentry>
1033         <varlistentry>
1034           <term><filename>remote-fs-pre.target</filename></term>
1035           <listitem>
1036             <para>This target unit is automatically ordered before all
1037             mount point units (see above) and cryptsetup/veritysetup devices
1038             marked with the <option>_netdev</option>. It can be used to run
1039             certain units before remote encrypted devices and mounts are established.
1040             Note that this unit is generally not part of the initial
1041             transaction, unless the unit that wants to be ordered before
1042             all remote mounts pulls it in via a
1043             <varname>Wants=</varname> type dependency. If the unit wants
1044             to be pulled in by the first remote mount showing up, it
1045             should use <filename>network-online.target</filename> (see
1046             above).</para>
1047           </listitem>
1048         </varlistentry>
1049         <varlistentry>
1050           <term><filename>rpcbind.target</filename></term>
1051           <listitem>
1052             <para>The portmapper/rpcbind pulls in this target and orders
1053             itself before it, to indicate its availability. systemd
1054             automatically adds dependencies of type
1055             <varname>After=</varname> for this target unit to all SysV
1056             init script service units with an LSB header referring to
1057             the <literal>$portmap</literal> facility.</para>
1058           </listitem>
1059         </varlistentry>
1060         <varlistentry>
1061           <term><filename>time-set.target</filename></term>
1062           <listitem>
1063             <para>Services responsible for setting the system clock (<constant>CLOCK_REALTIME</constant>)
1064             from a local source (such as a maintained timestamp file or imprecise real-time clock) should
1065             pull in this target and order themselves before it. Services where approximate, roughly monotonic
1066             time is desired should be ordered after this unit, but not pull it in.</para>
1067
1068             <para>This target does not provide the accuracy guarantees of
1069             <filename>time-sync.target</filename> (see below), however does not depend on remote clock
1070             sources to be reachable, i.e. the target is typically not delayed by network problems and
1071             similar. Use of this target is recommended for services where approximate clock accuracy and
1072             rough monotonicity is desired but activation shall not be delayed for possibly unreliable network
1073             communication.</para>
1074
1075             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1076             this target unit to all timer units with at least one <varname>OnCalendar=</varname>
1077             directive.</para>
1078
1079             <para>The
1080             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1081             service is a simple daemon that pulls in this target and orders itself before it. Besides
1082             implementing the SNTP network protocol it maintains a timestamp file on disk whose modification
1083             time is regularlary updated. At service start-up the local system clock is set from that modification time,
1084             ensuring it increases roughly monotonically.</para>
1085
1086             <para>Note that ordering a unit after <filename>time-set.target</filename> only has effect if
1087             there's actually a service ordered before it that delays it until the clock is adjusted for rough
1088             monotonicity. Otherwise, this target might get reached before the clock is adjusted to be roughly
1089             monotonic. Enable
1090             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1091             or an alternative NTP implementation to delay the target.</para>
1092           </listitem>
1093         </varlistentry>
1094         <varlistentry>
1095           <term><filename>time-sync.target</filename></term>
1096           <listitem>
1097             <para>Services indicating completed synchronization of the system clock
1098             (<constant>CLOCK_REALTIME</constant>) to a remote source should pull in this target and order
1099             themselves before it. Services where accurate time is essential should be ordered after this
1100             unit, but not pull it in.</para>
1101
1102             <para>The service manager automatically adds dependencies of type <varname>After=</varname> for
1103             this target unit to all SysV init script service units with an LSB header referring to the
1104             <literal>$time</literal> facility, as well to all timer units with at least one
1105             <varname>OnCalendar=</varname> directive.</para>
1106
1107             <para>This target provides stricter clock accuracy guarantees than
1108             <filename>time-set.target</filename> (see above), but likely requires
1109             network communication and thus introduces unpredictable delays.
1110             Services that require clock accuracy and where network
1111             communication delays are acceptable should use this target. Services that require a less accurate
1112             clock, and only approximate and roughly monotonic clock behaviour should use
1113             <filename>time-set.target</filename> instead.</para>
1114
1115             <para>Note that ordering a unit after <filename>time-sync.target</filename> only has effect if
1116             there's actually a service ordered before it that delays it until clock synchronization is
1117             reached. Otherwise, this target might get reached before the clock is synchronized to any remote
1118             accurate reference clock. When using
1119             <citerefentry><refentrytitle>systemd-timesyncd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1120             enable
1121             <citerefentry><refentrytitle>systemd-time-wait-sync.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1122             to delay the target; or use an equivalent service for other NTP implementations.</para>
1123
1124             <table>
1125               <title>Comparison</title>
1126               <tgroup cols='2' align='left' colsep='1' rowsep='1'>
1127                 <colspec colname="time-set" />
1128                 <colspec colname="time-sync" />
1129                 <thead>
1130                   <row>
1131                     <entry><filename>time-set.target</filename></entry>
1132                     <entry><filename>time-sync.target</filename></entry>
1133                   </row>
1134                 </thead>
1135                 <tbody>
1136                   <row>
1137                     <entry>"quick" to reach</entry>
1138                     <entry>"slow" to reach</entry>
1139                   </row>
1140                   <row>
1141                     <entry>typically uses local clock sources, boot process not affected by availability of external resources</entry>
1142                     <entry>typically uses remote clock sources, inserts dependencies on remote resources into boot process</entry>
1143                   </row>
1144                   <row>
1145                     <entry>reliable, because local</entry>
1146                     <entry>unreliable, because typically network involved</entry>
1147                   </row>
1148                   <row>
1149                     <entry>typically guarantees an approximate and roughly monotonic clock only</entry>
1150                     <entry>typically guarantees an accurate clock</entry>
1151                   </row>
1152                   <row>
1153                     <entry>implemented by <filename>systemd-timesyncd.service</filename></entry>
1154                     <entry>implemented by <filename>systemd-time-wait-sync.service</filename></entry>
1155                   </row>
1156                 </tbody>
1157               </tgroup>
1158             </table>
1159
1160           </listitem>
1161         </varlistentry>
1162       </variablelist>
1163     </refsect2>
1164
1165     <refsect2>
1166       <title>Special Slice Units</title>
1167
1168       <para>There are four <literal>.slice</literal> units which form the basis of the hierarchy for
1169       assignment of resources for services, users, and virtual machines or containers. See
1170       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1171       for details about slice units.</para>
1172
1173       <variablelist>
1174         <varlistentry>
1175           <term><filename>-.slice</filename></term>
1176           <listitem>
1177             <para>The root slice is the root of the slice hierarchy. It usually does not contain
1178             units directly, but may be used to set defaults for the whole tree.</para>
1179           </listitem>
1180         </varlistentry>
1181
1182         <varlistentry>
1183           <term><filename>system.slice</filename></term>
1184           <listitem>
1185             <para>By default, all system services started by
1186             <command>systemd</command> are found in this slice.</para>
1187           </listitem>
1188         </varlistentry>
1189
1190         <varlistentry>
1191           <term><filename>user.slice</filename></term>
1192           <listitem>
1193             <para>By default, all user processes and services started on
1194             behalf of the user, including the per-user systemd instance
1195             are found in this slice.  This is pulled in by
1196             <filename>systemd-logind.service</filename>.</para>
1197           </listitem>
1198         </varlistentry>
1199
1200         <varlistentry>
1201           <term><filename>machine.slice</filename></term>
1202           <listitem>
1203             <para>By default, all virtual machines and containers
1204             registered with <command>systemd-machined</command> are
1205             found in this slice. This is pulled in by
1206             <filename>systemd-machined.service</filename>.</para>
1207           </listitem>
1208         </varlistentry>
1209       </variablelist>
1210     </refsect2>
1211   </refsect1>
1212
1213   <refsect1>
1214     <title>Units managed by the user service manager</title>
1215
1216     <refsect2>
1217       <title>Special User Units</title>
1218
1219       <para>When systemd runs as a user instance, the following special
1220       units are available:</para>
1221
1222       <variablelist>
1223         <varlistentry>
1224           <term><filename>default.target</filename></term>
1225           <listitem>
1226             <para>This is the main target of the user session, started by default. Various services that
1227             compose the normal user session should be pulled into this target. In this regard,
1228             <filename>default.target</filename> is similar to <filename>multi-user.target</filename> in the
1229             system instance, but it is a real unit, not an alias.</para>
1230           </listitem>
1231         </varlistentry>
1232       </variablelist>
1233
1234       <para>In addition, the following units are available which have definitions similar to their
1235       system counterparts:
1236       <filename>exit.target</filename>,
1237       <filename>shutdown.target</filename>,
1238       <filename>sockets.target</filename>,
1239       <filename>timers.target</filename>,
1240       <filename>paths.target</filename>,
1241       <filename>bluetooth.target</filename>,
1242       <filename>printer.target</filename>,
1243       <filename>smartcard.target</filename>,
1244       <filename>sound.target</filename>.</para>
1245     </refsect2>
1246
1247     <refsect2>
1248       <title>Special Passive User Units</title>
1249
1250       <variablelist>
1251         <varlistentry>
1252           <term><filename>graphical-session.target</filename></term>
1253           <listitem>
1254             <para>This target is active whenever any graphical session is running. It is used to
1255             stop user services which only apply to a graphical (X, Wayland, etc.) session when the
1256             session is terminated. Such services should have
1257             <literal>PartOf=graphical-session.target</literal> in their [Unit]
1258             section. A target for a particular session (e. g.
1259             <filename>gnome-session.target</filename>) starts and stops
1260             <literal>graphical-session.target</literal> with
1261             <literal>BindsTo=graphical-session.target</literal>.</para>
1262
1263             <para>Which services are started by a session target is determined by the
1264             <literal>Wants=</literal> and <literal>Requires=</literal> dependencies.  For services
1265             that can be enabled independently, symlinks in <literal>.wants/</literal> and
1266             <literal>.requires/</literal> should be used, see
1267             <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
1268             Those symlinks should either be shipped in packages, or should be added dynamically
1269             after installation, for example using <literal>systemctl add-wants</literal>, see
1270             <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
1271             </para>
1272
1273             <example>
1274               <title>Nautilus as part of a GNOME session</title>
1275
1276               <para><literal>gnome-session.target</literal> pulls in Nautilus as top-level service:</para>
1277
1278               <programlisting>[Unit]
1279 Description=User systemd services for GNOME graphical session
1280 Wants=nautilus.service
1281 BindsTo=graphical-session.target</programlisting>
1282
1283               <para><literal>nautilus.service</literal> gets stopped when the session stops:</para>
1284
1285               <programlisting>[Unit]
1286 Description=Render the desktop icons with Nautilus
1287 PartOf=graphical-session.target
1288
1289 [Service]
1290 …</programlisting>
1291             </example>
1292           </listitem>
1293         </varlistentry>
1294
1295         <varlistentry>
1296           <term><filename>graphical-session-pre.target</filename></term>
1297           <listitem>
1298             <para>This target contains services which set up the environment or global configuration
1299             of a graphical session, such as SSH/GPG agents (which need to export an environment
1300             variable into all desktop processes) or migration of obsolete d-conf keys after an OS
1301             upgrade (which needs to happen before starting any process that might use them). This
1302             target must be started before starting a graphical session like
1303             <filename>gnome-session.target</filename>.</para>
1304           </listitem>
1305         </varlistentry>
1306
1307         <varlistentry>
1308           <term><filename>xdg-desktop-autostart.target</filename></term>
1309           <listitem>
1310             <para>The XDG specification defines a way to autostart applications using XDG desktop files.
1311             systemd ships
1312             <citerefentry><refentrytitle>systemd-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
1313             for the XDG desktop files in autostart directories. Desktop Environments can opt-in to use this
1314             service by adding a <varname>Wants=</varname> dependency on
1315             <filename>xdg-desktop-autostart.target</filename>.</para>
1316           </listitem>
1317         </varlistentry>
1318       </variablelist>
1319     </refsect2>
1320
1321     <refsect2>
1322       <title>Special User Slice Units</title>
1323
1324       <para>There are four <literal>.slice</literal> units which form the basis of the user hierarchy for
1325       assignment of resources for user applications and services. See
1326       <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>7</manvolnum></citerefentry>
1327       for details about slice units and the documentation about
1328       <ulink url="https://systemd.io/DESKTOP_ENVIRONMENTS">Desktop Environments</ulink>
1329       for further information.</para>
1330
1331       <variablelist>
1332         <varlistentry>
1333           <term><filename>-.slice</filename></term>
1334           <listitem>
1335             <para>The root slice is the root of the user's slice hierarchy.
1336             It usually does not contain units directly, but may be used to set defaults for the whole tree.</para>
1337           </listitem>
1338         </varlistentry>
1339
1340         <varlistentry>
1341           <term><filename>app.slice</filename></term>
1342           <listitem>
1343             <para>By default, all user services and applications managed by
1344             <command>systemd</command> are found in this slice.
1345             All interactively launched applications like web browsers and text editors
1346             as well as non-critical services should be placed into this slice.</para>
1347           </listitem>
1348         </varlistentry>
1349
1350         <varlistentry>
1351           <term><filename>session.slice</filename></term>
1352           <listitem>
1353             <para>All essential services and applications required for the
1354             session should use this slice.
1355             These are services that either cannot be restarted easily
1356             or where latency issues may affect the interactivity of the system and applications.
1357             This includes the display server, screen readers and other services such as DBus or XDG portals.
1358             Such services should be configured to be part of this slice by
1359             adding <varname>Slice=session.slice</varname> to their unit files.</para>
1360           </listitem>
1361         </varlistentry>
1362
1363         <varlistentry>
1364           <term><filename>background.slice</filename></term>
1365           <listitem>
1366             <para>All services running low-priority background tasks should use this slice.
1367             This permits resources to be preferentially assigned to the other slices.
1368             Examples include non-interactive tasks like file indexing or backup operations
1369             where latency is not important.</para>
1370           </listitem>
1371         </varlistentry>
1372       </variablelist>
1373     </refsect2>
1374   </refsect1>
1375
1376   <refsect1>
1377       <title>See Also</title>
1378       <para>
1379         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
1380         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1381         <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1382         <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1383         <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1384         <citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
1385         <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
1386         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
1387         <citerefentry><refentrytitle>user@.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
1388       </para>
1389   </refsect1>
1390
1391 </refentry>