systemd.unit

SYSTEMD.UNIT(5)                  systemd.unit                  SYSTEMD.UNIT(5)



NAME
       systemd.unit - Unit configuration

SYNOPSIS
       service.service, socket.socket, device.device, mount.mount,
       automount.automount, swap.swap, target.target, path.path, timer.timer,
       slice.slice, scope.scope

   System Unit Search Path
       /etc/systemd/system.control/*
       /run/systemd/system.control/*
       /run/systemd/transient/*
       /run/systemd/generator.early/*
       /etc/systemd/system/*
       /etc/systemd/systemd.attached/*
       /run/systemd/system/*
       /run/systemd/systemd.attached/*
       /run/systemd/generator/*
       ...
       /usr/lib/systemd/system/*
       /run/systemd/generator.late/*

   User Unit Search Path
       ~/.config/systemd/user.control/*
       $XDG_RUNTIME_DIR/systemd/user.control/*
       $XDG_RUNTIME_DIR/systemd/transient/*
       $XDG_RUNTIME_DIR/systemd/generator.early/*
       ~/.config/systemd/user/*
       /etc/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/user/*
       /run/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/generator/*
       ~/.local/share/systemd/user/*
       ...
       /usr/lib/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/generator.late/*

DESCRIPTION
       A unit file is a plain text ini-style file that encodes information
       about a service, a socket, a device, a mount point, an automount point,
       a swap file or partition, a start-up target, a watched file system
       path, a timer controlled and supervised by systemd(1), a resource
       management slice or a group of externally created processes. See
       systemd.syntax(5) for a general description of the syntax.

       This man page lists the common configuration options of all the unit
       types. These options need to be configured in the [Unit] or [Install]
       sections of the unit files.

       In addition to the generic [Unit] and [Install] sections described
       here, each unit may have a type-specific section, e.g. [Service] for a
       service unit. See the respective man pages for more information:
       systemd.service(5), systemd.socket(5), systemd.device(5),
       systemd.mount(5), systemd.automount(5), systemd.swap(5),
       systemd.target(5), systemd.path(5), systemd.timer(5), systemd.slice(5),
       systemd.scope(5).

       Unit files are loaded from a set of paths determined during
       compilation, described in the next section.

       Unit files can be parameterized by a single argument called the
       "instance name". The unit is then constructed based on a "template
       file" which serves as the definition of multiple services or other
       units. A template unit must have a single "@" at the end of the name
       (right before the type suffix). The name of the full unit is formed by
       inserting the instance name between "@" and the unit type suffix. In
       the unit file itself, the instance parameter may be referred to using
       "%i" and other specifiers, see below.

       Unit files may contain additional options on top of those listed here.
       If systemd encounters an unknown option, it will write a warning log
       message but continue loading the unit. If an option or section name is
       prefixed with X-, it is ignored completely by systemd. Options within
       an ignored section do not need the prefix. Applications may use this to
       include additional information in the unit files.

       Units can be aliased (have an alternative name), by creating a symlink
       from the new name to the existing name in one of the unit search paths.
       For example, systemd-networkd.service has the alias
       dbus-org.freedesktop.network1.service, created during installation as
       the symlink
       /usr/lib/systemd/system/dbus-org.freedesktop.network1.service. In
       addition, unit files may specify aliases through the Alias= directive
       in the [Install] section; those aliases are only effective when the
       unit is enabled. When the unit is enabled, symlinks will be created for
       those names, and removed when the unit is disabled. For example,
       reboot.target specifies Alias=ctrl-alt-del.target, so when enabled it
       will be invoked whenever CTRL+ALT+DEL is pressed. Alias names may be
       used in commands like enable, disable, start, stop, status, ..., and in
       unit dependency directives Wants=, Requires=, Before=, After=, ...,
       with the limitation that aliases specified through Alias= are only
       effective when the unit is enabled. Aliases cannot be used with the
       preset command.

       Along with a unit file foo.service, the directory foo.service.wants/
       may exist. All unit files symlinked from such a directory are
       implicitly added as dependencies of type Wants= to the unit. This is
       useful to hook units into the start-up of other units, without having
       to modify their unit files. For details about the semantics of Wants=,
       see below. The preferred way to create symlinks in the .wants/
       directory of a unit file is with the enable command of the systemctl(1)
       tool which reads information from the [Install] section of unit files
       (see below). A similar functionality exists for Requires= type
       dependencies as well, the directory suffix is .requires/ in this case.

       Along with a unit file foo.service, a "drop-in" directory
       foo.service.d/ may exist. All files with the suffix ".conf" from this
       directory will be parsed after the unit file itself is parsed. This is
       useful to alter or add configuration settings for a unit, without
       having to modify unit files. Drop-in files must contain appropriate
       section headers. For instantiated units, this logic will first look for
       the instance ".d/" subdirectory (e.g.  "foo@bar.service.d/") and read
       its ".conf" files, followed by the template ".d/" subdirectory (e.g.
       "foo@.service.d/") and the ".conf" files there. Moreover for units
       names containing dashes ("-"), the set of directories generated by
       truncating the unit name after all dashes is searched too.
       Specifically, for a unit name foo-bar-baz.service not only the regular
       drop-in directory foo-bar-baz.service.d/ is searched but also both
       foo-bar-.service.d/ and foo-.service.d/. This is useful for defining
       common drop-ins for a set of related units, whose names begin with a
       common prefix. This scheme is particularly useful for mount, automount
       and slice units, whose systematic naming structure is built around
       dashes as component separators. Note that equally named drop-in files
       further down the prefix hierarchy override those further up, i.e.
       foo-bar-.service.d/10-override.conf overrides
       foo-.service.d/10-override.conf.

       In addition to /etc/systemd/system, the drop-in ".d/" directories for
       system services can be placed in /usr/lib/systemd/system or
       /run/systemd/system directories. Drop-in files in /etc take precedence
       over those in /run which in turn take precedence over those in
       /usr/lib. Drop-in files under any of these directories take precedence
       over unit files wherever located. Multiple drop-in files with different
       names are applied in lexicographic order, regardless of which of the
       directories they reside in.

       Note that while systemd offers a flexible dependency system between
       units it is recommended to use this functionality only sparingly and
       instead rely on techniques such as bus-based or socket-based activation
       which make dependencies implicit, resulting in a both simpler and more
       flexible system.

       As mentioned above, a unit may be instantiated from a template file.
       This allows creation of multiple units from a single configuration
       file. If systemd looks for a unit configuration file, it will first
       search for the literal unit name in the file system. If that yields no
       success and the unit name contains an "@" character, systemd will look
       for a unit template that shares the same name but with the instance
       string (i.e. the part between the "@" character and the suffix)
       removed. Example: if a service getty@tty3.service is requested and no
       file by that name is found, systemd will look for getty@.service and
       instantiate a service from that configuration file if it is found.

       To refer to the instance string from within the configuration file you
       may use the special "%i" specifier in many of the configuration
       options. See below for details.

       If a unit file is empty (i.e. has the file size 0) or is symlinked to
       /dev/null, its configuration will not be loaded and it appears with a
       load state of "masked", and cannot be activated. Use this as an
       effective way to fully disable a unit, making it impossible to start it
       even manually.

       The unit file format is covered by the Interface Stability Promise[1].

STRING ESCAPING FOR INCLUSION IN UNIT NAMES
       Sometimes it is useful to convert arbitrary strings into unit names. To
       facilitate this, a method of string escaping is used, in order to map
       strings containing arbitrary byte values (except NUL) into valid unit
       names and their restricted character set. A common special case are
       unit names that reflect paths to objects in the file system hierarchy.
       Example: a device unit dev-sda.device refers to a device with the
       device node /dev/sda in the file system.

       The escaping algorithm operates as follows: given a string, any "/"
       character is replaced by "-", and all other characters which are not
       ASCII alphanumerics or "_" are replaced by C-style "\x2d" escapes. In
       addition, "."  is replaced with such a C-style escape when it would
       appear as the first character in the escaped string.

       When the input qualifies as absolute file system path, this algorithm
       is extended slightly: the path to the root directory "/" is encoded as
       single dash "-". In addition, any leading, trailing or duplicate "/"
       characters are removed from the string before transformation. Example:
       /foo//bar/baz/ becomes "foo-bar-baz".

       This escaping is fully reversible, as long as it is known whether the
       escaped string was a path (the unescaping results are different for
       paths and non-path strings). The systemd-escape(1) command may be used
       to apply and reverse escaping on arbitrary strings. Use systemd-escape
       --path to escape path strings, and systemd-escape without --path
       otherwise.

AUTOMATIC DEPENDENCIES
   Implicit Dependencies
       A number of unit dependencies are implicitly established, depending on
       unit type and unit configuration. These implicit dependencies can make
       unit configuration file cleaner. For the implicit dependencies in each
       unit type, please refer to section "Implicit Dependencies" in
       respective man pages.

       For example, service units with Type=dbus automatically acquire
       dependencies of type Requires= and After= on dbus.socket. See
       systemd.service(5) for details.

   Default Dependencies
       Default dependencies are similar to implicit dependencies, but can be
       turned on and off by setting DefaultDependencies= to yes (the default)
       and no, while implicit dependencies are always in effect. See section
       "Default Dependencies" in respective man pages for the effect of
       enabling DefaultDependencies= in each unit types.

       For example, target units will complement all configured dependencies
       of type Wants= or Requires= with dependencies of type After= unless
       DefaultDependencies=no is set in the specified units. See
       systemd.target(5) for details. Note that this behavior can be turned
       off by setting DefaultDependencies=no.

UNIT FILE LOAD PATH
       Unit files are loaded from a set of paths determined during
       compilation, described in the two tables below. Unit files found in
       directories listed earlier override files with the same name in
       directories lower in the list.

       When the variable $SYSTEMD_UNIT_PATH is set, the contents of this
       variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with
       an empty component (":"), the usual unit load path will be appended to
       the contents of the variable.

       Table 1.  Load path when running in system mode (--system).
       ┌──────────────────────────────┬────────────────────────────┐
       │Path                          Description                │
       ├──────────────────────────────┼────────────────────────────┤
       │/etc/systemd/system.control   │ Persistent and transient   │
       ├──────────────────────────────┤ configuration created      │
       │/run/systemd/system.control   │ using the dbus API         │
       ├──────────────────────────────┼────────────────────────────┤
       │/run/systemd/transient        │ Dynamic configuration for  │
       │                              │ transient units            │
       ├──────────────────────────────┼────────────────────────────┤
       │/run/systemd/generator.early  │ Generated units with high  │
       │                              │ priority (see early-dir in │
       │                              │ systemd.generator(7))      │
       ├──────────────────────────────┼────────────────────────────┤
       │/etc/systemd/system           │ System units created by    │
       │                              │ the administrator          │
       ├──────────────────────────────┼────────────────────────────┤
       │/run/systemd/system           │ Runtime units              │
       ├──────────────────────────────┼────────────────────────────┤
       │/run/systemd/generator        │ Generated units with       │
       │                              │ medium priority (see       │
       │                              │ normal-dir in              │
       │                              │ systemd.generator(7))      │
       ├──────────────────────────────┼────────────────────────────┤
       │/usr/local/lib/systemd/system │ System units installed by  │
       │                              │ the administrator          │
       ├──────────────────────────────┼────────────────────────────┤
       │/usr/lib/systemd/system       │ System units installed by  │
       │                              │ the distribution package   │
       │                              │ manager                    │
       ├──────────────────────────────┼────────────────────────────┤
       │/run/systemd/generator.late   │ Generated units with low   │
       │                              │ priority (see late-dir in  │
       │                              │ systemd.generator(7))      │
       └──────────────────────────────┴────────────────────────────┘

       Table 2.  Load path when running in user mode (--user).
       ┌────────────────────────────────────────┬────────────────────────────┐
       │Path                                    Description                │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_CONFIG_HOME/systemd/user.control   │ Persistent and transient   │
       │or                                      │ configuration created      │
       │~/.config/systemd/user.control          │ using the dbus API         │
       ├────────────────────────────────────────┤ ($XDG_CONFIG_HOME is used  │
       │$XDG_RUNTIME_DIR/systemd/user.control   │ if set, ~/.config          │
       │                                        │ otherwise)                 │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/run/systemd/transient                  │ Dynamic configuration for  │
       │                                        │ transient units            │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/run/systemd/generator.early            │ Generated units with high  │
       │                                        │ priority (see early-dir in │
       │                                        │ systemd.generator(7))      │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_CONFIG_HOME/systemd/user or        │ User configuration         │
       │$HOME/.config/systemd/user              │ ($XDG_CONFIG_HOME is used  │
       │                                        │ if set, ~/.config          │
       │                                        │ otherwise)                 │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/etc/systemd/user                       │ User units created by the  │
       │                                        │ administrator              │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_RUNTIME_DIR/systemd/user           │ Runtime units (only used   │
       │                                        │ when $XDG_RUNTIME_DIR is   │
       │                                        │ set)                       │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/run/systemd/user                       │ Runtime units              │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_RUNTIME_DIR/systemd/generator      │ Generated units with       │
       │                                        │ medium priority (see       │
       │                                        │ normal-dir in              │
       │                                        │ systemd.generator(7))      │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_DATA_HOME/systemd/user or          │ Units of packages that     │
       │$HOME/.local/share/systemd/user         │ have been installed in the │
       │                                        │ home directory             │
       │                                        │ ($XDG_DATA_HOME is used if │
       │                                        │ set, ~/.local/share        │
       │                                        │ otherwise)                 │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$dir/systemd/user for each $dir in      │ Additional locations for   │
       │$XDG_DATA_DIRS                          │ installed user units, one  │
       │                                        │ for each entry in          │
       │                                        │ $XDG_DATA_DIRS             │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/usr/local/lib/systemd/user             │ User units installed by    │
       │                                        │ the administrator          │
       ├────────────────────────────────────────┼────────────────────────────┤
       │/usr/lib/systemd/user                   │ User units installed by    │
       │                                        │ the distribution package   │
       │                                        │ manager                    │
       ├────────────────────────────────────────┼────────────────────────────┤
       │$XDG_RUNTIME_DIR/systemd/generator.late │ Generated units with low   │
       │                                        │ priority (see late-dir in  │
       │                                        │ systemd.generator(7))      │
       └────────────────────────────────────────┴────────────────────────────┘

       The set of load paths for the user manager instance may be augmented or
       changed using various environment variables. And environment variables
       may in turn be set using environment generators, see
       systemd.environment-generator(7). In particular, $XDG_DATA_HOME and
       $XDG_DATA_DIRS may be easily set using systemd-environment-d-
       generator(8). Thus, directories listed here are just the defaults. To
       see the actual list that would be used based on compilation options and
       current environment use

           systemd-analyze --user unit-paths

       Moreover, additional units might be loaded into systemd ("linked") from
       directories not on the unit load path. See the link command for
       systemctl(1).

UNIT GARBAGE COLLECTION
       The system and service manager loads a unit's configuration
       automatically when a unit is referenced for the first time. It will
       automatically unload the unit configuration and state again when the
       unit is not needed anymore ("garbage collection"). A unit may be
       referenced through a number of different mechanisms:

        1. Another loaded unit references it with a dependency such as After=,
           Wants=, ...

        2. The unit is currently starting, running, reloading or stopping.

        3. The unit is currently in the failed state. (But see below.)

        4. A job for the unit is pending.

        5. The unit is pinned by an active IPC client program.

        6. The unit is a special "perpetual" unit that is always active and
           loaded. Examples for perpetual units are the root mount unit
           -.mount or the scope unit init.scope that the service manager
           itself lives in.

        7. The unit has running processes associated with it.

       The garbage collection logic may be altered with the CollectMode=
       option, which allows configuration whether automatic unloading of units
       that are in failed state is permissible, see below.

       Note that when a unit's configuration and state is unloaded, all
       execution results, such as exit codes, exit signals, resource
       consumption and other statistics are lost, except for what is stored in
       the log subsystem.

       Use systemctl daemon-reload or an equivalent command to reload unit
       configuration while the unit is already loaded. In this case all
       configuration settings are flushed out and replaced with the new
       configuration (which however might not be in effect immediately),
       however all runtime state is saved/restored.

[UNIT] SECTION OPTIONS
       The unit file may include a [Unit] section, which carries generic
       information about the unit that is not dependent on the type of unit:

       Description=
           A human readable name for the unit. This is used by systemd (and
           other UIs) as the label for the unit, so this string should
           identify the unit rather than describe it, despite the name.
           "Apache2 Web Server" is a good example. Bad examples are
           "high-performance light-weight HTTP server" (too generic) or
           "Apache2" (too specific and meaningless for people who do not know
           Apache).  systemd will use this string as a noun in status messages
           ("Starting description...", "Started description.", "Reached target
           description.", "Failed to start description."), so it should be
           capitalized, and should not be a full sentence or a phrase with a
           continuous verb. Bad examples include "exiting the container" or
           "updating the database once per day.".

       Documentation=
           A space-separated list of URIs referencing documentation for this
           unit or its configuration. Accepted are only URIs of the types
           "http://", "https://", "file:", "info:", "man:". For more
           information about the syntax of these URIs, see uri(7). The URIs
           should be listed in order of relevance, starting with the most
           relevant. It is a good idea to first reference documentation that
           explains what the unit's purpose is, followed by how it is
           configured, followed by any other related documentation. This
           option may be specified more than once, in which case the specified
           list of URIs is merged. If the empty string is assigned to this
           option, the list is reset and all prior assignments will have no
           effect.

       Requires=
           Configures requirement dependencies on other units. If this unit
           gets activated, the units listed here will be activated as well. If
           one of the other units fails to activate, and an ordering
           dependency After= on the failing unit is set, this unit will not be
           started. Besides, with or without specifying After=, this unit will
           be stopped if one of the other units is explicitly stopped. This
           option may be specified more than once or multiple space-separated
           units may be specified in one option in which case requirement
           dependencies for all listed names will be created. Note that
           requirement dependencies do not influence the order in which
           services are started or stopped. This has to be configured
           independently with the After= or Before= options. If a unit
           foo.service requires a unit bar.service as configured with
           Requires= and no ordering is configured with After= or Before=,
           then both units will be started simultaneously and without any
           delay between them if foo.service is activated. Often, it is a
           better choice to use Wants= instead of Requires= in order to
           achieve a system that is more robust when dealing with failing
           services.

           Note that this dependency type does not imply that the other unit
           always has to be in active state when this unit is running.
           Specifically: failing condition checks (such as
           ConditionPathExists=, ConditionPathIsSymbolicLink=, ... — see
           below) do not cause the start job of a unit with a Requires=
           dependency on it to fail. Also, some unit types may deactivate on
           their own (for example, a service process may decide to exit
           cleanly, or a device may be unplugged by the user), which is not
           propagated to units having a Requires= dependency. Use the BindsTo=
           dependency type together with After= to ensure that a unit may
           never be in active state without a specific other unit also in
           active state (see below).

           Note that dependencies of this type may also be configured outside
           of the unit configuration file by adding a symlink to a .requires/
           directory accompanying the unit file. For details, see above.

       Requisite=
           Similar to Requires=. However, if the units listed here are not
           started already, they will not be started and the starting of this
           unit will fail immediately.  Requisite= does not imply an ordering
           dependency, even if both units are started in the same transaction.
           Hence this setting should usually be combined with After=, to
           ensure this unit is not started before the other unit.

           When Requisite=b.service is used on a.service, this dependency will
           show as RequisiteOf=a.service in property listing of b.service.
           RequisiteOf= dependency cannot be specified directly.

       Wants=
           A weaker version of Requires=. Units listed in this option will be
           started if the configuring unit is. However, if the listed units
           fail to start or cannot be added to the transaction, this has no
           impact on the validity of the transaction as a whole. This is the
           recommended way to hook start-up of one unit to the start-up of
           another unit.

           Note that dependencies of this type may also be configured outside
           of the unit configuration file by adding symlinks to a .wants/
           directory accompanying the unit file. For details, see above.

       BindsTo=
           Configures requirement dependencies, very similar in style to
           Requires=. However, this dependency type is stronger: in addition
           to the effect of Requires= it declares that if the unit bound to is
           stopped, this unit will be stopped too. This means a unit bound to
           another unit that suddenly enters inactive state will be stopped
           too. Units can suddenly, unexpectedly enter inactive state for
           different reasons: the main process of a service unit might
           terminate on its own choice, the backing device of a device unit
           might be unplugged or the mount point of a mount unit might be
           unmounted without involvement of the system and service manager.

           When used in conjunction with After= on the same unit the behaviour
           of BindsTo= is even stronger. In this case, the unit bound to
           strictly has to be in active state for this unit to also be in
           active state. This not only means a unit bound to another unit that
           suddenly enters inactive state, but also one that is bound to
           another unit that gets skipped due to a failed condition check
           (such as ConditionPathExists=, ConditionPathIsSymbolicLink=, ... —
           see below) will be stopped, should it be running. Hence, in many
           cases it is best to combine BindsTo= with After=.

           When BindsTo=b.service is used on a.service, this dependency will
           show as BoundBy=a.service in property listing of b.service.
           BoundBy= dependency cannot be specified directly.

       PartOf=
           Configures dependencies similar to Requires=, but limited to
           stopping and restarting of units. When systemd stops or restarts
           the units listed here, the action is propagated to this unit. Note
           that this is a one-way dependency — changes to this unit do not
           affect the listed units.

           When PartOf=b.service is used on a.service, this dependency will
           show as ConsistsOf=a.service in property listing of b.service.
           ConsistsOf= dependency cannot be specified directly.

       Conflicts=
           A space-separated list of unit names. Configures negative
           requirement dependencies. If a unit has a Conflicts= setting on
           another unit, starting the former will stop the latter and vice
           versa. Note that this setting is independent of and orthogonal to
           the After= and Before= ordering dependencies.

           If a unit A that conflicts with a unit B is scheduled to be started
           at the same time as B, the transaction will either fail (in case
           both are required parts of the transaction) or be modified to be
           fixed (in case one or both jobs are not a required part of the
           transaction). In the latter case, the job that is not required will
           be removed, or in case both are not required, the unit that
           conflicts will be started and the unit that is conflicted is
           stopped.

       Before=, After=
           These two settings expect a space-separated list of unit names.
           They configure ordering dependencies between units. If a unit
           foo.service contains a setting Before=bar.service and both units
           are being started, bar.service's start-up is delayed until
           foo.service has finished starting up. Note that this setting is
           independent of and orthogonal to the requirement dependencies as
           configured by Requires=, Wants= or BindsTo=. It is a common pattern
           to include a unit name in both the After= and Requires= options, in
           which case the unit listed will be started before the unit that is
           configured with these options. This option may be specified more
           than once, in which case ordering dependencies for all listed names
           are created.  After= is the inverse of Before=, i.e. while After=
           ensures that the configured unit is started after the listed unit
           finished starting up, Before= ensures the opposite, that the
           configured unit is fully started up before the listed unit is
           started. Note that when two units with an ordering dependency
           between them are shut down, the inverse of the start-up order is
           applied. i.e. if a unit is configured with After= on another unit,
           the former is stopped before the latter if both are shut down.
           Given two units with any ordering dependency between them, if one
           unit is shut down and the other is started up, the shutdown is
           ordered before the start-up. It doesn't matter if the ordering
           dependency is After= or Before=, in this case. It also doesn't
           matter which of the two is shut down, as long as one is shut down
           and the other is started up. The shutdown is ordered before the
           start-up in all cases. If two units have no ordering dependencies
           between them, they are shut down or started up simultaneously, and
           no ordering takes place. It depends on the unit type when precisely
           a unit has finished starting up. Most importantly, for service
           units start-up is considered completed for the purpose of
           Before=/After= when all its configured start-up commands have been
           invoked and they either failed or reported start-up success.

       OnFailure=
           A space-separated list of one or more units that are activated when
           this unit enters the "failed" state. A service unit using Restart=
           enters the failed state only after the start limits are reached.

       PropagatesReloadTo=, ReloadPropagatedFrom=
           A space-separated list of one or more units where reload requests
           on this unit will be propagated to, or reload requests on the other
           unit will be propagated to this unit, respectively. Issuing a
           reload request on a unit will automatically also enqueue a reload
           request on all units that the reload request shall be propagated to
           via these two settings.

       JoinsNamespaceOf=
           For units that start processes (such as service units), lists one
           or more other units whose network and/or temporary file namespace
           to join. This only applies to unit types which support the
           PrivateNetwork=, NetworkNamespacePath= and PrivateTmp= directives
           (see systemd.exec(5) for details). If a unit that has this setting
           set is started, its processes will see the same /tmp, /var/tmp and
           network namespace as one listed unit that is started. If multiple
           listed units are already started, it is not defined which namespace
           is joined. Note that this setting only has an effect if
           PrivateNetwork=/NetworkNamespacePath= and/or PrivateTmp= is enabled
           for both the unit that joins the namespace and the unit whose
           namespace is joined.

       RequiresMountsFor=
           Takes a space-separated list of absolute paths. Automatically adds
           dependencies of type Requires= and After= for all mount units
           required to access the specified path.

           Mount points marked with noauto are not mounted automatically
           through local-fs.target, but are still honored for the purposes of
           this option, i.e. they will be pulled in by this unit.

       OnFailureJobMode=
           Takes a value of "fail", "replace", "replace-irreversibly",
           "isolate", "flush", "ignore-dependencies" or "ignore-requirements".
           Defaults to "replace". Specifies how the units listed in OnFailure=
           will be enqueued. See systemctl(1)'s --job-mode= option for details
           on the possible values. If this is set to "isolate", only a single
           unit may be listed in OnFailure=..

       IgnoreOnIsolate=
           Takes a boolean argument. If true, this unit will not be stopped
           when isolating another unit. Defaults to false for service, target,
           socket, busname, timer, and path units, and true for slice, scope,
           device, swap, mount, and automount units.

       StopWhenUnneeded=
           Takes a boolean argument. If true, this unit will be stopped when
           it is no longer used. Note that, in order to minimize the work to
           be executed, systemd will not stop units by default unless they are
           conflicting with other units, or the user explicitly requested
           their shut down. If this option is set, a unit will be
           automatically cleaned up if no other active unit requires it.
           Defaults to false.

       RefuseManualStart=, RefuseManualStop=
           Takes a boolean argument. If true, this unit can only be activated
           or deactivated indirectly. In this case, explicit start-up or
           termination requested by the user is denied, however if it is
           started or stopped as a dependency of another unit, start-up or
           termination will succeed. This is mostly a safety feature to ensure
           that the user does not accidentally activate units that are not
           intended to be activated explicitly, and not accidentally
           deactivate units that are not intended to be deactivated. These
           options default to false.

       AllowIsolate=
           Takes a boolean argument. If true, this unit may be used with the
           systemctl isolate command. Otherwise, this will be refused. It
           probably is a good idea to leave this disabled except for target
           units that shall be used similar to runlevels in SysV init systems,
           just as a precaution to avoid unusable system states. This option
           defaults to false.

       DefaultDependencies=
           Takes a boolean argument. If true, (the default), a few default
           dependencies will implicitly be created for the unit. The actual
           dependencies created depend on the unit type. For example, for
           service units, these dependencies ensure that the service is
           started only after basic system initialization is completed and is
           properly terminated on system shutdown. See the respective man
           pages for details. Generally, only services involved with early
           boot or late shutdown should set this option to false. It is highly
           recommended to leave this option enabled for the majority of common
           units. If set to false, this option does not disable all implicit
           dependencies, just non-essential ones.

       CollectMode=
           Tweaks the "garbage collection" algorithm for this unit. Takes one
           of inactive or inactive-or-failed. If set to inactive the unit will
           be unloaded if it is in the inactive state and is not referenced by
           clients, jobs or other units — however it is not unloaded if it is
           in the failed state. In failed mode, failed units are not unloaded
           until the user invoked systemctl reset-failed on them to reset the
           failed state, or an equivalent command. This behaviour is altered
           if this option is set to inactive-or-failed: in this case the unit
           is unloaded even if the unit is in a failed state, and thus an
           explicitly resetting of the failed state is not necessary. Note
           that if this mode is used unit results (such as exit codes, exit
           signals, consumed resources, ...) are flushed out immediately after
           the unit completed, except for what is stored in the logging
           subsystem. Defaults to inactive.

       FailureAction=, SuccessAction=
           Configure the action to take when the unit stops and enters a
           failed state or inactive state. Takes one of none, reboot,
           reboot-force, reboot-immediate, poweroff, poweroff-force,
           poweroff-immediate, exit, and exit-force. In system mode, all
           options are allowed. In user mode, only none, exit, and exit-force
           are allowed. Both options default to none.

           If none is set, no action will be triggered.  reboot causes a
           reboot following the normal shutdown procedure (i.e. equivalent to
           systemctl reboot).  reboot-force causes a forced reboot which will
           terminate all processes forcibly but should cause no dirty file
           systems on reboot (i.e. equivalent to systemctl reboot -f) and
           reboot-immediate causes immediate execution of the reboot(2) system
           call, which might result in data loss (i.e. equivalent to systemctl
           reboot -ff). Similarly, poweroff, poweroff-force,
           poweroff-immediate have the effect of powering down the system with
           similar semantics.  exit causes the manager to exit following the
           normal shutdown procedure, and exit-force causes it terminate
           without shutting down services. When exit or exit-force is used by
           default the exit status of the main process of the unit (if this
           applies) is returned from the service manager. However, this may be
           overridden with FailureActionExitStatus=/SuccessActionExitStatus=,
           see below.

       FailureActionExitStatus=, SuccessActionExitStatus=
           Controls the exit status to propagate back to an invoking container
           manager (in case of a system service) or service manager (in case
           of a user manager) when the FailureAction=/SuccessAction= are set
           to exit or exit-force and the action is triggered. By default the
           exit status of the main process of the triggering unit (if this
           applies) is propagated. Takes a value in the range 0...255 or the
           empty string to request default behaviour.

       JobTimeoutSec=, JobRunningTimeoutSec=
           When a job for this unit is queued, a timeout JobTimeoutSec= may be
           configured. Similarly, JobRunningTimeoutSec= starts counting when
           the queued job is actually started. If either time limit is
           reached, the job will be cancelled, the unit however will not
           change state or even enter the "failed" mode. This value defaults
           to "infinity" (job timeouts disabled), except for device units
           (JobRunningTimeoutSec= defaults to DefaultTimeoutStartSec=). NB:
           this timeout is independent from any unit-specific timeout (for
           example, the timeout set with TimeoutStartSec= in service units) as
           the job timeout has no effect on the unit itself, only on the job
           that might be pending for it. Or in other words: unit-specific
           timeouts are useful to abort unit state changes, and revert them.
           The job timeout set with this option however is useful to abort
           only the job waiting for the unit state to change.

       JobTimeoutAction=, JobTimeoutRebootArgument=
           JobTimeoutAction= optionally configures an additional action to
           take when the timeout is hit, see description of JobTimeoutSec= and
           JobRunningTimeoutSec= above. It takes the same values as
           StartLimitAction=. Defaults to none.  JobTimeoutRebootArgument=
           configures an optional reboot string to pass to the reboot(2)
           system call.

       StartLimitIntervalSec=interval, StartLimitBurst=burst
           Configure unit start rate limiting. Units which are started more
           than burst times within an interval time interval are not permitted
           to start any more. Use StartLimitIntervalSec= to configure the
           checking interval (defaults to DefaultStartLimitIntervalSec= in
           manager configuration file, set it to 0 to disable any kind of rate
           limiting). Use StartLimitBurst= to configure how many starts per
           interval are allowed (defaults to DefaultStartLimitBurst= in
           manager configuration file). These configuration options are
           particularly useful in conjunction with the service setting
           Restart= (see systemd.service(5)); however, they apply to all kinds
           of starts (including manual), not just those triggered by the
           Restart= logic. Note that units which are configured for Restart=
           and which reach the start limit are not attempted to be restarted
           anymore; however, they may still be restarted manually at a later
           point, after the interval has passed. From this point on, the
           restart logic is activated again. Note that systemctl reset-failed
           will cause the restart rate counter for a service to be flushed,
           which is useful if the administrator wants to manually start a unit
           and the start limit interferes with that. Note that this
           rate-limiting is enforced after any unit condition checks are
           executed, and hence unit activations with failing conditions do not
           count towards this rate limit. This setting does not apply to
           slice, target, device, and scope units, since they are unit types
           whose activation may either never fail, or may succeed only a
           single time.

           When a unit is unloaded due to the garbage collection logic (see
           above) its rate limit counters are flushed out too. This means that
           configuring start rate limiting for a unit that is not referenced
           continuously has no effect.

       StartLimitAction=
           Configure an additional action to take if the rate limit configured
           with StartLimitIntervalSec= and StartLimitBurst= is hit. Takes the
           same values as the setting FailureAction=/SuccessAction= settings
           and executes the same actions. If none is set, hitting the rate
           limit will trigger no action besides that the start will not be
           permitted. Defaults to none.

       RebootArgument=
           Configure the optional argument for the reboot(2) system call if
           StartLimitAction= or FailureAction= is a reboot action. This works
           just like the optional argument to systemctl reboot command.

       ConditionArchitecture=, ConditionVirtualization=, ConditionHost=,
       ConditionKernelCommandLine=, ConditionKernelVersion=,
       ConditionSecurity=, ConditionCapability=, ConditionACPower=,
       ConditionNeedsUpdate=, ConditionFirstBoot=, ConditionPathExists=,
       ConditionPathExistsGlob=, ConditionPathIsDirectory=,
       ConditionPathIsSymbolicLink=, ConditionPathIsMountPoint=,
       ConditionPathIsReadWrite=, ConditionDirectoryNotEmpty=,
       ConditionFileNotEmpty=, ConditionFileIsExecutable=, ConditionUser=,
       ConditionGroup=, ConditionControlGroupController=, ConditionMemory=,
       ConditionCPUs=
           Before starting a unit, verify that the specified condition is
           true. If it is not true, the starting of the unit will be (mostly
           silently) skipped, however all ordering dependencies of it are
           still respected. A failing condition will not result in the unit
           being moved into the "failed" state. The condition is checked at
           the time the queued start job is to be executed. Use condition
           expressions in order to silently skip units that do not apply to
           the local running system, for example because the kernel or runtime
           environment doesn't require their functionality. Use the various
           AssertArchitecture=, AssertVirtualization=, ... options for a
           similar mechanism that causes the job to fail (instead of being
           skipped) and results in logging about the failed check (instead of
           being silently processed). For details about assertion conditions
           see below.

           ConditionArchitecture= may be used to check whether the system is
           running on a specific architecture. Takes one of "x86", "x86-64",
           "ppc", "ppc-le", "ppc64", "ppc64-le", "ia64", "parisc", "parisc64",
           "s390", "s390x", "sparc", "sparc64", "mips", "mips-le", "mips64",
           "mips64-le", "alpha", "arm", "arm-be", "arm64", "arm64-be", "sh",
           "sh64", "m68k", "tilegx", "cris", "arc", "arc-be" to test against a
           specific architecture. The architecture is determined from the
           information returned by uname(2) and is thus subject to
           personality(2). Note that a Personality= setting in the same unit
           file has no effect on this condition. A special architecture name
           "native" is mapped to the architecture the system manager itself is
           compiled for. The test may be negated by prepending an exclamation
           mark.

           ConditionVirtualization= may be used to check whether the system is
           executed in a virtualized environment and optionally test whether
           it is a specific implementation. Takes either boolean value to
           check if being executed in any virtualized environment, or one of
           "vm" and "container" to test against a generic type of
           virtualization solution, or one of "qemu", "kvm", "zvm", "vmware",
           "microsoft", "oracle", "xen", "bochs", "uml", "bhyve", "qnx",
           "openvz", "lxc", "lxc-libvirt", "systemd-nspawn", "docker", "rkt",
           "wsl", "acrn" to test against a specific implementation, or
           "private-users" to check whether we are running in a user
           namespace. See systemd-detect-virt(1) for a full list of known
           virtualization technologies and their identifiers. If multiple
           virtualization technologies are nested, only the innermost is
           considered. The test may be negated by prepending an exclamation
           mark.

           ConditionHost= may be used to match against the hostname or machine
           ID of the host. This either takes a hostname string (optionally
           with shell style globs) which is tested against the locally set
           hostname as returned by gethostname(2), or a machine ID formatted
           as string (see machine-id(5)). The test may be negated by
           prepending an exclamation mark.

           ConditionKernelCommandLine= may be used to check whether a specific
           kernel command line option is set (or if prefixed with the
           exclamation mark unset). The argument must either be a single word,
           or an assignment (i.e. two words, separated "="). In the former
           case the kernel command line is searched for the word appearing as
           is, or as left hand side of an assignment. In the latter case, the
           exact assignment is looked for with right and left hand side
           matching.

           ConditionKernelVersion= may be used to check whether the kernel
           version (as reported by uname -r) matches a certain expression (or
           if prefixed with the exclamation mark does not match it). The
           argument must be a single string. If the string starts with one of
           "<", "<=", "=", "!=", ">=", ">" a relative version comparison is
           done, otherwise the specified string is matched with shell-style
           globs.

           Note that using the kernel version string is an unreliable way to
           determine which features are supported by a kernel, because of the
           widespread practice of backporting drivers, features, and fixes
           from newer upstream kernels into older versions provided by
           distributions. Hence, this check is inherently unportable and
           should not be used for units which may be used on different
           distributions.

           ConditionSecurity= may be used to check whether the given security
           technology is enabled on the system. Currently, the recognized
           values are "selinux", "apparmor", "tomoyo", "ima", "smack", "audit"
           and "uefi-secureboot". The test may be negated by prepending an
           exclamation mark.

           ConditionCapability= may be used to check whether the given
           capability exists in the capability bounding set of the service
           manager (i.e. this does not check whether capability is actually
           available in the permitted or effective sets, see capabilities(7)
           for details). Pass a capability name such as "CAP_MKNOD", possibly
           prefixed with an exclamation mark to negate the check.

           ConditionACPower= may be used to check whether the system has AC
           power, or is exclusively battery powered at the time of activation
           of the unit. This takes a boolean argument. If set to "true", the
           condition will hold only if at least one AC connector of the system
           is connected to a power source, or if no AC connectors are known.
           Conversely, if set to "false", the condition will hold only if
           there is at least one AC connector known and all AC connectors are
           disconnected from a power source.

           ConditionNeedsUpdate= takes one of /var or /etc as argument,
           possibly prefixed with a "!"  (for inverting the condition). This
           condition may be used to conditionalize units on whether the
           specified directory requires an update because /usr's modification
           time is newer than the stamp file .updated in the specified
           directory. This is useful to implement offline updates of the
           vendor operating system resources in /usr that require updating of
           /etc or /var on the next following boot. Units making use of this
           condition should order themselves before systemd-update-
           done.service(8), to make sure they run before the stamp file's
           modification time gets reset indicating a completed update.

           ConditionFirstBoot= takes a boolean argument. This condition may be
           used to conditionalize units on whether the system is booting up
           with an unpopulated /etc directory (specifically: an /etc with no
           /etc/machine-id). This may be used to populate /etc on the first
           boot after factory reset, or when a new system instance boots up
           for the first time.

           With ConditionPathExists= a file existence condition is checked
           before a unit is started. If the specified absolute path name does
           not exist, the condition will fail. If the absolute path name
           passed to ConditionPathExists= is prefixed with an exclamation mark
           ("!"), the test is negated, and the unit is only started if the
           path does not exist.

           ConditionPathExistsGlob= is similar to ConditionPathExists=, but
           checks for the existence of at least one file or directory matching
           the specified globbing pattern.

           ConditionPathIsDirectory= is similar to ConditionPathExists= but
           verifies whether a certain path exists and is a directory.

           ConditionPathIsSymbolicLink= is similar to ConditionPathExists= but
           verifies whether a certain path exists and is a symbolic link.

           ConditionPathIsMountPoint= is similar to ConditionPathExists= but
           verifies whether a certain path exists and is a mount point.

           ConditionPathIsReadWrite= is similar to ConditionPathExists= but
           verifies whether the underlying file system is readable and
           writable (i.e. not mounted read-only).

           ConditionDirectoryNotEmpty= is similar to ConditionPathExists= but
           verifies whether a certain path exists and is a non-empty
           directory.

           ConditionFileNotEmpty= is similar to ConditionPathExists= but
           verifies whether a certain path exists and refers to a regular file
           with a non-zero size.

           ConditionFileIsExecutable= is similar to ConditionPathExists= but
           verifies whether a certain path exists, is a regular file and
           marked executable.

           ConditionUser= takes a numeric "UID", a UNIX user name, or the
           special value "@system". This condition may be used to check
           whether the service manager is running as the given user. The
           special value "@system" can be used to check if the user id is
           within the system user range. This option is not useful for system
           services, as the system manager exclusively runs as the root user,
           and thus the test result is constant.

           ConditionGroup= is similar to ConditionUser= but verifies that the
           service manager's real or effective group, or any of its auxiliary
           groups match the specified group or GID. This setting does not have
           a special value "@system".

           ConditionControlGroupController= takes a cgroup controller name
           (eg.  "cpu"), verifying that it is available for use on the system.
           For example, a particular controller may not be available if it was
           disabled on the kernel command line with cgroup_disable=controller.
           Multiple controllers may be passed with a space separating them; in
           this case the condition will only pass if all listed controllers
           are available for use. Controllers unknown to systemd are ignored.
           Valid controllers are "cpu", "cpuacct", "io", "blkio", "memory",
           "devices", and "pids".

           ConditionMemory= verifies if the specified amount of system memory
           is available to the current system. Takes a memory size in bytes as
           argument, optionally prefixed with a comparison operator "<", "<=",
           "=", "!=", ">=", ">". On bare-metal systems compares the amount of
           physical memory in the system with the specified size, adhering to
           the specified comparison operator. In containers compares the
           amount of memory assigned to the container instead.

           ConditionCPUs= verifies if the specified number of CPUs is
           available to the current system. Takes a number of CPUs as
           argument, optionally prefixed with a comparison operator "<", "<=",
           "=", "!=", ">=", ">". Compares the number of CPUs in the CPU
           affinity mask configured of the service manager itself with the
           specified number, adhering to the specified comparison operator. On
           physical systems the number of CPUs in the affinity mask of the
           service manager usually matches the number of physical CPUs, but in
           special and virtual environments might differ. In particular, in
           containers the affinity mask usually matches the number of CPUs
           assigned to the container and not the physically available ones.

           If multiple conditions are specified, the unit will be executed if
           all of them apply (i.e. a logical AND is applied). Condition checks
           can be prefixed with a pipe symbol (|) in which case a condition
           becomes a triggering condition. If at least one triggering
           condition is defined for a unit, then the unit will be executed if
           at least one of the triggering conditions apply and all of the
           non-triggering conditions. If you prefix an argument with the pipe
           symbol and an exclamation mark, the pipe symbol must be passed
           first, the exclamation second. Except for
           ConditionPathIsSymbolicLink=, all path checks follow symlinks. If
           any of these options is assigned the empty string, the list of
           conditions is reset completely, all previous condition settings (of
           any kind) will have no effect.

       AssertArchitecture=, AssertVirtualization=, AssertHost=,
       AssertKernelCommandLine=, AssertKernelVersion=, AssertSecurity=,
       AssertCapability=, AssertACPower=, AssertNeedsUpdate=,
       AssertFirstBoot=, AssertPathExists=, AssertPathExistsGlob=,
       AssertPathIsDirectory=, AssertPathIsSymbolicLink=,
       AssertPathIsMountPoint=, AssertPathIsReadWrite=,
       AssertDirectoryNotEmpty=, AssertFileNotEmpty=, AssertFileIsExecutable=,
       AssertUser=, AssertGroup=, AssertControlGroupController=
           Similar to the ConditionArchitecture=, ConditionVirtualization=,
           ..., condition settings described above, these settings add
           assertion checks to the start-up of the unit. However, unlike the
           conditions settings, any assertion setting that is not met results
           in failure of the start job (which means this is logged loudly).
           Note that hitting a configured assertion does not cause the unit to
           enter the "failed" state (or in fact result in any state change of
           the unit), it affects only the job queued for it. Use assertion
           expressions for units that cannot operate when specific
           requirements are not met, and when this is something the
           administrator or user should look into.

           Note that neither assertion nor condition expressions result in
           unit state changes. Also note that both are checked at the time the
           job is to be executed, i.e. long after depending jobs and it itself
           were queued. Thus, neither condition nor assertion expressions are
           suitable for conditionalizing unit dependencies.

       SourcePath=
           A path to a configuration file this unit has been generated from.
           This is primarily useful for implementation of generator tools that
           convert configuration from an external configuration file format
           into native unit files. This functionality should not be used in
           normal units.

MAPPING OF UNIT PROPERTIES TO THEIR INVERSES
       Unit settings that create a relationship with a second unit usually
       show up in properties of both units, for example in systemctl show
       output. In some cases the name of the property is the same as the name
       of the configuration setting, but not always. This table lists the
       properties that are shown on two units which are connected through some
       dependency, and shows which property on "source" unit corresponds to
       which property on the "target" unit.

       Table 3.  Forward and reverse unit properties
       ┌──────────────────────┬───────────────────────┬─────────────────────┐
       │"Forward" property    "Reverse" property    Where used          │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Before=               After=                │ Both are unit file  │
       ├──────────────────────┼───────────────────────┤ options             │
       │After=                Before=               │                     │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Requires=             RequiredBy=           │ A unit file option; │
       │                      │                       │ an option in the    │
       │                      │                       │ [Install] section   │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Wants=                WantedBy=             │ A unit file option; │
       │                      │                       │ an option in the    │
       │                      │                       │ [Install] section   │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │PartOf=               ConsistsOf=           │ A unit file option; │
       │                      │                       │ an automatic        │
       │                      │                       │ property            │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │BindsTo=              BoundBy=              │ A unit file option; │
       │                      │                       │ an automatic        │
       │                      │                       │ property            │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Requisite=            RequisiteOf=          │ A unit file option; │
       │                      │                       │ an automatic        │
       │                      │                       │ property            │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Triggers=             TriggeredBy=          │ Automatic           │
       │                      │                       │ properties, see     │
       │                      │                       │ notes below         │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Conflicts=            ConflictedBy=         │ A unit file option; │
       │                      │                       │ an automatic        │
       │                      │                       │ property            │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │PropagatesReloadTo=   ReloadPropagatedFrom= │ Both are unit file  │
       ├──────────────────────┼───────────────────────┤ options             │
       │ReloadPropagatedFrom= PropagatesReloadTo=   │                     │
       ├──────────────────────┼───────────────────────┼─────────────────────┤
       │Following=            │ n/a                   │ An automatic        │
       │                      │                       │ property            │
       └──────────────────────┴───────────────────────┴─────────────────────┘

       Note: WantedBy= and RequiredBy= are used in the [Install] section to
       create symlinks in .wants/ and .requires/ directories. They cannot be
       used directly as a unit configuration setting.

       Note: ConsistsOf=, BoundBy=, RequisiteOf=, ConflictedBy= are created
       implicitly along with their reverse and cannot be specified directly.

       Note: Triggers= is created implicitly between a socket, path unit, or
       an automount unit, and the unit they activate. By default a unit with
       the same name is triggered, but this can be overridden using Sockets=,
       Service=, and Unit= settings. See systemd.service(5),
       systemd.socket(5), systemd.path(5), and systemd.automount(5) for
       details.  TriggersBy= is created implicitly on the triggered unit.

       Note: Following= is used to group device aliases and points to the
       "primary" device unit that systemd is using to track device state,
       usually corresponding to a sysfs path. It does not show up in the
       "target" unit.

[INSTALL] SECTION OPTIONS
       Unit files may include an "[Install]" section, which carries
       installation information for the unit. This section is not interpreted
       by systemd(1) during runtime; it is used by the enable and disable
       commands of the systemctl(1) tool during installation of a unit.

       Alias=
           A space-separated list of additional names this unit shall be
           installed under. The names listed here must have the same suffix
           (i.e. type) as the unit filename. This option may be specified more
           than once, in which case all listed names are used. At installation
           time, systemctl enable will create symlinks from these names to the
           unit filename. Note that not all unit types support such alias
           names, and this setting is not supported for them. Specifically,
           mount, slice, swap, and automount units do not support aliasing.

       WantedBy=, RequiredBy=
           This option may be used more than once, or a space-separated list
           of unit names may be given. A symbolic link is created in the
           .wants/ or .requires/ directory of each of the listed units when
           this unit is installed by systemctl enable. This has the effect
           that a dependency of type Wants= or Requires= is added from the
           listed unit to the current unit. The primary result is that the
           current unit will be started when the listed unit is started. See
           the description of Wants= and Requires= in the [Unit] section for
           details.

           WantedBy=foo.service in a service bar.service is mostly equivalent
           to Alias=foo.service.wants/bar.service in the same file. In case of
           template units, systemctl enable must be called with an instance
           name, and this instance will be added to the .wants/ or .requires/
           list of the listed unit. E.g.  WantedBy=getty.target in a service
           getty@.service will result in systemctl enable getty@tty2.service
           creating a getty.target.wants/getty@tty2.service link to
           getty@.service.

       Also=
           Additional units to install/deinstall when this unit is
           installed/deinstalled. If the user requests
           installation/deinstallation of a unit with this option configured,
           systemctl enable and systemctl disable will automatically
           install/uninstall units listed in this option as well.

           This option may be used more than once, or a space-separated list
           of unit names may be given.

       DefaultInstance=
           In template unit files, this specifies for which instance the unit
           shall be enabled if the template is enabled without any explicitly
           set instance. This option has no effect in non-template unit files.
           The specified string must be usable as instance identifier.

       The following specifiers are interpreted in the Install section: %n,
       %N, %p, %i, %j, %g, %G, %U, %u, %m, %H, %b, %v. For their meaning see
       the next section.

SPECIFIERS
       Many settings resolve specifiers which may be used to write generic
       unit files referring to runtime or unit parameters that are replaced
       when the unit files are loaded. Specifiers must be known and resolvable
       for the setting to be valid. The following specifiers are understood:

       Table 4. Specifiers available in unit files
       ┌──────────┬─────────────────────┬─────────────────────┐
       │Specifier Meaning             Details             │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%b"      │ Boot ID             │ The boot ID of the  │
       │          │                     │ running system,     │
       │          │                     │ formatted as        │
       │          │                     │ string. See         │
       │          │                     │ random(4) for more  │
       │          │                     │ information.        │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%C"      │ Cache directory     │ This is either      │
       │          │ root                │ /var/cache (for the │
       │          │                     │ system manager) or  │
       │          │                     │ the path            │
       │          │                     │ "$XDG_CACHE_HOME"   │
       │          │                     │ resolves to (for    │
       │          │                     │ user managers).     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%E"      │ Configuration       │ This is either /etc │
       │          │ directory root      │ (for the system     │
       │          │                     │ manager) or the     │
       │          │                     │ path                │
       │          │                     │ "$XDG_CONFIG_HOME"  │
       │          │                     │ resolves to (for    │
       │          │                     │ user managers).     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%f"      │ Unescaped filename  │ This is either the  │
       │          │                     │ unescaped instance  │
       │          │                     │ name (if            │
       │          │                     │ applicable) with /  │
       │          │                     │ prepended (if       │
       │          │                     │ applicable), or the │
       │          │                     │ unescaped prefix    │
       │          │                     │ name prepended with │
       │          │                     │ /. This implements  │
       │          │                     │ unescaping          │
       │          │                     │ according to the    │
       │          │                     │ rules for escaping  │
       │          │                     │ absolute file       │
       │          │                     │ system paths        │
       │          │                     │ discussed above.    │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%h"      │ User home directory │ This is the home    │
       │          │                     │ directory of the    │
       │          │                     │ user running the    │
       │          │                     │ service manager     │
       │          │                     │ instance. In case   │
       │          │                     │ of the system       │
       │          │                     │ manager this        │
       │          │                     │ resolves to         │
       │          │                     │ "/root".            │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%H"      │ Host name           │ The hostname of the │
       │          │                     │ running system at   │
       │          │                     │ the point in time   │
       │          │                     │ the unit            │
       │          │                     │ configuration is    │
       │          │                     │ loaded.             │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%i"      │ Instance name       │ For instantiated    │
       │          │                     │ units this is the   │
       │          │                     │ string between the  │
       │          │                     │ first "@" character │
       │          │                     │ and the type        │
       │          │                     │ suffix. Empty for   │
       │          │                     │ non-instantiated    │
       │          │                     │ units.              │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%I"      │ Unescaped instance  │ Same as "%i", but   │
       │          │ name                │ with escaping       │
       │          │                     │ undone.             │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%j"      │ Final component of  │ This is the string  │
       │          │ the prefix          │ between the last    │
       │          │                     │ "-" and the end of  │
       │          │                     │ the prefix name. If │
       │          │                     │ there is no "-",    │
       │          │                     │ this is the same as │
       │          │                     │ "%p".               │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%J"      │ Unescaped final     │ Same as "%j", but   │
       │          │ component of the    │ with escaping       │
       │          │ prefix              │ undone.             │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%L"      │ Log directory root  │ This is either      │
       │          │                     │ /var/log (for the   │
       │          │                     │ system manager) or  │
       │          │                     │ the path            │
       │          │                     │ "$XDG_CONFIG_HOME"  │
       │          │                     │ resolves to with    │
       │          │                     │ /log appended (for  │
       │          │                     │ user managers).     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%m"      │ Machine ID          │ The machine ID of   │
       │          │                     │ the running system, │
       │          │                     │ formatted as        │
       │          │                     │ string. See         │
       │          │                     │ machine-id(5) for   │
       │          │                     │ more information.   │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%n"      │ Full unit name      │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%N"      │ Full unit name      │ Same as "%n", but   │
       │          │                     │ with the type       │
       │          │                     │ suffix removed.     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%p"      │ Prefix name         │ For instantiated    │
       │          │                     │ units, this refers  │
       │          │                     │ to the string       │
       │          │                     │ before the first    │
       │          │                     │ "@" character of    │
       │          │                     │ the unit name. For  │
       │          │                     │ non-instantiated    │
       │          │                     │ units, same as      │
       │          │                     │ "%N".               │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%P"      │ Unescaped prefix    │ Same as "%p", but   │
       │          │ name                │ with escaping       │
       │          │                     │ undone.             │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%s"      │ User shell          │ This is the shell   │
       │          │                     │ of the user running │
       │          │                     │ the service manager │
       │          │                     │ instance. In case   │
       │          │                     │ of the system       │
       │          │                     │ manager this        │
       │          │                     │ resolves to         │
       │          │                     │ "/bin/sh".          │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%S"      │ State directory     │ This is either      │
       │          │ root                │ /var/lib (for the   │
       │          │                     │ system manager) or  │
       │          │                     │ the path            │
       │          │                     │ "$XDG_CONFIG_HOME"  │
       │          │                     │ resolves to (for    │
       │          │                     │ user managers).     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%t"      │ Runtime directory   │ This is either /run │
       │          │ root                │ (for the system     │
       │          │                     │ manager) or the     │
       │          │                     │ path                │
       │          │                     │ "$XDG_RUNTIME_DIR"  │
       │          │                     │ resolves to (for    │
       │          │                     │ user managers).     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%T"      │ Directory for       │ This is either /tmp │
       │          │ temporary files     │ or the path         │
       │          │                     │ "$TMPDIR", "$TEMP"  │
       │          │                     │ or "$TMP" are set   │
       │          │                     │ to.                 │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%g"      │ User group          │ This is the name of │
       │          │                     │ the group running   │
       │          │                     │ the service manager │
       │          │                     │ instance. In case   │
       │          │                     │ of the system       │
       │          │                     │ manager this        │
       │          │                     │ resolves to "root". │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%G"      │ User GID            │ This is the numeric │
       │          │                     │ GID of the user     │
       │          │                     │ running the service │
       │          │                     │ manager instance.   │
       │          │                     │ In case of the      │
       │          │                     │ system manager this │
       │          │                     │ resolves to "0".    │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%u"      │ User name           │ This is the name of │
       │          │                     │ the user running    │
       │          │                     │ the service manager │
       │          │                     │ instance. In case   │
       │          │                     │ of the system       │
       │          │                     │ manager this        │
       │          │                     │ resolves to "root". │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%U"      │ User UID            │ This is the numeric │
       │          │                     │ UID of the user     │
       │          │                     │ running the service │
       │          │                     │ manager instance.   │
       │          │                     │ In case of the      │
       │          │                     │ system manager this │
       │          │                     │ resolves to "0".    │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%v"      │ Kernel release      │ Identical to uname  │
       │          │                     │ -r output           │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%V"      │ Directory for       │ This is either      │
       │          │ larger and          │ /var/tmp or the     │
       │          │ persistent          │ path "$TMPDIR",     │
       │          │ temporary files     │ "$TEMP" or "$TMP"   │
       │          │                     │ are set to.         │
       ├──────────┼─────────────────────┼─────────────────────┤
       │"%%"      │ Single percent sign │ Use "%%" in place   │
       │          │                     │ of "%" to specify a │
       │          │                     │ single percent      │
       │          │                     │ sign.               │
       └──────────┴─────────────────────┴─────────────────────┘

EXAMPLES
       Example 1. Allowing units to be enabled

       The following snippet (highlighted) allows a unit (e.g.  foo.service)
       to be enabled via systemctl enable:

           [Unit]
           Description=Foo

           [Service]
           ExecStart=/usr/sbin/foo-daemon

           [Install]
           WantedBy=multi-user.target

       After running systemctl enable, a symlink
       /etc/systemd/system/multi-user.target.wants/foo.service linking to the
       actual unit will be created. It tells systemd to pull in the unit when
       starting multi-user.target. The inverse systemctl disable will remove
       that symlink again.

       Example 2. Overriding vendor settings

       There are two methods of overriding vendor settings in unit files:
       copying the unit file from /usr/lib/systemd/system to
       /etc/systemd/system and modifying the chosen settings. Alternatively,
       one can create a directory named unit.d/ within /etc/systemd/system and
       place a drop-in file name.conf there that only changes the specific
       settings one is interested in. Note that multiple such drop-in files
       are read if present, processed in lexicographic order of their
       filename.

       The advantage of the first method is that one easily overrides the
       complete unit, the vendor unit is not parsed at all anymore. It has the
       disadvantage that improvements to the unit file by the vendor are not
       automatically incorporated on updates.

       The advantage of the second method is that one only overrides the
       settings one specifically wants, where updates to the unit by the
       vendor automatically apply. This has the disadvantage that some future
       updates by the vendor might be incompatible with the local changes.

       This also applies for user instances of systemd, but with different
       locations for the unit files. See the section on unit load paths for
       further details.

       Suppose there is a vendor-supplied unit
       /usr/lib/systemd/system/httpd.service with the following contents:

           [Unit]
           Description=Some HTTP server
           After=remote-fs.target sqldb.service
           Requires=sqldb.service
           AssertPathExists=/srv/webserver

           [Service]
           Type=notify
           ExecStart=/usr/sbin/some-fancy-httpd-server
           Nice=5

           [Install]
           WantedBy=multi-user.target

       Now one wants to change some settings as an administrator: firstly, in
       the local setup, /srv/webserver might not exist, because the HTTP
       server is configured to use /srv/www instead. Secondly, the local
       configuration makes the HTTP server also depend on a memory cache
       service, memcached.service, that should be pulled in (Requires=) and
       also be ordered appropriately (After=). Thirdly, in order to harden the
       service a bit more, the administrator would like to set the PrivateTmp=
       setting (see systemd.exec(5) for details). And lastly, the
       administrator would like to reset the niceness of the service to its
       default value of 0.

       The first possibility is to copy the unit file to
       /etc/systemd/system/httpd.service and change the chosen settings:

           [Unit]
           Description=Some HTTP server
           After=remote-fs.target sqldb.service memcached.service
           Requires=sqldb.service memcached.service
           AssertPathExists=/srv/www

           [Service]
           Type=notify
           ExecStart=/usr/sbin/some-fancy-httpd-server
           Nice=0
           PrivateTmp=yes

           [Install]
           WantedBy=multi-user.target

       Alternatively, the administrator could create a drop-in file
       /etc/systemd/system/httpd.service.d/local.conf with the following
       contents:

           [Unit]
           After=memcached.service
           Requires=memcached.service
           # Reset all assertions and then re-add the condition we want
           AssertPathExists=
           AssertPathExists=/srv/www

           [Service]
           Nice=0
           PrivateTmp=yes

       Note that for drop-in files, if one wants to remove entries from a
       setting that is parsed as a list (and is not a dependency), such as
       AssertPathExists= (or e.g.  ExecStart= in service units), one needs to
       first clear the list before re-adding all entries except the one that
       is to be removed. Dependencies (After=, etc.) cannot be reset to an
       empty list, so dependencies can only be added in drop-ins. If you want
       to remove dependencies, you have to override the entire unit.

SEE ALSO
       systemd(1), systemctl(1), systemd-system.conf(5), systemd.special(7),
       systemd.service(5), systemd.socket(5), systemd.device(5),
       systemd.mount(5), systemd.automount(5), systemd.swap(5),
       systemd.target(5), systemd.path(5), systemd.timer(5), systemd.scope(5),
       systemd.slice(5), systemd.time(7), systemd-analyze(1), capabilities(7),
       systemd.directives(7), uname(1)

NOTES
        1. Interface Stability Promise
           https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise



systemd 242                                                    SYSTEMD.UNIT(5)