man > systemd.unit(5)

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/system.attached/*
       /run/systemd/system/*
       /run/systemd/system.attached/*
       /run/systemd/generator/*
       ...
       /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/*
       $XDG_CONFIG_DIRS/systemd/user/*
       /etc/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/user/*
       /run/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/generator/*
       $XDG_DATA_HOME/systemd/user/*
       $XDG_DATA_DIRS/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(7)
       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.

       Valid unit names consist of a "name prefix" and a dot and a suffix specifying the unit type.
       The "unit prefix" must consist of one or more valid characters (ASCII letters, digits, ":",
       "-", "_", ".", and "\"). The total length of the unit name including the suffix must not
       exceed 256 characters. The type suffix must be one of ".service", ".socket", ".device",
       ".mount", ".automount", ".swap", ".target", ".path", ".timer", ".slice", or ".scope".

       Units names 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. To access those options, applications need to parse
       the unit files on their own.

       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 a symlink, so
       when systemd is asked through D-Bus to load dbus-org.freedesktop.network1.service, it'll load
       systemd-networkd.service. As another example, default.target — the default system target
       started at boot — is commonly symlinked (aliased) to either multi-user.target or
       graphical.target to select what is started by default. Alias names may be used in commands
       like disable, start, stop, status, and similar, and in all unit dependency directives,
       including Wants=, Requires=, Before=, After=. Aliases cannot be used with the preset command.

       Aliases obey the following restrictions: a unit of a certain type (".service", ".socket",
       ...) can only be aliased by a name with the same type suffix. A plain unit (not a template or
       an instance), may only be aliased by a plain name. A template instance may only be aliased by
       another template instance, and the instance part must be identical. A template may be aliased
       by another template (in which case the alias applies to all instances of the template). As a
       special case, a template instance (e.g.  "alias AT inst.service") may be a symlink to different
       template (e.g.  "template AT inst.service"). In that case, just this specific instance is
       aliased, while other instances of the template (e.g.  "alias AT foo.service",
       "alias AT bar.service") are not aliased. Those rule preserve the requirement that the instance
       (if any) is always uniquely defined for a given unit and all its aliases.

       Unit files may specify aliases through the Alias= directive in the [Install] section. 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,
       the symlink /etc/systemd/system/ctrl-alt-del.service pointing to the reboot.target file will
       be created, and when Ctrl+Alt+Del is invoked, systemd will look for the ctrl-alt-del.service
       and execute reboot.service.  systemd does not look at the [Install] section at all during
       normal operation, so any directives in that section only have an effect through the symlinks
       created during enablement.

       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. Similar functionality exists for Requires= type dependencies as well, the directory
       suffix is .requires/ in this case. This functionality 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/ or
       .requires/ directory of a unit file is by embedding the dependency in [Install] section of
       the target unit, and creating the symlink in the file system with the enable or preset
       commands of systemctl(1).

       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 merged in the alphanumeric order and
       parsed after the main unit file itself has been parsed. This is useful to alter or add
       configuration settings for a unit, without having to modify unit files. Each drop-in file
       must contain appropriate section headers. For instantiated units, this logic will first look
       for the instance ".d/" subdirectory (e.g.  "foo AT bar.d/") and read its ".conf" files,
       followed by the template ".d/" subdirectory (e.g.  "foo@.service.d/") and the ".conf" files
       there. Moreover for unit names containing dashes ("-"), the set of directories generated by
       repeatedly 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 cases of unit aliases (described above), dropins for the aliased name and all aliases are
       loaded. In the example of default.target aliasing graphical.target, default.target.d/,
       default.target.wants/, default.target.requires/, graphical.target.d/,
       graphical.target.wants/, graphical.target.requires/ would all be read. For templates, dropins
       for the template, any template aliases, the template instance, and all alias instances are
       read. When just a specific template instance is aliased, then the dropins for the target
       template, the target template instance, and the alias template instance are read.

       In addition to /etc/systemd/system, the drop-in ".d/" directories for system services can be
       placed in /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 /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.

       Units also support a top-level drop-in with type.d/, where type may be e.g.  "service" or
       "socket", that allows altering or adding to the settings of all corresponding unit files on
       the system. The formatting and precedence of applying drop-in configurations follow what is
       defined above. Files in type.d/ have lower precedence compared to files in name-specific
       override directories. The usual rules apply: multiple drop-in files with different names are
       applied in lexicographic order, regardless of which of the directories they reside in, so a
       file in type.d/ applies to a unit only if there are no drop-ins or masks with that name in
       directories with higher precedence. See Examples.

       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 AT 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 Portability and 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 using the   │
       │/run/systemd/system.control   │ 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                     │
       ├──────────────────────────────┼───────────────────────────────────┤
       │/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                       │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$XDG_CONFIG_HOME/systemd/user.control   │ Persistent and transient          │
       │or ~/.config/systemd/user.control       │ configuration created using the   │
       ├────────────────────────────────────────┤ dbus API ($XDG_CONFIG_HOME is     │
       │$XDG_RUNTIME_DIR/systemd/user.control   │ used if set, ~/.config otherwise) │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │/run/systemd/transient                  │ Dynamic configuration for         │
       │                                        │ transient units                   │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │/run/systemd/generator.early            │ Generated units with high         │
       │                                        │ priority (see early-dir in        │
       │                                        │ systemd.generator(7))             │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$XDG_CONFIG_HOME/systemd/user or        │ User configuration                │
       │$HOME/.config/systemd/user              │ ($XDG_CONFIG_HOME is used if set, │
       │                                        │ ~/.config otherwise)              │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$XDG_CONFIG_DIRS/systemd/user or        │ Additional configuration          │
       │/etc/xdg/systemd/user                   │ directories as specified by the   │
       │                                        │ XDG base directory specification  │
       │                                        │ ($XDG_CONFIG_DIRS is used if set, │
       │                                        │ /etc/xdg 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))             │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$XDG_DATA_HOME/systemd/user or          │ Units of packages that have been  │
       │$HOME/.local/share/systemd/user         │ installed in the home directory   │
       │                                        │ ($XDG_DATA_HOME is used if set,   │
       │                                        │ ~/.local/share otherwise)         │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$XDG_DATA_DIRS/systemd/user or          │ Additional data directories as    │
       │/usr/local/share/systemd/user and       │ specified by the XDG base         │
       │/usr/share/systemd/user                 │ directory specification           │
       │                                        │ ($XDG_DATA_DIRS is used if set,   │
       │                                        │ /usr/local/share and /usr/share   │
       │                                        │ otherwise)                        │
       ├────────────────────────────────────────┼───────────────────────────────────┤
       │$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 from directories not on the unit load
       path by creating a symlink pointing to a unit file in the directories. You can use systemctl
       link for this operation. See systemctl(1) for its usage and precaution.

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 short human readable title of the unit. This may be used by systemd (and other UIs) as
           a user-visible label for the unit, so this string should identify the unit rather than
           describe it, despite the name. This string also shouldn't just repeat the unit name.
           "Apache2 Web Server" is a good example. Bad examples are "high-performance light-weight
           HTTP server" (too generic) or "Apache2" (meaningless for people who do not know Apache,
           duplicates the unit name).  systemd may 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.

       Wants=
           Configures (weak) requirement dependencies on other units. This option may be specified
           more than once or multiple space-separated units may be specified in one option in which
           case dependencies for all listed names will be created. Dependencies of this type may
           also be configured outside of the unit configuration file by adding a symlink to a
           .wants/ directory accompanying the unit file. For details, see above.

           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, and this unit will still be started. This is
           the recommended way to hook the start-up of one unit to the start-up of another unit.

           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 unit foo.service pulls in unit bar.service as configured with Wants= 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.

       Requires=
           Similar to Wants=, but declares a stronger requirement dependency. Dependencies of this
           type may also be configured by adding a symlink to a .requires/ directory accompanying
           the unit file.

           If this unit gets activated, the units listed 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.

           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).

       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.

       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.

       Upholds=
           Configures dependencies similar to Wants=, but as long a this unit is up, all units
           listed in Upholds= are started whenever found to be inactive or failed, and no job is
           queued for them. While a Wants= dependency on another unit has a one-time effect when
           this units started, a Upholds= dependency on it has a continuous effect, constantly
           restarting the unit if necessary. This is an alternative to the Restart= setting of
           service units, to ensure they are kept running whatever happens.

           When Upholds=b.service is used on a.service, this dependency will show as
           UpheldBy=a.service in the property listing of b.service. The UpheldBy= 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 does not imply an ordering dependency, similarly to the Wants= and
           Requires= dependencies described above. This means that to ensure that the conflicting
           unit is stopped before the other unit is started, an After= or Before= dependency must be
           declared. It doesn't matter which of the two ordering dependencies is used, because stop
           jobs are always ordered before start jobs, see the discussion in Before=/After= below.

           If unit A that conflicts with 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 may be specified
           more than once, in which case dependencies for all listed names are created.

           Those two settings configure ordering dependencies between units. If unit foo.service
           contains the setting Before=bar.service and both units are being started, bar.service's
           start-up is delayed until foo.service has finished starting up.  After= is the inverse of
           Before=, i.e. while Before= ensures that the configured unit is started before the listed
           unit begins starting up, After= ensures the opposite, that the listed unit is fully
           started up before the configured unit is started.

           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. Note that this does includes ExecStartPost= (or ExecStopPost= for the
           shutdown case).

           Note that those settings are independent of and orthogonal to the requirement
           dependencies as configured by Requires=, Wants=, Requisite=, or BindsTo=. It is a common
           pattern to include a unit name in both the After= and Wants= options, in which case the
           unit listed will be started before the unit that is configured with these options.

           Note that Before= dependencies on device units have no effect and are not supported.
           Devices generally become available as a result of an external hotplug event, and systemd
           creates the corresponding device unit without delay.

       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.

       OnSuccess=
           A space-separated list of one or more units that are activated when this unit enters the
           "inactive" state.

       PropagatesReloadTo=, ReloadPropagatedFrom=
           A space-separated list of one or more units to which reload requests from this unit shall
           be propagated to, or units from which reload requests shall be propagated to this unit,
           respectively. Issuing a reload request on a unit will automatically also enqueue reload
           requests on all units that are linked to it using these two settings.

       PropagatesStopTo=, StopPropagatedFrom=
           A space-separated list of one or more units to which stop requests from this unit shall
           be propagated to, or units from which stop requests shall be propagated to this unit,
           respectively. Issuing a stop request on a unit will automatically also enqueue stop
           requests on all units that are linked to it using 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=, PrivateIPC=, IPCNamespacePath=,
           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/, IPC namespace
           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=, PrivateIPC=/IPCNamespacePath=
           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, 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 yes, (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 no. It is highly
           recommended to leave this option enabled for the majority of common units. If set to no,
           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=
           JobTimeoutSec= specifies a timeout for the whole job that starts running when the job is
           queued.  JobRunningTimeoutSec= specifies a timeout that starts running when the queued
           job is actually started. If either limit is reached, the job will be cancelled, the unit
           however will not change state or even enter the "failed" mode.

           Both settings take a time span with the default unit of seconds, but other units may be
           specified, see systemd.time(5). The default is "infinity" (job timeouts disabled), except
           for device units where JobRunningTimeoutSec= defaults to DefaultTimeoutStartSec=.

           Note: these timeouts are independent from any unit-specific timeouts (for example, the
           timeout set with TimeoutStartSec= in service units). The job timeout has no effect on the
           unit itself. 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 span are not permitted to start any more. Use StartLimitIntervalSec= to
           configure the checking interval and StartLimitBurst= to configure how many starts per
           interval are allowed.

           interval is a time span with the default unit of seconds, but other units may be
           specified, see systemd.time(5). Defaults to DefaultStartLimitIntervalSec= in manager
           configuration file, and may be set to 0 to disable any kind of rate limiting.  burst is a
           number and 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 or
           from a timer or socket at a later point, after the interval has passed. From that point
           on, the restart logic is activated again.  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. Rate-limiting is enforced
           after any unit condition checks are executed, and hence unit activations with failing
           conditions do not count towards the rate limit.

           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.

           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.

       StartLimitAction=
           Configure an additional action to take if the rate limit configured with
           StartLimitIntervalSec= and StartLimitBurst= is hit. Takes the same values as the
           FailureAction=/SuccessAction= settings. If none is set, hitting the rate limit will
           trigger no action except 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.

       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.

   Conditions and Asserts
       Unit files may also include a number of Condition...= and Assert...= settings. Before the
       unit is started, systemd will verify that the specified conditions and asserts are true. If
       not, the starting of the unit will be (mostly silently) skipped (in case of conditions), or
       aborted with an error message (in case of asserts). Failing conditions or asserts will not
       result in the unit being moved into the "failed" state. The conditions and asserts are
       checked at the time the queued start job is to be executed. The ordering dependencies are
       still respected, so other units are still pulled in and ordered as if this unit was
       successfully activated, and the conditions and asserts are executed the precise moment the
       unit would normally start and thus can validate system state after the units ordered before
       completed initialization. Use condition expressions for skipping units that do not apply to
       the local system, for example because the kernel or runtime environment doesn't require their
       functionality.

       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 use a pipe symbol ("|") after the equals sign
       ("Condition...=|..."), which causes the condition to become a triggering condition. If at
       least one triggering condition is defined for a unit, then the unit will be started if at
       least one of the triggering conditions of the unit applies and all of the regular (i.e.
       non-triggering) conditions apply. If you prefix an argument with the pipe symbol and an
       exclamation mark, the pipe symbol must be passed first, the exclamation second. 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.

       The AssertArchitecture=, AssertVirtualization=, ... options are similar to conditions but
       cause the start job to fail (instead of being skipped). The failed check is logged. Units
       with failed conditions are considered to be in a clean state and will be garbage collected if
       they are not referenced. This means that when queried, the condition failure may or may not
       show up in the state of the unit.

       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.

       The condition verb of systemd-analyze(1) can be used to test condition and assert
       expressions.

       Except for ConditionPathIsSymbolicLink=, all path checks follow symlinks.

       ConditionArchitecture=
           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",
           or "native".

           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.

       ConditionFirmware=
           Check whether the system's firmware is of a certain type. Possible values are: "uefi"
           (for systems with EFI), "device-tree" (for systems with a device tree) and
           "device-tree-compatible(xyz)" (for systems with a device tree that is compatible to
           "xyz").

       ConditionVirtualization=
           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", "amazon", "zvm",
           "vmware", "microsoft", "oracle", "powervm", "xen", "bochs", "uml", "bhyve", "qnx",
           "openvz", "lxc", "lxc-libvirt", "systemd-nspawn", "docker", "podman", "rkt", "wsl",
           "proot", "pouch", "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=
           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=
           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 by "="). 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. This operates on the kernel command line communicated
           to userspace via /proc/cmdline, except when the service manager is invoked as payload of
           a container manager, in which case the command line of PID 1 is used instead (i.e.
           /proc/1/cmdline).

       ConditionKernelVersion=
           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 list of (potentially quoted) expressions. For each of
           the expressions, if it 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.

       ConditionEnvironment=
           ConditionEnvironment= may be used to check whether a specific environment variable is set
           (or if prefixed with the exclamation mark — unset) in the service manager's environment
           block. The argument may be a single word, to check if the variable with this name is
           defined in the environment block, or an assignment ("name=value"), to check if the
           variable with this exact value is defined. Note that the environment block of the service
           manager itself is checked, i.e. not any variables defined with Environment= or
           EnvironmentFile=, as described above. This is particularly useful when the service
           manager runs inside a containerized environment or as per-user service manager, in order
           to check for variables passed in by the enclosing container manager or PAM.

       ConditionSecurity=
           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", "uefi-secureboot" and "tpm2". The test may be negated by
           prepending an exclamation mark.

       ConditionCapability=
           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=
           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 "!"  (to invert 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.

           If the systemd.condition-needs-update= option is specified on the kernel command line
           (taking a boolean), it will override the result of this condition check, taking
           precedence over any file modification time checks. If the kernel command line option is
           used, systemd-update-done.service will not have immediate effect on any following
           ConditionNeedsUpdate= checks, until the system is rebooted where the kernel command line
           option is not specified anymore.

           Note that to make this scheme effective, the timestamp of /usr/ should be explicitly
           updated after its contents are modified. The kernel will automatically update
           modification timestamp on a directory only when immediate children of a directory are
           modified; an modification of nested files will not automatically result in mtime of /usr/
           being updated.

           Also note that if the update method includes a call to execute appropriate post-update
           steps itself, it should not touch the timestamp of /usr/. In a typical distribution
           packaging scheme, packages will do any required update steps as part of the installation
           or upgrade, to make package contents immediately usable.  ConditionNeedsUpdate= should be
           used with other update mechanisms where such an immediate update does not happen.

       ConditionFirstBoot=
           Takes a boolean argument. This condition may be used to conditionalize units on whether
           the system is booting up for the first time. This roughly means that /etc/ is unpopulated
           (for details, see "First Boot Semantics" in machine-id(5)). 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.

           For robustness, units with ConditionFirstBoot=yes should order themselves before
           first-boot-complete.target and pull in this passive target with Wants=. This ensures that
           in a case of an aborted first boot, these units will be re-run during the next system
           startup.

           If the systemd.condition-first-boot= option is specified on the kernel command line
           (taking a boolean), it will override the result of this condition check, taking
           precedence over /etc/machine-id existence checks.

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

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

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

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

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

       ConditionPathIsEncrypted=
           ConditionPathIsEncrypted= is similar to ConditionPathExists= but verifies that the
           underlying file system's backing block device is encrypted using dm-crypt/LUKS. Note that
           this check does not cover ext4 per-directory encryption, and only detects block level
           encryption. Moreover, if the specified path resides on a file system on top of a loopback
           block device, only encryption above the loopback device is detected. It is not detected
           whether the file system backing the loopback block device is encrypted.

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

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

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

       ConditionUser=
           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=
           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 support the special value "@system".

       ConditionControlGroupController=
           Check whether given cgroup controllers (e.g.  "cpu") are available for use on the system
           or whether the legacy v1 cgroup or the modern v2 cgroup hierarchy is used.

           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". Even if available in the kernel, a particular controller
           may not be available if it was disabled on the kernel command line with
           cgroup_disable=controller.

           Alternatively, two special strings "v1" and "v2" may be specified (without any controller
           names).  "v2" will pass if the unified v2 cgroup hierarchy is used, and "v1" will pass if
           the legacy v1 hierarchy or the hybrid hierarchy are used (see the discussion of
           systemd.unified_cgroup_hierarchy and systemd.legacy_systemd_cgroup_controller in
           systemd.service(5) for more information).

       ConditionMemory=
           Verify that 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=
           Verify that 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.

       ConditionCPUFeature=
           Verify that a given CPU feature is available via the "CPUID" instruction. This condition
           only does something on i386 and x86-64 processors. On other processors it is assumed that
           the CPU does not support the given feature. It checks the leaves "1", "7", "0x80000001",
           and "0x80000007". Valid values are: "fpu", "vme", "de", "pse", "tsc", "msr", "pae",
           "mce", "cx8", "apic", "sep", "mtrr", "pge", "mca", "cmov", "pat", "pse36", "clflush",
           "mmx", "fxsr", "sse", "sse2", "ht", "pni", "pclmul", "monitor", "ssse3", "fma3", "cx16",
           "sse4_1", "sse4_2", "movbe", "popcnt", "aes", "xsave", "osxsave", "avx", "f16c",
           "rdrand", "bmi1", "avx2", "bmi2", "rdseed", "adx", "sha_ni", "syscall", "rdtscp", "lm",
           "lahf_lm", "abm", "constant_tsc".

       ConditionOSRelease=
           Verify that a specific "key=value" pair is set in the host's os-release(5).

           Other than exact matching with "=", and "!=", relative comparisons are supported for
           versioned parameters (e.g.  "VERSION_ID"). The comparator can be one of "<", "<=", "=",
           "!=", ">=" and ">".

       AssertArchitecture=, AssertVirtualization=, AssertHost=, AssertKernelCommandLine=,
       AssertKernelVersion=, AssertEnvironment=, AssertSecurity=, AssertCapability=, AssertACPower=,
       AssertNeedsUpdate=, AssertFirstBoot=, AssertPathExists=, AssertPathExistsGlob=,
       AssertPathIsDirectory=, AssertPathIsSymbolicLink=, AssertPathIsMountPoint=,
       AssertPathIsReadWrite=, AssertPathIsEncrypted=, AssertDirectoryNotEmpty=,
       AssertFileNotEmpty=, AssertFileIsExecutable=, AssertUser=, AssertGroup=,
       AssertControlGroupController=, AssertMemory=, AssertCPUs=, AssertOSRelease=
           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.

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=                │                                       │
       ├──────────────────────┼───────────────────────┤ [Unit] section                        │
       │After=                │ Before=               │                                       │
       ├──────────────────────┼───────────────────────┼──────────────────┬────────────────────┤
       │Requires=             │ RequiredBy=           │ [Unit] section   │[Install] section   │
       ├──────────────────────┼───────────────────────┼──────────────────┼────────────────────┤
       │Wants=                │ WantedBy=             │ [Unit] section   │[Install] section   │
       ├──────────────────────┼───────────────────────┼──────────────────┼────────────────────┤
       │PartOf=               │ ConsistsOf=           │ [Unit] section   │an automatic        │
       │                      │                       │                  │property            │
       ├──────────────────────┼───────────────────────┼──────────────────┼────────────────────┤
       │BindsTo=              │ BoundBy=              │ [Unit] section   │an automatic        │
       │                      │                       │                  │property            │
       ├──────────────────────┼───────────────────────┼──────────────────┼────────────────────┤
       │Requisite=            │ RequisiteOf=          │ [Unit] section   │an automatic        │
       │                      │                       │                  │property            │
       ├──────────────────────┼───────────────────────┼──────────────────┴────────────────────┤
       │Triggers=             │ TriggeredBy=          │ Automatic properties, see notes below │
       ├──────────────────────┼───────────────────────┼──────────────────┬────────────────────┤
       │Conflicts=            │ ConflictedBy=         │ [Unit] section   │an automatic        │
       │                      │                       │                  │property            │
       ├──────────────────────┼───────────────────────┼──────────────────┴────────────────────┤
       │PropagatesReloadTo=   │ ReloadPropagatedFrom= │                                       │
       ├──────────────────────┼───────────────────────┤ [Unit] section                        │
       │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 reverses 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.  TriggeredBy= 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 AT tty2.service creating a
           getty.target.wants/getty AT 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: %a, %b, %B, %g, %G, %H, %i,
       %j, %l, %m, %n, %N, %o, %p, %u, %U, %v, %w, %W, %%. 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                   │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%a"      │ Architecture              │ A short string            │
       │          │                           │ identifying the           │
       │          │                           │ architecture of the local │
       │          │                           │ system. A string such as  │
       │          │                           │ x86, x86-64 or arm64. See │
       │          │                           │ the architectures defined │
       │          │                           │ for                       │
       │          │                           │ ConditionArchitecture=    │
       │          │                           │ above for a full list.    │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%A"      │ Operating system image    │ The operating system      │
       │          │ version                   │ image version identifier  │
       │          │                           │ of the running system, as │
       │          │                           │ read from the             │
       │          │                           │ IMAGE_VERSION= field of   │
       │          │                           │ /etc/os-release. If not   │
       │          │                           │ set, resolves to an empty │
       │          │                           │ string. See os-release(5) │
       │          │                           │ for more information.     │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%b"      │ Boot ID                   │ The boot ID of the        │
       │          │                           │ running system, formatted │
       │          │                           │ as string. See random(4)  │
       │          │                           │ for more information.     │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%B"      │ Operating system build ID │ The operating system      │
       │          │                           │ build identifier of the   │
       │          │                           │ running system, as read   │
       │          │                           │ from the BUILD_ID= field  │
       │          │                           │ of /etc/os-release. If    │
       │          │                           │ not set, resolves to an   │
       │          │                           │ empty string. See os-     │
       │          │                           │ release(5) for more       │
       │          │                           │ information.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%C"      │ Cache directory root      │ This is either /var/cache │
       │          │                           │ (for the system manager)  │
       │          │                           │ or the path               │
       │          │                           │ "$XDG_CACHE_HOME"         │
       │          │                           │ resolves to (for user     │
       │          │                           │ managers).                │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%E"      │ Configuration directory   │ This is either /etc/ (for │
       │          │ root                      │ the system manager) or    │
       │          │                           │ the path                  │
       │          │                           │ "$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.          │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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".                      │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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". │
       │          │                           │                           │
       │          │                           │ Note that this setting is │
       │          │                           │ not influenced by the     │
       │          │                           │ User= setting             │
       │          │                           │ configurable in the       │
       │          │                           │ [Service] section of the  │
       │          │                           │ service unit.             │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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 name   │ Same as "%i", but with    │
       │          │                           │ escaping undone.          │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%j"      │ Final component of the    │ This is the string        │
       │          │ prefix                    │ between the last "-" and  │
       │          │                           │ the end of the prefix     │
       │          │                           │ name. If there is no "-", │
       │          │                           │ this is the same as "%p". │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%J"      │ Unescaped final component │ Same as "%j", but with    │
       │          │ of the prefix             │ escaping undone.          │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%l"      │ Short host name           │ The hostname of the       │
       │          │                           │ running system at the     │
       │          │                           │ point in time the unit    │
       │          │                           │ configuration is loaded,  │
       │          │                           │ truncated at the first    │
       │          │                           │ dot to remove any domain  │
       │          │                           │ component.                │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%M"      │ Operating system image    │ The operating system      │
       │          │ identifier                │ image identifier of the   │
       │          │                           │ running system, as read   │
       │          │                           │ from the IMAGE_ID= field  │
       │          │                           │ of /etc/os-release. If    │
       │          │                           │ not set, resolves to an   │
       │          │                           │ empty string. See os-     │
       │          │                           │ release(5) for more       │
       │          │                           │ information.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%n"      │ Full unit name            │                           │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%N"      │ Full unit name            │ Same as "%n", but with    │
       │          │                           │ the type suffix removed.  │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%o"      │ Operating system ID       │ The operating system      │
       │          │                           │ identifier of the running │
       │          │                           │ system, as read from the  │
       │          │                           │ ID= field of              │
       │          │                           │ /etc/os-release. See os-  │
       │          │                           │ release(5) for more       │
       │          │                           │ information.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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 name     │ Same as "%p", but 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 root      │ This is either /var/lib   │
       │          │                           │ (for the system manager)  │
       │          │                           │ or the path               │
       │          │                           │ "$XDG_CONFIG_HOME"        │
       │          │                           │ resolves to (for user     │
       │          │                           │ managers).                │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%t"      │ Runtime directory root    │ This is either /run/ (for │
       │          │                           │ the system manager) or    │
       │          │                           │ the path                  │
       │          │                           │ "$XDG_RUNTIME_DIR"        │
       │          │                           │ resolves to (for user     │
       │          │                           │ managers).                │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%T"      │ Directory for temporary   │ This is either /tmp or    │
       │          │ files                     │ the path "$TMPDIR",       │
       │          │                           │ "$TEMP" or "$TMP" are set │
       │          │                           │ to. (Note that the        │
       │          │                           │ directory may be          │
       │          │                           │ specified without a       │
       │          │                           │ trailing slash.)          │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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".  │
       │          │                           │                           │
       │          │                           │ Note that this setting is │
       │          │                           │ not influenced by the     │
       │          │                           │ User= setting             │
       │          │                           │ configurable in the       │
       │          │                           │ [Service] section of the  │
       │          │                           │ service unit.             │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%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".                      │
       │          │                           │                           │
       │          │                           │ Note that this setting is │
       │          │                           │ not influenced by the     │
       │          │                           │ User= setting             │
       │          │                           │ configurable in the       │
       │          │                           │ [Service] section of the  │
       │          │                           │ service unit.             │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%v"      │ Kernel release            │ Identical to uname -r     │
       │          │                           │ output.                   │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%V"      │ Directory for larger and  │ This is either /var/tmp   │
       │          │ persistent temporary      │ or the path "$TMPDIR",    │
       │          │ files                     │ "$TEMP" or "$TMP" are set │
       │          │                           │ to. (Note that the        │
       │          │                           │ directory may be          │
       │          │                           │ specified without a       │
       │          │                           │ trailing slash.)          │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%w"      │ Operating system version  │ The operating system      │
       │          │ ID                        │ version identifier of the │
       │          │                           │ running system, as read   │
       │          │                           │ from the VERSION_ID=      │
       │          │                           │ field of /etc/os-release. │
       │          │                           │ If not set, resolves to   │
       │          │                           │ an empty string. See os-  │
       │          │                           │ release(5) for more       │
       │          │                           │ information.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%W"      │ Operating system variant  │ The operating system      │
       │          │ ID                        │ variant identifier of the │
       │          │                           │ running system, as read   │
       │          │                           │ from the VARIANT_ID=      │
       │          │                           │ field of /etc/os-release. │
       │          │                           │ If not set, resolves to   │
       │          │                           │ an empty string. See os-  │
       │          │                           │ release(5) for more       │
       │          │                           │ information.              │
       ├──────────┼───────────────────────────┼───────────────────────────┤
       │"%%"      │ 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
       /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 /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.

       Example 3. Top level drop-ins with template units

       Top level per-type drop-ins can be used to change some aspect of all units of a particular
       type. For example by creating the /etc/systemd/system/service.d/ directory with a drop-in
       file, the contents of the drop-in file can be applied to all service units. We can take this
       further by having the top-level drop-in instantiate a secondary helper unit. Consider for
       example the following set of units and drop-in files where we install an OnFailure=
       dependency for all service units.

       /etc/systemd/system/failure-handler@.service:

           [Unit]
           Description=My failure handler for %i

           [Service]
           Type=oneshot
           # Perform some special action for when %i exits unexpectedly.
           ExecStart=/usr/sbin/myfailurehandler %i


       We can then add an instance of failure-handler@.service as an OnFailure= dependency for all
       service units.

       /etc/systemd/system/service.d/10-all.conf:

           [Unit]
           OnFailure=failure-handler@%N.service


       Now, after running systemctl daemon-reload all services will have acquired an OnFailure=
       dependency on failure-handler@%N.service. The template instance units will also have gained
       the dependency which results in the creation of a recursive dependency chain. We can break
       the chain by disabling the drop-in for the template instance units via a symlink to
       /dev/null:

           mkdir /etc/systemd/system/failure-handler@.service.d/
           ln -s /dev/null /etc/systemd/system/failure-handler@.service.d/10-all.conf
           systemctl daemon-reload


       This ensures that if a failure-handler@.service instance fails it will not trigger an
       instance named failure-handler AT failure-handler.service.

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 Portability and Stability Promise
           https://systemd.io/PORTABILITY_AND_STABILITY/



systemd 249                                                                          SYSTEMD.UNIT(5)