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systemd.unit


SYNOPSIS

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

       /etc/systemd/system/*
       /run/systemd/system/*
       /usr/lib/systemd/system/*
       ...

       $XDG_CONFIG_HOME/systemd/user/*
       $HOME/.config/systemd/user/*
       /etc/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/user/*
       /run/systemd/user/*
       $XDG_DATA_HOME/systemd/user/*
       $HOME/.local/share/systemd/user/*
       /usr/lib/systemd/user/*
       ...


DESCRIPTION

       A unit configuration file 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 temporary system state
       snapshot, a resource management slice or a group of externally created
       processes. The syntax is inspired by XDG Desktop Entry
       Specification[1].desktop files, which are in turn inspired by Microsoft
       Windows .ini files.

       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.snapshot(5).  systemd.slice(5).  systemd.scope(5).

       Various settings are allowed to be specified more than once, in which
       case the interpretation depends on the setting. Often, multiple
       settings form a list, and setting to an empty value "resets", which
       means that previous assignments are ignored. When this is allowed, it
       is mentioned in the description of the setting. Note that using
       multiple assignments to the same value makes the unit file incompatible
       with parsers for the XDG .desktop file format.

       Time span values encoded in unit files can be written in various
       formats. A stand-alone number specifies a time in seconds. If suffixed
       with a time unit, the unit is honored. A concatenation of multiple
       values with units is supported, in which case the values are added up.
       Example: "50" refers to 50 seconds; "2min 200ms" refers to 2 minutes
       plus 200 milliseconds, i.e. 120200ms. The following time units are
       understood: s, min, h, d, w, ms, us. For details see systemd.time(7).

       Empty lines and lines starting with # or ; are ignored. This may be
       used for commenting. Lines ending in a backslash are concatenated with
       the following line while reading and the backslash is replaced by a
       space character. This may be used to wrap long lines.

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

       Along with a unit file foo.service, a directory foo.service.d/ may
       exist. All files with the suffix ".conf" from this directory will be
       parsed after the file itself is parsed. This is useful to alter or add
       configuration settings to a unit, without having to modify their unit
       files. Make sure that the file that is included has the appropriate
       section headers before any directive. Note that for instanced units
       this logic will first look for the instance ".d/" subdirectory and read
       its ".conf" files, followed by the template ".d/" subdirectory and
       reads its ".conf" files.

       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.

       Some unit names reflect paths existing in the file system namespace.
       Example: a device unit dev-sda.device refers to a device with the
       device node /dev/sda in the file system namespace. If this applies, a
       special way to escape the path name is used, so that the result is
       usable as part of a filename. Basically, given a path, "/" is replaced
       by "-" and all other characters which are not ASCII alphanumerics are
       replaced by C-style "\x2d" escapes (except that "_" is never replaced
       and "." is only replaced when it would be the first character in the
       escaped path). The root directory "/" is encoded as single dash, while
       otherwise the initial and ending "/" are removed from all paths during
       transformation. This escaping is reversible. Properly escaped paths can
       be generated using the systemd-escape(1) command.
       options. See below for details.

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

       The unit file format is covered by the Interface Stability Promise[2].


UNIT 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.

       Table 1.  Load path when running in system mode (--system).
       +------------------------+---------------------+
       |Path                    | Description         |
       +------------------------+---------------------+
       |/etc/systemd/system     | Local configuration |
       +------------------------+---------------------+
       |/run/systemd/system     | Runtime units       |
       +------------------------+---------------------+
       |/usr/lib/systemd/system | Units of installed  |
       |                        | packages            |
       +------------------------+---------------------+

       Additional units might be loaded into systemd ("linked") from
       directories not on the unit load path. See the link command for
       systemctl(1). Also, some units are dynamically created via a
       systemd.generator(7).


[UNIT] SECTION OPTIONS

       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 free-form string describing the unit. This is intended for use in
           UIs to show descriptive information along with the unit name. The
           description should contain a name that means something to the end
           user.  "Apache2 Web Server" is a good example. Bad examples are
           "high-performance light-weight HTTP server" (too generic) or
           "Apache2" (too specific and meaningless for people who do not know
           Apache).

       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
           option in which case requirement dependencies for all listed names
           will be created. Note that requirement dependencies do not
           influence the order in which services are started or stopped. This
           has to be configured independently with the After= or Before=
           options. If a unit foo.service requires a unit bar.service as
           configured with Requires= and no ordering is configured with After=
           or Before=, then both units will be started simultaneously and
           without any delay between them if foo.service is activated. Often
           it is a better choice to use Wants= instead of Requires= in order
           to achieve a system that is more robust when dealing with failing
           services.

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

       RequiresOverridable=
           Similar to Requires=. Dependencies listed in RequiresOverridable=
           which cannot be fulfilled or fail to start are ignored if the
           startup was explicitly requested by the user. If the start-up was
           pulled in indirectly by some dependency or automatic start-up of
           units that is not requested by the user, this dependency must be
           fulfilled and otherwise the transaction fails. Hence, this option
           may be used to configure dependencies that are normally honored
           unless the user explicitly starts up the unit, in which case
           whether they failed or not is irrelevant.

       Requisite=, RequisiteOverridable=
           Similar to Requires= and RequiresOverridable=, respectively.
           However, if the units listed here are not started already, they
           will not be started and the transaction will fail immediately.

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

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

       BindsTo=
           Configures requirement dependencies, very similar in style to
           Requires=, however in addition to this behavior, it also declares
           that this unit is stopped when any of the units listed suddenly
           disappears. Units can suddenly, unexpectedly disappear if a service
           terminates on its own choice, a device is unplugged or a mount
           point unmounted without involvement of systemd.

           If a unit A that conflicts with a unit B is scheduled to be started
           at the same time as B, the transaction will either fail (in case
           both are required part 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 the 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=
           A space-separated list of unit names. Configures ordering
           dependencies between units. If a unit foo.service contains a
           setting Before=bar.service and both units are being started,
           bar.service's start-up is delayed until foo.service is started up.
           Note that this setting is independent of and orthogonal to the
           requirement dependencies as configured by Requires=. It is a common
           pattern to include a unit name in both the After= and Requires=
           option, in which case the unit listed will be started before the
           unit that is configured with these options. This option may be
           specified more than once, in which case ordering dependencies for
           all listed names are created.  After= is the inverse of Before=,
           i.e. while After= ensures that the configured unit is started after
           the listed unit finished starting up, Before= ensures the opposite,
           i.e. that the configured unit is fully started up before the listed
           unit is started. Note that when two units with an ordering
           dependency between them are shut down, the inverse of the start-up
           order is applied. i.e. if a unit is configured with After= on
           another unit, the former is stopped before the latter if both are
           shut down. If one unit with an ordering dependency on another unit
           is shut down while the latter is started up, the shut down is
           ordered before the start-up regardless of whether the ordering
           dependency is actually of type After= or Before=. If two units have
           no ordering dependencies between them, they are shut down or
           started up simultaneously, and no ordering takes place.

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

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

       JoinsNamespaceOf=
           For units that start processes (such as service units), lists one
           or more other units whose network and/or temporary file namespace
           to join. This only applies to unit types which support the
           PrivateNetwork= and PrivateTmp= directives (see systemd.exec(5) for
           will be ignored for the purposes of this option. If such a mount
           should be a requirement for this unit, direct dependencies on the
           mount units may be added (Requires= and After= or some other
           combination).

       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.

       IgnoreOnSnapshot=
           Takes a boolean argument. If true, this unit will not be included
           in snapshots. Defaults to true for device and snapshot units, false
           for the others.

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

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

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

       DefaultDependencies=
           Takes a boolean argument. If true, (the default), a few default
           however will not change state or even enter the "failed" mode. This
           value defaults to 0 (job timeouts disabled), except for device
           units. NB: this timeout is independent from any unit-specific
           timeout (for example, the timeout set with StartTimeoutSec= in
           service units) as the job timeout has no effect on the unit itself,
           only on the job that might be pending for it. Or in other words:
           unit-specific timeouts are useful to abort unit state changes, and
           revert them. The job timeout set with this option however is useful
           to abort only the job waiting for the unit state to change.

           JobTimeoutAction= optionally configures an additional action to
           take when the time-out is hit. It takes the same values as the
           per-service StartLimitAction= setting, see systemd.service(5) for
           details. Defaults to none.  JobTimeoutRebootArgument= configures an
           optional reboot string to pass to the reboot(2) system call.

       ConditionArchitecture=, ConditionVirtualization=, ConditionHost=,
       ConditionKernelCommandLine=, ConditionSecurity=, ConditionCapability=,
       ConditionACPower=, ConditionNeedsUpdate=, ConditionFirstBoot=,
       ConditionPathExists=, ConditionPathExistsGlob=,
       ConditionPathIsDirectory=, ConditionPathIsSymbolicLink=,
       ConditionPathIsMountPoint=, ConditionPathIsReadWrite=,
       ConditionDirectoryNotEmpty=, ConditionFileNotEmpty=,
       ConditionFileIsExecutable=
           Before starting a unit verify that the specified condition is true.
           If it is not true, the starting of the unit will be skipped,
           however all ordering dependencies of it are still respected. A
           failing condition will not result in the unit being moved into a
           failure state. The condition is checked at the time the queued
           start job is to be executed.

           ConditionArchitecture= may be used to check whether the system is
           running on a specific architecture. Takes one of x86, x86-64, ppc,
           ppc-le, ppc64, ppc64-le, ia64, parisc, parisc64, s390, s390x,
           sparc, sparc64, mips, mips-le, mips64, mips64-le, alpha, arm,
           arm-be, arm64, arm64-be, sh, sh64, m86k, tilegx, cris to test
           against a specific architecture. The architecture is determined
           from the information returned by uname(2) and is thus subject to
           personality(2). Note that a Personality= setting in the same unit
           file has no effect on this condition. A special architecture name
           native is mapped to the architecture the system manager itself is
           compiled for. The test may be negated by prepending an exclamation
           mark.

           ConditionVirtualization= may be used to check whether the system is
           executed in a virtualized environment and optionally test whether
           it is a specific implementation. Takes either boolean value to
           check if being executed in any virtualized environment, or one of
           vm and container to test against a generic type of virtualization
           solution, or one of qemu, kvm, zvm, vmware, microsoft, oracle, xen,
           bochs, uml, openvz, lxc, lxc-libvirt, systemd-nspawn, docker to
           test against a specific implementation. See systemd-detect-virt(1)
           exclamation mark unset). The argument must either be a single word,
           or an assignment (i.e. two words, separated "="). In the former
           case the kernel command line is searched for the word appearing as
           is, or as left hand side of an assignment. In the latter case, the
           exact assignment is looked for with right and left hand side
           matching.

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

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

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

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

           ConditionFirstBoot= takes a boolean argument. This condition may be
           used to conditionalize units on whether the system is booting up
           with an unpopulated /etc directory. This may be used to populate
           /etc on the first boot after factory reset, or when a new system
           instances boots up for the first time.

           With ConditionPathExists= a file existence condition is checked
           before a unit is started. If the specified absolute path name does
           not exist, the condition will fail. If the absolute path name
           passed to ConditionPathExists= is prefixed with an exclamation mark
           ("!"), the test is negated, and the unit is only started if the
           path does not exist.
           ConditionPathIsReadWrite= is similar to ConditionPathExists= but
           verifies whether the underlying file system is readable and
           writable (i.e. not mounted read-only).

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

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

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

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

       AssertArchitecture=, AssertVirtualization=, AssertHost=,
       AssertKernelCommandLine=, AssertSecurity=, AssertCapability=,
       AssertACPower=, AssertNeedsUpdate=, AssertFirstBoot=,
       AssertPathExists=, AssertPathExistsGlob=, AssertPathIsDirectory=,
       AssertPathIsSymbolicLink=, AssertPathIsMountPoint=,
       AssertPathIsReadWrite=, AssertDirectoryNotEmpty=, AssertFileNotEmpty=,
       AssertFileIsExecutable=
           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 it was triggered by.

       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.


[INSTALL] SECTION OPTIONS

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

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

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

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

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

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


SPECIFIERS

       Many settings resolve specifiers which may be used to write generic
       unit files referring to runtime or unit parameters that are replaced
       when the unit files are loaded. The following specifiers are
       understood:

       Table 2. Specifiers available in unit files
       +----------+---------------------+-------------------------+
       |Specifier | Meaning             | Details                 |
       +----------+---------------------+-------------------------+
       |"%n"      | Full unit name      |                         |
       +----------+---------------------+-------------------------+
       |"%N"      | Unescaped full unit | Same as "%n", but       |
       +----------+---------------------+-------------------------+
       |"%P"      | Unescaped prefix    | Same as "%p", but       |
       |          | name                | with escaping           |
       |          |                     | undone                  |
       +----------+---------------------+-------------------------+
       |"%i"      | Instance name       | For instantiated        |
       |          |                     | units: this is the      |
       |          |                     | string between the      |
       |          |                     | "@" character and       |
       |          |                     | the suffix of the       |
       |          |                     | unit name.              |
       +----------+---------------------+-------------------------+
       |"%I"      | Unescaped instance  | Same as "%i", but       |
       |          | name                | with escaping           |
       |          |                     | undone                  |
       +----------+---------------------+-------------------------+
       |"%f"      | Unescaped filename  | This is either the      |
       |          |                     | unescaped instance      |
       |          |                     | name (if                |
       |          |                     | applicable) with /      |
       |          |                     | prepended (if           |
       |          |                     | applicable), or the     |
       |          |                     | prefix name             |
       |          |                     | prepended with /.       |
       +----------+---------------------+-------------------------+
       |"%c"      | Control group path  | This path does not      |
       |          | of the unit         | include the             |
       |          |                     | /sys/fs/cgroup/systemd/ |
       |          |                     | prefix.                 |
       +----------+---------------------+-------------------------+
       |"%r"      | Control group path  | This usually maps to    |
       |          | of the slice the    | the parent cgroup path  |
       |          | unit is placed in   | of "%c".                |
       +----------+---------------------+-------------------------+
       |"%R"      | Root control group  | For system instances,   |
       |          | path below which    | this resolves to /,     |
       |          | slices and units    | except in containers,   |
       |          | are placed          | where this maps to the  |
       |          |                     | container's root        |
       |          |                     | control group path.     |
       +----------+---------------------+-------------------------+
       |"%t"      | Runtime directory   | This is either /run     |
       |          |                     | (for the system         |
       |          |                     | manager) or the path    |
       |          |                     | "$XDG_RUNTIME_DIR"      |
       |          |                     | resolves to (for user   |
       |          |                     | managers).              |
       +----------+---------------------+-------------------------+
       |"%u"      | User name           | This is the name of the |
       |          |                     | configured user of the  |
       |          |                     | unit, or (if none is    |
       |          |                     | set) the user running   |
       |          |                     | instance), unless the   |
       |          |                     | user has been           |
       |          |                     | configured as a numeric |
       |          |                     | UID in the first place  |
       |          |                     | or the configured user  |
       |          |                     | is the root user.       |
       +----------+---------------------+-------------------------+
       |"%h"      | User home directory | This is the home        |
       |          |                     | directory of the        |
       |          |                     | configured user of the  |
       |          |                     | unit, or (if none is    |
       |          |                     | set) the user running   |
       |          |                     | the systemd user        |
       |          |                     | instance. Similar to    |
       |          |                     | "%U", this specifier is |
       |          |                     | not available for units |
       |          |                     | run by the systemd      |
       |          |                     | system instance, unless |
       |          |                     | the configured user is  |
       |          |                     | the root user.          |
       +----------+---------------------+-------------------------+
       |"%s"      | User shell          | This is the shell of    |
       |          |                     | the configured user of  |
       |          |                     | the unit, or (if none   |
       |          |                     | is set) the user        |
       |          |                     | running the systemd     |
       |          |                     | user instance. Similar  |
       |          |                     | to "%U", this specifier |
       |          |                     | is not available for    |
       |          |                     | units run by the        |
       |          |                     | systemd system          |
       |          |                     | instance, unless the    |
       |          |                     | configured user is the  |
       |          |                     | root user.              |
       +----------+---------------------+-------------------------+
       |"%m"      | Machine ID          | The machine ID of the   |
       |          |                     | running system,         |
       |          |                     | formatted as string.    |
       |          |                     | See machine-id(5) for   |
       |          |                     | more information.       |
       +----------+---------------------+-------------------------+
       |"%b"      | Boot ID             | The boot ID of the      |
       |          |                     | running system,         |
       |          |                     | formatted as string.    |
       |          |                     | See random(4) for more  |
       |          |                     | information.            |
       +----------+---------------------+-------------------------+
       |"%H"      | Host name           | The hostname of the     |
       |          |                     | running system at the   |
       |          |                     | point in time the unit  |
       |          |                     | configuation is loaded. |
       +----------+---------------------+-------------------------+
       Example 1. Allowing units to be enabled

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

           [Unit]
           Description=Foo

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

           [Install]
           WantedBy=multi-user.target

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

       Example 2. Overriding vendor settings

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

       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.

       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
       ConditionPathExists= (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. See below for an example.

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

           [Unit]
           Description=Some HTTP server
           After=remote-fs.target sqldb.service

       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.service(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 dependencies (After=, etc.) cannot be reset to an empty list,
       so dependencies can only be added in drop-ins. If you want to remove
       dependencies, you have to override the entire unit.


SEE ALSO

       systemd(1), systemctl(1), systemd.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.snapshot(5),
       systemd.scope(5), systemd.slice(5), systemd.time(7), systemd-
       analyze(1), capabilities(7), systemd.directives(7), uname(1)
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