Name
systemd.service — Service unit configuration



Synopsis
service.service



Description
A unit configuration file whose name ends in ".service" encodes information 
about a process controlled and supervised by systemd.

This man page lists the configuration options specific to this unit type.  See 
systemd.unit(5) for the common options of all unit configuration files. The
common configuration items are configured in the generic [Unit] and [Install]
sections. The service specific configuration options are configured in the 
[Service] section.

Additional options are listed in systemd.exec(5), which define the execution
environment the commands are executed in, and in systemd.kill(5), which define 
the way the processes of the service are terminated, and in 
systemd.resource-control(5), which configure resource control settings for the 
processes of the service.

If SysV init compat is enabled, systemd automatically creates service units that
wrap SysV init scripts (the service name is the same as the name of the script,
with a ".service" suffix added); see systemd-sysv-generator(8).

The systemd-run(1) command allows creating .service and .scope units dynamically
and transiently from the command line.



Service Templates
It is possible for systemd services to take a single argument via the 
"service@argument.service" syntax. Such services are called "instantiated" 
services, while the unit definition without the argument parameter is called a 
"template". An example could be a dhcpcd@.service service template which takes 
a network interface as a parameter to form an instantiated service. Within the 
service file, this parameter or "instance name" can be accessed with 
%-specifiers. See systemd.unit(5) for details.



Automatic Dependencies



Implicit Dependencies
The following dependencies are implicitly added:

Services with Type=dbus set automatically acquire dependencies of type 
Requires= and After= on dbus.socket.

Socket activated services are automatically ordered after their activating
.socket units via an automatic After= dependency. Services also pull in all 
.socket units listed in Sockets= via automatic Wants= and After= dependencies.

Additional implicit dependencies may be added as result of execution and 
resource control parameters as documented in systemd.exec(5) and 
systemd.resource-control(5).



Default Dependencies
The following dependencies are added unless DefaultDependencies=no is set:

Service units will have dependencies of type Requires= and After= on 
sysinit.target, a dependency of type After= on basic.target as well as 
dependencies of type Conflicts= and Before= on shutdown.target. These 
ensure that normal service units pull in basic system initialization, and are 
terminated cleanly prior to system shutdown. Only services involved with early 
boot or late system shutdown should disable this option.

Instanced service units (i.e. service units with an "@" in their name) are 
assigned by default a per-template slice unit (see systemd.slice(5)), named
after the template unit, containing all instances of the specific template.
This slice is normally stopped at shutdown, together with all template instances.
If that is not desired, set DefaultDependencies=no in the template unit, and
either define your own per-template slice unit file that also sets 
DefaultDependencies=no, or set Slice=system.slice (or another suitable slice) 
in the template unit. Also see systemd.resource-control(5).



Options
Service unit files may include [Unit] and [Install] sections, which are 
described in systemd.unit(5).

Service unit files must include a [Service] section, which carries information 
about the service and the process it supervises. A number of options that may 
be used in this section are shared with other unit types. These options are
documented in systemd.exec(5), systemd.kill(5) and systemd.resource-control(5).
The options specific to the [Service] section of service units are the following:


Type=

Configures the mechanism via which the service notifies the manager that the
service start-up has finished. One of simple, exec, forking, oneshot, dbus,
notify, notify-reload, or idle.

If set to simple (the default if ExecStart= is specified but neither 
Type= nor BusName= are), the service manager will consider the unit started
immediately after the main service process has been forked off (i.e. 
immediately after fork(), and before various process attributes have been 
configured and in particular before the new process has called execve() to 
invoke the actual service binary). Typically, Type=exec is the better choice, 
see below.

It is expected that the process configured with ExecStart= is the main 
process of the service. In this mode, if the process offers functionality to
other processes on the system, its communication channels should be installed 
before the service is started up (e.g. sockets set up by systemd, via socket
activation), as the service manager will immediately proceed starting follow-up
units, right after creating the main service process, and before executing the 
service's binary. Note that this means systemctl start command lines for simple
services will report success even if the service's binary cannot be invoked
successfully (for example because the selected User= doesn't exist, or the 
service binary is missing).

The exec type is similar to simple, but the service manager will consider
the unit started immediately after the main service binary has been executed.
The service manager will delay starting of follow-up units until that point. 
(Or in other words: simple proceeds with further jobs right after fork() 
returns, while exec will not proceed before both fork() and execve() in the 
service process succeeded.) Note that this means systemctl start command lines 
for exec services will report failure when the service's binary cannot be 
invoked successfully (for example because the selected User= doesn't exist, or 
the service binary is missing).

If set to forking, the manager will consider the unit started immediately
after the binary that forked off by the manager exits. The use of this type
is discouraged, use notify, notify-reload, or dbus instead.

It is expected that the process configured with ExecStart= will call fork()
as part of its start-up. The parent process is expected to exit when start-up
is complete and all communication channels are set up. The child
continues to run as the main service process, and the service manager will
consider the unit started when the parent process exits. This is the
behavior of traditional UNIX services. If this setting is used, it is
recommended to also use the PIDFile= option, so that systemd can reliably
identify the main process of the service. The manager will proceed with
starting follow-up units after the parent process exits.

Behavior of oneshot is similar to simple; however, the service manager will
consider the unit up after the main process exits. It will then start follow-up
units.  RemainAfterExit= is particularly useful for this type of service.
Type=oneshot is the implied default if neither Type= nor ExecStart= are
specified. Note that if this option is used without RemainAfterExit= the
service will never enter "active" unit state, but will directly transition from 
"activating" to "deactivating" or "dead", since no process is configured that 
shall run continuously. In particular this means that after a service of this 
type ran (and which has RemainAfterExit= not set) it will not show up as started
afterwards, but as dead.

Behavior of dbus is similar to simple; however, units of this type must
have the BusName= specified and the service manager will consider the unit
up when the specified bus name has been acquired. This type is the default
if BusName= is specified.

Service units with this option configured implicitly gain dependencies on
the dbus.socket unit. A service unit of this type is considered to be in the
activating state until the specified bus name is acquired. It is considered
activated while the bus name is taken. Once the bus name is released the
service is considered being no longer functional which has the effect that
the service manager attempts to terminate any remaining processes belonging
to the service. Services that drop their bus name as part of their shutdown
logic thus should be prepared to receive a SIGTERM (or whichever signal is
configured in KillSignal=) as result.

Behavior of notify is similar to exec; however, it is expected that the
service sends a "READY=1" notification message via sd_notify(3) or an 
equivalent call when it has finished starting up. systemd will proceed with 
starting follow -up units after this notification message has been sent. If 
this option is used, NotifyAccess= (see below) should be set to open access 
to the notification socket provided by systemd. If NotifyAccess= is missing 
or set to none, it will be forcibly set to main.

If the service supports reloading, and uses a signal to start the reload,
using notify-reload instead is recommended.

Behavior of notify-reload is similar to notify, with one difference: the
SIGHUP UNIX process signal is sent to the service's main process when the
service is asked to reload and the manager will wait for a notification
about the reload being finished.

When initiating the reload process the service is expected to reply with a
notification message via sd_notify(3) that contains the "RELOADING=1" field in
combination with "MONOTONIC_USEC=" set to the current monotonic time (i.e. 
CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as decimal string. Once 
reloading is complete another notification message must be sent, containing 
"READY=1" . Using this service type and implementing this reload protocol is 
an efficient alternative to providing an ExecReload= command for reloading of 
the service's configuration.

The signal to send can be tweaked via ReloadSignal=, see below.

Behavior of idle is very similar to simple; however, actual execution of
the service program is delayed until all active jobs are dispatched. This
may be used to avoid interleaving of output of shell services with the
status output on the console. Note that this type is useful only to improve
console output, it is not useful as a general unit ordering tool, and the
effect of this service type is subject to a 5s timeout, after which the
service program is invoked anyway.


It is recommended to use Type=exec for long-running services, as it ensures
that process setup errors (e.g. errors such as a missing service executable,
or missing user) are properly tracked. However, as this service type won't
propagate the failures in the service's own startup code (as opposed to
failures in the preparatory steps the service manager executes before
execve()) and doesn't allow ordering of other units against completion of
initialization of the service code itself (which for example is useful if
clients need to connect to the service through some form of IPC, and the IPC
channel is only established by the service itself — in contrast to doing
this ahead of time through socket or bus activation or similar), it might
not be sufficient for many cases. If so, notify, notify-reload, or dbus (the
latter only in case the service provides a D-Bus interface) are the
preferred options as they allow service program code to precisely schedule
when to consider the service started up successfully and when to proceed
with follow-up units. The notify/notify-reload service types require
explicit support in the service codebase (as sd_notify() or an equivalent
API needs to be invoked by the service at the appropriate time) — if it's
not supported, then forking is an alternative: it supports the traditional
heavy-weight UNIX service start-up protocol. Note that using any type other
than simple possibly delays the boot process, as the service manager needs
to wait for at least some service initialization to complete. (Also note it
is generally not recommended to use idle or oneshot for long-running
services.)

Note that various service settings (e.g.  User= , Group= through libc NSS)
might result in "hidden" blocking IPC calls to other services when used. 
Sometimes it might be advisable to use the simple service type to ensure 
that the service manager's transaction logic is not affected by such 
potentially slow operations and hidden dependencies, as this is the only 
service type where the service manager will not wait for such service 
execution setup operations to complete before proceeding.


ExitType=

Specifies when the manager should consider the service to be finished. One
of main or cgroup:

If set to main (the default), the service manager will consider the unit
stopped when the main process, which is determined according to the Type=,
exits. Consequently, it cannot be used with Type=oneshot.

If set to cgroup, the service will be considered running as long as at
least one process in the cgroup has not exited.

It is generally recommended to use ExitType=main when a service has a known
forking model and a main process can reliably be determined. ExitType=
cgroup is meant for applications whose forking model is not known ahead of
time and which might not have a specific main process. It is well suited for
transient or automatically generated services, such as graphical
applications inside of a desktop environment.

Added in version 250.


RemainAfterExit=

Takes a boolean value that specifies whether the service shall be
considered active even when all its processes exited. Defaults to no.


GuessMainPID=

Takes a boolean value that specifies whether systemd should try to guess
the main PID of a service if it cannot be determined reliably. This option
is ignored unless Type=forking is set and PIDFile= is unset because for the
other types or with an explicitly configured PID file, the main PID is
always known. The guessing algorithm might come to incorrect conclusions if
a daemon consists of more than one process. If the main PID cannot be
determined, failure detection and automatic restarting of a service will not
work reliably. Defaults to yes.


PIDFile=

Takes a path referring to the PID file of the service. Usage of this option
is recommended for services where Type= is set to forking. The path
specified typically points to a file below /run/. If a relative path is
specified it is hence prefixed with /run/. The service manager will read the
PID of the main process of the service from this file after start-up of the
service. The service manager will not write to the file configured here,
although it will remove the file after the service has shut down if it still
exists. The PID file does not need to be owned by a privileged user, but if
it is owned by an unprivileged user additional safety restrictions are
enforced: the file may not be a symlink to a file owned by a different user
(neither directly nor indirectly), and the PID file must refer to a process
already belonging to the service.

 Note that PID files should be avoided in modern projects. Use Type=notify,
Type=notify-reload or Type=simple where possible, which does not require use
of PID files to determine the main process of a service and avoids needless
forking.


BusName=

Takes a D-Bus destination name that this service shall use. This option is
mandatory for services where Type= is set to dbus. It is recommended to
always set this property if known to make it easy to map the service name to
the D-Bus destination. In particular, systemctl 
service-log-level/service-log-target verbs make use of this.


ExecStart=

Commands that are executed when this service is started. The value is split
into zero or more command lines according to the rules described in the
section "Command Lines" below.

Unless Type= is oneshot, exactly one command must be given. When
Type=oneshot is used, zero or more commands may be specified. Commands may
be specified by providing multiple command lines in the same directive, or
alternatively, this directive may be specified more than once with the same
effect. If the empty string is assigned to this option, the list of commands
to start is reset, prior assignments of this option will have no effect. If
no ExecStart= is specified, then the service must have RemainAfterExit=yes
and at least one ExecStop= line set. (Services lacking both ExecStart= and
ExecStop= are not valid.)

If more than one command is specified, the commands are invoked
sequentially in the order they appear in the unit file. If one of the
commands fails (and is not prefixed with "-"), other lines are not executed,
and the unit is considered failed.

Unless Type=forking is set, the process started via this command line will
be considered the main process of the daemon.  



ExecStartPre=, ExecStartPost=

Additional commands that are executed before or after the command in
ExecStart=, respectively. Syntax is the same as for ExecStart=, except that
multiple command lines are allowed and the commands are executed one after
the other, serially.

If any of those commands (not prefixed with "-") fail, the rest are not
executed and the unit is considered failed.

ExecStart= commands are only run after all ExecStartPre= commands that were
not prefixed with a "-" exit successfully.

ExecStartPost= commands are only run after the commands specified in
ExecStart= have been invoked successfully, as determined by Type= (i.e. the
process has been started for Type=simple or Type=idle, the last ExecStart=
process exited successfully for Type=oneshot, the initial process exited
successfully for Type=forking, "READY=1" is sent for Type=notify/Type=notify-reload,
or the BusName= has been taken for Type=dbus).

Note that ExecStartPre= may not be used to start long-running processes.
All processes forked off by processes invoked via ExecStartPre= will be
killed before the next service process is run.

Note that if any of the commands specified in ExecStartPre=, ExecStart=, or
ExecStartPost= fail (and are not prefixed with "-", see above) or time out
before the service is fully up, execution continues with commands specified
in ExecStopPost=, the commands in ExecStop= are skipped.

Note that the execution of ExecStartPost= is taken into account for the purpose
of Before=/After= ordering constraints.


ExecCondition=

Optional commands that are executed before the commands in ExecStartPre=.
Syntax is the same as for ExecStart=, except that multiple command lines are
allowed and the commands are executed one after the other, serially.

The behavior is like an ExecStartPre= and condition check hybrid: when an
ExecCondition= command exits with exit code 1 through 254 (inclusive), the
remaining commands are skipped and the unit is not marked as failed.
However, if an ExecCondition= command exits with 255 or abnormally (e.g.
timeout, killed by a signal, etc.), the unit will be considered failed (and
remaining commands will be skipped). Exit code of 0 or those matching
SuccessExitStatus= will continue execution to the next commands.

The same recommendations about not running long-running processes in
ExecStartPre= also applies to ExecCondition=. ExecCondition= will also run
the commands in ExecStopPost=, as part of stopping the service, in the case
of any non-zero or abnormal exits, like the ones described above.

Added in version 243.


ExecReload=

Commands to execute to trigger a configuration reload in the service. This
argument takes multiple command lines, following the same scheme as
described for ExecStart= above. Use of this setting is optional. Specifier
and environment variable substitution is supported here following the same
scheme as for ExecStart=.

One additional, special environment variable is set: if known, $MAINPID is
set to the main process of the daemon, and may be used for command lines
like the following:
	ExecReload=kill -HUP $MAINPID

Note however that reloading a daemon by enqueuing a signal (as with the
example line above) is usually not a good choice, because this is an
asynchronous operation and hence not suitable when ordering reloads of
multiple services against each other. It is thus strongly recommended to
either use Type=notify-reload in place of ExecReload=, or to set ExecReload=
to a command that not only triggers a configuration reload of the daemon,
but also synchronously waits for it to complete. For example, dbus-broker(1)
uses the following:
	ExecReload=busctl call org.freedesktop.DBus \
 /org/freedesktop/DBus org.freedesktop.DBus \
 ReloadConfig


ExecStop=

Commands to execute to stop the service started via ExecStart=. This
argument takes multiple command lines, following the same scheme as
described for ExecStart= above. Use of this setting is optional. After the
commands configured in this option are run, it is implied that the service
is stopped, and any processes remaining for it are terminated according to
the KillMode= setting (see systemd.kill(5)). If this option is not
specified, the process is terminated by sending the signal specified in
KillSignal= or RestartKillSignal= when service stop is requested. Specifier
and environment variable substitution is supported (including $MAINPID, see
above).

 Note that it is usually not sufficient to specify a command for this
setting that only asks the service to terminate (for example, by sending
some form of termination signal to it), but does not wait for it to do so.
Since the remaining processes of the services are killed according to
KillMode= and KillSignal= or RestartKillSignal= as described above
immediately after the command exited, this may not result in a clean stop.
The specified command should hence be a synchronous operation, not an
asynchronous one.

 Note that the commands specified in ExecStop= are only executed when the
service started successfully first. They are not invoked if the service was
never started at all, or in case its start-up failed, for example because
any of the commands specified in ExecStart=, ExecStartPre= or ExecStartPost=
failed (and weren't prefixed with "-", see above) or timed out. Use
ExecStopPost= to invoke commands when a service failed to start up correctly
and is shut down again. Also note that the stop operation is always
performed if the service started successfully, even if the processes in the
service terminated on their own or were killed. The stop commands must be
prepared to deal with that case. $MAINPID will be unset if systemd knows
that the main process exited by the time the stop commands are called.

 Service restart requests are implemented as stop operations followed by
start operations. This means that ExecStop= and ExecStopPost= are executed
during a service restart operation.

 It is recommended to use this setting for commands that communicate with
the service requesting clean termination. For post-mortem clean-up steps use
ExecStopPost= instead.


ExecStopPost=

Additional commands that are executed after the service is stopped. This
includes cases where the commands configured in ExecStop= were used, where
the service does not have any ExecStop= defined, or where the service exited
unexpectedly. This argument takes multiple command lines, following the same
scheme as described for ExecStart=. Use of these settings is optional.
Specifier and environment variable substitution is supported. Note that –
unlike ExecStop= – commands specified with this setting are invoked when a
service failed to start up correctly and is shut down again.

 It is recommended to use this setting for clean-up operations that shall be
executed even when the service failed to start up correctly. Commands
configured with this setting need to be able to operate even if the service
failed starting up half-way and left incompletely initialized data around.
As the service's processes have been terminated already when the commands
specified with this setting are executed they should not attempt to
communicate with them.

 Note that all commands that are configured with this setting are invoked
with the result code of the service, as well as the main process' exit code
and status, set in the $SERVICE_RESULT, $EXIT_CODE and $EXIT_STATUS
environment variables, see systemd.exec(5) for details.

 Note that the execution of ExecStopPost= is taken into account for the
purpose of Before=/After= ordering constraints.


RestartSec=
Configures the time to sleep before restarting a service (as configured
with Restart=). Takes a unit-less value in seconds, or a time span value
such as "5min 20s". Defaults to 100ms.


RestartSteps=

Configures the number of steps to take to increase the interval of auto-restarts
from RestartSec= to RestartMaxDelaySec=. Takes a positive integer or 0 to 
disable it. Defaults to 0.

 This setting is effective only if RestartMaxDelaySec= is also set.

 Added in version 254.


RestartMaxDelaySec=

Configures the longest time to sleep before restarting a service as the
interval goes up with RestartSteps=. Takes a value in the same format as
RestartSec=, or "infinity" to disable the setting. Defaults to "infinity".

This setting is effective only if RestartSteps= is also set.

Added in version 254.


TimeoutStartSec=

Configures the time to wait for start-up. If a daemon service does not
signal start-up completion within the configured time, the service will be
considered failed and will be shut down again. The precise action depends on
the TimeoutStartFailureMode= option. Takes a unit-less value in seconds, or
a time span value such as "5min 20s". Pass "infinity" to disable the timeout 
logic. Defaults to DefaultTimeoutStartSec= set in the manager, except when 
Type=oneshot is used, in which case the timeout is disabled by default (see 
systemd-system.conf(5)).

If a service of Type=notify/Type=notify-reload sends "EXTEND_TIMEOUT_USEC=…", 
this may cause the start time to be extended beyond TimeoutStartSec=. The 
first receipt of this message must occur before TimeoutStartSec= is exceeded,
and once the start time has extended beyond TimeoutStartSec=, the service 
manager will allow the service to continue to start, provided the service 
repeats "EXTEND_TIMEOUT_USEC=…" within the interval specified until the 
service startup status is finished by "READY=1". (see sd_notify(3)).

 Added in version 188.


TimeoutStopSec=

This option serves two purposes. First, it configures the time to wait for
each ExecStop= command. If any of them times out, subsequent ExecStop=
commands are skipped and the service will be terminated by SIGTERM. If no
ExecStop= commands are specified, the service gets the SIGTERM immediately.
This default behavior can be changed by the TimeoutStopFailureMode= option.
Second, it configures the time to wait for the service itself to stop. If it
doesn't terminate in the specified time, it will be forcibly terminated by
SIGKILL (see KillMode= in systemd.kill(5)). Takes a unit-less value in
seconds, or a time span value such as "5min 20s". Pass "infinity" to 
disable the timeout logic.
Defaults to DefaultTimeoutStopSec= from the manager configuration file (see
systemd-system.conf(5)).

If a service of Type=notify/Type=notify-reload sends "EXTEND_TIMEOUT_USEC=…", 
this may cause the stop time to be extended beyond TimeoutStopSec=. The 
first receipt of this message must occur before TimeoutStopSec= is exceeded, 
and once the stop time has extended beyond TimeoutStopSec=, the service 
manager will allow the service to continue to stop, provided the service 
repeats "EXTEND_TIMEOUT_USEC=…" within the interval specified, or terminates 
itself (see sd_notify(3)).

Added in version 188.


TimeoutAbortSec=

This option configures the time to wait for the service to terminate when
it was aborted due to a watchdog timeout (see WatchdogSec=). If the service
has a short TimeoutStopSec= this option can be used to give the system more
time to write a core dump of the service. Upon expiration the service will
be forcibly terminated by SIGKILL (see KillMode= in systemd.kill(5)). The
core file will be truncated in this case. Use TimeoutAbortSec= to set a
sensible timeout for the core dumping per service that is large enough to
write all expected data while also being short enough to handle the service
failure in due time.

Takes a unit-less value in seconds, or a time span value such as "5min 20s". 
Pass an empty value to skip the dedicated watchdog abort timeout handling 
and fall back TimeoutStopSec=. Pass "infinity" to disable the timeout logic.
Defaults to
DefaultTimeoutAbortSec= from the manager configuration file 
(see systemd-system.conf(5)).

If a service of Type=notify/Type=notify-reload handles SIGABRT itself
(instead of relying on the kernel to write a core dump) it can send 
"EXTEND_TIMEOUT_USEC=…" to extended the abort time beyond TimeoutAbortSec=. 
The first receipt of this message must occur before TimeoutAbortSec= is 
exceeded, and once the abort time has extended beyond TimeoutAbortSec=, the 
service manager will allow the service to continue to abort, provided the 
service repeats "EXTEND_TIMEOUT_USEC=…" within the interval specified, or
terminates itself (see sd_notify(3)).

Added in version 243.


TimeoutSec=

A shorthand for configuring both TimeoutStartSec= and TimeoutStopSec= to
the specified value.


TimeoutStartFailureMode=, TimeoutStopFailureMode=

These options configure the action that is taken in case a daemon service
does not signal start-up within its configured TimeoutStartSec=,
respectively if it does not stop within TimeoutStopSec=. Takes one of
terminate, abort and kill. Both options default to terminate.

 If terminate is set the service will be gracefully terminated by sending
the signal specified in KillSignal= (defaults to SIGTERM, see
systemd.kill(5)). If the service does not terminate the FinalKillSignal= is
sent after TimeoutStopSec=. If abort is set, WatchdogSignal= is sent instead
and TimeoutAbortSec= applies before sending FinalKillSignal=. This setting
may be used to analyze services that fail to start-up or shut-down
intermittently. By using kill the service is immediately terminated by
sending FinalKillSignal= without any further timeout. This setting can be
used to expedite the shutdown of failing services.

 Added in version 246.


RuntimeMaxSec=

Configures a maximum time for the service to run. If this is used and the
service has been active for longer than the specified time it is terminated
and put into a failure state. Note that this setting does not have any
effect on Type=oneshot services, as they terminate immediately after
activation completed. Pass "infinity" (the default) to configure no runtime limit.

 If a service of Type=notify/Type=notify-reload sends "EXTEND_TIMEOUT_USEC=…", this may cause the
runtime to be extended beyond RuntimeMaxSec=. The first receipt of this
message must occur before RuntimeMaxSec= is exceeded, and once the runtime
has extended beyond RuntimeMaxSec=, the service manager will allow the
service to continue to run, provided the service repeats "EXTEND_TIMEOUT_USEC=…" within the
interval specified until the service shutdown is achieved by "STOPPING=1" (or
termination). (see sd_notify(3)).

Added in version 229.


RuntimeRandomizedExtraSec=

This option modifies RuntimeMaxSec= by increasing the maximum runtime by an
evenly distributed duration between 0 and the specified value (in seconds).
If RuntimeMaxSec= is unspecified, then this feature will be disabled.

Added in version 250.


WatchdogSec=

Configures the watchdog timeout for a service. The watchdog is activated
when the start-up is completed. The service must call sd_notify(3) regularly
with "WATCHDOG=1" (i.e. the "keep-alive ping"). If the time between two such calls is larger than the
configured time, then the service is placed in a failed state and it will be
terminated with SIGABRT (or the signal specified by WatchdogSignal=). By
setting Restart= to on-failure, on-watchdog, on-abnormal or always, the
service will be automatically restarted. The time configured here will be
passed to the executed service process in the WATCHDOG_USEC= environment
variable. This allows daemons to automatically enable the keep-alive pinging
logic if watchdog support is enabled for the service. If this option is
used, NotifyAccess= (see below) should be set to open access to the
notification socket provided by systemd. If NotifyAccess= is not set, it
will be implicitly set to main. Defaults to 0, which disables this feature.
The service can check whether the service manager expects watchdog keep
-alive notifications. See sd_watchdog_enabled(3) for details.
sd_event_set_watchdog(3) may be used to enable automatic watchdog
notification support.


Restart=

Configures whether the service shall be restarted when the service process
exits, is killed, or a timeout is reached. The service process may be the
main service process, but it may also be one of the processes specified with
ExecStartPre=, ExecStartPost=, ExecStop=, ExecStopPost=, or ExecReload=.
When the death of the process is a result of systemd operation (e.g. service
stop or restart), the service will not be restarted. Timeouts include
missing the watchdog "keep-alive ping" deadline and a service start, reload, 
and stop operation timeouts.

Takes one of no, on-success, on-failure, on-abnormal, on-watchdog, on-abort,
or always. If set to no (the default), the service will not be restarted. If
set to on-success, it will be restarted only when the service process exits
 cleanly. In this context, a clean exit means any of the following:

exit code of 0;
for types other than Type=oneshot, one of the signals SIGHUP, SIGINT,
SIGTERM, or SIGPIPE;
exit statuses and signals specified in SuccessExitStatus=.

If set to on-failure, the service will be restarted when the process exits
with a non-zero exit code, is terminated by a signal (including on core
dump, but excluding the aforementioned four signals), when an operation
(such as service reload) times out, and when the configured watchdog timeout
is triggered. If set to on-abnormal, the service will be restarted when the
process is terminated by a signal (including on core dump, excluding the
aforementioned four signals), when an operation times out, or when the
watchdog timeout is triggered. If set to on-abort, the service will be
restarted only if the service process exits due to an uncaught signal not
specified as a clean exit status. If set to on-watchdog, the service will be
restarted only if the watchdog timeout for the service expires. If set to
always, the service will be restarted regardless of whether it exited
cleanly or not, got terminated abnormally by a signal, or hit a timeout.


Table 1. Exit causes and the effect of the Restart= settings


Restart settings/Exit causes
no
always
on-success
on-failure
on-abnormal
on-abort
on-watchdog
Clean exit code or signal
 
X
X
 
 
 
 
Unclean exit code
 
X
 
X
 
 
 
Unclean signal
 
X
 
X
X
X
 
Timeout
 
X
 
X
X
 
 
Watchdog
 
X
 
X
X
 
X


 As exceptions to the setting above, the service will not be restarted if
the exit code or signal is specified in RestartPreventExitStatus= (see
below) or the service is stopped with systemctl stop or an equivalent
operation. Also, the services will always be restarted if the exit code or
signal is specified in RestartForceExitStatus= (see below).

 Note that service restart is subject to unit start rate limiting configured
with StartLimitIntervalSec= and StartLimitBurst=, see systemd.unit(5) for
details.

 Setting this to on-failure is the recommended choice for long-running
services, in order to increase reliability by attempting automatic recovery
from errors. For services that shall be able to terminate on their own
choice (and avoid immediate restarting), on-abnormal is an alternative
choice.


RestartMode=

Takes a string value that specifies how a service should restart:

If set to normal (the default), the service restarts by going through a
failed/inactive state.

If set to direct, the service transitions to the activating state directly
during auto-restart, skipping failed/inactive state. ExecStopPost= is
invoked. OnSuccess= and OnFailure= are skipped.

This option is useful in cases where a dependency can fail temporarily but
we don't want these temporary failures to make the dependent units fail.
When this option is set to direct, dependent units are not notified of these
temporary failures.

 Added in version 254.


SuccessExitStatus=

Takes a list of exit status definitions that, when returned by the main
service process, will be considered successful termination, in addition to
the normal successful exit status 0 and, except for Type=oneshot, the
signals SIGHUP, SIGINT, SIGTERM, and SIGPIPE. Exit status definitions can be
numeric termination statuses, termination status names, or termination
signal names, separated by spaces. See the Process Exit Codes section in
systemd.exec(5) for a list of termination status names (for this setting
only the part without the "EXIT_" or "EX_" prefix should be used). See 
signal(7) for a list of signal names.

Note that this setting does not change the mapping between numeric exit
statuses and their names, i.e. regardless how this setting is used 0 will
still be mapped to "SUCCESS" (and thus typically shown as "0/SUCCESS" in
tool outputs) and 1 to "FAILURE" (and thus typically shown as "1/FAILURE"),
and so on. It only controls what happens as effect of these exit statuses,
 and how it propagates to the state of the service as a whole.

This option may appear more than once, in which case the list of successful
exit statuses is merged. If the empty string is assigned to this option, the
list is reset, all prior assignments of this option will have no effect.


Example 1. A service with the SuccessExitStatus= setting
	SuccessExitStatus=TEMPFAIL 250 SIGKILL

Exit status 75 (TEMPFAIL), 250, and the termination signal SIGKILL are
considered clean service terminations.

Note: systemd-analyze exit-status may be used to list exit statuses and
translate between numerical status values and names.

 Added in version 189.


RestartPreventExitStatus=

Takes a list of exit status definitions that, when returned by the main
service process, will prevent automatic service restarts, regardless of the
restart setting configured with Restart=. Exit status definitions can either
be numeric exit codes or termination signal names, and are separated by
spaces. Defaults to the empty list, so that, by default, no exit status is
excluded from the configured restart logic. For example:
	RestartPreventExitStatus=1 6 SIGABRT
ensures that exit codes 1 and 6 and the termination signal SIGABRT will not
result in automatic service restarting. This option may appear more than
once, in which case the list of restart-preventing statuses is merged. If
the empty string is assigned to this option, the list is reset and all prior
assignments of this option will have no effect.

Note that this setting has no effect on processes configured via
ExecStartPre=, ExecStartPost=, ExecStop=, ExecStopPost= or ExecReload=, but
only on the main service process, i.e. either the one invoked by ExecStart=
or (depending on Type=, PIDFile=, …) the otherwise configured main
process.

Added in version 189.


RestartForceExitStatus=

Takes a list of exit status definitions that, when returned by the main
service process, will force automatic service restarts, regardless of the
restart setting configured with Restart=. The argument format is similar to
RestartPreventExitStatus=.

Added in version 215.


RootDirectoryStartOnly=

Takes a boolean argument. If true, the root directory, as configured with
the RootDirectory= option (see systemd.exec(5) for more information), is
only applied to the process started with ExecStart=, and not to the various
other ExecStartPre=, ExecStartPost=, ExecReload=, ExecStop=, and
ExecStopPost= commands. If false, the setting is applied to all configured
commands the same way. Defaults to false.


NonBlocking=

Set the O_NONBLOCK flag for all file descriptors passed via socket-based
activation. If true, all file descriptors >= 3 (i.e. all except stdin,
stdout, stderr), excluding those passed in via the file descriptor storage
logic (see FileDescriptorStoreMax= for details), will have the O_NONBLOCK
flag set and hence are in non-blocking mode. This option is only useful in
conjunction with a socket unit, as described in systemd.socket(5) and has no
effect on file descriptors which were previously saved in the file
-descriptor store for example. Defaults to false.

 Note that if the same socket unit is configured to be passed to multiple
service units (via the Sockets= setting, see below), and these services have
different NonBlocking= configurations, the precise state of O_NONBLOCK
depends on the order in which these services are invoked, and will possibly
change after service code already took possession of the socket file
descriptor, simply because the O_NONBLOCK state of a socket is shared by all
file descriptors referencing it. Hence it is essential that all services
sharing the same socket use the same NonBlocking= configuration, and do not
change the flag in service code either.


NotifyAccess=

Controls access to the service status notification socket, as accessible
via the sd_notify(3) call. Takes one of none (the default), main, exec or
all. If none, no daemon status updates are accepted from the service
processes, all status update messages are ignored. If main, only service
updates sent from the main process of the service are accepted. If exec,
only service updates sent from any of the main or control processes
originating from one of the Exec*= commands are accepted. If all, all
services updates from all members of the service's control group are
accepted. This option should be set to open access to the notification
socket when using Type=notify/Type=notify-reload or WatchdogSec= (see
above). If those options are used but NotifyAccess= is not configured, it
will be implicitly set to main.

Note that sd_notify() notifications may be attributed to units correctly
only if either the sending process is still around at the time PID 1
processes the message, or if the sending process is explicitly runtime
-tracked by the service manager. The latter is the case if the service
manager originally forked off the process, i.e. on all processes that match
main or exec. Conversely, if an auxiliary process of the unit sends an
sd_notify() message and immediately exits, the service manager might not be
able to properly attribute the message to the unit, and thus will ignore it,
even if NotifyAccess=all is set for it.

 Hence, to eliminate all race conditions involving lookup of the client's
unit and attribution of notifications to units correctly,
sd_notify_barrier() may be used. This call acts as a synchronization point
and ensures all notifications sent before this call have been picked up by
the service manager when it returns successfully. Use of sd_notify_barrier()
is needed for clients which are not invoked by the service manager,
otherwise this synchronization mechanism is unnecessary for attribution of
notifications to the unit.


Sockets=

Specifies the name of the socket units this service shall inherit socket
file descriptors from when the service is started. Normally, it should not
be necessary to use this setting, as all socket file descriptors whose unit
shares the same name as the service (subject to the different unit name
suffix of course) are passed to the spawned process.

 Note that the same socket file descriptors may be passed to multiple
processes simultaneously. Also note that a different service may be
activated on incoming socket traffic than the one which is ultimately
configured to inherit the socket file descriptors. Or, in other words: the
Service= setting of .socket units does not have to match the inverse of the
Sockets= setting of the .service it refers to.

 This option may appear more than once, in which case the list of socket
units is merged. Note that once set, clearing the list of sockets again (for
example, by assigning the empty string to this option) is not supported.


FileDescriptorStoreMax=

Configure how many file descriptors may be stored in the service manager
for the service using sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages. This 
is useful for implementing services that can restart after an explicit request 
or a crash without losing state. Any open sockets and other file descriptors
which should not be closed during the restart may be stored this way.
Application state can either be serialized to a file in RuntimeDirectory=,
or stored in a memfd_create(2) memory file descriptor. Defaults to 0, i.e.
no file descriptors may be stored in the service manager. All file
descriptors passed to the service manager from a specific service are passed
back to the service's main process on the next service restart (see
sd_listen_fds(3) for details about the precise protocol used and the order
in which the file descriptors are passed). Any file descriptors passed to
the service manager are automatically closed when POLLHUP or POLLERR is seen
on them, or when the service is fully stopped and no job is queued or being
executed for it (the latter can be tweaked with File
DescriptorStorePreserve=, see below). If this option is used, NotifyAccess=
(see above) should be set to open access to the notification socket provided
by systemd. If NotifyAccess= is not set, it will be implicitly set to main.

 The fdstore command of systemd-analyze(1) may be used to list the current
contents of a service's file descriptor store.

 Note that the service manager will only pass file descriptors contained in
the file descriptor store to the service's own processes, never to other
clients via IPC or similar. However, it does allow unprivileged clients to
query the list of currently open file descriptors of a service. Sensitive
data may hence be safely placed inside the referenced files, but should not
be attached to the metadata (e.g. included in filenames) of the stored file
descriptors.

 If this option is set to a non-zero value the $FDSTORE environment variable
will be set for processes invoked for this service. See systemd.exec(5) for
details.

 For further information on the file descriptor store see the File
Descriptor Store overview.

Added in version 219.


FileDescriptorStorePreserve=

Takes one of no, yes, restart and controls when to release the service's
file descriptor store (i.e. when to close the contained file descriptors, if
any). If set to no the file descriptor store is automatically released when
the service is stopped; if restart (the default) it is kept around as long
as the unit is neither inactive nor failed, or a job is queued for the
service, or the service is expected to be restarted. If yes the file
descriptor store is kept around until the unit is removed from memory (i.e.
is not referenced anymore and inactive). The latter is useful to keep
entries in the file descriptor store pinned until the service manager
exits.

Use systemctl clean --what=fdstore … to release the file descriptor store
explicitly.

 Added in version 254.


USBFunctionDescriptors=

Configure the location of a file containing USB FunctionFS descriptors, for
implementation of USB gadget functions. This is used only in conjunction
with a socket unit with ListenUSBFunction= configured. The contents of this
file are written to the ep0 file after it is opened.

 Added in version 227.


USBFunctionStrings=

Configure the location of a file containing USB FunctionFS strings.
Behavior is similar to USBFunctionDescriptors= above.

Added in version 227.


OOMPolicy=

Configure the out-of-memory (OOM) killing policy for the kernel and the
userspace OOM killer systemd-oomd.service(8). On Linux, when memory becomes
scarce to the point that the kernel has trouble allocating memory for
itself, it might decide to kill a running process in order to free up memory
and reduce memory pressure. Note that systemd-oomd.service is a more
flexible solution that aims to prevent out-of-memory situations for the
userspace too, not just the kernel, by attempting to terminate services
earlier, before the kernel would have to act.

 This setting takes one of continue, stop or kill. If set to continue and a
process in the unit is killed by the OOM killer, this is logged but the unit
continues running. If set to stop the event is logged but the unit is
terminated cleanly by the service manager. If set to kill and one of the
unit's processes is killed by the OOM killer the kernel is instructed to
kill all remaining processes of the unit too, by setting the
memory.oom.group attribute to 1; also see kernel page Control Group v2.

 Defaults to the setting DefaultOOMPolicy= in systemd-system.conf(5) is set
to, except for units where Delegate= is turned on, where it defaults to
continue.

 Use the OOMScoreAdjust= setting to configure whether processes of the unit
shall be considered preferred or less preferred candidates for process
termination by the Linux OOM killer logic. See systemd.exec(5) for details.

 This setting also applies to systemd-oomd.service(8). Similarly to the
kernel OOM kills performed by the kernel, this setting determines the state
of the unit after systemd-oomd kills a cgroup associated with it.

Added in version 243.


OpenFile=

Takes an argument of the form "path[:fd-name:options]", where:

"path" is a path to a file or an AF_UNIX socket in the file system;
"fd-name" is a name that will be associated with the file descriptor; the 
name may contain any ASCII character, but must exclude control characters and 
":", and must be at most 255 characters in length; it is optional and, if not
provided, defaults to the file name;
"options" is a comma-separated list of access options; possible values are 
"read-only", "append", "truncate", "graceful";
if not specified, files will be opened in rw mode; if "graceful" is specified, 
errors during file/socket opening are ignored. Specifying the same option 
several times is treated as an error.
The file or socket is opened by the service manager and the file descriptor
is passed to the service. If the path is a socket, we call connect() on it.
See sd_listen_fds(3) for more details on how to retrieve these file
descriptors.

 This setting is useful to allow services to access files/sockets that they
can't access themselves (due to running in a separate mount namespace, not
having privileges, ...).

 This setting can be specified multiple times, in which case all the
specified paths are opened and the file descriptors passed to the service.
If the empty string is assigned, the entire list of open files defined prior
to this is reset.

Added in version 253.


ReloadSignal=

Configures the UNIX process signal to send to the service's main process
when asked to reload the service's configuration. Defaults to SIGHUP. This
option has no effect unless Type=notify-reload is used, see above.

Added in version 253.

Check systemd.unit(5), systemd.exec(5), and systemd.kill(5) for more
settings.



Command lines
This section describes command line parsing and variable and specifier
substitutions for ExecStart=, ExecStartPre=, ExecStartPost=, ExecReload=,
ExecStop=, and ExecStopPost= options.

Multiple command lines may be concatenated in a single directive by
separating them with semicolons (these semicolons must be passed as separate
words). Lone semicolons may be escaped as "\;".

Each command line is unquoted using the rules described in "Quoting" section in
systemd.syntax(7). The first item becomes the command to execute, and the
subsequent items the arguments.

This syntax is inspired by shell syntax, but only the meta-characters and
expansions described in the following paragraphs are understood, and the
expansion of variables is different. Specifically, redirection using "<", "<<", ">",
and ">>", pipes using "|", running programs in the background using "&", and other
elements of shell syntax are not supported.

The command to execute may contain spaces, but control characters are not
allowed.

Each command may be prefixed with a number of special characters:


Table 2. Special executable prefixes


Prefix
Effect
"@"
If the executable path is prefixed with "@", the second specified token will
be passed as argv[0] to the executed process (instead of the actual
filename), followed by the further arguments specified.
"-"
If the executable path is prefixed with "-", an exit code of the command
normally considered a failure (i.e. non-zero exit status or abnormal exit
due to signal) is recorded, but has no further effect and is considered
equivalent to success.
":"
If the executable path is prefixed with ":", environment variable substitution
(as described by the "Command Lines" section below) is not applied.
"+"
If the executable path is prefixed with "+" then the process is executed with
full privileges. In this mode privilege restrictions configured with User=,
Group=, CapabilityBoundingSet= or the various file system namespacing
options (such as PrivateDevices=, PrivateTmp=) are not applied to the
invoked command line (but still affect any other ExecStart=, ExecStop=, …
lines). However, note that this will not bypass options that apply to the
whole control group, such as DevicePolicy=, see systemd.resource-control(5)
for the full list.
"!"
Similar to the "+" character discussed above this permits invoking command
lines with elevated privileges. However, unlike "+" the "!" character exclusively
alters the effect of User=, Group= and SupplementaryGroups=, i.e. only the
stanzas that affect user and group credentials. Note that this setting may
be combined with DynamicUser=, in which case a dynamic user/group pair is
allocated before the command is invoked, but credential changing is left to
the executed process itself.
"!!"
This prefix is very similar to "!", however it only has an effect on systems
lacking support for ambient process capabilities, i.e. without support for
AmbientCapabilities=. It's intended to be used for unit files that take
benefit of ambient capabilities to run processes with minimal privileges
wherever possible while remaining compatible with systems that lack ambient
capabilities support. Note that when "!!" is used, and a system lacking ambient
capability support is detected any configured SystemCallFilter= and
CapabilityBoundingSet= stanzas are implicitly modified, in order to permit
spawned processes to drop credentials and capabilities themselves, even if
this is configured to not be allowed. Moreover, if this prefix is used and a
system lacking ambient capability support is detected AmbientCapabilities=
will be skipped and not be applied. On systems supporting ambient
capabilities, "!!" has no effect and is redundant.


"@", "-", ":", and one of "+"/"!"/"!!" may be used together and they can 
appear in any order.
However, only one of "+", "!", "!!" may be used at a time.

For each command, the first argument must be either an absolute path to an
executable or a simple file name without any slashes. If the command is not
a full (absolute) path, it will be resolved to a full path using a fixed
search path determined at compilation time. Searched directories include
/usr/local/bin/, /usr/bin/, /bin/ on systems using split /usr/bin/ and /bin/
directories, and their sbin/ counterparts on systems using split bin/ and
sbin/. It is thus safe to use just the executable name in case of
executables located in any of the "standard" directories, and an absolute 
path must be used in other cases. Hint: this search path may be queried 
using systemd-path search-binaries-default.

The command line accepts "%" specifiers as described in systemd.unit(5).

Basic environment variable substitution is supported. Use "${FOO}" as part of a
word, or as a word of its own, on the command line, in which case it will be
erased and replaced by the exact value of the environment variable (if any)
including all whitespace it contains, always resulting in exactly a single
argument. Use "$FOO" as a separate word on the command line, in which case it will
be replaced by the value of the environment variable split at whitespace,
resulting in zero or more arguments. For this type of expansion, quotes are
respected when splitting into words, and afterwards removed.


Example: A
	Environment="ONE=one" 'TWO=two two'
ExecStart=echo $ONE $TWO ${TWO}

This will execute /bin/echo with four arguments: "one", "two", "two", and "two two".


Example: B
	Environment=ONE='one' "TWO='two two' too" THREE=
ExecStart=/bin/echo ${ONE} ${TWO} ${THREE}
ExecStart=/bin/echo $ONE $TWO $THREE

This results in /bin/echo being called twice, the first time with arguments
"'one'", "'two two' too", "", and the second time with arguments "one", 
"two two", "too".

To pass a literal dollar sign, use "$$". Variables whose value is not known at
expansion time are treated as empty strings. Note that the first argument
(i.e. the program to execute) may not be a variable.

Variables to be used in this fashion may be defined through Environment= and
EnvironmentFile=. In addition, variables listed in the section "Environment 
variables in spawned processes" in systemd.exec(5), which are considered 
"static configuration", may be used (this includes e.g.  $USER, but not $TERM).

Note that shell command lines are not directly supported. If shell command
lines are to be used, they need to be passed explicitly to a shell
implementation of some kind.  

Example: C
	ExecStart=sh -c 'dmesg | tac'


Example: D
	ExecStart=echo one ; echo "two two"

This will execute echo two times, each time with one argument: "one" and "two two",
respectively. Because two commands are specified, Type=oneshot must be
used.


Example: E
	Type=oneshot
ExecStart=:echo $USER ; -false ; +:@true $TEST

This will execute /usr/bin/echo with the literal argument "$USER" (":" suppresses
variable expansion), and then /usr/bin/false (the return value will be
ignored because "-" suppresses checking of the return value), and /usr/bin/true
(with elevated privileges, with "$TEST" as argv[0]).


Example: F
	ExecStart=echo / >/dev/null & \; \
ls

This will execute echo with five arguments: "/", ">/dev/null", "&", ";", and "ls".



Examples



Example 2. Simple service
The following unit file creates a service that will execute /usr/sbin/foo
-daemon. Since no Type= is specified, the default Type=simple will be
assumed. systemd will assume the unit to be started immediately after the
program has begun executing.
	[Unit]
Description=Foo

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

[Install]
WantedBy=multi-user.target

Note that systemd assumes here that the process started by systemd will
continue running until the service terminates. If the program daemonizes
itself (i.e. forks), please use Type=forking instead.

Since no ExecStop= was specified, systemd will send SIGTERM to all processes
started from this service, and after a timeout also SIGKILL. This behavior
can be modified, see systemd.kill(5) for details.

Note that this unit type does not include any type of notification when a
service has completed initialization. For this, you should use other unit
types, such as Type=notify/Type=notify-reload if the service understands
systemd's notification protocol, Type=forking if the service can background
itself or Type=dbus if the unit acquires a DBus name once initialization is
complete. See below.



Example 3. Oneshot service
Sometimes, units should just execute an action without keeping active
processes, such as a filesystem check or a cleanup action on boot. For this,
Type=oneshot exists. Units of this type will wait until the process
specified terminates and then fall back to being inactive. The following
unit will perform a cleanup action:
	[Unit]
Description=Cleanup old Foo data

[Service]
Type=oneshot
ExecStart=/usr/sbin/foo-cleanup

[Install]
WantedBy=multi-user.target 

Note that systemd will consider the unit to be in the state "starting" until the
program has terminated, so ordered dependencies will wait for the program to
finish before starting themselves. The unit will revert to the "inactive" state 
after the execution is done, never reaching the "active" state. That means 
another request to start the unit will perform the action again.

Type=oneshot are the only service units that may have more than one
ExecStart= specified. For units with multiple commands (Type=oneshot), all
commands will be run again.

For Type=oneshot, Restart=always and Restart=on-success are not allowed.



Example 4. Stoppable oneshot service
Similarly to the oneshot services, there are sometimes units that need to
execute a program to set up something and then execute another to shut it
down, but no process remains active while they are considered "started". Network
configuration can sometimes fall into this category. Another use case is if
a oneshot service shall not be executed each time when they are pulled in as
a dependency, but only the first time.

For this, systemd knows the setting RemainAfterExit=yes, which causes
systemd to consider the unit to be active if the start action exited
successfully. This directive can be used with all types, but is most useful
with Type=oneshot and Type=simple. With Type=oneshot, systemd waits until
the start action has completed before it considers the unit to be active, so
dependencies start only after the start action has succeeded. With
Type=simple, dependencies will start immediately after the start action has
been dispatched. The following unit provides an example for a simple static
firewall.
	[Unit]
Description=Simple firewall

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/sbin/simple-firewall-start
ExecStop=/usr/local/sbin/simple-firewall-stop

[Install]
WantedBy=multi-user.target

Since the unit is considered to be running after the start action has
exited, invoking systemctl start on that unit again will cause no action to
be taken.



Example 5. Traditional forking services
Many traditional daemons/services background (i.e. fork, daemonize)
themselves when starting. Set Type=forking in the service's unit file to
support this mode of operation. systemd will consider the service to be in
the process of initialization while the original program is still running.
Once it exits successfully and at least a process remains (and 
RemainAfterExit=no), the service is considered started.

Often, a traditional daemon only consists of one process. Therefore, if only
one process is left after the original process terminates, systemd will
consider that process the main process of the service. In that case, the
$MAINPID variable will be available in ExecReload=, ExecStop=, etc.

In case more than one process remains, systemd will be unable to determine
the main process, so it will not assume there is one. In that case, $MAINPID
will not expand to anything. However, if the process decides to write a
traditional PID file, systemd will be able to read the main PID from there.
Please set PIDFile= accordingly. Note that the daemon should write that file
before finishing with its initialization. Otherwise, systemd might try to
read the file before it exists.

The following example shows a simple daemon that forks and just starts one
process in the background:
	[Unit]
Description=Some simple daemon

[Service]
Type=forking
ExecStart=/usr/sbin/my-simple-daemon -d

[Install]
WantedBy=multi-user.target

Please see systemd.kill(5) for details on how you can influence the way
systemd terminates the service.



Example 6. DBus services
For services that acquire a name on the DBus system bus, use Type=dbus and
set BusName= accordingly. The service should not fork (daemonize). systemd
will consider the service to be initialized once the name has been acquired
on the system bus. The following example shows a typical DBus service:
	[Unit]
Description=Simple DBus service

[Service]
Type=dbus
BusName=org.example.simple-dbus-service
ExecStart=/usr/sbin/simple-dbus-service

[Install]
WantedBy=multi-user.target

For bus-activatable services, do not include a [Install] section in the
systemd service file, but use the SystemdService= option in the
corresponding DBus service file, for example (/usr/share/dbus-1/system
-services/org.example.simple-dbus-service.service):
	[D-BUS Service]
Name=org.example.simple-dbus-service
Exec=/usr/sbin/simple-dbus-service
User=root
SystemdService=simple-dbus-service.service

Please see systemd.kill(5) for details on how you can influence the way
systemd terminates the service.



Example 7. Services that notify systemd 
about initialization

Type=simple services are really easy to write, but have the major
disadvantage of systemd not being able to tell when initialization of the
given service is complete. For this reason, systemd supports a simple
notification protocol that allows daemons to make systemd aware that they
are done initializing. Use Type=notify or Type=notify-reload for this. A
typical service file for such a daemon would look like this:
	[Unit]
Description=Simple notifying service

[Service]
Type=notify
ExecStart=/usr/sbin/simple-notifying-service

[Install]
WantedBy=multi-user.target

Note that the daemon has to support systemd's notification protocol, else
systemd will think the service has not started yet and kill it after a
timeout. For an example of how to update daemons to support this protocol
transparently, take a look at sd_notify(3). systemd will consider the unit
to be in the 'starting' state until a readiness notification has arrived.

Please see systemd.kill(5) for details on how you can influence the way
systemd terminates the service.



See Also
systemd(1), systemctl(1), systemd-system.conf(5), systemd.unit(5),
systemd.exec(5), systemd.resource-control(5), systemd.kill(5),
systemd.directives(7), systemd-run(1)