Provided by: systemd_256.5-2ubuntu3.1_amd64 
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)).
Note that the start timeout is also applied to service reloads, regardless if
implemented through ExecReload= or via the reload logic enabled via
Type=notify-reload. If the reload does not complete within the configured time, the
reload will be considered failed and the service will continue running with the old
configuration. This will not affect the running service, but will be logged and will
cause e.g. systemctl reload to fail.
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 (use TimeoutStartSec= to limit
their activation). 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. Note that Type=oneshot services will never be restarted on
a clean exit status, i.e. always and on-success are rejected for them.
Table 1. Exit causes and the effect of the Restart= settings
┌──────────────┬────┬────────┬────────────┬────────────┬─────────────┬──────────┬─────────────┐
│Restart │ no │ always │ on-success │ on-failure │ on-abnormal │ on-abort │ on-watchdog │
│settings/Exit │ │ │ │ │ │ │ │
│causes │ │ │ │ │ │ │ │
├──────────────┼────┼────────┼────────────┼────────────┼─────────────┼──────────┼─────────────┤
│Clean exit │ │ X │ X │ │ │ │ │
│code or │ │ │ │ │ │ │ │
│signal │ │ │ │ │ │ │ │
├──────────────┼────┼────────┼────────────┼────────────┼─────────────┼──────────┼─────────────┤
│Unclean exit │ │ X │ │ X │ │ │ │
│code │ │ │ │ │ │ │ │
├──────────────┼────┼────────┼────────────┼────────────┼─────────────┼──────────┼─────────────┤
│Unclean │ │ X │ │ X │ X │ X │ │
│signal │ │ │ │ │ │ │ │
├──────────────┼────┼────────┼────────────┼────────────┼─────────────┼──────────┼─────────────┤
│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 be numeric termination statuses,
termination status names, or termination signal names, separated by spaces. Defaults
to the empty list, so that, by default, no exit status is excluded from the configured
restart logic.
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=.
Note that for Type=oneshot services, a success exit status will prevent them from
auto-restarting, no matter whether the corresponding exit statuses are listed in this
option or not.
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
FileDescriptorStorePreserve=, 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[1]
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[2] 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[3].
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=, ExecStopPost=, and
ExecCondition= 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 below this table) 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:
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:
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:
ExecStart=sh -c 'dmesg | tac'
Example:
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:
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:
ExecStart=echo / >/dev/null & \; \
ls
This will execute echo with five arguments: "/", ">/dev/null", "&", ";", and "ls".
EXAMPLES
Example 3. 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 4. 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 5. 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 6. 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 7. 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 8. Services that notify systemd about their 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-reload
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.
To avoid code duplication, it is preferable to use sd_notify(3) when possible, especially
when other APIs provided by libsystemd(3) are also used, but note that the notification
protocol is very simple and guaranteed to be stable as per the Interface Portability and
Stability Promise[4], so it can be reimplemented by services with no external
dependencies. For a self-contained example, see sd_notify(3).
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)
NOTES
1. File Descriptor Store
https://systemd.io/FILE_DESCRIPTOR_STORE
2. USB FunctionFS
https://docs.kernel.org/usb/functionfs.html
3. Control Group v2
https://docs.kernel.org/admin-guide/cgroup-v2.html
4. Interface Portability and Stability Promise
https://systemd.io/PORTABILITY_AND_STABILITY/