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SYSTEMD.SERVICE(5) systemd.service SYSTEMD.SERVICE(5)
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:
o Services with Type=dbus set automatically acquire dependencies of
type Requires= and After= on dbus.socket.
o 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:
o 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.
o 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:
o 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 (see below) 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).
o 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).
o 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.
o 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.
o 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.
o 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 the a signal to
start the reload, using notify-reload instead is recommended.
o 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 <mu>s, 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.
o 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:
o 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.
o 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.
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.
Unless Type= is oneshot, exactly one command must be given. When
Type=oneshot is used, this setting may be used multiple times to
define multiple commands to execute. 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 configured, 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=.
Multiple command lines are allowed, regardless of the service type
(i.e. Type=), 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=. Multiple
command lines are allowed, regardless of the service type (i.e.
Type=), 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.
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 likely exited 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.
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.
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)).
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)).
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)).
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.
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)).
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.
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:
o exit code of 0;
o for types other than Type=oneshot, one of the signals SIGHUP,
SIGINT, SIGTERM, or SIGPIPE;
o 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:
o If set to normal (the default), the service restarts by going
through a failed/inactive state.
o 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.
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.
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.
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=.
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.
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.
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.
USBFunctionStrings=
Configure the location of a file containing USB FunctionFS strings.
Behavior is similar to USBFunctionDescriptors= above.
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 documentation[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.
OpenFile=
Takes an argument of the form "path[:fd-name:options]", where:
o "path" is a path to a file or an AF_UNIX socket in the file
system;
o "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;
o "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.
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.
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 specified by using the relevant setting
multiple times.
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. Using an
absolute path is recommended to avoid ambiguity. Hint: this search path
may be queried using systemd-path search-binaries-default.
The command line accepts "%" specifiers as described in
systemd.unit(5).
An argument solely consisting of ";" must be escaped, i.e. specified as
"\;"
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
ExecStart=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
ExecStart=-false
ExecStart=+:@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 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 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. kernel documentation
https://docs.kernel.org/admin-guide/cgroup-v2.html
4. Interface Portability and Stability Promise
https://systemd.io/PORTABILITY_AND_STABILITY/
systemd 254 SYSTEMD.SERVICE(5)
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