SYSTEMD.SERVICE(5)
Configure systemd service units. Static snapshot from Ubuntu 24.04.
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:
• Services with Type=dbus set automatically acquire dependencies of type
Requires= and After= on dbus.socket.
• Socket activated services are automatically ordered after their activating
.socket units via an automatic After= dependency. Services also pull in all
.socket units listed in Sockets= via automatic Wants= and After=
dependencies.
Additional implicit dependencies may be added as result of execution and resource
control parameters as documented in systemd.exec(5) and systemd.resource-
control(5).
Default Dependencies
The following dependencies are added unless DefaultDependencies=no is set:
• Service units will have dependencies of type Requires= and After= on
sysinit.target, a dependency of type After= on basic.target as well as
dependencies of type Conflicts= and Before= on shutdown.target. These ensure
that normal service units pull in basic system initialization, and are
terminated cleanly prior to system shutdown. Only services involved with
early boot or late system shutdown should disable this option.
• Instanced service units (i.e. service units with an "@" in their name) are
assigned by default a per-template slice unit (see systemd.slice(5)), named
after the template unit, containing all instances of the specific template.
This slice is normally stopped at shutdown, together with all template
instances. If that is not desired, set DefaultDependencies=no in the template
unit, and either define your own per-template slice unit file that also sets
DefaultDependencies=no, or set Slice=system.slice (or another suitable slice)
in the template unit. Also see systemd.resource-control(5).
OPTIONS
Service unit files may include [Unit] and [Install] sections, which are described
in systemd.unit(5).
Service unit files must include a [Service] section, which carries information
about the service and the process it supervises. A number of options that may be
used in this section are shared with other unit types. These options are
documented in systemd.exec(5), systemd.kill(5) and systemd.resource-control(5).
The options specific to the [Service] section of service units are the following:
Type=
Configures the mechanism via which the service notifies the manager that the
service start-up has finished. One of simple, exec, forking, oneshot, dbus,
notify, notify-reload, or idle:
• If set to simple (the default if ExecStart= is specified but neither
Type= nor BusName= are), the service manager will consider the unit
started immediately after the main service process has been forked off
(i.e. immediately after fork(), and before various process attributes
have been configured and in particular before the new process has called
execve() to invoke the actual service binary). Typically, Type=exec is
the better choice, see below.
It is expected that the process configured with ExecStart= is the main
process of the service. In this mode, if the process offers functionality
to other processes on the system, its communication channels should be
installed before the service is started up (e.g. sockets set up by
systemd, via socket activation), as the service manager will immediately
proceed starting follow-up units, right after creating the main service
process, and before executing the service's binary. Note that this means
systemctl start command lines for simple services will report success
even if the service's binary cannot be invoked successfully (for example
because the selected User= doesn't exist, or the service binary is
missing).
• The exec type is similar to simple, but the service manager will consider
the unit started immediately after the main service binary has been
executed. The service manager will delay starting of follow-up units
until that point. (Or in other words: simple proceeds with further jobs
right after fork() returns, while exec will not proceed before both
fork() and execve() in the service process succeeded.) Note that this
means systemctl start command lines for exec services will report failure
when the service's binary cannot be invoked successfully (for example
because the selected User= doesn't exist, or the service binary is
missing).
• If set to forking, the manager will consider the unit started immediately
after the binary that forked off by the manager exits. The use of this
type is discouraged, use notify, notify-reload, or dbus instead.
It is expected that the process configured with ExecStart= will call
fork() as part of its start-up. The parent process is expected to exit
when start-up is complete and all communication channels are set up. The
child continues to run as the main service process, and the service
manager will consider the unit started when the parent process exits.
This is the behavior of traditional UNIX services. If this setting is
used, it is recommended to also use the PIDFile= option, so that systemd
can reliably identify the main process of the service. The manager will
proceed with starting follow-up units after the parent process exits.
• Behavior of oneshot is similar to simple; however, the service manager
will consider the unit up after the main process exits. It will then
start follow-up units. RemainAfterExit= is particularly useful for this
type of service. Type=oneshot is the implied default if neither Type=
nor ExecStart= are specified. Note that if this option is used without
RemainAfterExit= the service will never enter "active" unit state, but
will directly transition from "activating" to "deactivating" or "dead",
since no process is configured that shall run continuously. In particular
this means that after a service of this type ran (and which has
RemainAfterExit= not set) it will not show up as started afterwards, but
as dead.
• Behavior of dbus is similar to simple; however, units of this type must
have the BusName= specified and the service manager will consider the
unit up when the specified bus name has been acquired. This type is the
default if BusName= is specified.
Service units with this option configured implicitly gain dependencies on
the dbus.socket unit. A service unit of this type is considered to be in
the activating state until the specified bus name is acquired. It is
considered activated while the bus name is taken. Once the bus name is
released the service is considered being no longer functional which has
the effect that the service manager attempts to terminate any remaining
processes belonging to the service. Services that drop their bus name as
part of their shutdown logic thus should be prepared to receive a SIGTERM
(or whichever signal is configured in KillSignal=) as result.
• Behavior of notify is similar to exec; however, it is expected that the
service sends a "READY=1" notification message via sd_notify(3) or an
equivalent call when it has finished starting up. systemd will proceed
with starting follow-up units after this notification message has been
sent. If this option is used, NotifyAccess= (see below) should be set to
open access to the notification socket provided by systemd. If
NotifyAccess= is missing or set to none, it will be forcibly set to main.
If the service supports reloading, and uses a signal to start the reload,
using notify-reload instead is recommended.
• Behavior of notify-reload is similar to notify, with one difference: the
SIGHUP UNIX process signal is sent to the service's main process when the
service is asked to reload and the manager will wait for a notification
about the reload being finished.
When initiating the reload process the service is expected to reply with
a notification message via sd_notify(3) that contains the "RELOADING=1"
field in combination with "MONOTONIC_USEC=" set to the current monotonic
time (i.e. CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as
decimal string. Once reloading is complete another notification message
must be sent, containing "READY=1". Using this service type and
implementing this reload protocol is an efficient alternative to
providing an ExecReload= command for reloading of the service's
configuration.
The signal to send can be tweaked via ReloadSignal=, see below.
• Behavior of idle is very similar to simple; however, actual execution of
the service program is delayed until all active jobs are dispatched. This
may be used to avoid interleaving of output of shell services with the
status output on the console. Note that this type is useful only to
improve console output, it is not useful as a general unit ordering tool,
and the effect of this service type is subject to a 5s timeout, after
which the service program is invoked anyway.
It is recommended to use Type=exec for long-running services, as it ensures
that process setup errors (e.g. errors such as a missing service executable,
or missing user) are properly tracked. However, as this service type won't
propagate the failures in the service's own startup code (as opposed to
failures in the preparatory steps the service manager executes before
execve()) and doesn't allow ordering of other units against completion of
initialization of the service code itself (which for example is useful if
clients need to connect to the service through some form of IPC, and the IPC
channel is only established by the service itself — in contrast to doing this
ahead of time through socket or bus activation or similar), it might not be
sufficient for many cases. If so, notify, notify-reload, or dbus (the latter
only in case the service provides a D-Bus interface) are the preferred
options as they allow service program code to precisely schedule when to
consider the service started up successfully and when to proceed with
follow-up units. The notify/notify-reload service types require explicit
support in the service codebase (as sd_notify() or an equivalent API needs to
be invoked by the service at the appropriate time) — if it's not supported,
then forking is an alternative: it supports the traditional heavy-weight UNIX
service start-up protocol. Note that using any type other than simple
possibly delays the boot process, as the service manager needs to wait for at
least some service initialization to complete. (Also note it is generally not
recommended to use idle or oneshot for long-running services.)
Note that various service settings (e.g. User=, Group= through libc NSS)
might result in "hidden" blocking IPC calls to other services when used.
Sometimes it might be advisable to use the simple service type to ensure that
the service manager's transaction logic is not affected by such potentially
slow operations and hidden dependencies, as this is the only service type
where the service manager will not wait for such service execution setup
operations to complete before proceeding.
ExitType=
Specifies when the manager should consider the service to be finished. One of
main or cgroup:
• If set to main (the default), the service manager will consider the unit
stopped when the main process, which is determined according to the
Type=, exits. Consequently, it cannot be used with Type=oneshot.
• If set to cgroup, the service will be considered running as long as at
least one process in the cgroup has not exited.
It is generally recommended to use ExitType=main when a service has a known
forking model and a main process can reliably be determined. ExitType=
cgroup is meant for applications whose forking model is not known ahead of
time and which might not have a specific main process. It is well suited for
transient or automatically generated services, such as graphical applications
inside of a desktop environment.
Added in version 250.
RemainAfterExit=
Takes a boolean value that specifies whether the service shall be considered
active even when all its processes exited. Defaults to no.
GuessMainPID=
Takes a boolean value that specifies whether systemd should try to guess the
main PID of a service if it cannot be determined reliably. This option is
ignored unless Type=forking is set and PIDFile= is unset because for the
other types or with an explicitly configured PID file, the main PID is always
known. The guessing algorithm might come to incorrect conclusions if a daemon
consists of more than one process. If the main PID cannot be determined,
failure detection and automatic restarting of a service will not work
reliably. Defaults to yes.
PIDFile=
Takes a path referring to the PID file of the service. Usage of this option
is recommended for services where Type= is set to forking. The path specified
typically points to a file below /run/. If a relative path is specified it is
hence prefixed with /run/. The service manager will read the PID of the main
process of the service from this file after start-up of the service. The
service manager will not write to the file configured here, although it will
remove the file after the service has shut down if it still exists. The PID
file does not need to be owned by a privileged user, but if it is owned by an
unprivileged user additional safety restrictions are enforced: the file may
not be a symlink to a file owned by a different user (neither directly nor
indirectly), and the PID file must refer to a process already belonging to
the service.
Note that PID files should be avoided in modern projects. Use Type=notify,
Type=notify-reload or Type=simple where possible, which does not require use
of PID files to determine the main process of a service and avoids needless
forking.
BusName=
Takes a D-Bus destination name that this service shall use. This option is
mandatory for services where Type= is set to dbus. It is recommended to
always set this property if known to make it easy to map the service name to
the D-Bus destination. In particular, systemctl
service-log-level/service-log-target verbs make use of this.
ExecStart=
Commands that are executed when this service is started. The value is split
into zero or more command lines according to the rules described in the
section "Command Lines" below.
Unless Type= is oneshot, exactly one command must be given. When Type=oneshot
is used, zero or more commands may be specified. Commands may be specified by
providing multiple command lines in the same directive, or alternatively,
this directive may be specified more than once with the same effect. If the
empty string is assigned to this option, the list of commands to start is
reset, prior assignments of this option will have no effect. If no ExecStart=
is specified, then the service must have RemainAfterExit=yes and at least one
ExecStop= line set. (Services lacking both ExecStart= and ExecStop= are not
valid.)
If more than one command is specified, the commands are invoked sequentially
in the order they appear in the unit file. If one of the commands fails (and
is not prefixed with "-"), other lines are not executed, and the unit is
considered failed.
Unless Type=forking is set, the process started via this command line will be
considered the main process of the daemon.
ExecStartPre=, ExecStartPost=
Additional commands that are executed before or after the command in
ExecStart=, respectively. Syntax is the same as for ExecStart=, except that
multiple command lines are allowed and the commands are executed one after
the other, serially.
If any of those commands (not prefixed with "-") fail, the rest are not
executed and the unit is considered failed.
ExecStart= commands are only run after all ExecStartPre= commands that were
not prefixed with a "-" exit successfully.
ExecStartPost= commands are only run after the commands specified in
ExecStart= have been invoked successfully, as determined by Type= (i.e. the
process has been started for Type=simple or Type=idle, the last ExecStart=
process exited successfully for Type=oneshot, the initial process exited
successfully for Type=forking, "READY=1" is sent for
Type=notify/Type=notify-reload, or the BusName= has been taken for
Type=dbus).
Note that ExecStartPre= may not be used to start long-running processes. All
processes forked off by processes invoked via ExecStartPre= will be killed
before the next service process is run.
Note that if any of the commands specified in ExecStartPre=, ExecStart=, or
ExecStartPost= fail (and are not prefixed with "-", see above) or time out
before the service is fully up, execution continues with commands specified
in ExecStopPost=, the commands in ExecStop= are skipped.
Note that the execution of ExecStartPost= is taken into account for the
purpose of Before=/After= ordering constraints.
ExecCondition=
Optional commands that are executed before the commands in ExecStartPre=.
Syntax is the same as for ExecStart=, except that multiple command lines are
allowed and the commands are executed one after the other, serially.
The behavior is like an ExecStartPre= and condition check hybrid: when an
ExecCondition= command exits with exit code 1 through 254 (inclusive), the
remaining commands are skipped and the unit is not marked as failed. However,
if an ExecCondition= command exits with 255 or abnormally (e.g. timeout,
killed by a signal, etc.), the unit will be considered failed (and remaining
commands will be skipped). Exit code of 0 or those matching
SuccessExitStatus= will continue execution to the next commands.
The same recommendations about not running long-running processes in
ExecStartPre= also applies to ExecCondition=. ExecCondition= will also run
the commands in ExecStopPost=, as part of stopping the service, in the case
of any non-zero or abnormal exits, like the ones described above.
Added in version 243.
ExecReload=
Commands to execute to trigger a configuration reload in the service. This
argument takes multiple command lines, following the same scheme as described
for ExecStart= above. Use of this setting is optional. Specifier and
environment variable substitution is supported here following the same scheme
as for ExecStart=.
One additional, special environment variable is set: if known, $MAINPID is
set to the main process of the daemon, and may be used for command lines like
the following:
ExecReload=kill -HUP $MAINPID
Note however that reloading a daemon by enqueuing a signal (as with the
example line above) is usually not a good choice, because this is an
asynchronous operation and hence not suitable when ordering reloads of
multiple services against each other. It is thus strongly recommended to
either use Type=notify-reload in place of ExecReload=, or to set ExecReload=
to a command that not only triggers a configuration reload of the daemon, but
also synchronously waits for it to complete. For example, dbus-broker(1) uses
the following:
ExecReload=busctl call org.freedesktop.DBus \
/org/freedesktop/DBus org.freedesktop.DBus \
ReloadConfig
ExecStop=
Commands to execute to stop the service started via ExecStart=. This argument
takes multiple command lines, following the same scheme as described for
ExecStart= above. Use of this setting is optional. After the commands
configured in this option are run, it is implied that the service is stopped,
and any processes remaining for it are terminated according to the KillMode=
setting (see systemd.kill(5)). If this option is not specified, the process
is terminated by sending the signal specified in KillSignal= or
RestartKillSignal= when service stop is requested. Specifier and environment
variable substitution is supported (including $MAINPID, see above).
Note that it is usually not sufficient to specify a command for this setting
that only asks the service to terminate (for example, by sending some form of
termination signal to it), but does not wait for it to do so. Since the
remaining processes of the services are killed according to KillMode= and
KillSignal= or RestartKillSignal= as described above immediately after the
command exited, this may not result in a clean stop. The specified command
should hence be a synchronous operation, not an asynchronous one.
Note that the commands specified in ExecStop= are only executed when the
service started successfully first. They are not invoked if the service was
never started at all, or in case its start-up failed, for example because any
of the commands specified in ExecStart=, ExecStartPre= or ExecStartPost=
failed (and weren't prefixed with "-", see above) or timed out. Use
ExecStopPost= to invoke commands when a service failed to start up correctly
and is shut down again. Also note that the stop operation is always performed
if the service started successfully, even if the processes in the service
terminated on their own or were killed. The stop commands must be prepared to
deal with that case. $MAINPID will be unset if systemd knows that the main
process exited by the time the stop commands are called.
Service restart requests are implemented as stop operations followed by start
operations. This means that ExecStop= and ExecStopPost= are executed during a
service restart operation.
It is recommended to use this setting for commands that communicate with the
service requesting clean termination. For post-mortem clean-up steps use
ExecStopPost= instead.
ExecStopPost=
Additional commands that are executed after the service is stopped. This
includes cases where the commands configured in ExecStop= were used, where
the service does not have any ExecStop= defined, or where the service exited
unexpectedly. This argument takes multiple command lines, following the same
scheme as described for ExecStart=. Use of these settings is optional.
Specifier and environment variable substitution is supported. Note that –
unlike ExecStop= – commands specified with this setting are invoked when a
service failed to start up correctly and is shut down again.
It is recommended to use this setting for clean-up operations that shall be
executed even when the service failed to start up correctly. Commands
configured with this setting need to be able to operate even if the service
failed starting up half-way and left incompletely initialized data around. As
the service's processes have been terminated already when the commands
specified with this setting are executed they should not attempt to
communicate with them.
Note that all commands that are configured with this setting are invoked with
the result code of the service, as well as the main process' exit code and
status, set in the $SERVICE_RESULT, $EXIT_CODE and $EXIT_STATUS environment
variables, see systemd.exec(5) for details.
Note that the execution of ExecStopPost= is taken into account for the
purpose of Before=/After= ordering constraints.
RestartSec=
Configures the time to sleep before restarting a service (as configured with
Restart=). Takes a unit-less value in seconds, or a time span value such as
"5min 20s". Defaults to 100ms.
RestartSteps=
Configures the number of steps to take to increase the interval of
auto-restarts from RestartSec= to RestartMaxDelaySec=. Takes a positive
integer or 0 to disable it. Defaults to 0.
This setting is effective only if RestartMaxDelaySec= is also set.
Added in version 254.
RestartMaxDelaySec=
Configures the longest time to sleep before restarting a service as the
interval goes up with RestartSteps=. Takes a value in the same format as
RestartSec=, or "infinity" to disable the setting. Defaults to "infinity".
This setting is effective only if RestartSteps= is also set.
Added in version 254.
TimeoutStartSec=
Configures the time to wait for start-up. If a daemon service does not signal
start-up completion within the configured time, the service will be
considered failed and will be shut down again. The precise action depends on
the TimeoutStartFailureMode= option. Takes a unit-less value in seconds, or a
time span value such as "5min 20s". Pass "infinity" to disable the timeout
logic. Defaults to DefaultTimeoutStartSec= set in the manager, except when
Type=oneshot is used, in which case the timeout is disabled by default (see
systemd-system.conf(5)).
If a service of Type=notify/Type=notify-reload sends
"EXTEND_TIMEOUT_USEC=...", this may cause the start time to be extended
beyond TimeoutStartSec=. The first receipt of this message must occur before
TimeoutStartSec= is exceeded, and once the start time has extended beyond
TimeoutStartSec=, the service manager will allow the service to continue to
start, provided the service repeats "EXTEND_TIMEOUT_USEC=..." within the
interval specified until the service startup status is finished by "READY=1".
(see sd_notify(3)).
Added in version 188.
TimeoutStopSec=
This option serves two purposes. First, it configures the time to wait for
each ExecStop= command. If any of them times out, subsequent ExecStop=
commands are skipped and the service will be terminated by SIGTERM. If no
ExecStop= commands are specified, the service gets the SIGTERM immediately.
This default behavior can be changed by the TimeoutStopFailureMode= option.
Second, it configures the time to wait for the service itself to stop. If it
doesn't terminate in the specified time, it will be forcibly terminated by
SIGKILL (see KillMode= in systemd.kill(5)). Takes a unit-less value in
seconds, or a time span value such as "5min 20s". Pass "infinity" to disable
the timeout logic. Defaults to DefaultTimeoutStopSec= from the manager
configuration file (see systemd-system.conf(5)).
If a service of Type=notify/Type=notify-reload sends
"EXTEND_TIMEOUT_USEC=...", this may cause the stop time to be extended beyond
TimeoutStopSec=. The first receipt of this message must occur before
TimeoutStopSec= is exceeded, and once the stop time has extended beyond
TimeoutStopSec=, the service manager will allow the service to continue to
stop, provided the service repeats "EXTEND_TIMEOUT_USEC=..." within the
interval specified, or terminates itself (see sd_notify(3)).
Added in version 188.
TimeoutAbortSec=
This option configures the time to wait for the service to terminate when it
was aborted due to a watchdog timeout (see WatchdogSec=). If the service has
a short TimeoutStopSec= this option can be used to give the system more time
to write a core dump of the service. Upon expiration the service will be
forcibly terminated by SIGKILL (see KillMode= in systemd.kill(5)). The core
file will be truncated in this case. Use TimeoutAbortSec= to set a sensible
timeout for the core dumping per service that is large enough to write all
expected data while also being short enough to handle the service failure in
due time.
Takes a unit-less value in seconds, or a time span value such as "5min 20s".
Pass an empty value to skip the dedicated watchdog abort timeout handling and
fall back TimeoutStopSec=. Pass "infinity" to disable the timeout logic.
Defaults to DefaultTimeoutAbortSec= from the manager configuration file (see
systemd-system.conf(5)).
If a service of Type=notify/Type=notify-reload handles SIGABRT itself
(instead of relying on the kernel to write a core dump) it can send
"EXTEND_TIMEOUT_USEC=..." to extended the abort time beyond
TimeoutAbortSec=. The first receipt of this message must occur before
TimeoutAbortSec= is exceeded, and once the abort time has extended beyond
TimeoutAbortSec=, the service manager will allow the service to continue to
abort, provided the service repeats "EXTEND_TIMEOUT_USEC=..." within the
interval specified, or terminates itself (see sd_notify(3)).
Added in version 243.
TimeoutSec=
A shorthand for configuring both TimeoutStartSec= and TimeoutStopSec= to the
specified value.
TimeoutStartFailureMode=, TimeoutStopFailureMode=
These options configure the action that is taken in case a daemon service
does not signal start-up within its configured TimeoutStartSec=, respectively
if it does not stop within TimeoutStopSec=. Takes one of terminate, abort and
kill. Both options default to terminate.
If terminate is set the service will be gracefully terminated by sending the
signal specified in KillSignal= (defaults to SIGTERM, see systemd.kill(5)).
If the service does not terminate the FinalKillSignal= is sent after
TimeoutStopSec=. If abort is set, WatchdogSignal= is sent instead and
TimeoutAbortSec= applies before sending FinalKillSignal=. This setting may be
used to analyze services that fail to start-up or shut-down intermittently.
By using kill the service is immediately terminated by sending
FinalKillSignal= without any further timeout. This setting can be used to
expedite the shutdown of failing services.
Added in version 246.
RuntimeMaxSec=
Configures a maximum time for the service to run. If this is used and the
service has been active for longer than the specified time it is terminated
and put into a failure state. Note that this setting does not have any effect
on Type=oneshot services, as they terminate immediately after activation
completed. Pass "infinity" (the default) to configure no runtime limit.
If a service of Type=notify/Type=notify-reload sends
"EXTEND_TIMEOUT_USEC=...", this may cause the runtime to be extended beyond
RuntimeMaxSec=. The first receipt of this message must occur before
RuntimeMaxSec= is exceeded, and once the runtime has extended beyond
RuntimeMaxSec=, the service manager will allow the service to continue to
run, provided the service repeats "EXTEND_TIMEOUT_USEC=..." within the
interval specified until the service shutdown is achieved by "STOPPING=1" (or
termination). (see sd_notify(3)).
Added in version 229.
RuntimeRandomizedExtraSec=
This option modifies RuntimeMaxSec= by increasing the maximum runtime by an
evenly distributed duration between 0 and the specified value (in seconds).
If RuntimeMaxSec= is unspecified, then this feature will be disabled.
Added in version 250.
WatchdogSec=
Configures the watchdog timeout for a service. The watchdog is activated when
the start-up is completed. The service must call sd_notify(3) regularly with
"WATCHDOG=1" (i.e. the "keep-alive ping"). If the time between two such calls
is larger than the configured time, then the service is placed in a failed
state and it will be terminated with SIGABRT (or the signal specified by
WatchdogSignal=). By setting Restart= to on-failure, on-watchdog, on-abnormal
or always, the service will be automatically restarted. The time configured
here will be passed to the executed service process in the WATCHDOG_USEC=
environment variable. This allows daemons to automatically enable the
keep-alive pinging logic if watchdog support is enabled for the service. If
this option is used, NotifyAccess= (see below) should be set to open access
to the notification socket provided by systemd. If NotifyAccess= is not set,
it will be implicitly set to main. Defaults to 0, which disables this
feature. The service can check whether the service manager expects watchdog
keep-alive notifications. See sd_watchdog_enabled(3) for details.
sd_event_set_watchdog(3) may be used to enable automatic watchdog
notification support.
Restart=
Configures whether the service shall be restarted when the service process
exits, is killed, or a timeout is reached. The service process may be the
main service process, but it may also be one of the processes specified with
ExecStartPre=, ExecStartPost=, ExecStop=, ExecStopPost=, or ExecReload=. When
the death of the process is a result of systemd operation (e.g. service stop
or restart), the service will not be restarted. Timeouts include missing the
watchdog "keep-alive ping" deadline and a service start, reload, and stop
operation timeouts.
Takes one of no, on-success, on-failure, on-abnormal, on-watchdog, on-abort,
or always. If set to no (the default), the service will not be restarted. If
set to on-success, it will be restarted only when the service process exits
cleanly. In this context, a clean exit means any of the following:
• exit code of 0;
• for types other than Type=oneshot, one of the signals SIGHUP, SIGINT,
SIGTERM, or SIGPIPE;
• exit statuses and signals specified in SuccessExitStatus=.
If set to on-failure, the service will be restarted when the process exits
with a non-zero exit code, is terminated by a signal (including on core dump,
but excluding the aforementioned four signals), when an operation (such as
service reload) times out, and when the configured watchdog timeout is
triggered. If set to on-abnormal, the service will be restarted when the
process is terminated by a signal (including on core dump, excluding the
aforementioned four signals), when an operation times out, or when the
watchdog timeout is triggered. If set to on-abort, the service will be
restarted only if the service process exits due to an uncaught signal not
specified as a clean exit status. If set to on-watchdog, the service will be
restarted only if the watchdog timeout for the service expires. If set to
always, the service will be restarted regardless of whether it exited cleanly
or not, got terminated abnormally by a signal, or hit a timeout. 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 allbox tab(:);
lB lB lB lB lB lB lB lB. T{ Restart settings/Exit causes T}:T{ no T}:T{
always T}:T{ on-success T}:T{ on-failure T}:T{ on-abnormal T}:T{ on-abort
T}:T{ on-watchdog T} l l l l l l l l l l l l l l l l l l l l l l l l l l l l
l l l l l l l l l l l l. T{ Clean exit code or signal T}:T{ T}:T{ X T}:T{
X T}:T{ T}:T{ T}:T{ T}:T{ T} T{ Unclean exit code T}:T{ T}:T{ X
T}:T{ T}:T{ X T}:T{ T}:T{ T}:T{ T} T{ Unclean signal T}:T{ T}:T{ X
T}:T{ T}:T{ X T}:T{ X T}:T{ X T}:T{ T} T{ Timeout T}:T{ T}:T{ X T}:T{
T}:T{ X T}:T{ X T}:T{ T}:T{ T} T{ Watchdog T}:T{ T}:T{ X T}:T{ T}:T{
X T}:T{ X T}:T{ T}:T{ X T}
As exceptions to the setting above, the service will not be restarted if the
exit code or signal is specified in RestartPreventExitStatus= (see below) or
the service is stopped with systemctl stop or an equivalent operation. Also,
the services will always be restarted if the exit code or signal is specified
in RestartForceExitStatus= (see below).
Note that service restart is subject to unit start rate limiting configured
with StartLimitIntervalSec= and StartLimitBurst=, see systemd.unit(5) for
details.
Setting this to on-failure is the recommended choice for long-running
services, in order to increase reliability by attempting automatic recovery
from errors. For services that shall be able to terminate on their own choice
(and avoid immediate restarting), on-abnormal is an alternative choice.
RestartMode=
Takes a string value that specifies how a service should restart:
• If set to normal (the default), the service restarts by going through a
failed/inactive state.
• If set to direct, the service transitions to the activating state
directly during auto-restart, skipping failed/inactive state.
ExecStopPost= is invoked. OnSuccess= and OnFailure= are skipped.
This option is useful in cases where a dependency can fail temporarily but we
don't want these temporary failures to make the dependent units fail. When
this option is set to direct, dependent units are not notified of these
temporary failures.
Added in version 254.
SuccessExitStatus=
Takes a list of exit status definitions that, when returned by the main
service process, will be considered successful termination, in addition to
the normal successful exit status 0 and, except for Type=oneshot, the signals
SIGHUP, SIGINT, SIGTERM, and SIGPIPE. Exit status definitions can be numeric
termination statuses, termination status names, or termination signal names,
separated by spaces. See the Process Exit Codes section in systemd.exec(5)
for a list of termination status names (for this setting only the part
without the "EXIT_" or "EX_" prefix should be used). See signal(7) for a list
of signal names.
Note that this setting does not change the mapping between numeric exit
statuses and their names, i.e. regardless how this setting is used 0 will
still be mapped to "SUCCESS" (and thus typically shown as "0/SUCCESS" in tool
outputs) and 1 to "FAILURE" (and thus typically shown as "1/FAILURE"), and so
on. It only controls what happens as effect of these exit statuses, and how
it propagates to the state of the service as a whole.
This option may appear more than once, in which case the list of successful
exit statuses is merged. If the empty string is assigned to this option, the
list is reset, all prior assignments of this option will have no effect.
Example 1. A service with the SuccessExitStatus= setting
SuccessExitStatus=TEMPFAIL 250 SIGKILL
Exit status 75 (TEMPFAIL), 250, and the termination signal SIGKILL are
considered clean service terminations.
Note: systemd-analyze exit-status may be used to list exit statuses and
translate between numerical status values and names.
Added in version 189.
RestartPreventExitStatus=
Takes a list of exit status definitions that, when returned by the main
service process, will prevent automatic service restarts, regardless of the
restart setting configured with Restart=. Exit status definitions can either
be numeric exit codes or termination signal names, and are separated by
spaces. Defaults to the empty list, so that, by default, no exit status is
excluded from the configured restart logic. For example:
RestartPreventExitStatus=1 6 SIGABRT
ensures that exit codes 1 and 6 and the termination signal SIGABRT will not
result in automatic service restarting. This option may appear more than
once, in which case the list of restart-preventing statuses is merged. If the
empty string is assigned to this option, the list is reset and all prior
assignments of this option will have no effect.
Note that this setting has no effect on processes configured via
ExecStartPre=, ExecStartPost=, ExecStop=, ExecStopPost= or ExecReload=, but
only on the main service process, i.e. either the one invoked by ExecStart=
or (depending on Type=, PIDFile=, ...) the otherwise configured main process.
Added in version 189.
RestartForceExitStatus=
Takes a list of exit status definitions that, when returned by the main
service process, will force automatic service restarts, regardless of the
restart setting configured with Restart=. The argument format is similar to
RestartPreventExitStatus=.
Added in version 215.
RootDirectoryStartOnly=
Takes a boolean argument. If true, the root directory, as configured with the
RootDirectory= option (see systemd.exec(5) for more information), is only
applied to the process started with ExecStart=, and not to the various other
ExecStartPre=, ExecStartPost=, ExecReload=, ExecStop=, and ExecStopPost=
commands. If false, the setting is applied to all configured commands the
same way. Defaults to false.
NonBlocking=
Set the O_NONBLOCK flag for all file descriptors passed via socket-based
activation. If true, all file descriptors >= 3 (i.e. all except stdin,
stdout, stderr), excluding those passed in via the file descriptor storage
logic (see FileDescriptorStoreMax= for details), will have the O_NONBLOCK
flag set and hence are in non-blocking mode. This option is only useful in
conjunction with a socket unit, as described in systemd.socket(5) and has no
effect on file descriptors which were previously saved in the file-descriptor
store for example. Defaults to false.
Note that if the same socket unit is configured to be passed to multiple
service units (via the Sockets= setting, see below), and these services have
different NonBlocking= configurations, the precise state of O_NONBLOCK
depends on the order in which these services are invoked, and will possibly
change after service code already took possession of the socket file
descriptor, simply because the O_NONBLOCK state of a socket is shared by all
file descriptors referencing it. Hence it is essential that all services
sharing the same socket use the same NonBlocking= configuration, and do not
change the flag in service code either.
NotifyAccess=
Controls access to the service status notification socket, as accessible via
the sd_notify(3) call. Takes one of none (the default), main, exec or all. If
none, no daemon status updates are accepted from the service processes, all
status update messages are ignored. If main, only service updates sent from
the main process of the service are accepted. If exec, only service updates
sent from any of the main or control processes originating from one of the
Exec*= commands are accepted. If all, all services updates from all members
of the service's control group are accepted. This option should be set to
open access to the notification socket when using
Type=notify/Type=notify-reload or WatchdogSec= (see above). If those options
are used but NotifyAccess= is not configured, it will be implicitly set to
main.
Note that sd_notify() notifications may be attributed to units correctly only
if either the sending process is still around at the time PID 1 processes the
message, or if the sending process is explicitly runtime-tracked by the
service manager. The latter is the case if the service manager originally
forked off the process, i.e. on all processes that match main or exec.
Conversely, if an auxiliary process of the unit sends an sd_notify() message
and immediately exits, the service manager might not be able to properly
attribute the message to the unit, and thus will ignore it, even if
NotifyAccess=all is set for it.
Hence, to eliminate all race conditions involving lookup of the client's unit
and attribution of notifications to units correctly, sd_notify_barrier() may
be used. This call acts as a synchronization point and ensures all
notifications sent before this call have been picked up by the service
manager when it returns successfully. Use of sd_notify_barrier() is needed
for clients which are not invoked by the service manager, otherwise this
synchronization mechanism is unnecessary for attribution of notifications to
the unit.
Sockets=
Specifies the name of the socket units this service shall inherit socket file
descriptors from when the service is started. Normally, it should not be
necessary to use this setting, as all socket file descriptors whose unit
shares the same name as the service (subject to the different unit name
suffix of course) are passed to the spawned process.
Note that the same socket file descriptors may be passed to multiple
processes simultaneously. Also note that a different service may be activated
on incoming socket traffic than the one which is ultimately configured to
inherit the socket file descriptors. Or, in other words: the Service= setting
of .socket units does not have to match the inverse of the Sockets= setting
of the .service it refers to.
This option may appear more than once, in which case the list of socket units
is merged. Note that once set, clearing the list of sockets again (for
example, by assigning the empty string to this option) is not supported.
FileDescriptorStoreMax=
Configure how many file descriptors may be stored in the service manager for
the service using sd_pid_notify_with_fds(3)'s "FDSTORE=1" messages. This is
useful for implementing services that can restart after an explicit request
or a crash without losing state. Any open sockets and other file descriptors
which should not be closed during the restart may be stored this way.
Application state can either be serialized to a file in RuntimeDirectory=, or
stored in a memfd_create(2) memory file descriptor. Defaults to 0, i.e. no
file descriptors may be stored in the service manager. All file descriptors
passed to the service manager from a specific service are passed back to the
service's main process on the next service restart (see sd_listen_fds(3) for
details about the precise protocol used and the order in which the file
descriptors are passed). Any file descriptors passed to the service manager
are automatically closed when POLLHUP or POLLERR is seen on them, or when the
service is fully stopped and no job is queued or being executed for it (the
latter can be tweaked with 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=, and ExecStopPost= options.
Multiple command lines may be concatenated in a single directive by separating
them with semicolons (these semicolons must be passed as separate words). Lone
semicolons may be escaped as "\;".
Each command line is unquoted using the rules described in "Quoting" section in
systemd.syntax(7). The first item becomes the command to execute, and the
subsequent items the arguments.
This syntax is inspired by shell syntax, but only the meta-characters and
expansions described in the following paragraphs are understood, and the
expansion of variables is different. Specifically, redirection using "<", "<<",
">", and ">>", pipes using "|", running programs in the background using "&", and
other elements of shell syntax are not supported.
The command to execute may contain spaces, but control characters are not
allowed.
Each command may be prefixed with a number of special characters:
Table 2. Special executable prefixes allbox tab(:); lB lB. T{ Prefix T}:T{
Effect T} l l l l l l l l l l l l. T{ "@" T}:T{ 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. T} T{ "-" T}:T{ 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. T} T{ ":" T}:T{ If the
executable path is prefixed with ":", environment variable substitution (as
described by the "Command Lines" section below) is not applied. T} T{ "+" T}:T{
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. T} T{ "!"
T}:T{ 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. T} T{ "!!" T}:T{ 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. T}
"@", "-", ":", 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 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
ExecStart=/usr/sbin/simple-notifying-service
[Install]
WantedBy=multi-user.target
Note that the daemon has to support systemd's notification protocol, else systemd
will think the service has not started yet and kill it after a timeout. For an
example of how to update daemons to support this protocol transparently, take a
look at sd_notify(3). systemd will consider the unit to be in the 'starting'
state until a readiness notification has arrived.
Please see systemd.kill(5) for details on how you can influence the way systemd
terminates the service.
SEE ALSO
systemd(1), systemctl(1), systemd-system.conf(5), systemd.unit(5),
systemd.exec(5), systemd.resource-control(5), systemd.kill(5),
systemd.directives(7), systemd-run(1)
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
systemd 255 SYSTEMD.SERVICE(5)
Generated 2026-11-03 from systemd.service.5 in the official Ubuntu 24.04 package systemd 255.4-1ubuntu8.18. Extracted without installing the package. Original copyright and license terms apply.