It is generally recommended to use Type=simple for long-running services whenever possible, as it is the simplest and fastest option. However, as this service type won't propagate service start-up failures and doesn't allow ordering of other units against completion of initialization of the service (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 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 service type requires 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 UNIX service start-up protocol. Finally, exec might be an option for cases where it is enough to ensure the service binary is invoked, and where the service binary itself executes no or little initialization on its own (and its initialization is unlikely to fail). Note that using any type other than simple possibly delays the boot process, as the service manager needs to wait for service initialization to complete. It is hence recommended not to needlessly use any types other than simple. (Also note it is generally not recommended to use idle or oneshot for long-running services.)

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.

Note that PID files should be avoided in modern projects. Use Type=notify 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.

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.)

For each of the specified commands, the first argument must be either an absolute path to an executable or a simple file name without any slashes. Optionally, this filename may be prefixed with a number of special characters:

Table 1. Special executable prefixes

"@",
"-", ":", and one of "+"/"!"/"!!" may be used together and they can appear in any order. However, only one of "+", "!", "!!" may be used at a time. Note that these prefixes are also supported for the other command line settings, i.e. ExecStartPre=, ExecStartPost=, ExecReload=, ExecStop= and ExecStopPost=.

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.

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, 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.

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.

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 sending a signal (as with the example line above) is usually not a good choice, because this is an asynchronous operation and hence not suitable to order reloads of multiple services against each other. It is strongly recommended 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

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.

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.

If a service of Type=notify 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)).

If a service of Type=notify 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)).

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 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)).

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.

If a service of Type=notify 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)).

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:

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 2. Exit causes and the effect of the Restart= settings

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.

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=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.

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.

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.

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.

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. This setting takes one of continue, stop or kill. If set to continue and a process of the service 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[2].

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. Similarly to the kernel OOM kills, this setting determines the state of the unit after systemd-oomd kills a cgroup associated with it.

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

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

ExecStart=sh -c 'dmesg | tac'

ExecStart=echo one ; echo "two two"

ExecStart=echo / >/dev/null & \; \
ls

[Unit]
Description=Foo
[Service]
ExecStart=/usr/sbin/foo-daemon
[Install]
WantedBy=multi-user.target

[Unit]
Description=Cleanup old Foo data
[Service]
Type=oneshot
ExecStart=/usr/sbin/foo-cleanup
[Install]
WantedBy=multi-user.target

[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

[Unit]
Description=Some simple daemon
[Service]
Type=forking
ExecStart=/usr/sbin/my-simple-daemon -d
[Install]
WantedBy=multi-user.target

[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

[D-BUS Service]
Name=org.example.simple-dbus-service
Exec=/usr/sbin/simple-dbus-service
User=root
SystemdService=simple-dbus-service.service

[Unit]
Description=Simple notifying service
[Service]
Type=notify
ExecStart=/usr/sbin/simple-notifying-service
[Install]
WantedBy=multi-user.target