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 [email protected] 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:

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:

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=

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= does not exist, or the service binary is missing).

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.

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.

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

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.

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 will not 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 does not 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 is 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=

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=

GuessMainPID=

PIDFile=

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=

ExecStart=

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=

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=

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=

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 \
\n
        /org/freedesktop/DBus org.freedesktop.DBus \
\n
        ReloadConfig

ExecStop=

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 were not 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=

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=

RestartSteps=

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

Added in version 254.

RestartMaxDelaySec=

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

Added in version 254.

TimeoutStartSec=

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

Note that the start timeout is also applied to service reloads, regardless if implemented through ExecReload= or via the reload logic enabled via Type=notify-reload. If the reload does not complete within the configured time, the reload will be considered failed and the service will continue running with the old configuration. This will not affect the running service, but will be logged and will cause e.g. systemctl reload to fail.

Added in version 188.

TimeoutStopSec=

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=

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=

TimeoutStartFailureMode=, TimeoutStopFailureMode=

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=

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=

Added in version 250.

WatchdogSec=

Restart=

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 1. 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.

RestartMode=

Added in version 254.

This option is useful in cases where a dependency can fail temporarily but we do not want these temporary failures to make the dependent units fail. Dependent units are not notified of these temporary failures.

Added in version 254.

Added in version 257.

Added in version 254.

SuccessExitStatus=

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 theSuccessExitStatus= 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=

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=

Note that for Type=oneshot services, a success exit status will prevent them from auto-restarting, no matter whether the corresponding exit statuses are listed in this option or not.

Added in version 215.

RootDirectoryStartOnly=

NonBlocking=

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=

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=

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=

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=

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

Added in version 254.

USBFunctionDescriptors=

Added in version 227.

USBFunctionStrings=

Added in version 227.

OOMPolicy=

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=

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 cannot 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=

Added in version 253.

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

COMMAND LINES#

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

Multiple command lines may be 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

"@", "-", ":", 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/, and their sbin/ counterparts (only 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).

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 3. Simple service

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

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

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

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

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

Example 4. Oneshot service

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

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

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

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

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

Example 5. Stoppable oneshot service

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

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

[Unit]
Description=Simple firewall
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/sbin/simple-firewall-start
ExecStop=/usr/local/sbin/simple-firewall-stop
[Install]
WantedBy=multi-user.target

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

Example 6. Traditional forking services

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

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

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

The following example shows a simple daemon that forks and just starts one process in the background:

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

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

Example 7. DBus services

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

[Unit]
Description=Simple DBus Service
[Service]
Type=dbus
BusName=org.example.simple-dbus-service
ExecStart=/usr/sbin/simple-dbus-service
[Install]
WantedBy=multi-user.target

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

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

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

Example 8. Services that notify systemd about their initialization

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

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

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

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

To avoid code duplication, it is preferable to use sd_notify(3) when possible, especially when other APIs provided by libsystemd(3) are also used, but note that the notification protocol is very simple and guaranteed to be stable as per the Interface Portability and Stability Promise[4], so it can be reimplemented by services with no external dependencies. For a self-contained example, see sd_notify(3).

SEE ALSO#

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

NOTES#

1.

File Descriptor Store

2.

USB FunctionFS

3.

Control Group v2

4.

Interface Portability and Stability Promise