systemd.service (5)

NAME

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 a service is requested under a certain name but no unit configuration file is found, systemd looks for a SysV init script by the same name (with the .service suffix removed) and dynamically creates a service unit from that script. This is useful for compatibility with SysV. Note that this compatibility is quite comprehensive but not 100%. For details about the incompatibilities, see the m[blue]Incompatibilities with SysVm[]

AUTOMATIC DEPENDENCIES

Services with Type=dbus set automatically acquire dependencies of type Requires= and After= on dbus.socket.

Socket activated service are automatically ordered after their activated .socket units via an automatic After= dependency.

Unless DefaultDependencies= is set to false, service units will implicitly 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).

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).

OPTIONS

Service 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) and systemd.kill(5). The options specific to the "[Service]" section of service units are the following:

Type=

Configures the process start-up type for this service unit. One of simple, forking, oneshot, dbus, notify or idle.

If set to simple (the default if neither Type= nor BusName=, but ExecStart= are specified), 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 daemon is started up (e.g. sockets set up by systemd, via socket activation), as systemd will immediately proceed starting follow-up units.

If set to forking, 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 daemon process. This is the behavior of traditional UNIX daemons. If this setting is used, it is recommended to also use the PIDFile= option, so that systemd can identify the main process of the daemon. systemd will proceed with starting follow-up units as soon as the parent process exits.

Behavior of oneshot is similar to simple; however, it is expected that the process has to exit before systemd starts follow-up units. RemainAfterExit= is particularly useful for this type of service. This is the implied default if neither Type= or ExecStart= are specified.

Behavior of dbus is similar to simple; however, it is expected that the daemon acquires a name on the D-Bus bus, as configured by BusName=. systemd will proceed with starting follow-up units after the D-Bus bus name has been acquired. Service units with this option configured implicitly gain dependencies on the dbus.socket unit. This type is the default if BusName= is specified.

Behavior of notify is similar to simple; however, it is expected that the daemon sends a 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 not set, it will be implicitly set to main. Note that currently Type=notify will not work if used in combination with PrivateNetwork=yes.

Behavior of idle is very similar to simple; however, actual execution of the service binary is delayed until all jobs are dispatched. This may be used to avoid interleaving of output of shell services with the status output on the console.

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 an absolute file name pointing to the PID file of this daemon. Use of this option is recommended for services where Type= is set to forking. systemd will read the PID of the main process of the daemon after start-up of the service. systemd will not write to the file configured here, although it will remove the file after the service has shut down if it still exists.

BusName=

Takes a D-Bus bus name that this service is reachable as. This option is mandatory for services where Type= is set to dbus.

BusPolicy=

If specified, a custom kdbus endpoint will be created and installed as the default bus node for the service. Such a custom endpoint can hold an own set of policy rules that are enforced on top of the bus-wide ones. The custom endpoint is named after the service it was created for, and its node will be bind-mounted over the default bus node location, so the service can only access the bus through its own endpoint. Note that custom bus endpoints default to a "deny all" policy. Hence, if at least one BusPolicy= directive is given, you have to make sure to add explicit rules for everything the service should be able to do.

The value of this directive is comprised of two parts; the bus name, and a verb to specify to granted access, which is one of see, talk, or own. talk implies see, and own implies both talk and see. If multiple access levels are specified for the same bus name, the most powerful one takes effect.

Examples:

BusPolicy=org.freedesktop.systemd1 talk

BusPolicy=org.foo.bar see

This option is only available on kdbus enabled systems.

ExecStart=

Commands with their arguments that are executed when this service is started. The value is split into zero or more command lines according to the rules described below (see section "Command Lines" below).

When Type= is not oneshot, only one command may and must be given. When Type=oneshot is used, zero or more commands may be specified. This can 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 set.

For each of the specified commands, the first argument must be an absolute path to an executable. Optionally, if this file name is prefixed with "@", the second token will be passed as "argv[0]" to the executed process, followed by the further arguments specified. If the absolute filename 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 ignored and considered success. If both "-" and "@" are used, they can appear in either order.

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 service has started successfully, as determined by Type= (i.e. the process has been started for Type=simple or Type=idle, the process exits successfully for Type=oneshot, the initial process exits 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.

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:

/bin/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.

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, all processes remaining for a service 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= 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 queuing some form of termination signal for it), but does not wait for it to do so. Since the remaining processes of the services are killed using SIGKILL immediately after the command exited, this would 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 successfuly 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.

It is recommended to use this setting for commands that communicate with the service requesting clean termination. When the commands specified with this option are executed it should be assumed that the service is still fully up and is able to react correctly to all commands. 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.

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.

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. 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= from the manager configuration file, except when Type=oneshot is used, in which case the timeout is disabled by default (see systemd-system.conf(5)).

TimeoutStopSec=

Configures the time to wait for stop. If a service is asked to stop, but does not terminate in the specified time, it will be terminated forcibly via SIGTERM, and after another timeout of equal duration with 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)).

TimeoutSec=

A shorthand for configuring both TimeoutStartSec= and TimeoutStopSec= to the specified value.

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.

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. 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 an exit code of 0, or one of the signals SIGHUP, SIGINT, SIGTERM or SIGPIPE, and additionally, 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.

Table 1. Exit causes and the effect of the Restart= settings on them

Restart settings/Exit causes	no	always	on-success	on-failure	on-abnormal	on-abort	on-watchdog
Clean exit code or signal		X	X
Unclean exit code		X		X
Unclean signal		X		X	X	X
Timeout		X		X	X
Watchdog		X		X	X		X

As exceptions to the setting above, the service will not be restarted if the exit code or signal is specified in RestartPreventExitStatus= (see below). Also, the services will always be restarted if the exit code or signal is specified in RestartForceExitStatus= (see below).

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.

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 code 0 and the signals SIGHUP, SIGINT, SIGTERM, and SIGPIPE. Exit status definitions can either be numeric exit codes or termination signal names, separated by spaces. For example:

SuccessExitStatus=1 2 8 SIGKILL

ensures that exit codes 1, 2, 8 and the termination signal SIGKILL are considered clean service terminations.

Note that if a process has a signal handler installed and exits by calling _exit(2) in response to a signal, the information about the signal is lost. Programs should instead perform cleanup and kill themselves with the same signal instead. See m[blue]Proper handling of SIGINT/SIGQUIT --- How to be a proper programm[]

[2]

.

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.

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.

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=.

PermissionsStartOnly=

Takes a boolean argument. If true, the permission-related execution options, as configured with User= and similar options (see systemd.exec(5) for more information), are 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.

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, and stderr) 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). Defaults to false.

NotifyAccess=

Controls access to the service status notification socket, as accessible via the sd_notify(3) call. Takes one of none (the default), main 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 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 or WatchdogSec= (see above). If those options are used but NotifyAccess= is not configured, it will be implicitly set to main.

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. If the empty string is assigned to this option, the list of sockets is reset, and all prior uses of this setting will have no effect.

FailureAction=

Configure the action to take when the service enters a failed state. Takes the same values as the unit setting StartLimitAction= and executes the same actions (see systemd.unit(5)). Defaults to none.

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 service restart schemes where the state is serialized to /run and the file descriptors passed to the service manager, to allow restarts without losing state. Defaults to 0, i.e. no file descriptors may be stored in the service manager by default. 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. 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 queued or being executed for it.

USBFunctionDescriptors=

Configure the location of a file containing m[blue]USB FunctionFSm[]

[3]

descriptors, for implementation of USB gadget functions. This is used only in conjunction with a socket unit with ListenUSBFunction= configured. The contents of this file are written to the ep0 file after it is opened.

USBFunctionStrings=

Configure the location of a file containing USB FunctionFS strings. Behavior is similar to USBFunctionDescriptors= above.

Check 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 split on whitespace, with the first item being the command to execute, and the subsequent items being the arguments. Double quotes ("...") and single quotes ('...') may be used, in which case everything until the next matching quote becomes part of the same argument. C-style escapes are also supported. The table below contains the list of allowed escape patterns. Only patterns which match the syntax in the table are allowed; others will result in an error, and must be escaped by doubling the backslash. Quotes themselves are removed after parsing and escape sequences substituted. In addition, a trailing backslash ("\") may be used to merge lines.

This syntax is intended to be very similar to shell syntax, but only the meta-characters and expansions described in the following paragraphs are understood. 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 must be an absolute path name. It may contain spaces, but control characters are not allowed.

The command line accepts "%" specifiers as described in systemd.unit(5). Note that the first argument of the command line (i.e. the program to execute) may not include specifiers.

Basic environment variable substitution is supported. Use "${FOO}" as part of a word, or as a word of its own, on the command line, in which case it will be replaced by the value of the environment variable including all whitespace it contains, resulting in a single argument. Use "$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=/bin/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 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=/bin/sh -c 'dmesg | tac'

Example:

ExecStart=/bin/echo one ; /bin/echo "two two"

This will execute /bin/echo two times, each time with one argument: "one" and "two two", respectively. Because two commands are specified, Type=oneshot must be used.

Example:

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

This will execute /bin/echo with five arguments: "/", ">/dev/null", "&", ";", and "/bin/ls".

Table 2. C escapes supported in command lines and environment variables

EXAMPLES

Example 1. 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 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 2. 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. They will be executed in order until either they are all successful or one of them fails.

Example 3. 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 4. 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 5. 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 6. 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 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.unit(5), systemd.exec(5), systemd.resource-control(5), systemd.kill(5), systemd.directives(7)

NOTES

1.

Incompatibilities with SysV

www.freedesktop.org/wiki/Software/systemd/Incompatibilities

2.

Proper handling of SIGINT/SIGQUIT --- How to be a proper program

www.cons.org/cracauer/sigint.html

3.

USB FunctionFS

www.kernel.org/doc/Documentation/usb/functionfs.txt