SYSTEMD.SERVICE(5) systemd.service SYSTEMD.SERVICE(5)
NAME
systemd.service - Service unit configuration
SYNOPSIS
service.service
DESCRIPTION
A unit configuration file whose name ends in .service encodes
information about a process controlled and supervised by systemd.
This man page lists the configuration options specific to this unit
type. See systemd.unit(5) for the common options of all unit
configuration files. The common configuration items are configured in
the generic "[Unit]" and "[Install]" sections. The service specific
configuration options are configured in the "[Service]" section.
Additional options are listed in systemd.exec(5), which define the
execution environment the commands are executed in, and in
systemd.kill(5), which define the way the processes of the service are
terminated, and in systemd.resource-control(5), which configure
resource control settings for the processes of the service.
Unless DefaultDependencies= is set to false, service units will
implicitly have dependencies of type Requires= and 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.
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 Incompatibilities with SysV[1] document.
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 path referring to the PID file of the service.
Usage of this option is recommended for services where Type= is set
to forking. The service manager will read the PID of the main
process of the service from this file after start-up of the
service. The service manager will not write to the file configured
here, although it will remove the file after the service has shut
down if it still exists. The PID file does not need to be owned by
a privileged user, but if it is owned by an unprivileged user
additional safety restrictions are enforced: the file may not be a
symlink to a file owned by a different user (neither directly nor
indirectly), and the PID file must refer to a process already
belonging to the service.
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[2] 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 is
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.
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.
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 immediately when service stop
is requested. Specifier and environment variable substitution is
supported (including $MAINPID, see above).
ExecStopPost=
Additional commands that are executed after the service was
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.
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 "0" 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=
This option serves two purposes. First, it configures the time to
wait for each ExecStop= command. If any of them times out,
subsequent ExecStop= commands are skipped and the service will be
terminated by SIGTERM. If no ExecStop= commands are specified, the
service gets the SIGTERM immediately. Second, it configures the
time to wait for the service itself to stop. If it doesn't
terminate in the specified time, it will be forcibly terminated by
SIGKILL (see KillMode= in systemd.kill(5)). Takes a unit-less value
in seconds, or a time span value such as "5min 20s". Pass "0" 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.
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 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.
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 aforementiond 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 | no | always | on-success | on-failure | on-abnormal | on-abort | on-watchdog |
|settings/Exit | | | | | | | |
|causes | | | | | | | |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
|Clean exit | | X | X | | | | |
|code or | | | | | | | |
|signal | | | | | | | |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
|Unclean exit | | X | | X | | | |
|code | | | | | | | |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
|Unclean | | X | | X | X | X | |
|signal | | | | | | | |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
|Timeout | | X | | X | X | | |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
|Watchdog | | X | | X | X | | X |
+--------------+----+--------+------------+------------+-------------+----------+-------------+
As exceptions to the setting above the service will not be
restarted if the exit code or signal is specified in
RestartPreventExitStatus= (see below). 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 Proper handling of
SIGINT/SIGQUIT -- How to be a proper program[3].
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.
StartLimitInterval=, StartLimitBurst=
Configure service start rate limiting. By default, services which
are started more than 5 times within 10 seconds are not permitted
to start any more times until the 10 second interval ends. With
these two options, this rate limiting may be modified. Use
StartLimitInterval= to configure the checking interval (defaults to
DefaultStartLimitInterval= in manager configuration file, set to 0
to disable any kind of rate limiting). Use StartLimitBurst= to
configure how many starts per interval are allowed (defaults to
DefaultStartLimitBurst= in manager configuration file). These
configuration options are particularly useful in conjunction with
Restart=; however, they apply to all kinds of starts (including
manual), not just those triggered by the Restart= logic. Note that
units which are configured for Restart= and which reach the start
limit are not attempted to be restarted anymore; however, they may
still be restarted manually at a later point, from which point on,
the restart logic is again activated. Note that systemctl
reset-failed will cause the restart rate counter for a service to
be flushed, which is useful if the administrator wants to manually
start a service and the start limit interferes with that.
StartLimitAction=
Configure the action to take if the rate limit configured with
StartLimitInterval= and StartLimitBurst= is hit. Takes one of none,
reboot, reboot-force, reboot-immediate, poweroff, poweroff-force or
poweroff-immediate. If none is set, hitting the rate limit will
trigger no action besides that the start will not be permitted.
reboot causes a reboot following the normal shutdown procedure
(i.e. equivalent to systemctl reboot). reboot-force causes a
forced reboot which will terminate all processes forcibly but
should cause no dirty file systems on reboot (i.e. equivalent to
systemctl reboot -f) and reboot-immediate causes immediate
execution of the reboot(2) system call, which might result in data
loss. Similar, poweroff, poweroff-force, poweroff-immediate have
the effect of powering down the system with similar semantics.
Defaults to none.
FailureAction=
Configure the action to take when the service enters a failed
state. Takes the same values as StartLimitAction= and executes the
same actions. Defaults to none.
RebootArgument=
Configure the optional argument for the reboot(2) system call if
StartLimitAction= or FailureAction= is a reboot action. This works
just like the optional argument to systemctl reboot command.
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.
Check systemd.exec(5) and systemd.kill(5) for more settings.
COMMAND LINES
This section describes command line parsing and variable and specifier
substitions 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, see table below. 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 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 and 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
+--------+-------------------------+
|Literal | Actual value |
+--------+-------------------------+
|"\a" | bell |
+--------+-------------------------+
|"\b" | backspace |
+--------+-------------------------+
|"\f" | form feed |
+--------+-------------------------+
|"\n" | newline |
+--------+-------------------------+
|"\r" | carriage return |
+--------+-------------------------+
|"\t" | tab |
+--------+-------------------------+
|"\v" | vertical tab |
+--------+-------------------------+
|"\\" | backslash |
+--------+-------------------------+
|"\"" | double quotation mark |
+--------+-------------------------+
|"\'" | single quotation mark |
+--------+-------------------------+
|"\s" | space |
+--------+-------------------------+
|"\xxx" | character number xx in |
| | hexadecimal encoding |
+--------+-------------------------+
|"\nnn" | character number nnn in |
| | octal encoding |
+--------+-------------------------+
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 clenaup 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 a 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, don't 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 hasn't 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
http://www.freedesktop.org/wiki/Software/systemd/Incompatibilities
2. kdbus
https://code.google.com/p/d-bus/
3. Proper handling of SIGINT/SIGQUIT -- How to be a proper program
http://www.cons.org/cracauer/sigint.html
systemd 219 SYSTEMD.SERVICE(5)
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