systemd.unit - Unit configuration
service.service, socket.socket, device.device, mount.mount, automount.automount, swap.swap, target.target, path.path, timer.timer, slice.slice, scope.scope
System Unit Search Path
/etc/systemd/system.control/* /run/systemd/system.control/* /run/systemd/transient/* /run/systemd/generator.early/* /etc/systemd/system/* /etc/systemd/system.attached/* /run/systemd/system/* /run/systemd/system.attached/* /run/systemd/generator/* ... /usr/lib/systemd/system/* /run/systemd/generator.late/*
User Unit Search Path
~/.config/systemd/user.control/* $XDG_RUNTIME_DIR/systemd/user.control/* $XDG_RUNTIME_DIR/systemd/transient/* $XDG_RUNTIME_DIR/systemd/generator.early/* ~/.config/systemd/user/* $XDG_CONFIG_DIRS/systemd/user/* /etc/systemd/user/* $XDG_RUNTIME_DIR/systemd/user/* /run/systemd/user/* $XDG_RUNTIME_DIR/systemd/generator/* $XDG_DATA_HOME/systemd/user/* $XDG_DATA_DIRS/systemd/user/* ... /usr/lib/systemd/user/* $XDG_RUNTIME_DIR/systemd/generator.late/*
A unit file is a plain text ini-style file that encodes information about a service, a socket, a device, a mount point, an automount point, a swap file or partition, a start-up target, a watched file system path, a timer controlled and supervised by systemd(1), a resource management slice or a group of externally created processes. See systemd.syntax(7) for a general description of the syntax.
This man page lists the common configuration options of all the unit types. These options need to be configured in the [Unit] or [Install] sections of the unit files.
In addition to the generic [Unit] and [Install] sections described here, each unit may have a type-specific section, e.g. [Service] for a service unit. See the respective man pages for more information: systemd.service(5), systemd.socket(5), systemd.device(5), systemd.mount(5), systemd.automount(5), systemd.swap(5), systemd.target(5), systemd.path(5), systemd.timer(5), systemd.slice(5), systemd.scope(5).
Unit files are loaded from a set of paths determined during compilation, described in the next section.
Valid unit names consist of a "unit name prefix", and a suffix specifying the unit type which begins with a dot. The "unit name prefix" must consist of one or more valid characters (ASCII letters, digits, ":", "-", "_", ".", and "\"). The total length of the unit name including the suffix must not exceed 255 characters. The unit type suffix must be one of ".service", ".socket", ".device", ".mount", ".automount", ".swap", ".target", ".path", ".timer", ".slice", or ".scope".
Unit names can be parameterized by a single argument called the "instance name". The unit is then constructed based on a "template file" which serves as the definition of multiple services or other units. A template unit must have a single "@" at the end of the unit name prefix (right before the type suffix). The name of the full unit is formed by inserting the instance name between "@" and the unit type suffix. In the unit file itself, the instance parameter may be referred to using "%i" and other specifiers, see below.
Unit files may contain additional options on top of those listed here. If systemd encounters an unknown option, it will write a warning log message but continue loading the unit. If an option or section name is prefixed with X-, it is ignored completely by systemd. Options within an ignored section do not need the prefix. Applications may use this to include additional information in the unit files. To access those options, applications need to parse the unit files on their own.
Units can be aliased (have an alternative name), by creating a symlink from the new name to the existing name in one of the unit search paths. For example, systemd-networkd.service has the alias dbus-org.freedesktop.network1.service, created during installation as a symlink, so when systemd is asked through D-Bus to load dbus-org.freedesktop.network1.service, it'll load systemd-networkd.service. As another example, default.target — the default system target started at boot — is commonly aliased to either multi-user.target or graphical.target to select what is started by default. Alias names may be used in commands like disable, start, stop, status, and similar, and in all unit dependency directives, including Wants=, Requires=, Before=, After=. Aliases cannot be used with the preset command.
Aliases obey the following restrictions: a unit of a certain type (".service", ".socket", ...) can only be aliased by a name with the same type suffix. A plain unit (not a template or an instance), may only be aliased by a plain name. A template instance may only be aliased by another template instance, and the instance part must be identical. A template may be aliased by another template (in which case the alias applies to all instances of the template). As a special case, a template instance (e.g. "firstname.lastname@example.org") may be a symlink to different template (e.g. "email@example.com"). In that case, just this specific instance is aliased, while other instances of the template (e.g. "firstname.lastname@example.org", "email@example.com") are not aliased. Those rules preserve the requirement that the instance (if any) is always uniquely defined for a given unit and all its aliases. The target of alias symlink must point to a valid unit file location, i.e. the symlink target name must match the symlink source name as described, and the destination path must be in one of the unit search paths, see UNIT FILE LOAD PATH section below for more details. Note that the target file may not exist, i.e. the symlink may be dangling.
Unit files may specify aliases through the Alias= directive in the [Install] section. When the unit is enabled, symlinks will be created for those names, and removed when the unit is disabled. For example, reboot.target specifies Alias=ctrl-alt-del.target, so when enabled, the symlink /etc/systemd/system/ctrl-alt-del.service pointing to the reboot.target file will be created, and when Ctrl+Alt+Del is invoked, systemd will look for the ctrl-alt-del.service and execute reboot.service. systemd does not look at the [Install] section at all during normal operation, so any directives in that section only have an effect through the symlinks created during enablement.
Along with a unit file foo.service, the directory foo.service.wants/ may exist. All unit files symlinked from such a directory are implicitly added as dependencies of type Wants= to the unit. Similar functionality exists for Requires= type dependencies as well, the directory suffix is .requires/ in this case. This functionality is useful to hook units into the start-up of other units, without having to modify their unit files. For details about the semantics of Wants= and Requires=, see below. The preferred way to create symlinks in the .wants/ or .requires/ directories is by specifying the dependency in [Install] section of the target unit, and creating the symlink in the file system with the enable or preset commands of systemctl(1). The target can be a normal unit (either plain or a specific instance of a template unit). In case when the source unit is a template, the target can also be a template, in which case the instance will be "propagated" to the target unit to form a valid unit instance. The target of symlinks in .wants/ or .requires/ must thus point to a valid unit file location, i.e. the symlink target name must satisfy the described requirements, and the destination path must be in one of the unit search paths, see UNIT FILE LOAD PATH section below for more details. Note that the target file may not exist, i.e. the symlink may be dangling.
Along with a unit file foo.service, a "drop-in" directory foo.service.d/ may exist. All files with the suffix ".conf" from this directory will be merged in the alphanumeric order and parsed after the main unit file itself has been parsed. This is useful to alter or add configuration settings for a unit, without having to modify unit files. Each drop-in file must contain appropriate section headers. For instantiated units, this logic will first look for the instance ".d/" subdirectory (e.g. "firstname.lastname@example.org/") and read its ".conf" files, followed by the template ".d/" subdirectory (e.g. "foo@.service.d/") and the ".conf" files there. Moreover for unit names containing dashes ("-"), the set of directories generated by repeatedly truncating the unit name after all dashes is searched too. Specifically, for a unit name foo-bar-baz.service not only the regular drop-in directory foo-bar-baz.service.d/ is searched but also both foo-bar-.service.d/ and foo-.service.d/. This is useful for defining common drop-ins for a set of related units, whose names begin with a common prefix. This scheme is particularly useful for mount, automount and slice units, whose systematic naming structure is built around dashes as component separators. Note that equally named drop-in files further down the prefix hierarchy override those further up, i.e. foo-bar-.service.d/10-override.conf overrides foo-.service.d/10-override.conf.
In cases of unit aliases (described above), dropins for the aliased name and all aliases are loaded. In the example of default.target aliasing graphical.target, default.target.d/, default.target.wants/, default.target.requires/, graphical.target.d/, graphical.target.wants/, graphical.target.requires/ would all be read. For templates, dropins for the template, any template aliases, the template instance, and all alias instances are read. When just a specific template instance is aliased, then the dropins for the target template, the target template instance, and the alias template instance are read.
In addition to /etc/systemd/system, the drop-in ".d/" directories for system services can be placed in /usr/lib/systemd/system or /run/systemd/system directories. Drop-in files in /etc/ take precedence over those in /run/ which in turn take precedence over those in /usr/lib/. Drop-in files under any of these directories take precedence over unit files wherever located. Multiple drop-in files with different names are applied in lexicographic order, regardless of which of the directories they reside in.
Units also support a top-level drop-in with type.d/, where type may be e.g. "service" or "socket", that allows altering or adding to the settings of all corresponding unit files on the system. The formatting and precedence of applying drop-in configurations follow what is defined above. Files in type.d/ have lower precedence compared to files in name-specific override directories. The usual rules apply: multiple drop-in files with different names are applied in lexicographic order, regardless of which of the directories they reside in, so a file in type.d/ applies to a unit only if there are no drop-ins or masks with that name in directories with higher precedence. See Examples.
Note that while systemd offers a flexible dependency system between units it is recommended to use this functionality only sparingly and instead rely on techniques such as bus-based or socket-based activation which make dependencies implicit, resulting in a both simpler and more flexible system.
As mentioned above, a unit may be instantiated from a template file. This allows creation of multiple units from a single configuration file. If systemd looks for a unit configuration file, it will first search for the literal unit name in the file system. If that yields no success and the unit name contains an "@" character, systemd will look for a unit template that shares the same name but with the instance string (i.e. the part between the "@" character and the suffix) removed. Example: if a service email@example.com is requested and no file by that name is found, systemd will look for getty@.service and instantiate a service from that configuration file if it is found.
To refer to the instance string from within the configuration file you may use the special "%i" specifier in many of the configuration options. See below for details.
If a unit file is empty (i.e. has the file size 0) or is symlinked to /dev/null, its configuration will not be loaded and it appears with a load state of "masked", and cannot be activated. Use this as an effective way to fully disable a unit, making it impossible to start it even manually.
The unit file format is covered by the Interface Portability and Stability Promise.
STRING ESCAPING FOR INCLUSION IN UNIT NAMES
Sometimes it is useful to convert arbitrary strings into unit names. To facilitate this, a method of string escaping is used, in order to map strings containing arbitrary byte values (except NUL) into valid unit names and their restricted character set. A common special case are unit names that reflect paths to objects in the file system hierarchy. Example: a device unit dev-sda.device refers to a device with the device node /dev/sda in the file system.
The escaping algorithm operates as follows: given a string, any "/" character is replaced by "-", and all other characters which are not ASCII alphanumerics, ":", "_" or "." are replaced by C-style "\x2d" escapes. In addition, "." is replaced with such a C-style escape when it would appear as the first character in the escaped string.
When the input qualifies as absolute file system path, this algorithm is extended slightly: the path to the root directory "/" is encoded as single dash "-". In addition, any leading, trailing or duplicate "/" characters are removed from the string before transformation. Example: /foo//bar/baz/ becomes "foo-bar-baz".
This escaping is fully reversible, as long as it is known whether the escaped string was a path (the unescaping results are different for paths and non-path strings). The systemd-escape(1) command may be used to apply and reverse escaping on arbitrary strings. Use systemd-escape --path to escape path strings, and systemd-escape without --path otherwise.
A number of unit dependencies are implicitly established, depending on unit type and unit configuration. These implicit dependencies can make unit configuration file cleaner. For the implicit dependencies in each unit type, please refer to section "Implicit Dependencies" in respective man pages.
For example, service units with Type=dbus automatically acquire dependencies of type Requires= and After= on dbus.socket. See systemd.service(5) for details.
Default dependencies are similar to implicit dependencies, but can be turned on and off by setting DefaultDependencies= to yes (the default) and no, while implicit dependencies are always in effect. See section "Default Dependencies" in respective man pages for the effect of enabling DefaultDependencies= in each unit types.
For example, target units will complement all configured dependencies of type Wants= or Requires= with dependencies of type After=. See systemd.target(5) for details. Note that this behavior can be opted out by setting DefaultDependencies=no in the specified units, or it can be selectively overridden via an explicit Before= dependency.
UNIT FILE LOAD PATH
Unit files are loaded from a set of paths determined during compilation, described in the two tables below. Unit files found in directories listed earlier override files with the same name in directories lower in the list.
When the variable $SYSTEMD_UNIT_PATH is set, the contents of this variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.
Table 1. Load path when running in system mode (--system).
|/etc/systemd/system.control||Persistent and transient configuration created using the dbus API|
|/run/systemd/transient||Dynamic configuration for transient units|
|/run/systemd/generator.early||Generated units with high priority (see early-dir in systemd.generator(7))|
|/etc/systemd/system||System units created by the administrator|
|/run/systemd/generator||Generated units with medium priority (see normal-dir in systemd.generator(7))|
|/usr/local/lib/systemd/system||System units installed by the administrator|
|/usr/lib/systemd/system||System units installed by the distribution package manager|
|/run/systemd/generator.late||Generated units with low priority (see late-dir in systemd.generator(7))|
Table 2. Load path when running in user mode (--user).
|$XDG_CONFIG_HOME/systemd/user.control or ~/.config/systemd/user.control||Persistent and transient configuration created using the dbus API ($XDG_CONFIG_HOME is used if set, ~/.config otherwise)|
|$XDG_RUNTIME_DIR/systemd/transient||Dynamic configuration for transient units|
|$XDG_RUNTIME_DIR/systemd/generator.early||Generated units with high priority (see early-dir in systemd.generator(7))|
|$XDG_CONFIG_HOME/systemd/user or $HOME/.config/systemd/user||User configuration ($XDG_CONFIG_HOME is used if set, ~/.config otherwise)|
|$XDG_CONFIG_DIRS/systemd/user or /etc/xdg/systemd/user||Additional configuration directories as specified by the XDG base directory specification ($XDG_CONFIG_DIRS is used if set, /etc/xdg otherwise)|
|/etc/systemd/user||User units created by the administrator|
|$XDG_RUNTIME_DIR/systemd/user||Runtime units (only used when $XDG_RUNTIME_DIR is set)|
|$XDG_RUNTIME_DIR/systemd/generator||Generated units with medium priority (see normal-dir in systemd.generator(7))|
|$XDG_DATA_HOME/systemd/user or $HOME/.local/share/systemd/user||Units of packages that have been installed in the home directory ($XDG_DATA_HOME is used if set, ~/.local/share otherwise)|
|$XDG_DATA_DIRS/systemd/user or /usr/local/share/systemd/user and /usr/share/systemd/user||Additional data directories as specified by the XDG base directory specification ($XDG_DATA_DIRS is used if set, /usr/local/share and /usr/share otherwise)|
|$dir/systemd/user for each $dir in $XDG_DATA_DIRS||Additional locations for installed user units, one for each entry in $XDG_DATA_DIRS|
|/usr/local/lib/systemd/user||User units installed by the administrator|
|/usr/lib/systemd/user||User units installed by the distribution package manager|
|$XDG_RUNTIME_DIR/systemd/generator.late||Generated units with low priority (see late-dir in systemd.generator(7))|
The set of load paths for the user manager instance may be
augmented or changed using various environment variables. And environment
variables may in turn be set using environment generators, see
systemd.environment-generator(7). In particular, $XDG_DATA_HOME and $XDG_DATA_DIRS may be easily set using systemd-environment-d-generator(8). Thus, directories listed here are just the defaults. To see the actual list that would be used based on compilation options and current environment use
systemd-analyze --user unit-paths
Moreover, additional units might be loaded into systemd from directories not on the unit load path by creating a symlink pointing to a unit file in the directories. You can use systemctl link for this; see systemctl(1). The file system where the linked unit files are located must be accessible when systemd is started (e.g. anything underneath /home/ or /var/ is not allowed, unless those directories are located on the root file system).
It is important to distinguish "linked unit files" from "unit file aliases": any symlink where the symlink target is within the unit load path becomes an alias: the source name and the target file name must satisfy specific constraints listed above in the discussion of aliases, but the symlink target doesn't have to exist, and in fact the symlink target path is not used, except to check whether the target is within the unit load path. In contrast, a symlink which goes outside of the unit load path signifies a linked unit file. The symlink is followed when loading the file, but the destination name is otherwise unused (and may even not be a valid unit file name). For example, symlinks /etc/systemd/system/alias1.service → service1.service, /etc/systemd/system/alias2.service → /usr/lib/systemd/service1.service, /etc/systemd/system/alias3.service → /etc/systemd/system/service1.service are all valid aliases and service1.service will have four names, even if the unit file is located at /run/systemd/system/service1.service. In contrast, a symlink /etc/systemd/system/link1.service → ../link1_service_file means that link1.service is a "linked unit" and the contents of /etc/systemd/link1_service_file provide its configuration.
UNIT GARBAGE COLLECTION
The system and service manager loads a unit's configuration automatically when a unit is referenced for the first time. It will automatically unload the unit configuration and state again when the unit is not needed anymore ("garbage collection"). A unit may be referenced through a number of different mechanisms:
The garbage collection logic may be altered with the CollectMode= option, which allows configuration whether automatic unloading of units that are in failed state is permissible, see below.
Note that when a unit's configuration and state is unloaded, all execution results, such as exit codes, exit signals, resource consumption and other statistics are lost, except for what is stored in the log subsystem.
Use systemctl daemon-reload or an equivalent command to reload unit configuration while the unit is already loaded. In this case all configuration settings are flushed out and replaced with the new configuration (which however might not be in effect immediately), however all runtime state is saved/restored.
[UNIT] SECTION OPTIONS
The unit file may include a [Unit] section, which carries generic information about the unit that is not dependent on the type of unit:
Units listed in this option will be started if the configuring unit is. However, if the listed units fail to start or cannot be added to the transaction, this has no impact on the validity of the transaction as a whole, and this unit will still be started. This is the recommended way to hook the start-up of one unit to the start-up of another unit.
Note that requirement dependencies do not influence the order in which services are started or stopped. This has to be configured independently with the After= or Before= options. If unit foo.service pulls in unit bar.service as configured with Wants= and no ordering is configured with After= or Before=, then both units will be started simultaneously and without any delay between them if foo.service is activated.
If this unit gets activated, the units listed will be activated as well. If one of the other units fails to activate, and an ordering dependency After= on the failing unit is set, this unit will not be started. Besides, with or without specifying After=, this unit will be stopped (or restarted) if one of the other units is explicitly stopped (or restarted).
Often, it is a better choice to use Wants= instead of Requires= in order to achieve a system that is more robust when dealing with failing services.
Note that this dependency type does not imply that the other unit always has to be in active state when this unit is running. Specifically: failing condition checks (such as ConditionPathExists=, ConditionPathIsSymbolicLink=, ... — see below) do not cause the start job of a unit with a Requires= dependency on it to fail. Also, some unit types may deactivate on their own (for example, a service process may decide to exit cleanly, or a device may be unplugged by the user), which is not propagated to units having a Requires= dependency. Use the BindsTo= dependency type together with After= to ensure that a unit may never be in active state without a specific other unit also in active state (see below).
When Requisite=b.service is used on a.service, this dependency will show as RequisiteOf=a.service in property listing of b.service. RequisiteOf= dependency cannot be specified directly.
When used in conjunction with After= on the same unit the behaviour of BindsTo= is even stronger. In this case, the unit bound to strictly has to be in active state for this unit to also be in active state. This not only means a unit bound to another unit that suddenly enters inactive state, but also one that is bound to another unit that gets skipped due to an unmet condition check (such as ConditionPathExists=, ConditionPathIsSymbolicLink=, ... — see below) will be stopped, should it be running. Hence, in many cases it is best to combine BindsTo= with After=.
When BindsTo=b.service is used on a.service, this dependency will show as BoundBy=a.service in property listing of b.service. BoundBy= dependency cannot be specified directly.
When PartOf=b.service is used on a.service, this dependency will show as ConsistsOf=a.service in property listing of b.service. ConsistsOf= dependency cannot be specified directly.
When Upholds=b.service is used on a.service, this dependency will show as UpheldBy=a.service in the property listing of b.service. The UpheldBy= dependency cannot be specified directly.
Note that this setting does not imply an ordering dependency, similarly to the Wants= and Requires= dependencies described above. This means that to ensure that the conflicting unit is stopped before the other unit is started, an After= or Before= dependency must be declared. It doesn't matter which of the two ordering dependencies is used, because stop jobs are always ordered before start jobs, see the discussion in Before=/After= below.
If unit A that conflicts with unit B is scheduled to be started at the same time as B, the transaction will either fail (in case both are required parts of the transaction) or be modified to be fixed (in case one or both jobs are not a required part of the transaction). In the latter case, the job that is not required will be removed, or in case both are not required, the unit that conflicts will be started and the unit that is conflicted is stopped.
Those two settings configure ordering dependencies between units. If unit foo.service contains the setting Before=bar.service and both units are being started, bar.service's start-up is delayed until foo.service has finished starting up. After= is the inverse of Before=, i.e. while Before= ensures that the configured unit is started before the listed unit begins starting up, After= ensures the opposite, that the listed unit is fully started up before the configured unit is started.
When two units with an ordering dependency between them are shut down, the inverse of the start-up order is applied. I.e. if a unit is configured with After= on another unit, the former is stopped before the latter if both are shut down. Given two units with any ordering dependency between them, if one unit is shut down and the other is started up, the shutdown is ordered before the start-up. It doesn't matter if the ordering dependency is After= or Before=, in this case. It also doesn't matter which of the two is shut down, as long as one is shut down and the other is started up; the shutdown is ordered before the start-up in all cases. If two units have no ordering dependencies between them, they are shut down or started up simultaneously, and no ordering takes place. It depends on the unit type when precisely a unit has finished starting up. Most importantly, for service units start-up is considered completed for the purpose of Before=/After= when all its configured start-up commands have been invoked and they either failed or reported start-up success. Note that this does includes ExecStartPost= (or ExecStopPost= for the shutdown case).
Note that those settings are independent of and orthogonal to the requirement dependencies as configured by Requires=, Wants=, Requisite=, or BindsTo=. It is a common pattern to include a unit name in both the After= and Wants= options, in which case the unit listed will be started before the unit that is configured with these options.
Note that Before= dependencies on device units have no effect and are not supported. Devices generally become available as a result of an external hotplug event, and systemd creates the corresponding device unit without delay.
Mount points marked with noauto are not mounted automatically through local-fs.target, but are still honored for the purposes of this option, i.e. they will be pulled in by this unit.
If none is set, no action will be triggered. 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 (i.e. equivalent to systemctl reboot -ff). Similarly, poweroff, poweroff-force, poweroff-immediate have the effect of powering down the system with similar semantics. exit causes the manager to exit following the normal shutdown procedure, and exit-force causes it terminate without shutting down services. When exit or exit-force is used by default the exit status of the main process of the unit (if this applies) is returned from the service manager. However, this may be overridden with FailureActionExitStatus=/SuccessActionExitStatus=, see below.
Both settings take a time span with the default unit of seconds, but other units may be specified, see systemd.time(5). The default is "infinity" (job timeouts disabled), except for device units where JobRunningTimeoutSec= defaults to DefaultTimeoutStartSec=.
Note: these timeouts are independent from any unit-specific timeouts (for example, the timeout set with TimeoutStartSec= in service units). The job timeout has no effect on the unit itself. Or in other words: unit-specific timeouts are useful to abort unit state changes, and revert them. The job timeout set with this option however is useful to abort only the job waiting for the unit state to change.
JobTimeoutRebootArgument= configures an optional reboot string to pass to the reboot(2) system call.
interval is a time span with the default unit of seconds, but other units may be specified, see systemd.time(5). Defaults to DefaultStartLimitIntervalSec= in manager configuration file, and may be set to 0 to disable any kind of rate limiting. burst is a number and defaults to DefaultStartLimitBurst= in manager configuration file.
These configuration options are particularly useful in conjunction with the service setting Restart= (see systemd.service(5)); 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 or from a timer or socket at a later point, after the interval has passed. From that point on, the restart logic is activated again. 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 unit and the start limit interferes with that. Rate-limiting is enforced after any unit condition checks are executed, and hence unit activations with failing conditions do not count towards the rate limit.
When a unit is unloaded due to the garbage collection logic (see above) its rate limit counters are flushed out too. This means that configuring start rate limiting for a unit that is not referenced continuously has no effect.
This setting does not apply to slice, target, device, and scope units, since they are unit types whose activation may either never fail, or may succeed only a single time.
Conditions and Asserts
Unit files may also include a number of Condition...= and Assert...= settings. Before the unit is started, systemd will verify that the specified conditions and asserts are true. If not, the starting of the unit will be (mostly silently) skipped (in case of conditions), or aborted with an error message (in case of asserts). Failing conditions or asserts will not result in the unit being moved into the "failed" state. The conditions and asserts are checked at the time the queued start job is to be executed. The ordering dependencies are still respected, so other units are still pulled in and ordered as if this unit was successfully activated, and the conditions and asserts are executed the precise moment the unit would normally start and thus can validate system state after the units ordered before completed initialization. Use condition expressions for skipping units that do not apply to the local system, for example because the kernel or runtime environment doesn't require their functionality.
If multiple conditions are specified, the unit will be executed if all of them apply (i.e. a logical AND is applied). Condition checks can use a pipe symbol ("|") after the equals sign ("Condition...=|..."), which causes the condition to become a triggering condition. If at least one triggering condition is defined for a unit, then the unit will be started if at least one of the triggering conditions of the unit applies and all of the regular (i.e. non-triggering) conditions apply. If you prefix an argument with the pipe symbol and an exclamation mark, the pipe symbol must be passed first, the exclamation second. If any of these options is assigned the empty string, the list of conditions is reset completely, all previous condition settings (of any kind) will have no effect.
The AssertArchitecture=, AssertVirtualization=, ... options are similar to conditions but cause the start job to fail (instead of being skipped). The failed check is logged. Units with unmet conditions are considered to be in a clean state and will be garbage collected if they are not referenced. This means that when queried, the condition failure may or may not show up in the state of the unit.
Note that neither assertion nor condition expressions result in unit state changes. Also note that both are checked at the time the job is to be executed, i.e. long after depending jobs and it itself were queued. Thus, neither condition nor assertion expressions are suitable for conditionalizing unit dependencies.
The condition verb of systemd-analyze(1) can be used to test condition and assert expressions.
Except for ConditionPathIsSymbolicLink=, all path checks follow symlinks.
The architecture is determined from the information returned by uname(2) and is thus subject to personality(2). Note that a Personality= setting in the same unit file has no effect on this condition. A special architecture name "native" is mapped to the architecture the system manager itself is compiled for. The test may be negated by prepending an exclamation mark.
Note that using the kernel version string is an unreliable way to determine which features are supported by a kernel, because of the widespread practice of backporting drivers, features, and fixes from newer upstream kernels into older versions provided by distributions. Hence, this check is inherently unportable and should not be used for units which may be used on different distributions.
If the systemd.condition-needs-update= option is specified on the kernel command line (taking a boolean), it will override the result of this condition check, taking precedence over any file modification time checks. If the kernel command line option is used, systemd-update-done.service will not have immediate effect on any following ConditionNeedsUpdate= checks, until the system is rebooted where the kernel command line option is not specified anymore.
Note that to make this scheme effective, the timestamp of /usr/ should be explicitly updated after its contents are modified. The kernel will automatically update modification timestamp on a directory only when immediate children of a directory are modified; an modification of nested files will not automatically result in mtime of /usr/ being updated.
Also note that if the update method includes a call to execute appropriate post-update steps itself, it should not touch the timestamp of /usr/. In a typical distribution packaging scheme, packages will do any required update steps as part of the installation or upgrade, to make package contents immediately usable. ConditionNeedsUpdate= should be used with other update mechanisms where such an immediate update does not happen.
This condition may be used to populate /etc/ on the first boot after factory reset, or when a new system instance boots up for the first time.
For robustness, units with ConditionFirstBoot=yes should order themselves before first-boot-complete.target and pull in this passive target with Wants=. This ensures that in a case of an aborted first boot, these units will be re-run during the next system startup.
If the systemd.condition-first-boot= option is specified on the kernel command line (taking a boolean), it will override the result of this condition check, taking precedence over /etc/machine-id existence checks.
Multiple controllers may be passed with a space separating them; in this case the condition will only pass if all listed controllers are available for use. Controllers unknown to systemd are ignored. Valid controllers are "cpu", "io", "memory", and "pids". Even if available in the kernel, a particular controller may not be available if it was disabled on the kernel command line with cgroup_disable=controller.
Alternatively, two special strings "v1" and "v2" may be specified (without any controller names). "v2" will pass if the unified v2 cgroup hierarchy is used, and "v1" will pass if the legacy v1 hierarchy or the hybrid hierarchy are used. Note that legacy or hybrid hierarchies have been deprecated. See systemd(1) for more information.
Other than exact string matching (with "=" and "!="), relative comparisons are supported for versioned parameters (e.g. "VERSION_ID"; with "<", "<=", "==", "<>", ">=", ">"), and shell-style wildcard comparisons ("*", "?", "") are supported with the "$=" (match) and "!$=" (non-match).
ConditionMemoryPressure=, ConditionCPUPressure=, ConditionIOPressure=
Optionally, the threshold value can be prefixed with the slice unit under which the pressure will be checked, followed by a ":". If the slice unit is not specified, the overall system pressure will be measured, instead of a particular cgroup's.
AssertArchitecture=, AssertVirtualization=, AssertHost=, AssertKernelCommandLine=, AssertKernelVersion=, AssertCredential=, AssertEnvironment=, AssertSecurity=, AssertCapability=, AssertACPower=, AssertNeedsUpdate=, AssertFirstBoot=, AssertPathExists=, AssertPathExistsGlob=, AssertPathIsDirectory=, AssertPathIsSymbolicLink=, AssertPathIsMountPoint=, AssertPathIsReadWrite=, AssertPathIsEncrypted=, AssertDirectoryNotEmpty=, AssertFileNotEmpty=, AssertFileIsExecutable=, AssertUser=, AssertGroup=, AssertControlGroupController=, AssertMemory=, AssertCPUs=, AssertCPUFeature=, AssertOSRelease=, AssertMemoryPressure=, AssertCPUPressure=, AssertIOPressure=
MAPPING OF UNIT PROPERTIES TO THEIR INVERSES
Unit settings that create a relationship with a second unit usually show up in properties of both units, for example in systemctl show output. In some cases the name of the property is the same as the name of the configuration setting, but not always. This table lists the properties that are shown on two units which are connected through some dependency, and shows which property on "source" unit corresponds to which property on the "target" unit.
Table 3. Forward and reverse unit properties
|"Forward" property||"Reverse" property||Where used|
|Requires=||RequiredBy=||[Unit] section||[Install] section|
|Wants=||WantedBy=||[Unit] section||[Install] section|
|PartOf=||ConsistsOf=||[Unit] section||an automatic property|
|BindsTo=||BoundBy=||[Unit] section||an automatic property|
|Requisite=||RequisiteOf=||[Unit] section||an automatic property|
|Triggers=||TriggeredBy=||Automatic properties, see notes below|
|Conflicts=||ConflictedBy=||[Unit] section||an automatic property|
|Following=||n/a||An automatic property|
WantedBy= and RequiredBy= are used in the [Install] section to create symlinks in .wants/ and .requires/ directories. They cannot be used directly as a unit configuration setting.
Note: ConsistsOf=, BoundBy=, RequisiteOf=, ConflictedBy= are created implicitly along with their reverses and cannot be specified directly.
Note: Triggers= is created implicitly between a socket, path unit, or an automount unit, and the unit they activate. By default a unit with the same name is triggered, but this can be overridden using Sockets=, Service=, and Unit= settings. See systemd.service(5), systemd.socket(5), systemd.path(5), and systemd.automount(5) for details. TriggeredBy= is created implicitly on the triggered unit.
Note: Following= is used to group device aliases and points to the "primary" device unit that systemd is using to track device state, usually corresponding to a sysfs path. It does not show up in the "target" unit.
[INSTALL] SECTION OPTIONS
Unit files may include an [Install] section, which carries installation information for the unit. This section is not interpreted by systemd(1) during runtime; it is used by the enable and disable commands of the systemctl(1) tool during installation of a unit.
In case of template units listing non template units, the listing unit must have DefaultInstance= set, or systemctl enable must be called with an instance name. The instance (default or specified) will be added to the .wants/ or .requires/ list of the listed unit. For example, WantedBy=getty.target in a service getty@.service will result in systemctl enable firstname.lastname@example.org creating a email@example.com link to getty@.service. This also applies to listing specific instances of templated units: this specific instance will gain the dependency. A template unit may also list a template unit, in which case a generic dependency will be added where each instance of the listing unit will have a dependency on an instance of the listed template with the same instance value. For example, WantedBy=container@.target in a service monitor@.service will result in systemctl enable monitor@.service creating a container@.target.wants/monitor@.service link to monitor@.service, which applies to all instances of container@.target.
This option may be used more than once, or a space-separated list of unit names may be given.
The following specifiers are interpreted in the Install section: %a, %b, %B, %g, %G, %H, %i, %j, %l, %m, %n, %N, %o, %p, %u, %U, %v, %w, %W, %%. For their meaning see the next section.
Many settings resolve specifiers which may be used to write generic unit files referring to runtime or unit parameters that are replaced when the unit files are loaded. Specifiers must be known and resolvable for the setting to be valid. The following specifiers are understood:
Table 4. Specifiers available in unit files
|"%a"||Architecture||A short string identifying the architecture of the local system. A string such as x86, x86-64 or arm64. See the architectures defined for ConditionArchitecture= above for a full list.|
|"%A"||Operating system image version||The operating system image version identifier of the running system, as read from the IMAGE_VERSION= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.|
|"%b"||Boot ID||The boot ID of the running system, formatted as string. See random(4) for more information.|
|"%B"||Operating system build ID||The operating system build identifier of the running system, as read from the BUILD_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.|
|"%C"||Cache directory root||This is either /var/cache (for the system manager) or the path "$XDG_CACHE_HOME" resolves to (for user managers).|
|"%d"||Credentials directory||This is the value of the "$CREDENTIALS_DIRECTORY" environment variable if available. See section "Credentials" in systemd.exec(5) for more information.|
|"%E"||Configuration directory root||This is either /etc/ (for the system manager) or the path "$XDG_CONFIG_HOME" resolves to (for user managers).|
|"%f"||Unescaped filename||This is either the unescaped instance name (if applicable) with / prepended (if applicable), or the unescaped prefix name prepended with /. This implements unescaping according to the rules for escaping absolute file system paths discussed above.|
|"%g"||User group||This is the name of the group running the service manager instance. In case of the system manager this resolves to "root".|
|"%G"||User GID||This is the numeric GID of the user running the service manager instance. In case of the system manager this resolves to "0".|
|"%h"||User home directory||This is the home directory of the user running the service manager instance. In case of the system manager this resolves to "/root". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%H"||Host name||The hostname of the running system at the point in time the unit configuration is loaded.|
|"%i"||Instance name||For instantiated units this is the string between the first "@" character and the type suffix. Empty for non-instantiated units.|
|"%I"||Unescaped instance name||Same as "%i", but with escaping undone.|
|"%j"||Final component of the prefix||This is the string between the last "-" and the end of the prefix name. If there is no "-", this is the same as "%p".|
|"%J"||Unescaped final component of the prefix||Same as "%j", but with escaping undone.|
|"%l"||Short host name||The hostname of the running system at the point in time the unit configuration is loaded, truncated at the first dot to remove any domain component.|
|"%L"||Log directory root||This is either /var/log (for the system manager) or the path "$XDG_CONFIG_HOME" resolves to with /log appended (for user managers).|
|"%m"||Machine ID||The machine ID of the running system, formatted as string. See machine-id(5) for more information.|
|"%M"||Operating system image identifier||The operating system image identifier of the running system, as read from the IMAGE_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.|
|"%n"||Full unit name|
|"%N"||Full unit name||Same as "%n", but with the type suffix removed.|
|"%o"||Operating system ID||The operating system identifier of the running system, as read from the ID= field of /etc/os-release. See os-release(5) for more information.|
|"%p"||Prefix name||For instantiated units, this refers to the string before the first "@" character of the unit name. For non-instantiated units, same as "%N".|
|"%P"||Unescaped prefix name||Same as "%p", but with escaping undone.|
|"%q"||Pretty host name||The pretty hostname of the running system at the point in time the unit configuration is loaded, as read from the PRETTY_HOSTNAME= field of /etc/machine-info. If not set, resolves to the short hostname. See machine-info(5) for more information.|
|"%s"||User shell||This is the shell of the user running the service manager instance.|
|"%S"||State directory root||This is either /var/lib (for the system manager) or the path "$XDG_CONFIG_HOME" resolves to (for user managers).|
|"%t"||Runtime directory root||This is either /run/ (for the system manager) or the path "$XDG_RUNTIME_DIR" resolves to (for user managers).|
|"%T"||Directory for temporary files||This is either /tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to. (Note that the directory may be specified without a trailing slash.)|
|"%u"||User name||This is the name of the user running the service manager instance. In case of the system manager this resolves to "root". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%U"||User UID||This is the numeric UID of the user running the service manager instance. In case of the system manager this resolves to "0". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%v"||Kernel release||Identical to uname -r output.|
|"%V"||Directory for larger and persistent temporary files||This is either /var/tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to. (Note that the directory may be specified without a trailing slash.)|
|"%w"||Operating system version ID||The operating system version identifier of the running system, as read from the VERSION_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.|
|"%W"||Operating system variant ID||The operating system variant identifier of the running system, as read from the VARIANT_ID= field of /etc/os-release. If not set, resolves to an empty string. See os-release(5) for more information.|
|"%y"||The path to the fragment||This is the path where the main part of the unit file is located. For linked unit files, the real path outside of the unit search directories is used. For units that don't have a fragment file, this specifier will raise an error.|
|"%Y"||The directory of the fragment||This is the directory part of "%y".|
|"%%"||Single percent sign||Use "%%" in place of "%" to specify a single percent sign.|
Example 1. Allowing units to be enabled
The following snippet (highlighted) allows a unit (e.g. foo.service) to be enabled via systemctl enable:
[Unit] Description=Foo [Service] ExecStart=/usr/sbin/foo-daemon [Install] WantedBy=multi-user.target
After running systemctl enable, a symlink /etc/systemd/system/multi-user.target.wants/foo.service linking to the actual unit will be created. It tells systemd to pull in the unit when starting multi-user.target. The inverse systemctl disable will remove that symlink again.
Example 2. Overriding vendor settings
There are two methods of overriding vendor settings in unit files: copying the unit file from /usr/lib/systemd/system to /etc/systemd/system and modifying the chosen settings. Alternatively, one can create a directory named unit.d/ within /etc/systemd/system and place a drop-in file name.conf there that only changes the specific settings one is interested in. Note that multiple such drop-in files are read if present, processed in lexicographic order of their filename.
The advantage of the first method is that one easily overrides the complete unit, the vendor unit is not parsed at all anymore. It has the disadvantage that improvements to the unit file by the vendor are not automatically incorporated on updates.
The advantage of the second method is that one only overrides the settings one specifically wants, where updates to the unit by the vendor automatically apply. This has the disadvantage that some future updates by the vendor might be incompatible with the local changes.
This also applies for user instances of systemd, but with different locations for the unit files. See the section on unit load paths for further details.
Suppose there is a vendor-supplied unit /usr/lib/systemd/system/httpd.service with the following contents:
[Unit] Description=Some HTTP server After=remote-fs.target sqldb.service Requires=sqldb.service AssertPathExists=/srv/webserver [Service] Type=notify ExecStart=/usr/sbin/some-fancy-httpd-server Nice=5 [Install] WantedBy=multi-user.target
Now one wants to change some settings as an administrator: firstly, in the local setup, /srv/webserver might not exist, because the HTTP server is configured to use /srv/www instead. Secondly, the local configuration makes the HTTP server also depend on a memory cache service, memcached.service, that should be pulled in (Requires=) and also be ordered appropriately (After=). Thirdly, in order to harden the service a bit more, the administrator would like to set the PrivateTmp= setting (see systemd.exec(5) for details). And lastly, the administrator would like to reset the niceness of the service to its default value of 0.
The first possibility is to copy the unit file to /etc/systemd/system/httpd.service and change the chosen settings:
[Unit] Description=Some HTTP server After=remote-fs.target sqldb.service memcached.service Requires=sqldb.service memcached.service AssertPathExists=/srv/www [Service] Type=notify ExecStart=/usr/sbin/some-fancy-httpd-server Nice=0 PrivateTmp=yes [Install] WantedBy=multi-user.target
Alternatively, the administrator could create a drop-in file /etc/systemd/system/httpd.service.d/local.conf with the following contents:
[Unit] After=memcached.service Requires=memcached.service # Reset all assertions and then re-add the condition we want AssertPathExists= AssertPathExists=/srv/www [Service] Nice=0 PrivateTmp=yes
Note that for drop-in files, if one wants to remove entries from a setting that is parsed as a list (and is not a dependency), such as AssertPathExists= (or e.g. ExecStart= in service units), one needs to first clear the list before re-adding all entries except the one that is to be removed. Dependencies (After=, etc.) cannot be reset to an empty list, so dependencies can only be added in drop-ins. If you want to remove dependencies, you have to override the entire unit.
Example 3. Top level drop-ins with template units
Top level per-type drop-ins can be used to change some aspect of all units of a particular type. For example, by creating the /etc/systemd/system/service.d/ directory with a drop-in file, the contents of the drop-in file can be applied to all service units. We can take this further by having the top-level drop-in instantiate a secondary helper unit. Consider for example the following set of units and drop-in files where we install an OnFailure= dependency for all service units.
[Unit] Description=My failure handler for %i [Service] Type=oneshot # Perform some special action for when %i exits unexpectedly. ExecStart=/usr/sbin/myfailurehandler %i
We can then add an instance of failure-handler@.service as an OnFailure= dependency for all service units.
Now, after running systemctl daemon-reload all services will have acquired an OnFailure= dependency on failure-handler@%N.service. The template instance units will also have gained the dependency which results in the creation of a recursive dependency chain. systemd will try to detect these recursive dependency chains where a template unit directly and recursively depends on itself and will remove such dependencies automatically if it finds them. If systemd doesn't detect the recursive dependency chain, we can break the chain ourselves by disabling the drop-in for the template instance units via a symlink to /dev/null:
mkdir /etc/systemd/system/failure-handler@.service.d/ ln -s /dev/null /etc/systemd/system/failure-handler@.service.d/10-all.conf systemctl daemon-reload
This ensures that if a failure-handler@.service instance fails it will not trigger an instance named firstname.lastname@example.org.
systemd(1), systemctl(1), systemd-system.conf(5), systemd.special(7), systemd.service(5), systemd.socket(5), systemd.device(5), systemd.mount(5), systemd.automount(5), systemd.swap(5), systemd.target(5), systemd.path(5), systemd.timer(5), systemd.scope(5), systemd.slice(5), systemd.time(7), systemd-analyze(1), capabilities(7), systemd.directives(7), uname(1)
- Interface Portability and Stability Promise
- System and Service Credentials
- PSI (Pressure Stall Information)