NAME¶

systemd.unit - Unit configuration

SYNOPSIS¶

service.service, socket.socket, device.device, mount.mount, automount.automount, swap.swap, target.target, path.path, timer.timer, slice.slice, scope.scope

DESCRIPTION¶

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. "alias@inst.service") may be a symlink to different template (e.g. "template@inst.service"). In that case, just this specific instance is aliased, while other instances of the template (e.g. "alias@foo.service", "alias@bar.service") 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 might 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.target pointing to the reboot.target file will be created, and when Ctrl+Alt+Del is invoked, systemd will look for ctrl-alt-del.target, follow the symlink to reboot.target, and execute reboot.service as part of that target. 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 might 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. "foo@bar.service.d/") 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 getty@tty3.service 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[1].

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.

AUTOMATIC DEPENDENCIES¶

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 [2].

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).

Table 2.  Load path when running in user mode (--user).

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:

Description=

Documentation=

Wants=

Requires=

Requisite=

BindsTo=

PartOf=

Upholds=

Conflicts=

Before=, After=

OnFailure=

OnSuccess=

PropagatesReloadTo=, ReloadPropagatedFrom=

PropagatesStopTo=, StopPropagatedFrom=

JoinsNamespaceOf=

RequiresMountsFor=

WantsMountsFor=

OnSuccessJobMode=, OnFailureJobMode=

IgnoreOnIsolate=

StopWhenUnneeded=

RefuseManualStart=, RefuseManualStop=

AllowIsolate=

DefaultDependencies=

SurviveFinalKillSignal=

CollectMode=

FailureAction=, SuccessAction=

FailureActionExitStatus=, SuccessActionExitStatus=

JobTimeoutSec=, JobRunningTimeoutSec=

JobTimeoutAction=, JobTimeoutRebootArgument=

StartLimitIntervalSec=interval, StartLimitBurst=burst

StartLimitAction=

RebootArgument=

SourcePath=

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 4.  Forward and reverse unit properties

Note:
WantedBy=, RequiredBy=, and UpheldBy= are used in the [Install] section to create symlinks in .wants/, .requires/, and .upholds/ 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.

Alias=

WantedBy=, RequiredBy=, UpheldBy=

Also=

DefaultInstance=

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.

SPECIFIERS¶

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 5. Specifiers available in unit files

EXAMPLES¶

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.

/etc/systemd/system/failure-handler@.service:

[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.

/etc/systemd/system/service.d/10-all.conf:

[Unit]
OnFailure=failure-handler@%N.service


    

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 failure-handler@failure-handler.service.

SEE ALSO¶

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)

NOTES¶

1.

Interface Portability and Stability Promise

2.

💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If /usr/local/ is a separate partition, it may not be available during early boot, and must not be used for configuration.

3.

systemd and Storage Daemons for the Root File System

4.

System and Service Credentials

5.

PSI (Pressure Stall Information)