Provided by: systemd_253.5-1ubuntu6.1_amd64 
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
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/*
...
/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/*
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 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. "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 /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
/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
Implicit Dependencies
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
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).
┌──────────────────────────────┬─────────────────────────────────┐
│Path │ Description │
├──────────────────────────────┼─────────────────────────────────┤
│/etc/systemd/system.control │ Persistent and transient │
├──────────────────────────────┤ configuration created using the │
│/run/systemd/system.control │ 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/system │ Runtime units │
├──────────────────────────────┼─────────────────────────────────┤
│/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 │
├──────────────────────────────┼─────────────────────────────────┤
│/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).
┌─────────────────────────────────────────┬──────────────────────────────────┐
│Path │ Description │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_CONFIG_HOME/systemd/user.control │ Persistent and transient │
│or │ configuration created using the │
│~/.config/systemd/user.control │ dbus API ($XDG_CONFIG_HOME is │
├─────────────────────────────────────────┤ used if set, ~/.config │
│$XDG_RUNTIME_DIR/systemd/user.control │ 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)) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_CONFIG_HOME/systemd/user or │ User configuration │
│$HOME/.config/systemd/user │ ($XDG_CONFIG_HOME is used if │
│ │ set, ~/.config otherwise) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_CONFIG_DIRS/systemd/user or │ Additional configuration │
│/etc/xdg/systemd/user │ 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 │
│ │ $XDG_RUNTIME_DIR is set) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│/run/systemd/user │ Runtime units │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_RUNTIME_DIR/systemd/generator │ Generated units with medium │
│ │ priority (see normal-dir in │
│ │ systemd.generator(7)) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_DATA_HOME/systemd/user or │ Units of packages that have been │
│$HOME/.local/share/systemd/user │ installed in the home directory │
│ │ ($XDG_DATA_HOME is used if set, │
│ │ ~/.local/share otherwise) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$XDG_DATA_DIRS/systemd/user or │ Additional data directories as │
│/usr/local/share/systemd/user and │ specified by the XDG base │
│/usr/share/systemd/user │ directory specification │
│ │ ($XDG_DATA_DIRS is used if set, │
│ │ /usr/local/share and /usr/share │
│ │ otherwise) │
├─────────────────────────────────────────┼──────────────────────────────────┤
│$dir/systemd/user for each $dir in │ Additional locations for │
│$XDG_DATA_DIRS │ 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:
1. Another loaded unit references it with a dependency such as After=, Wants=, ...
2. The unit is currently starting, running, reloading or stopping.
3. The unit is currently in the failed state. (But see below.)
4. A job for the unit is pending.
5. The unit is pinned by an active IPC client program.
6. The unit is a special "perpetual" unit that is always active and loaded. Examples for
perpetual units are the root mount unit -.mount or the scope unit init.scope that the
service manager itself lives in.
7. The unit has running processes associated with it.
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=
A short human readable title of the unit. This may be used by systemd (and other UIs)
as a user-visible label for the unit, so this string should identify the unit rather
than describe it, despite the name. This string also shouldn't just repeat the unit
name. "Apache2 Web Server" is a good example. Bad examples are "high-performance
light-weight HTTP server" (too generic) or "Apache2" (meaningless for people who do
not know Apache, duplicates the unit name). systemd may use this string as a noun in
status messages ("Starting description...", "Started description.", "Reached target
description.", "Failed to start description."), so it should be capitalized, and
should not be a full sentence, or a phrase with a continuous verb. Bad examples
include "exiting the container" or "updating the database once per day.".
Documentation=
A space-separated list of URIs referencing documentation for this unit or its
configuration. Accepted are only URIs of the types "http://", "https://", "file:",
"info:", "man:". For more information about the syntax of these URIs, see uri(7). The
URIs should be listed in order of relevance, starting with the most relevant. It is a
good idea to first reference documentation that explains what the unit's purpose is,
followed by how it is configured, followed by any other related documentation. This
option may be specified more than once, in which case the specified list of URIs is
merged. If the empty string is assigned to this option, the list is reset and all
prior assignments will have no effect.
Wants=
Configures (weak) requirement dependencies on other units. This option may be
specified more than once or multiple space-separated units may be specified in one
option in which case dependencies for all listed names will be created. Dependencies
of this type may also be configured outside of the unit configuration file by adding a
symlink to a .wants/ directory accompanying the unit file. For details, see above.
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.
Requires=
Similar to Wants=, but declares a stronger requirement dependency. Dependencies of
this type may also be configured by adding a symlink to a .requires/ directory
accompanying the unit file.
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).
Requisite=
Similar to Requires=. However, if the units listed here are not started already, they
will not be started and the starting of this unit will fail immediately. Requisite=
does not imply an ordering dependency, even if both units are started in the same
transaction. Hence this setting should usually be combined with After=, to ensure this
unit is not started before the other unit.
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.
BindsTo=
Configures requirement dependencies, very similar in style to Requires=. However, this
dependency type is stronger: in addition to the effect of Requires= it declares that
if the unit bound to is stopped, this unit will be stopped too. This means a unit
bound to another unit that suddenly enters inactive state will be stopped too. Units
can suddenly, unexpectedly enter inactive state for different reasons: the main
process of a service unit might terminate on its own choice, the backing device of a
device unit might be unplugged or the mount point of a mount unit might be unmounted
without involvement of the system and service manager.
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.
PartOf=
Configures dependencies similar to Requires=, but limited to stopping and restarting
of units. When systemd stops or restarts the units listed here, the action is
propagated to this unit. Note that this is a one-way dependency — changes to this unit
do not affect the listed units.
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.
Upholds=
Configures dependencies similar to Wants=, but as long as this unit is up, all units
listed in Upholds= are started whenever found to be inactive or failed, and no job is
queued for them. While a Wants= dependency on another unit has a one-time effect when
this units started, a Upholds= dependency on it has a continuous effect, constantly
restarting the unit if necessary. This is an alternative to the Restart= setting of
service units, to ensure they are kept running whatever happens.
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.
Conflicts=
A space-separated list of unit names. Configures negative requirement dependencies. If
a unit has a Conflicts= setting on another unit, starting the former will stop the
latter and vice versa.
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.
Before=, After=
These two settings expect a space-separated list of unit names. They may be specified
more than once, in which case dependencies for all listed names are created.
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.
OnFailure=
A space-separated list of one or more units that are activated when this unit enters
the "failed" state.
OnSuccess=
A space-separated list of one or more units that are activated when this unit enters
the "inactive" state.
PropagatesReloadTo=, ReloadPropagatedFrom=
A space-separated list of one or more units to which reload requests from this unit
shall be propagated to, or units from which reload requests shall be propagated to
this unit, respectively. Issuing a reload request on a unit will automatically also
enqueue reload requests on all units that are linked to it using these two settings.
PropagatesStopTo=, StopPropagatedFrom=
A space-separated list of one or more units to which stop requests from this unit
shall be propagated to, or units from which stop requests shall be propagated to this
unit, respectively. Issuing a stop request on a unit will automatically also enqueue
stop requests on all units that are linked to it using these two settings.
JoinsNamespaceOf=
For units that start processes (such as service units), lists one or more other units
whose network and/or temporary file namespace to join. This only applies to unit types
which support the PrivateNetwork=, NetworkNamespacePath=, PrivateIPC=,
IPCNamespacePath=, and PrivateTmp= directives (see systemd.exec(5) for details). If a
unit that has this setting set is started, its processes will see the same /tmp/,
/var/tmp/, IPC namespace and network namespace as one listed unit that is started. If
multiple listed units are already started, it is not defined which namespace is
joined. Note that this setting only has an effect if
PrivateNetwork=/NetworkNamespacePath=, PrivateIPC=/IPCNamespacePath= and/or
PrivateTmp= is enabled for both the unit that joins the namespace and the unit whose
namespace is joined.
RequiresMountsFor=
Takes a space-separated list of absolute paths. Automatically adds dependencies of
type Requires= and After= for all mount units required to access the specified path.
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.
OnFailureJobMode=
Takes a value of "fail", "replace", "replace-irreversibly", "isolate", "flush",
"ignore-dependencies" or "ignore-requirements". Defaults to "replace". Specifies how
the units listed in OnFailure= will be enqueued. See systemctl(1)'s --job-mode= option
for details on the possible values. If this is set to "isolate", only a single unit
may be listed in OnFailure=.
IgnoreOnIsolate=
Takes a boolean argument. If true, this unit will not be stopped when isolating
another unit. Defaults to false for service, target, socket, timer, and path units,
and true for slice, scope, device, swap, mount, and automount units.
StopWhenUnneeded=
Takes a boolean argument. If true, this unit will be stopped when it is no longer
used. Note that, in order to minimize the work to be executed, systemd will not stop
units by default unless they are conflicting with other units, or the user explicitly
requested their shut down. If this option is set, a unit will be automatically cleaned
up if no other active unit requires it. Defaults to false.
RefuseManualStart=, RefuseManualStop=
Takes a boolean argument. If true, this unit can only be activated or deactivated
indirectly. In this case, explicit start-up or termination requested by the user is
denied, however if it is started or stopped as a dependency of another unit, start-up
or termination will succeed. This is mostly a safety feature to ensure that the user
does not accidentally activate units that are not intended to be activated explicitly,
and not accidentally deactivate units that are not intended to be deactivated. These
options default to false.
AllowIsolate=
Takes a boolean argument. If true, this unit may be used with the systemctl isolate
command. Otherwise, this will be refused. It probably is a good idea to leave this
disabled except for target units that shall be used similar to runlevels in SysV init
systems, just as a precaution to avoid unusable system states. This option defaults to
false.
DefaultDependencies=
Takes a boolean argument. If yes, (the default), a few default dependencies will
implicitly be created for the unit. The actual dependencies created depend on the unit
type. For example, for service units, these dependencies ensure that the service is
started only after basic system initialization is completed and is properly terminated
on system shutdown. See the respective man pages for details. Generally, only services
involved with early boot or late shutdown should set this option to no. It is highly
recommended to leave this option enabled for the majority of common units. If set to
no, this option does not disable all implicit dependencies, just non-essential ones.
CollectMode=
Tweaks the "garbage collection" algorithm for this unit. Takes one of inactive or
inactive-or-failed. If set to inactive the unit will be unloaded if it is in the
inactive state and is not referenced by clients, jobs or other units — however it is
not unloaded if it is in the failed state. In failed mode, failed units are not
unloaded until the user invoked systemctl reset-failed on them to reset the failed
state, or an equivalent command. This behaviour is altered if this option is set to
inactive-or-failed: in this case the unit is unloaded even if the unit is in a failed
state, and thus an explicitly resetting of the failed state is not necessary. Note
that if this mode is used unit results (such as exit codes, exit signals, consumed
resources, ...) are flushed out immediately after the unit completed, except for what
is stored in the logging subsystem. Defaults to inactive.
FailureAction=, SuccessAction=
Configure the action to take when the unit stops and enters a failed state or inactive
state. Takes one of none, reboot, reboot-force, reboot-immediate, poweroff,
poweroff-force, poweroff-immediate, exit, and exit-force. In system mode, all options
are allowed. In user mode, only none, exit, and exit-force are allowed. Both options
default to none.
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.
FailureActionExitStatus=, SuccessActionExitStatus=
Controls the exit status to propagate back to an invoking container manager (in case
of a system service) or service manager (in case of a user manager) when the
FailureAction=/SuccessAction= are set to exit or exit-force and the action is
triggered. By default the exit status of the main process of the triggering unit (if
this applies) is propagated. Takes a value in the range 0...255 or the empty string to
request default behaviour.
JobTimeoutSec=, JobRunningTimeoutSec=
JobTimeoutSec= specifies a timeout for the whole job that starts running when the job
is queued. JobRunningTimeoutSec= specifies a timeout that starts running when the
queued job is actually started. If either limit is reached, the job will be cancelled,
the unit however will not change state or even enter the "failed" mode.
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.
JobTimeoutAction=, JobTimeoutRebootArgument=
JobTimeoutAction= optionally configures an additional action to take when the timeout
is hit, see description of JobTimeoutSec= and JobRunningTimeoutSec= above. It takes
the same values as StartLimitAction=. Defaults to none.
JobTimeoutRebootArgument= configures an optional reboot string to pass to the
reboot(2) system call.
StartLimitIntervalSec=interval, StartLimitBurst=burst
Configure unit start rate limiting. Units which are started more than burst times
within an interval time span are not permitted to start any more. Use
StartLimitIntervalSec= to configure the checking interval and StartLimitBurst= to
configure how many starts per interval are allowed.
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.
StartLimitAction=
Configure an additional action to take if the rate limit configured with
StartLimitIntervalSec= and StartLimitBurst= is hit. Takes the same values as the
FailureAction=/SuccessAction= settings. If none is set, hitting the rate limit will
trigger no action except that the start will not be permitted. 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.
SourcePath=
A path to a configuration file this unit has been generated from. This is primarily
useful for implementation of generator tools that convert configuration from an
external configuration file format into native unit files. This functionality should
not be used in normal units.
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.
ConditionArchitecture=
Check whether the system is running on a specific architecture. Takes one of "x86",
"x86-64", "ppc", "ppc-le", "ppc64", "ppc64-le", "ia64", "parisc", "parisc64", "s390",
"s390x", "sparc", "sparc64", "mips", "mips-le", "mips64", "mips64-le", "alpha", "arm",
"arm-be", "arm64", "arm64-be", "sh", "sh64", "m68k", "tilegx", "cris", "arc",
"arc-be", or "native".
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.
ConditionFirmware=
Check whether the system's firmware is of a certain type. The following values are
possible:
• "uefi" matches systems with EFI.
• "device-tree" matches systems with a device tree.
• "device-tree-compatible(value)" matches systems with a device tree that are
compatible with "value".
• "smbios-field(field operator value)" matches systems with a SMBIOS field
containing a certain value. field is the name of the SMBIOS field exposed as
"sysfs" attribute file below /sys/class/dmi/id/. operator is one of "<", "<=",
">=", ">", "==", "<>" for version comparisons, "=" and "!=" for literal string
comparisons, or "$=", "!$=" for shell-style glob comparisons. value is the
expected value of the SMBIOS field value (possibly containing shell style globs in
case "$="/"!$=" is used).
ConditionVirtualization=
Check whether the system is executed in a virtualized environment and optionally test
whether it is a specific implementation. Takes either boolean value to check if being
executed in any virtualized environment, or one of "vm" and "container" to test
against a generic type of virtualization solution, or one of "qemu", "kvm", "amazon",
"zvm", "vmware", "microsoft", "oracle", "powervm", "xen", "bochs", "uml", "bhyve",
"qnx", "apple", "sre", "openvz", "lxc", "lxc-libvirt", "systemd-nspawn", "docker",
"podman", "rkt", "wsl", "proot", "pouch", "acrn" to test against a specific
implementation, or "private-users" to check whether we are running in a user
namespace. See systemd-detect-virt(1) for a full list of known virtualization
technologies and their identifiers. If multiple virtualization technologies are
nested, only the innermost is considered. The test may be negated by prepending an
exclamation mark.
ConditionHost=
ConditionHost= may be used to match against the hostname or machine ID of the host.
This either takes a hostname string (optionally with shell style globs) which is
tested against the locally set hostname as returned by gethostname(2), or a machine ID
formatted as string (see machine-id(5)). The test may be negated by prepending an
exclamation mark.
ConditionKernelCommandLine=
ConditionKernelCommandLine= may be used to check whether a specific kernel command
line option is set (or if prefixed with the exclamation mark — unset). The argument
must either be a single word, or an assignment (i.e. two words, separated by "="). In
the former case the kernel command line is searched for the word appearing as is, or
as left hand side of an assignment. In the latter case, the exact assignment is looked
for with right and left hand side matching. This operates on the kernel command line
communicated to userspace via /proc/cmdline, except when the service manager is
invoked as payload of a container manager, in which case the command line of PID 1 is
used instead (i.e. /proc/1/cmdline).
ConditionKernelVersion=
ConditionKernelVersion= may be used to check whether the kernel version (as reported
by uname -r) matches a certain expression, or if prefixed with the exclamation mark,
does not match. The argument must be a list of (potentially quoted) expressions. Each
expression starts with one of "=" or "!=" for string comparisons, "<", "<=", "==",
"<>", ">=", ">" for version comparisons, or "$=", "!$=" for a shell-style glob match.
If no operator is specified, "$=" is implied.
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.
ConditionCredential=
ConditionCredential= may be used to check whether a credential by the specified name
was passed into the service manager. See System and Service Credentials[2] for details
about credentials. If used in services for the system service manager this may be used
to conditionalize services based on system credentials passed in. If used in services
for the per-user service manager this may be used to conditionalize services based on
credentials passed into the unit@.service service instance belonging to the user. The
argument must be a valid credential name.
ConditionEnvironment=
ConditionEnvironment= may be used to check whether a specific environment variable is
set (or if prefixed with the exclamation mark — unset) in the service manager's
environment block. The argument may be a single word, to check if the variable with
this name is defined in the environment block, or an assignment ("name=value"), to
check if the variable with this exact value is defined. Note that the environment
block of the service manager itself is checked, i.e. not any variables defined with
Environment= or EnvironmentFile=, as described above. This is particularly useful when
the service manager runs inside a containerized environment or as per-user service
manager, in order to check for variables passed in by the enclosing container manager
or PAM.
ConditionSecurity=
ConditionSecurity= may be used to check whether the given security technology is
enabled on the system. Currently, the recognized values are "selinux", "apparmor",
"tomoyo", "ima", "smack", "audit", "uefi-secureboot" and "tpm2". The test may be
negated by prepending an exclamation mark.
ConditionCapability=
Check whether the given capability exists in the capability bounding set of the
service manager (i.e. this does not check whether capability is actually available in
the permitted or effective sets, see capabilities(7) for details). Pass a capability
name such as "CAP_MKNOD", possibly prefixed with an exclamation mark to negate the
check.
ConditionACPower=
Check whether the system has AC power, or is exclusively battery powered at the time
of activation of the unit. This takes a boolean argument. If set to "true", the
condition will hold only if at least one AC connector of the system is connected to a
power source, or if no AC connectors are known. Conversely, if set to "false", the
condition will hold only if there is at least one AC connector known and all AC
connectors are disconnected from a power source.
ConditionNeedsUpdate=
Takes one of /var/ or /etc/ as argument, possibly prefixed with a "!" (to invert the
condition). This condition may be used to conditionalize units on whether the
specified directory requires an update because /usr/'s modification time is newer than
the stamp file .updated in the specified directory. This is useful to implement
offline updates of the vendor operating system resources in /usr/ that require
updating of /etc/ or /var/ on the next following boot. Units making use of this
condition should order themselves before systemd-update-done.service(8), to make sure
they run before the stamp file's modification time gets reset indicating a completed
update.
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.
ConditionFirstBoot=
Takes a boolean argument. This condition may be used to conditionalize units on
whether the system is booting up for the first time. This roughly means that /etc/ was
unpopulated when the system started booting (for details, see "First Boot Semantics"
in machine-id(5)). First boot is considered finished (this condition will evaluate as
false) after the manager has finished the startup phase.
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.
ConditionPathExists=
Check for the existence of a file. If the specified absolute path name does not exist,
the condition will fail. If the absolute path name passed to ConditionPathExists= is
prefixed with an exclamation mark ("!"), the test is negated, and the unit is only
started if the path does not exist.
ConditionPathExistsGlob=
ConditionPathExistsGlob= is similar to ConditionPathExists=, but checks for the
existence of at least one file or directory matching the specified globbing pattern.
ConditionPathIsDirectory=
ConditionPathIsDirectory= is similar to ConditionPathExists= but verifies that a
certain path exists and is a directory.
ConditionPathIsSymbolicLink=
ConditionPathIsSymbolicLink= is similar to ConditionPathExists= but verifies that a
certain path exists and is a symbolic link.
ConditionPathIsMountPoint=
ConditionPathIsMountPoint= is similar to ConditionPathExists= but verifies that a
certain path exists and is a mount point.
ConditionPathIsReadWrite=
ConditionPathIsReadWrite= is similar to ConditionPathExists= but verifies that the
underlying file system is readable and writable (i.e. not mounted read-only).
ConditionPathIsEncrypted=
ConditionPathIsEncrypted= is similar to ConditionPathExists= but verifies that the
underlying file system's backing block device is encrypted using dm-crypt/LUKS. Note
that this check does not cover ext4 per-directory encryption, and only detects block
level encryption. Moreover, if the specified path resides on a file system on top of a
loopback block device, only encryption above the loopback device is detected. It is
not detected whether the file system backing the loopback block device is encrypted.
ConditionDirectoryNotEmpty=
ConditionDirectoryNotEmpty= is similar to ConditionPathExists= but verifies that a
certain path exists and is a non-empty directory.
ConditionFileNotEmpty=
ConditionFileNotEmpty= is similar to ConditionPathExists= but verifies that a certain
path exists and refers to a regular file with a non-zero size.
ConditionFileIsExecutable=
ConditionFileIsExecutable= is similar to ConditionPathExists= but verifies that a
certain path exists, is a regular file, and marked executable.
ConditionUser=
ConditionUser= takes a numeric "UID", a UNIX user name, or the special value
"@system". This condition may be used to check whether the service manager is running
as the given user. The special value "@system" can be used to check if the user id is
within the system user range. This option is not useful for system services, as the
system manager exclusively runs as the root user, and thus the test result is
constant.
ConditionGroup=
ConditionGroup= is similar to ConditionUser= but verifies that the service manager's
real or effective group, or any of its auxiliary groups, match the specified group or
GID. This setting does not support the special value "@system".
ConditionControlGroupController=
Check whether given cgroup controllers (e.g. "cpu") are available for use on the
system or whether the legacy v1 cgroup or the modern v2 cgroup hierarchy is used.
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.
ConditionMemory=
Verify that the specified amount of system memory is available to the current system.
Takes a memory size in bytes as argument, optionally prefixed with a comparison
operator "<", "<=", "=" (or "=="), "!=" (or "<>"), ">=", ">". On bare-metal systems
compares the amount of physical memory in the system with the specified size, adhering
to the specified comparison operator. In containers compares the amount of memory
assigned to the container instead.
ConditionCPUs=
Verify that the specified number of CPUs is available to the current system. Takes a
number of CPUs as argument, optionally prefixed with a comparison operator "<", "<=",
"=" (or "=="), "!=" (or "<>"), ">=", ">". Compares the number of CPUs in the CPU
affinity mask configured of the service manager itself with the specified number,
adhering to the specified comparison operator. On physical systems the number of CPUs
in the affinity mask of the service manager usually matches the number of physical
CPUs, but in special and virtual environments might differ. In particular, in
containers the affinity mask usually matches the number of CPUs assigned to the
container and not the physically available ones.
ConditionCPUFeature=
Verify that a given CPU feature is available via the "CPUID" instruction. This
condition only does something on i386 and x86-64 processors. On other processors it is
assumed that the CPU does not support the given feature. It checks the leaves "1",
"7", "0x80000001", and "0x80000007". Valid values are: "fpu", "vme", "de", "pse",
"tsc", "msr", "pae", "mce", "cx8", "apic", "sep", "mtrr", "pge", "mca", "cmov", "pat",
"pse36", "clflush", "mmx", "fxsr", "sse", "sse2", "ht", "pni", "pclmul", "monitor",
"ssse3", "fma3", "cx16", "sse4_1", "sse4_2", "movbe", "popcnt", "aes", "xsave",
"osxsave", "avx", "f16c", "rdrand", "bmi1", "avx2", "bmi2", "rdseed", "adx", "sha_ni",
"syscall", "rdtscp", "lm", "lahf_lm", "abm", "constant_tsc".
ConditionOSRelease=
Verify that a specific "key=value" pair is set in the host's os-release(5).
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=
Verify that the overall system (memory, CPU or IO) pressure is below or equal to a
threshold. This setting takes a threshold value as argument. It can be specified as a
simple percentage value, suffixed with "%", in which case the pressure will be
measured as an average over the last five minutes before the attempt to start the unit
is performed. Alternatively, the average timespan can also be specified using "/" as a
separator, for example: "10%/1min". The supported timespans match what the kernel
provides, and are limited to "10sec", "1min" and "5min". The "full" PSI will be
checked first, and if not found "some" will be checked. For more details, see the
documentation on PSI (Pressure Stall Information)[3].
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=
Similar to the ConditionArchitecture=, ConditionVirtualization=, ..., condition
settings described above, these settings add assertion checks to the start-up of the
unit. However, unlike the conditions settings, any assertion setting that is not met
results in failure of the start job (which means this is logged loudly). Note that
hitting a configured assertion does not cause the unit to enter the "failed" state (or
in fact result in any state change of the unit), it affects only the job queued for
it. Use assertion expressions for units that cannot operate when specific requirements
are not met, and when this is something the administrator or user should look into.
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 │
├──────────────────────┼───────────────────────┼───────────────────────────────────────┤
│Before= │ After= │ │
├──────────────────────┼───────────────────────┤ [Unit] section │
│After= │ Before= │ │
├──────────────────────┼───────────────────────┼──────────────────┬────────────────────┤
│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 │
├──────────────────────┼───────────────────────┼──────────────────┴────────────────────┤
│PropagatesReloadTo= │ ReloadPropagatedFrom= │ │
├──────────────────────┼───────────────────────┤ [Unit] section │
│ReloadPropagatedFrom= │ PropagatesReloadTo= │ │
├──────────────────────┼───────────────────────┼──────────────────┬────────────────────┤
│Following= │ n/a │ An automatic │ │
│ │ │ property │ │
└──────────────────────┴───────────────────────┴──────────────────┴────────────────────┘
Note: 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.
Alias=
A space-separated list of additional names this unit shall be installed under. The
names listed here must have the same suffix (i.e. type) as the unit filename. This
option may be specified more than once, in which case all listed names are used. At
installation time, systemctl enable will create symlinks from these names to the unit
filename. Note that not all unit types support such alias names, and this setting is
not supported for them. Specifically, mount, slice, swap, and automount units do not
support aliasing.
WantedBy=, RequiredBy=
This option may be used more than once, or a space-separated list of unit names may be
given. A symbolic link is created in the .wants/ or .requires/ directory of each of
the listed units when this unit is installed by systemctl enable. This has the effect
of a dependency of type Wants= or Requires= being added from the listed unit to the
current unit. The primary result is that the current unit will be started when the
listed unit is started, see the description of Wants= and Requires= in the [Unit]
section for details.
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 getty@tty2.service creating a
getty.target.wants/getty@tty2.service 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.
Also=
Additional units to install/deinstall when this unit is installed/deinstalled. If the
user requests installation/deinstallation of a unit with this option configured,
systemctl enable and systemctl disable will automatically install/uninstall units
listed in this option as well.
This option may be used more than once, or a space-separated list of unit names may be
given.
DefaultInstance=
In template unit files, this specifies for which instance the unit shall be enabled if
the template is enabled without any explicitly set instance. This option has no effect
in non-template unit files. The specified string must be usable as instance
identifier.
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 4. Specifiers available in unit files
┌──────────┬──────────────────────────┬──────────────────────────┐
│Specifier │ Meaning │ Details │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%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 │ The operating system │
│ │ version │ 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 │ The operating system │
│ │ ID │ 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 │ This is either /etc/ │
│ │ root │ (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 │ This is the string │
│ │ prefix │ between the last "-" and │
│ │ │ the end of the prefix │
│ │ │ name. If there is no │
│ │ │ "-", this is the same as │
│ │ │ "%p". │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%J" │ Unescaped final │ Same as "%j", but with │
│ │ component of the prefix │ 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 │ The operating system │
│ │ identifier │ 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 │ This is either /tmp or │
│ │ files │ the path "$TMPDIR", │
│ │ │ "$TEMP" or "$TMP" are │
│ │ │ set to. (Note that the │
│ │ │ directory may be │
│ │ │ specified without a │
│ │ │ trailing slash.) │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%u" │ User name │ This is the name of the │
│ │ │ user running the service │
│ │ │ manager instance. In │
│ │ │ case of the system │
│ │ │ manager this resolves to │
│ │ │ "root". │
│ │ │ │
│ │ │ Note that this setting │
│ │ │ is not influenced by the │
│ │ │ User= setting │
│ │ │ configurable in the │
│ │ │ [Service] section of the │
│ │ │ service unit. │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%U" │ User UID │ This is the numeric UID │
│ │ │ of the user running the │
│ │ │ service manager │
│ │ │ instance. In case of the │
│ │ │ system manager this │
│ │ │ resolves to "0". │
│ │ │ │
│ │ │ Note that this setting │
│ │ │ is not influenced by the │
│ │ │ User= setting │
│ │ │ configurable in the │
│ │ │ [Service] section of the │
│ │ │ service unit. │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%v" │ Kernel release │ Identical to uname -r │
│ │ │ output. │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%V" │ Directory for larger and │ This is either /var/tmp │
│ │ persistent temporary │ or the path "$TMPDIR", │
│ │ files │ "$TEMP" or "$TMP" are │
│ │ │ set to. (Note that the │
│ │ │ directory may be │
│ │ │ specified without a │
│ │ │ trailing slash.) │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%w" │ Operating system version │ The operating system │
│ │ ID │ 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 │ The operating system │
│ │ ID │ 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 │ This is the directory │
│ │ fragment │ part of "%y". │
├──────────┼──────────────────────────┼──────────────────────────┤
│"%%" │ Single percent sign │ Use "%%" in place of "%" │
│ │ │ to specify a single │
│ │ │ percent sign. │
└──────────┴──────────────────────────┴──────────────────────────┘
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 /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 /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
https://systemd.io/PORTABILITY_AND_STABILITY/
2. System and Service Credentials
https://systemd.io/CREDENTIALS
3. PSI (Pressure Stall Information)
https://docs.kernel.org/accounting/psi.html