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systemd.unit 文档

[root@rockylinux docs]# man systemd.unit

SYSTEMD.UNIT(5) systemd.unit SYSTEMD.UNIT(5)

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

/etc/systemd/system.control/*
/run/systemd/system.control/*
/run/systemd/transient/*
/run/systemd/generator.early/*
/etc/systemd/system/*
/run/systemd/system/*
/run/systemd/generator/*
...
/usr/lib/systemd/system/*
/run/systemd/generator.late/*

~/.config/systemd/user.control/*
$XDG_RUNTIME_DIR/systemd/user.control/*
$XDG_RUNTIME_DIR/systemd/transient/*
$XDG_RUNTIME_DIR/systemd/generator.early/*
~/.config/systemd/user/*
/etc/systemd/user/*
$XDG_RUNTIME_DIR/systemd/user/*
/run/systemd/user/*
$XDG_RUNTIME_DIR/systemd/generator/*
~/.local/share/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(5) 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.

Unit files 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 name (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.

Boolean arguments used in unit files can be written in various formats. For
positive settings the strings 1, yes, true and on are equivalent. For negative
settings, the strings 0, no, false and off are equivalent.

Time span values encoded in unit files can be written in various formats. A
stand-alone number specifies a time in seconds. If suffixed with a time unit,
the unit is honored. A concatenation of multiple values with units is
supported, in which case the values are added up. Example: "50" refers to 50
seconds; "2min 200ms" refers to 2 minutes and 200 milliseconds, i.e. 120200 ms.
The following time units are understood: "s", "min", "h", "d", "w", "ms", "us".
For details see systemd.time(7).

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 the symlink
/usr/lib/systemd/system/dbus-org.freedesktop.network1.service. In addition,
unit files may specify aliases through the Alias= directive in the [Install]
section; those aliases are only effective when the unit is enabled. 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 it will be invoked whenever
CTRL+ALT+DEL is pressed. Alias names may be used in commands like enable,
disable, start, stop, status, ..., and in unit dependency directives Wants=,
Requires=, Before=, After=, ..., with the limitation that aliases specified
through Alias= are only effective when the unit is enabled. Aliases cannot be
used with the preset command.

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. This 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=, see below. The preferred way to create symlinks
in the .wants/ directory of a unit file is with the enable command of the
systemctl(1) tool which reads information from the [Install] section of unit
files (see below). A similar functionality exists for Requires= type
dependencies as well, the directory suffix is .requires/ in this case.

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 parsed
after the unit file itself is parsed. This is useful to alter or add
configuration settings for a unit, without having to modify unit files. Drop-in
files 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 units names containing dashes ("-"), the set of directories
generated by 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 addition to /etc/systemd/system, the drop-in ".d/" directories for system
services can be placed in /usr/lib/systemd/system or /run/systemd/system
directories. Drop-in files in /etc take precedence over those in /run which in
turn take precedence over those in /usr/lib. Drop-in files under any of these
directories take precedence over unit files wherever located. Multiple drop-in
files with different names are applied in lexicographic order, regardless of
which of the directories they reside in.

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 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= unless
DefaultDependencies=no is set in the specified units. See systemd.target(5) for
details. Note that this behavior can be turned off by setting
DefaultDependencies=no.

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 │
│/run/systemd/system.control │ the dbus API │
├──────────────────────────────┼─────────────────────────────┤
│/run/systemd/transient │ Dynamic configuration for │
│ │ transient units │
├──────────────────────────────┼─────────────────────────────┤
│/run/systemd/generator.early │ Generated units with high │
│ │ priority (see early-dir in │
│ │ system.generator(7)) │
├──────────────────────────────┼─────────────────────────────┤
│/etc/systemd/system │ Local configuration │
├──────────────────────────────┼─────────────────────────────┤
│/run/systemd/system │ Runtime units │
├──────────────────────────────┼─────────────────────────────┤
│/run/systemd/generator │ Generated units with medium │
│ │ priority (see normal-dir in │
│ │ system.generator(7)) │
├──────────────────────────────┼─────────────────────────────┤
│/usr/local/lib/systemd/system │ │
├──────────────────────────────┤ Units of installed packages │
│/usr/lib/systemd/system │ │
├──────────────────────────────┼─────────────────────────────┤
│/run/systemd/generator.late │ Generated units with low │
│ │ priority (see late-dir in │
│ │ system.generator(7)) │
└──────────────────────────────┴─────────────────────────────┘

Table 2. Load path when running in user mode (--user).
┌────────────────────────────────────────┬───────────────────────────────┐
│Path │ Description │
├────────────────────────────────────────┼───────────────────────────────┤
│$XDG_CONFIG_HOME/systemd/user.control │ Persistent and transient │
│or │ configuration created using │
│~/.config/systemd/user.control │ the dbus API │
├────────────────────────────────────────┤ ($XDG_CONFIG_HOME is used if │
│$XDG_RUNTIME_DIR/systemd/user.control │ set, ~/.config otherwise) │
├────────────────────────────────────────┼───────────────────────────────┤
│/run/systemd/transient │ Dynamic configuration for │
│ │ transient units │
├────────────────────────────────────────┼───────────────────────────────┤
│/run/systemd/generator.early │ Generated units with high │
│ │ priority (see early-dir in │
│ │ system.generator(7)) │
├────────────────────────────────────────┼───────────────────────────────┤
│$XDG_CONFIG_HOME/systemd/user or │ User configuration │
│$HOME/.config/systemd/user │ ($XDG_CONFIG_HOME is used if │
│ │ set, ~/.config otherwise) │
├────────────────────────────────────────┼───────────────────────────────┤
│/etc/systemd/user │ Local configuration │
├────────────────────────────────────────┼───────────────────────────────┤
│$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 │
│ │ system.generator(7)) │
├────────────────────────────────────────┼───────────────────────────────┤
│$XDG_DATA_HOME/systemd/user or │ Units of packages that have │
│$HOME/.local/share/systemd/user │ been installed in the home │
│ │ directory ($XDG_DATA_HOME is │
│ │ used if set, ~/.local/share │
│ │ otherwise) │
├────────────────────────────────────────┼───────────────────────────────┤
│$dir/systemd/user for each $dir in │ Additional locations for │
│$XDG_DATA_DIRS │ installed user units, one for │
│ │ each entry in $XDG_DATA_DIRS │
├────────────────────────────────────────┼───────────────────────────────┤
│/usr/local/lib/systemd/user │ Units of packages that have │
├────────────────────────────────────────┤ been installed system-wide │
│/usr/lib/systemd/user │ │
├────────────────────────────────────────┼───────────────────────────────┤
│$XDG_RUNTIME_DIR/systemd/generator.late │ Generated units with low │
│ │ priority (see late-dir in │
│ │ system.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 ("linked") from
directories not on the unit load path. See the link command for systemctl(1).

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 free-form string describing the unit. This is intended for use in UIs to
show descriptive information along with the unit name. The description
should contain a name that means something to the end user. "Apache2 Web
Server" is a good example. Bad examples are "high-performance light-weight
HTTP server" (too generic) or "Apache2" (too specific and meaningless for
people who do not know Apache).

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.

Requires=
Configures requirement dependencies on other units. If this unit gets
activated, the units listed here 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 if one of the other
units is explicitly stopped. This option may be specified more than once or
multiple space-separated units may be specified in one option in which case
requirement dependencies for all listed names will be created. 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 a unit foo.service requires a unit bar.service as
configured with Requires= 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. 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).

Note that dependencies of this type may also be configured outside of the
unit configuration file by adding a symlink to a .requires/ directory
accompanying the unit file. For details, see above.

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.

Wants=
A weaker version of Requires=. 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. This is the recommended way to hook start-up of
one unit to the start-up of another unit.

Note that dependencies of this type may also be configured outside of the
unit configuration file by adding symlinks to a .wants/ directory
accompanying the unit file. For details, see above.

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 a failed
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.

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 is
independent of and orthogonal to the After= and Before= ordering
dependencies.

If a unit A that conflicts with a unit B is scheduled to be started at the
same time as B, the transaction will either fail (in case both are required
part 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 the 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
configure ordering dependencies between units. If a unit foo.service
contains a setting Before=bar.service and both units are being started,
bar.service's start-up is delayed until foo.service has finished starting
up. Note that this setting is independent of and orthogonal to the
requirement dependencies as configured by Requires=, Wants= or BindsTo=. It
is a common pattern to include a unit name in both the After= and Requires=
options, in which case the unit listed will be started before the unit that
is configured with these options. This option may be specified more than
once, in which case ordering dependencies for all listed names are created.
After= is the inverse of Before=, i.e. while After= ensures that the
configured unit is started after the listed unit finished starting up,
Before= ensures the opposite, that the configured unit is fully started up
before the listed unit is started. Note that 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.

OnFailure=
A space-separated list of one or more units that are activated when this
unit enters the "failed" state. A service unit using Restart= enters the
failed state only after the start limits are reached.

PropagatesReloadTo=, ReloadPropagatedFrom=
A space-separated list of one or more units where reload requests on this
unit will be propagated to, or reload requests on the other unit will be
propagated to this unit, respectively. Issuing a reload request on a unit
will automatically also enqueue a reload request on all units that the
reload request shall be propagated to via 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= 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 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= 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,
busname, 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 true, (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 false. It is highly recommended to leave this option enabled for the
majority of common units. If set to false, 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.

JobTimeoutSec=, JobRunningTimeoutSec=, JobTimeoutAction=,
JobTimeoutRebootArgument=
When a job for this unit is queued, a time-out JobTimeoutSec= may be
configured. Similarly, JobRunningTimeoutSec= starts counting when the
queued job is actually started. If either time limit is reached, the job
will be cancelled, the unit however will not change state or even enter the
"failed" mode. This value defaults to "infinity" (job timeouts disabled),
except for device units (JobRunningTimeoutSec= defaults to
DefaultTimeoutStartSec=). NB: this timeout is independent from any
unit-specific timeout (for example, the timeout set with TimeoutStartSec=
in service units) as the job timeout has no effect on the unit itself, only
on the job that might be pending for it. 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= optionally configures an additional action to take when
the time-out is hit. 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 interval are not permitted to start any more.
Use StartLimitIntervalSec= to configure the checking interval (defaults to
DefaultStartLimitIntervalSec= in manager configuration file, set it to 0 to
disable any kind of rate limiting). Use StartLimitBurst= to configure how
many starts per interval are allowed (defaults to DefaultStartLimitBurst=
in manager configuration file). These configuration options are
particularly useful in conjunction with 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 at a later point, after the interval has passed. From this point
on, the restart logic is activated again. Note that systemctl reset-failed
will cause the restart rate counter for a service to be flushed, which is
useful if the administrator wants to manually start a unit and the start
limit interferes with that. Note that this rate-limiting is enforced after
any unit condition checks are executed, and hence unit activations with
failing conditions do not count towards this rate limit. 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.

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.

StartLimitAction=
Configure the action to take if the rate limit configured with
StartLimitIntervalSec= and StartLimitBurst= is hit. Takes one of none,
reboot, reboot-force, reboot-immediate, poweroff, poweroff-force or
poweroff-immediate. If none is set, hitting the rate limit will trigger no
action besides that the start will not be permitted. reboot causes a
reboot following the normal shutdown procedure (i.e. equivalent to
systemctl reboot). reboot-force causes a forced reboot which will
terminate all processes forcibly but should cause no dirty file systems on
reboot (i.e. equivalent to systemctl reboot -f) and reboot-immediate causes
immediate execution of the reboot(2) system call, which might result in
data loss. Similarly, poweroff, poweroff-force, poweroff-immediate have the
effect of powering down the system with similar semantics. Defaults to
none.

FailureAction=, SuccessAction=
Configure the action to take when the unit stops and enters a failed state
or inactive state. Takes the same values as the setting StartLimitAction=
setting and executes the same actions. Both options default 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.

ConditionArchitecture=, ConditionVirtualization=, ConditionHost=,
ConditionKernelCommandLine=, ConditionKernelVersion=, ConditionSecurity=,
ConditionCapability=, ConditionACPower=, ConditionNeedsUpdate=,
ConditionFirstBoot=, ConditionPathExists=, ConditionPathExistsGlob=,
ConditionPathIsDirectory=, ConditionPathIsSymbolicLink=,
ConditionPathIsMountPoint=, ConditionPathIsReadWrite=,
ConditionDirectoryNotEmpty=, ConditionFileNotEmpty=,
ConditionFileIsExecutable=, ConditionUser=, ConditionGroup=,
ConditionControlGroupController=
Before starting a unit, verify that the specified condition is true. If it
is not true, the starting of the unit will be (mostly silently) skipped,
however all ordering dependencies of it are still respected. A failing
condition will not result in the unit being moved into a failure state. The
condition is checked at the time the queued start job is to be executed.
Use condition expressions in order to silently skip units that do not apply
to the local running system, for example because the kernel or runtime
environment doesn't require its functionality. Use the various
AssertArchitecture=, AssertVirtualization=, ... options for a similar
mechanism that puts the unit in a failure state and logs about the failed
check (see below).

ConditionArchitecture= may be used to 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 to test against a specific architecture.
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.

ConditionVirtualization= may be used to 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,
zvm, vmware, microsoft, oracle, xen, bochs, uml, bhyve, qnx, openvz, lxc,
lxc-libvirt, systemd-nspawn, docker, rkt 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= 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= 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 "="). 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.

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 it). The argument must be a single string.
If the string starts with one of "<", "<=", "=", ">=", ">" a relative
version comparison is done, otherwise the specified string is matched with
shell-style globs.

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.

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 and uefi-secureboot. The test
may be negated by prepending an exclamation mark.

ConditionCapability= may be used to 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= may be used to 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 "!" (for inverting 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.

ConditionFirstBoot= takes a boolean argument. This condition may be used to
conditionalize units on whether the system is booting up with an
unpopulated /etc directory (specifically: an /etc with no /etc/machine-id).
This 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.

With ConditionPathExists= a file existence condition is checked before a
unit is started. 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= is similar to ConditionPathExists=, but checks for
the existence of at least one file or directory matching the specified
globbing pattern.

ConditionPathIsDirectory= is similar to ConditionPathExists= but verifies
whether a certain path exists and is a directory.

ConditionPathIsSymbolicLink= is similar to ConditionPathExists= but
verifies whether a certain path exists and is a symbolic link.

ConditionPathIsMountPoint= is similar to ConditionPathExists= but verifies
whether a certain path exists and is a mount point.

ConditionPathIsReadWrite= is similar to ConditionPathExists= but verifies
whether the underlying file system is readable and writable (i.e. not
mounted read-only).

ConditionDirectoryNotEmpty= is similar to ConditionPathExists= but verifies
whether a certain path exists and is a non-empty directory.

ConditionFileNotEmpty= is similar to ConditionPathExists= but verifies
whether a certain path exists and refers to a regular file with a non-zero
size.

ConditionFileIsExecutable= is similar to ConditionPathExists= but verifies
whether a certain path exists, is a regular file and marked executable.

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= 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 have a special value
"@system".

ConditionControlGroupController= takes a cgroup controller name (eg. cpu),
verifying that it is available for use on the system. For example, a
particular controller may not be available if it was disabled on the kernel
command line with "cgroup_disable="controller. 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, cpuacct, io, blkio,
memory, devices, and pids.

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 be
prefixed with a pipe symbol (|) in which case a condition becomes a
triggering condition. If at least one triggering condition is defined for a
unit, then the unit will be executed if at least one of the triggering
conditions apply and all of the non-triggering conditions. If you prefix an
argument with the pipe symbol and an exclamation mark, the pipe symbol must
be passed first, the exclamation second. Except for
ConditionPathIsSymbolicLink=, all path checks follow symlinks. 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.

AssertArchitecture=, AssertVirtualization=, AssertHost=,
AssertKernelCommandLine=, AssertKernelVersion=, AssertSecurity=,
AssertCapability=, AssertACPower=, AssertNeedsUpdate=, AssertFirstBoot=,
AssertPathExists=, AssertPathExistsGlob=, AssertPathIsDirectory=,
AssertPathIsSymbolicLink=, AssertPathIsMountPoint=, AssertPathIsReadWrite=,
AssertDirectoryNotEmpty=, AssertFileNotEmpty=, AssertFileIsExecutable=,
AssertUser=, AssertGroup=, AssertControlGroupController=
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). 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.

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.

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= │ Both are unit file │
├──────────────────────┼───────────────────────┤ options │
│After= │ Before= │ │
├──────────────────────┼───────────────────────┼───────────────────────┤
│Requires= │ RequiredBy= │ A unit file option; │
│ │ │ an option in the │
│ │ │ [Install] section │
├──────────────────────┼───────────────────────┼───────────────────────┤
│Wants= │ WantedBy= │ A unit file option; │
│ │ │ an option in the │
│ │ │ [Install] section │
├──────────────────────┼───────────────────────┼───────────────────────┤
│PartOf= │ ConsistsOf= │ A unit file option; │
│ │ │ an automatic property │
├──────────────────────┼───────────────────────┼───────────────────────┤
│BindsTo= │ BoundBy= │ A unit file option; │
│ │ │ an automatic property │
├──────────────────────┼───────────────────────┼───────────────────────┤
│Requisite= │ RequisiteOf= │ A unit file option; │
│ │ │ an automatic property │
├──────────────────────┼───────────────────────┼───────────────────────┤
│Triggers= │ TriggeredBy= │ Automatic properties, │
│ │ │ see notes below │
├──────────────────────┼───────────────────────┼───────────────────────┤
│Conflicts= │ ConflictedBy= │ A unit file option; │
│ │ │ an automatic property │
├──────────────────────┼───────────────────────┼───────────────────────┤
│PropagatesReloadTo= │ ReloadPropagatedFrom= │ Both are unit file │
├──────────────────────┼───────────────────────┤ options │
│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 reverse 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. TriggersBy= 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 that a dependency of type Wants= or
Requires= is 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.

WantedBy=foo.service in a service bar.service is mostly equivalent to
Alias=foo.service.wants/bar.service in the same file. In case of template
units, systemctl enable must be called with an instance name, and this
instance will be added to the .wants/ or .requires/ list of the listed
unit. E.g. 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.

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: %n, %N, %p,
%i, %j, %U, %u, %m, %H, %b, %v. 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 │
├──────────┼───────────────────────┼───────────────────────┤
│"%b" │ Boot ID │ The boot ID of the │
│ │ │ running system, │
│ │ │ formatted as string. │
│ │ │ See random(4) for │
│ │ │ more information. │
├──────────┼───────────────────────┼───────────────────────┤
│"%C" │ Cache directory root │ This is either │
│ │ │ /var/cache (for the │
│ │ │ system manager) or │
│ │ │ the path │
│ │ │ "$XDG_CACHE_HOME" │
│ │ │ resolves to (for user │
│ │ │ managers). │
├──────────┼───────────────────────┼───────────────────────┤
│"%E" │ Configuration │ This is either /etc │
│ │ directory root │ (for the system │
│ │ │ manager) or the path │
│ │ │ "$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. │
├──────────┼───────────────────────┼───────────────────────┤
│"%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". │
├──────────┼───────────────────────┼───────────────────────┤
│"%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 │ Same as "%i", but │
│ │ name │ with escaping undone. │
├──────────┼───────────────────────┼───────────────────────┤
│"%j" │ Final component of │ This is the string │
│ │ the 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 │
│ │ component of the │ with escaping undone. │
│ │ prefix │ │
├──────────┼───────────────────────┼───────────────────────┤
│"%L" │ Log directory root │ This is either │
│ │ │ /var/log (for the │
│ │ │ system manager) or │
│ │ │ the path │
│ │ │ "$XDG_CONFIG_HOME" │
│ │ │ resolves to with /log │
│ │ │ appended (for user │
│ │ │ managers). │
├──────────┼───────────────────────┼───────────────────────┤
│"%m" │ Machine ID │ The machine ID of the │
│ │ │ running system, │
│ │ │ formatted as string. │
│ │ │ See machine-id(5) for │
│ │ │ more information. │
├──────────┼───────────────────────┼───────────────────────┤
│"%n" │ Full unit name │ │
├──────────┼───────────────────────┼───────────────────────┤
│"%N" │ Full unit name │ Same as "%n", but │
│ │ │ with the type suffix │
│ │ │ removed. │
├──────────┼───────────────────────┼───────────────────────┤
│"%p" │ Prefix name │ For instantiated │
│ │ │ units, this refers to │
│ │ │ the string before the │
│ │ │ first "@" character │
│ │ │ of the unit name. For │
│ │ │ non-instantiated │
│ │ │ units, same as "%N". │
├──────────┼───────────────────────┼───────────────────────┤
│"%P" │ Unescaped prefix name │ Same as "%p", but │
│ │ │ with escaping undone. │
├──────────┼───────────────────────┼───────────────────────┤
│"%s" │ User shell │ This is the shell of │
│ │ │ the user running the │
│ │ │ service manager │
│ │ │ instance. In case of │
│ │ │ the system manager │
│ │ │ this resolves to │
│ │ │ "/bin/sh". │
├──────────┼───────────────────────┼───────────────────────┤
│"%S" │ State directory root │ This is either │
│ │ │ /var/lib (for the │
│ │ │ system manager) or │
│ │ │ the path │
│ │ │ "$XDG_CONFIG_HOME" │
│ │ │ resolves to (for user │
│ │ │ managers). │
├──────────┼───────────────────────┼───────────────────────┤
│"%t" │ Runtime directory │ This is either /run │
│ │ root │ (for the system │
│ │ │ manager) or the path │
│ │ │ "$XDG_RUNTIME_DIR" │
│ │ │ resolves to (for user │
│ │ │ managers). │
├──────────┼───────────────────────┼───────────────────────┤
│"%T" │ Directory for │ This is either /tmp │
│ │ temporary files │ or the path │
│ │ │ "$TMPDIR", "$TEMP" or │
│ │ │ "$TMP" are set to. │
├──────────┼───────────────────────┼───────────────────────┤
│"%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". │
├──────────┼───────────────────────┼───────────────────────┤
│"%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". │
├──────────┼───────────────────────┼───────────────────────┤
│"%v" │ Kernel release │ Identical to uname -r │
│ │ │ output │
├──────────┼───────────────────────┼───────────────────────┤
│"%V" │ Directory for larger │ This is either │
│ │ and persistent │ /var/tmp or the path │
│ │ temporary files │ "$TMPDIR", "$TEMP" or │
│ │ │ "$TMP" are set to. │
├──────────┼───────────────────────┼───────────────────────┤
│"%%" │ 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 /usr/lib/systemd/system to /etc/systemd/system and modifying the
chosen settings. Alternatively, one can create a directory named unit.d/ within
/etc/systemd/system and place a drop-in file name.conf there that only changes
the specific settings one is interested in. Note that multiple such drop-in
files are read if present, processed in lexicographic order of their filename.

The advantage of the first method is that one easily overrides the complete
unit, the vendor unit is not parsed at all anymore. It has the disadvantage
that improvements to the unit file by the vendor are not automatically
incorporated on updates.

The advantage of the second method is that one only overrides the settings one
specifically wants, where updates to the unit by the vendor automatically
apply. This has the disadvantage that some future updates by the vendor might
be incompatible with the local changes.

This also applies for user instances of systemd, but with different locations
for the unit files. See the section on unit load paths for further details.

Suppose there is a vendor-supplied unit /usr/lib/systemd/system/httpd.service
with the following contents:

[Unit]
Description=Some HTTP server
After=remote-fs.target sqldb.service
Requires=sqldb.service
AssertPathExists=/srv/webserver

[Service]
Type=notify
ExecStart=/usr/sbin/some-fancy-httpd-server
Nice=5

[Install]
WantedBy=multi-user.target

Now one wants to change some settings as an administrator: firstly, in the
local setup, /srv/webserver might not exist, because the HTTP server is
configured to use /srv/www instead. Secondly, the local configuration makes the
HTTP server also depend on a memory cache service, memcached.service, that
should be pulled in (Requires=) and also be ordered appropriately (After=).
Thirdly, in order to harden the service a bit more, the administrator would
like to set the PrivateTmp= setting (see systemd.exec(5) for details). And
lastly, the administrator would like to reset the niceness of the service to
its default value of 0.

The first possibility is to copy the unit file to
/etc/systemd/system/httpd.service and change the chosen settings:

[Unit]
Description=Some HTTP server
After=remote-fs.target sqldb.service memcached.service
Requires=sqldb.service memcached.service
AssertPathExists=/srv/www

[Service]
Type=notify
ExecStart=/usr/sbin/some-fancy-httpd-server
Nice=0
PrivateTmp=yes

[Install]
WantedBy=multi-user.target

Alternatively, the administrator could create a drop-in file
/etc/systemd/system/httpd.service.d/local.conf with the following contents:

[Unit]
After=memcached.service
Requires=memcached.service
# Reset all assertions and then re-add the condition we want
AssertPathExists=
AssertPathExists=/srv/www

[Service]
Nice=0
PrivateTmp=yes

Note that for drop-in files, if one wants to remove entries from a setting that
is parsed as a list (and is not a dependency), such as AssertPathExists= (or
e.g. ExecStart= in service units), one needs to first clear the list before
re-adding all entries except the one that is to be removed. Dependencies
(After=, etc.) cannot be reset to an empty list, so dependencies can only be
added in drop-ins. If you want to remove dependencies, you have to override the
entire unit.

SEE ALSO
systemd(1), systemctl(1), 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 Stability Promise
https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise

systemd 239 SYSTEMD.UNIT(5)

posted on 2022-02-16 15:06 lnlidawei 阅读(29) 评论(0) 编辑收藏举报

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