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/systemd.attached/* /run/systemd/system/* /run/systemd/systemd.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/* /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.
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 /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. "firstname.lastname@example.org/") and read its ".conf" files, followed by the template ".d/" subdirectory (e.g. "foo@.service.d/") and the ".conf" files there. Moreover for 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 /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.
Note that while systemd offers a flexible dependency system between units it is recommended to use this functionality only sparingly and instead rely on techniques such as bus-based or socket-based activation which make dependencies implicit, resulting in a both simpler and more flexible system.
As mentioned above, a unit may be instantiated from a template file. This allows creation of multiple units from a single configuration file. If systemd looks for a unit configuration file, it will first search for the literal unit name in the file system. If that yields no success and the unit name contains an "@" character, systemd will look for a unit template that shares the same name but with the instance string (i.e. the part between the "@" character and the suffix) removed. Example: if a service email@example.com is requested and no file by that name is found, systemd will look for getty@.service and instantiate a service from that configuration file if it is found.
To refer to the instance string from within the configuration file you may use the special "%i" specifier in many of the configuration options. See below for details.
If a unit file is empty (i.e. has the file size 0) or is symlinked to /dev/null, its configuration will not be loaded and it appears with a load state of "masked", and cannot be activated. Use this as an effective way to fully disable a unit, making it impossible to start it even manually.
The unit file format is covered by the Interface Stability Promise.
STRING ESCAPING FOR INCLUSION IN UNIT NAMES¶Sometimes it is useful to convert arbitrary strings into unit names. To facilitate this, a method of string escaping is used, in order to map strings containing arbitrary byte values (except NUL) into valid unit names and their restricted character set. A common special case are unit names that reflect paths to objects in the file system hierarchy. Example: a device unit dev-sda.device refers to a device with the device node /dev/sda in the file system.
The escaping algorithm operates as follows: given a string, any "/" character is replaced by "-", and all other characters which are not ASCII alphanumerics or "_" are replaced by C-style "\x2d" escapes. In addition, "." is replaced with such a C-style escape when it would appear as the first character in the escaped string.
When the input qualifies as absolute file system path, this algorithm is extended slightly: the path to the root directory "/" is encoded as single dash "-". In addition, any leading, trailing or duplicate "/" characters are removed from the string before transformation. Example: /foo//bar/baz/ becomes "foo-bar-baz".
This escaping is fully reversible, as long as it is known whether the escaped string was a path (the unescaping results are different for paths and non-path strings). The systemd-escape(1) command may be used to apply and reverse escaping on arbitrary strings. Use systemd-escape --path to escape path strings, and systemd-escape without --path otherwise.
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 $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.
Table 1. Load path when running in system mode (--system).
|/etc/systemd/system.control||Persistent and transient configuration created using the dbus API|
|/run/systemd/transient||Dynamic configuration for transient units|
|/run/systemd/generator.early||Generated units with high priority (see early-dir in systemd.generator(7))|
|/etc/systemd/system||System units created by the administrator|
|/run/systemd/generator||Generated units with medium priority (see normal-dir in systemd.generator(7))|
|/usr/local/lib/systemd/system||System units installed by the administrator|
|/lib/systemd/system||System units installed by the distribution package manager|
|/run/systemd/generator.late||Generated units with low priority (see late-dir in systemd.generator(7))|
Table 2. Load path when running in user mode (--user).
|$XDG_CONFIG_HOME/systemd/user.control or ~/.config/systemd/user.control||Persistent and transient configuration created using the dbus API ($XDG_CONFIG_HOME is used if set, ~/.config otherwise)|
|/run/systemd/transient||Dynamic configuration for transient units|
|/run/systemd/generator.early||Generated units with high priority (see early-dir in systemd.generator(7))|
|$XDG_CONFIG_HOME/systemd/user or $HOME/.config/systemd/user||User configuration ($XDG_CONFIG_HOME is used if set, ~/.config otherwise)|
|/etc/systemd/user||User units created by the administrator|
|$XDG_RUNTIME_DIR/systemd/user||Runtime units (only used when $XDG_RUNTIME_DIR is set)|
|$XDG_RUNTIME_DIR/systemd/generator||Generated units with medium priority (see normal-dir in systemd.generator(7))|
|$XDG_DATA_HOME/systemd/user or $HOME/.local/share/systemd/user||Units of packages that have been installed in the home directory ($XDG_DATA_HOME is used if set, ~/.local/share otherwise)|
|$dir/systemd/user for each $dir in $XDG_DATA_DIRS||Additional locations for installed user units, one for each entry in $XDG_DATA_DIRS|
|/usr/local/lib/systemd/user||User units installed by the administrator|
|/usr/lib/systemd/user||User units installed by the distribution package manager|
|$XDG_RUNTIME_DIR/systemd/generator.late||Generated units with low priority (see late-dir in systemd.generator(7))|
The set of load paths for the user manager instance may be
augmented or changed using various environment variables. And environment
variables may in turn be set using environment generators, see
systemd.environment-generator(7). In particular, $XDG_DATA_HOME and $XDG_DATA_DIRS may be easily set using systemd-environment-d-generator(8). Thus, directories listed here are just the defaults. To see the actual list that would be used based on compilation options and current environment use
systemd-analyze --user unit-paths
Moreover, additional units might be loaded into systemd ("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:
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:
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.
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.
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.
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.
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.
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 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.
Mount points marked with noauto are not mounted automatically through local-fs.target, but are still honored for the purposes of this option, i.e. they will be pulled in by this unit.
If none is set, no action will be triggered. reboot causes a reboot following the normal shutdown procedure (i.e. equivalent to systemctl reboot). reboot-force causes a forced reboot which will terminate all processes forcibly but should cause no dirty file systems on reboot (i.e. equivalent to systemctl reboot -f) and reboot-immediate causes immediate execution of the reboot(2) system call, which might result in data loss (i.e. equivalent to systemctl reboot -ff). Similarly, poweroff, poweroff-force, poweroff-immediate have the effect of powering down the system with similar semantics. exit causes the manager to exit following the normal shutdown procedure, and exit-force causes it terminate without shutting down services. When exit or exit-force is used by default the exit status of the main process of the unit (if this applies) is returned from the service manager. However, this may be overriden with FailureActionExitStatus=/SuccessActionExitStatus=, see below.
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.
ConditionArchitecture=, ConditionVirtualization=, ConditionHost=, ConditionKernelCommandLine=, ConditionKernelVersion=, ConditionSecurity=, ConditionCapability=, ConditionACPower=, ConditionNeedsUpdate=, ConditionFirstBoot=, ConditionPathExists=, ConditionPathExistsGlob=, ConditionPathIsDirectory=, ConditionPathIsSymbolicLink=, ConditionPathIsMountPoint=, ConditionPathIsReadWrite=, ConditionDirectoryNotEmpty=, ConditionFileNotEmpty=, ConditionFileIsExecutable=, ConditionUser=, ConditionGroup=, ConditionControlGroupController=, ConditionMemory=, ConditionCPUs=
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", "wsl", "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= 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".
ConditionMemory= verifies if 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 "<", "<=", "=", "!=", ">=", ">". 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= verifies if the specified number of CPUs is available to the current system. Takes a number of CPUs as argument, optionally prefixed with a comparison operator "<", "<=", "=", "!=", ">=", ">". Compares the number of CPUs in the CPU affinity mask configured of the service manager itself with the specified number, adhering to the specified comparision 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.
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=
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.
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|
|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|
|Following=||n/a||An automatic property|
WantedBy= and RequiredBy= are used in the [Install] section to create symlinks in .wants/ and .requires/ directories. They cannot be used directly as a unit configuration setting.
Note: ConsistsOf=, BoundBy=, RequisiteOf=, ConflictedBy= are created implicitly along with their 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.
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 firstname.lastname@example.org creating a email@example.com link to getty@.service.
This option may be used more than once, or a space-separated list of unit names may be given.
The following specifiers are interpreted in the Install section: %n, %N, %p, %i, %j, %g, %G, %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
|"%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 directory root||This is either /etc (for the system manager) or the path "$XDG_CONFIG_HOME" resolves to (for user managers).|
|"%f"||Unescaped filename||This is either the unescaped instance name (if applicable) with / prepended (if applicable), or the unescaped prefix name prepended with /. This implements unescaping according to the rules for escaping absolute file system paths discussed above.|
|"%h"||User home directory||This is the home directory of the user running the service manager instance. In case of the system manager this resolves to "/root". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%H"||Host name||The hostname of the running system at the point in time the unit configuration is loaded.|
|"%i"||Instance name||For instantiated units this is the string between the first "@" character and the type suffix. Empty for non-instantiated units.|
|"%I"||Unescaped instance name||Same as "%i", but with escaping undone.|
|"%j"||Final component of the prefix||This is the string between the last "-" and the end of the prefix name. If there is no "-", this is the same as "%p".|
|"%J"||Unescaped final component of the prefix||Same as "%j", but with escaping undone.|
|"%L"||Log directory root||This is either /var/log (for the system manager) or the path "$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 "$XDG_CONFIG_HOME" resolves to (for user managers).|
|"%t"||Runtime directory root||This is either /run (for the system manager) or the path "$XDG_RUNTIME_DIR" resolves to (for user managers).|
|"%T"||Directory for temporary files||This is either /tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to.|
|"%g"||User group||This is the name of the group running the service manager instance. In case of the system manager this resolves to "root".|
|"%G"||User GID||This is the numeric GID of the user running the service manager instance. In case of the system manager this resolves to "0".|
|"%u"||User name||This is the name of the user running the service manager instance. In case of the system manager this resolves to "root". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%U"||User UID||This is the numeric UID of the user running the service manager instance. In case of the system manager this resolves to "0". Note that this setting is not influenced by the User= setting configurable in the [Service] section of the service unit.|
|"%v"||Kernel release||Identical to uname -r output|
|"%V"||Directory for larger and persistent temporary files||This is either /var/tmp or the path "$TMPDIR", "$TEMP" or "$TMP" are set to.|
|"%%"||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.
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)
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