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/* /run/systemd/system/* /lib/systemd/system/* ...
~/.config/systemd/user/* /etc/systemd/user/* $XDG_RUNTIME_DIR/systemd/user/* /run/systemd/user/* ~/.local/share/systemd/user/* /usr/lib/systemd/user/* ...
DESCRIPTION¶A unit configuration file 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. The syntax is inspired by XDG Desktop Entry Specification .desktop files, which are in turn inspired by Microsoft Windows .ini files.
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).
Various settings are allowed to be specified more than once, in which case the interpretation depends on the setting. Often, multiple settings form a list, and setting to an empty value "resets", which means that previous assignments are ignored. When this is allowed, it is mentioned in the description of the setting. Note that using multiple assignments to the same value makes the unit file incompatible with parsers for the XDG .desktop file format.
Unit files are loaded from a set of paths determined during compilation, described in the next section.
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).
Empty lines and lines starting with "#" or ";" are ignored. This may be used for commenting. Lines ending in a backslash are concatenated with the following line while reading and the backslash is replaced by a space character. This may be used to wrap long lines.
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 file itself is parsed. This is useful to alter or add configuration settings for a unit, without having to modify unit files. Each drop-in file must have appropriate section headers. Note that for instantiated units, this logic will first look for the instance ".d/" subdirectory and read its ".conf" files, followed by the template ".d/" subdirectory and the ".conf" files there. Also note that settings from the "[Install]" section are not honored in drop-in unit files, and have no effect.
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.
Some unit names reflect paths existing in the file system namespace. Example: a device unit dev-sda.device refers to a device with the device node /dev/sda in the file system namespace. If this applies, a special way to escape the path name is used, so that the result is usable as part of a filename. Basically, given a path, "/" is replaced by "-", and all other characters which are not ASCII alphanumerics are replaced by C-style "\x2d" escapes (except that "_" is never replaced and "." is only replaced when it would be the first character in the escaped path). The root directory "/" is encoded as single dash, while otherwise the initial and ending "/" are removed from all paths during transformation. This escaping is reversible. Properly escaped paths can be generated using the systemd-escape(1) command.
Optionally, units may be instantiated from a template file at runtime. 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 firstname.lastname@example.org 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.
AUTOMATIC DEPENDENCIES¶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.
A number of unit dependencies are automatically established, depending on unit configuration. On top of that, for units with DefaultDependencies=yes (the default) a couple of additional dependencies are added. The precise effect of DefaultDependencies=yes depends on the unit type (see below).
If DefaultDependencies=yes is set, units that are referenced by other units of type .target via a Wants= or Requires= dependency might automatically gain an Before= dependency too. See systemd.target(5) for details.
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 $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.
Table 1. Load path when running in system mode (--system).
|/lib/systemd/system||Units of installed packages|
Table 2. Load path when running in user mode (--user).
|$XDG_CONFIG_HOME/systemd/user||User configuration (only used when $XDG_CONFIG_HOME is set)|
|$HOME/.config/systemd/user||User configuration (only used when $XDG_CONFIG_HOME is not set)|
|$XDG_RUNTIME_DIR/systemd/user||Runtime units (only used when $XDG_RUNTIME_DIR is set)|
|$XDG_DATA_HOME/systemd/user||Units of packages that have been installed in the home directory (only used when $XDG_DATA_HOME is set)|
|$HOME/.local/share/systemd/user||Units of packages that have been installed in the home directory (only used when $XDG_DATA_HOME is not set)|
|/usr/lib/systemd/user||Units of packages that have been installed system-wide|
Additional units might be loaded into systemd ("linked")
from directories not on the unit load path. See the
link command for systemctl(1). Also, some units are dynamically created via a systemd.generator(7).
[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 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.
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.
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.
Mount points marked with noauto are not mounted automatically and will be ignored for the purposes of this option. If such a mount should be a requirement for this unit, direct dependencies on the mount units may be added (Requires= and After= or some other combination).
JobTimeoutSec=, JobTimeoutAction=, JobTimeoutRebootArgument=
JobTimeoutAction= optionally configures an additional action to take when the time-out is hit. It takes the same values as the per-service StartLimitAction= setting, see systemd.service(5) for details. Defaults to none. JobTimeoutRebootArgument= configures an optional reboot string to pass to the reboot(2) system call.
ConditionArchitecture=, ConditionVirtualization=, ConditionHost=, ConditionKernelCommandLine=, ConditionSecurity=, ConditionCapability=, ConditionACPower=, ConditionNeedsUpdate=, ConditionFirstBoot=, ConditionPathExists=, ConditionPathExistsGlob=, ConditionPathIsDirectory=, ConditionPathIsSymbolicLink=, ConditionPathIsMountPoint=, ConditionPathIsReadWrite=, ConditionDirectoryNotEmpty=, ConditionFileNotEmpty=, ConditionFileIsExecutable=
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, m86k, tilegx, cris 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, 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.
ConditionSecurity= may be used to check whether the given security module is enabled on the system. Currently, the recognized values are selinux, apparmor, ima, smack and audit. 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. This may be used to populate /etc on the first boot after factory reset, or when a new system instances 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.
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=, AssertSecurity=, AssertCapability=, AssertACPower=, AssertNeedsUpdate=, AssertFirstBoot=, AssertPathExists=, AssertPathExistsGlob=, AssertPathIsDirectory=, AssertPathIsSymbolicLink=, AssertPathIsMountPoint=, AssertPathIsReadWrite=, AssertDirectoryNotEmpty=, AssertFileNotEmpty=, AssertFileIsExecutable=
[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. Note that settings in the "[Install]" section may not appear in .d/*.conf unit file drop-ins (see above).
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 email@example.com creating a firstname.lastname@example.org 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, %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. The following specifiers are understood:
Table 3. Specifiers available in unit files
|"%n"||Full unit name|
|"%N"||Unescaped full unit name||Same as "%n", but with escaping undone|
|"%p"||Prefix name||For instantiated units, this refers to the string before the "@" character of the unit name. For non-instantiated units, this refers to the name of the unit with the type suffix removed.|
|"%P"||Unescaped prefix name||Same as "%p", but with escaping undone|
|"%i"||Instance name||For instantiated units: this is the string between the "@" character and the suffix of the unit name.|
|"%I"||Unescaped instance name||Same as "%i", but with escaping undone|
|"%f"||Unescaped filename||This is either the unescaped instance name (if applicable) with / prepended (if applicable), or the unescaped prefix name prepended with /.|
|"%t"||Runtime directory||This is either /run (for the system manager) or the path "$XDG_RUNTIME_DIR" resolves to (for user managers).|
|"%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".|
|"%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".|
|"%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".|
|"%m"||Machine ID||The machine ID of the running system, formatted as string. See machine-id(5) for more information.|
|"%b"||Boot ID||The boot ID of the running system, formatted as string. See random(4) for more information.|
|"%H"||Host name||The hostname of the running system at the point in time the unit configuration is loaded.|
|"%v"||Kernel release||Identical to uname -r output|
|"%%"||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.
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.
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 ConditionPathExists= (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. See below for an example.
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.service(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 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)
- XDG Desktop Entry Specification
- Interface Stability Promise