/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/* ... /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/*
This man page lists the common configuration options of all the unit types. These options need to be configured in the [Unit] or [Install] sections of the unit files.
In addition to the generic [Unit] and [Install] sections described here, each unit may have a type-specific section, e.g. [Service] for a service unit. See the respective man pages for more information: systemd.service(5), systemd.socket(5), systemd.device(5), systemd.mount(5), systemd.automount(5), systemd.swap(5), systemd.target(5), systemd.path(5), systemd.timer(5), systemd.slice(5), systemd.scope(5).
Unit files are loaded from a set of paths determined during compilation, described in the next section.
Valid unit names consist of a "name prefix" and a dot and a suffix specifying the unit type. The "unit prefix" must consist of one or more valid characters (ASCII letters, digits, ":", "-", "_", ".", and "\"). The total length of the unit name including the suffix must not exceed 256 characters. The type suffix must be one of ".service", ".socket", ".device", ".mount", ".automount", ".swap", ".target", ".path", ".timer", ".slice", or ".scope".
Units names can be parameterized by a single argument called the "instance name". The unit is then constructed based on a "template file" which serves as the definition of multiple services or other units. A template unit must have a single "@" at the end of the 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 a symlink, so when systemd is asked through D-Bus to load dbus-org.freedesktop.network1.service, it'll load systemd-networkd.service. Alias names may be used in commands like disable, start, stop, status, and similar, and in all unit dependency directives, including Wants=, Requires=, Before=, After=. Aliases cannot be used with the preset command.
Unit files may specify aliases through the Alias= directive in the [Install] section. When the unit is enabled, symlinks will be created for those names, and removed when the unit is disabled. For example, reboot.target specifies Alias=ctrl-alt-del.target, so when enabled, the symlink /etc/systemd/systemd/ctrl-alt-del.service pointing to the reboot.target file will be created, and when Ctrl+Alt+Del is invoked, systemd will look for the ctrl-alt-del.service and execute reboot.service. systemd does not look at the [Install] section at all during normal operation, so any directives in that section only have an effect through the symlinks created during enablement.
Along with a unit file foo.service, the directory foo.service.wants/ may exist. All unit files symlinked from such a directory are implicitly added as dependencies of type Wants= to the unit. Similar functionality exists for Requires= type dependencies as well, the directory suffix is .requires/ in this case. This functionality is useful to hook units into the start-up of other units, without having to modify their unit files. For details about the semantics of Wants=, see below. The preferred way to create symlinks in the .wants/ or .requires/ directory of a unit file is by embedding the dependency in [Install] section of the target unit, and creating the symlink in the file system with the enable or preset commands of systemctl(1).
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 /usr/lib/systemd/system or /run/systemd/system directories. Drop-in files in /etc take precedence over those in /run which in turn take precedence over those in /usr/lib. Drop-in files under any of these directories take precedence over unit files wherever located. Multiple drop-in files with different names are applied in lexicographic order, regardless of which of the directories they reside in.
Units also support a top-level drop-in with type.d/, where type may be e.g. "service" or "socket", that allows altering or adding to the settings of all corresponding unit files on the system. The formatting and precedence of applying drop-in configurations follow what is defined above. Configurations in type.d/ have the lowest precedence compared to settings in the name specific override directories. So the contents of foo-.service.d/10-override.conf would override service.d/10-override.conf.
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.
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.
For example, service units with Type=dbus automatically acquire dependencies of type Requires= and After= on dbus.socket. See systemd.service(5) for details.
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.
When the variable $SYSTEMD_UNIT_PATH is set, the contents of this variable overrides the unit load path. If $SYSTEMD_UNIT_PATH ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable.
Table 1. Load path when running in system mode (--system).
|/etc/systemd/system.control||Persistent and transient configuration created using the dbus API|
|/run/systemd/transient||Dynamic configuration for transient units|
|/run/systemd/generator.early||Generated units with high priority (see early-dir in systemd.generator(7))|
|/etc/systemd/system||System units created by the administrator|
|/run/systemd/generator||Generated units with medium priority (see normal-dir in systemd.generator(7))|
|/usr/local/lib/systemd/system||System units installed by the administrator|
|/usr/lib/systemd/system||System units installed by the distribution package manager|
|/run/systemd/generator.late||Generated units with low priority (see late-dir in systemd.generator(7))|
Table 2. Load path when running in user mode (--user).
|$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).
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.
Units listed in this option will be started if the configuring unit is. However, if the listed units fail to start or cannot be added to the transaction, this has no impact on the validity of the transaction as a whole, and this unit will still be started. This is the recommended way to hook start-up of one unit to the start-up of another unit.
Note that requirement dependencies do not influence the order in which services are started or stopped. This has to be configured independently with the After= or Before= options. If unit foo.service pulls in unit bar.service as configured with Wants= and no ordering is configured with After= or Before=, then both units will be started simultaneously and without any delay between them if foo.service is activated.
If this unit gets activated, the units listed will be activated as well. If one of the other units fails to activate, and an ordering dependency After= on the failing unit is set, this unit will not be started. Besides, with or without specifying After=, this unit will be stopped if one of the other units is explicitly stopped.
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).
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.
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.
Note that this setting does not imply an ordering dependency, similarly to the Wants= and Requires= dependencies described above. This means that to ensure that the conflicting unit is stopped before the other unit is started, an After= or Before= dependency must be declared. It doesn't matter which of the two ordering dependencies is used, because stop jobs are always ordered before start jobs, see the discussion in Before=/After= below.
If unit A that conflicts with unit B is scheduled to be started at the same time as B, the transaction will either fail (in case both are required parts of the transaction) or be modified to be fixed (in case one or both jobs are not a required part of the transaction). In the latter case, the job that is not required will be removed, or in case both are not required, the unit that conflicts will be started and the unit that is conflicted is stopped.
Those two setttings configure ordering dependencies between units. If unit foo.service contains the setting Before=bar.service and both units are being started, bar.service's start-up is delayed until foo.service has finished starting up. After= is the inverse of Before=, i.e. while Before= ensures that the configured unit is started before the listed unit begins starting up, After= ensures the opposite, that the listed unit is fully started up before the configured unit is started.
When two units with an ordering dependency between them are shut down, the inverse of the start-up order is applied. i.e. if a unit is configured with After= on another unit, the former is stopped before the latter if both are shut down. Given two units with any ordering dependency between them, if one unit is shut down and the other is started up, the shutdown is ordered before the start-up. It doesn't matter if the ordering dependency is After= or Before=, in this case. It also doesn't matter which of the two is shut down, as long as one is shut down and the other is started up; the shutdown is ordered before the start-up in all cases. If two units have no ordering dependencies between them, they are shut down or started up simultaneously, and no ordering takes place. It depends on the unit type when precisely a unit has finished starting up. Most importantly, for service units start-up is considered completed for the purpose of Before=/After= when all its configured start-up commands have been invoked and they either failed or reported start-up success.
Note that those settings are independent of and orthogonal to the requirement dependencies as configured by Requires=, Wants=, Requisite=, or BindsTo=. It is a common pattern to include a unit name in both the After= and Wants= options, in which case the unit listed will be started before the unit that is configured with these options.
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 overridden 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.
If multiple conditions are specified, the unit will be executed if all of them apply (i.e. a logical AND is applied). Condition checks can use a pipe symbol ("|") after the equals sign ("Condition...=|..."), which causes the condition 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. If any of these options is assigned the empty string, the list of conditions is reset completely, all previous condition settings (of any kind) will have no effect.
The AssertArchitecture=, AssertVirtualization=, ... options provide a similar mechanism that causes the job to fail (instead of being skipped). The failed check is logged. Units with failed conditions are considered to be in a clean state and will be garbage collected if they are not referenced. This means that when queried, the condition failure may or may not show up in the state of the unit.
Note that neither assertion nor condition expressions result in unit state changes. Also note that both are checked at the time the job is to be executed, i.e. long after depending jobs and it itself were queued. Thus, neither condition nor assertion expressions are suitable for conditionalizing unit dependencies.
The condition verb of systemd-analyze(1) can be used to test condition and assert expressions.
Except for ConditionPathIsSymbolicLink=, all path checks follow symlinks.
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.
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.
AssertArchitecture=, AssertVirtualization=, AssertHost=, AssertKernelCommandLine=, AssertKernelVersion=, AssertSecurity=, AssertCapability=, AssertACPower=, AssertNeedsUpdate=, AssertFirstBoot=, AssertPathExists=, AssertPathExistsGlob=, AssertPathIsDirectory=, AssertPathIsSymbolicLink=, AssertPathIsMountPoint=, AssertPathIsReadWrite=, AssertDirectoryNotEmpty=, AssertFileNotEmpty=, AssertFileIsExecutable=, AssertUser=, AssertGroup=, AssertControlGroupController=
Table 3. Forward and reverse unit properties
|"Forward" property||"Reverse" property||Where used|
|Requires=||RequiredBy=||[Unit] section||[Install] section|
|Wants=||WantedBy=||[Unit] section||[Install] section|
|PartOf=||ConsistsOf=||[Unit] section||an automatic property|
|BindsTo=||BoundBy=||[Unit] section||an automatic property|
|Requisite=||RequisiteOf=||[Unit] section||an automatic property|
|Triggers=||TriggeredBy=||Automatic properties, see notes below|
|Conflicts=||ConflictedBy=||[Unit] section||an automatic property|
|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.
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.
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.|
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.
- Interface Stability Promise