systemd.unit

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/*
…
       /etc/systemd/user/*
       $XDG_RUNTIME_DIR/systemd/user/*
       /run/systemd/user/*
       ~/.config/systemd/user/*
       ~/.local/share/systemd/user/*
       /usr/lib/systemd/user/*
Unit files are loaded from paths defined during compilation.

aliases to unit files are created using 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. Unit files may specify Alias= 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 it will be invoked when CTRL+ALT+DEL is pressed.
Alias names through Alias= are only effective when the unit is enabled.
Aliases cannot be used with preset

Unit files may have corresponding directores with files symlinked as Wants=. This starts other units, without having to modify their unit files. Create symlinks in .wants/ with systemctl enable which reads information from the [Install] section. Requires= is similar.

"drop-in" directores, .d/, contain .conf files which are parsed after the file . Useful to alter or add configuration settings without modifng unit files. "drop-in" file must have section headers. 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.
[Install] sections are ignored.
drop-in directories for system services placed in /etc/systemd/system have precedene over those in /run/systemd/system which have precedene over those in /lib/systemd/system which take precedence over unit files wherever located.

Some unit names reflect paths existing in the file system. Example: a device unit dev-sda.device refers to /dev/sda.
"/" is replaced by "-", and non-alnum characters are replaced by \x2dd (for example \x2d for -)
The root directory (/) is encoded as single dash, otherwise initial and ending "/" are removed . systemd-escape generates properly escaped paths.

Units may be instantiated from a template file allowing of multiple units from a single configuration file.
First search is for the literal unit name.
If the unit name contains @, systemd will look for a unit template with the same name but with the instance string (i.e. the part between "@" and the suffix) removed. Example: if a service getty@tty3.service is requested and no file by that name is found, systemd will look for getty@.service

An empty unit file or one symlinked to /dev/null, will not be loaded, is masked, and cannot be activated. This fully disables a unit, and it cannot be started manually.

Automatic Dependencies

Dependency between units should be used sparingly and rather rely on techniques such as bus-based or socket-based activation which make dependencies implicit¿

A number of unit dependencies are automatically established, depending on unit configuration.
units with DefaultDependencies=yes (the default) additional dependencies are added depending on the unit type. If DefaultDependencies=yes units that are referenced by other units of type .target via a Wants= or Requires= dependency might automatically gain an Before= dependency.

Unit File Load Path

Unit files are loaded from a set of paths determined during compilation, Unit files found in directories listed earlier override files with the same name in directories lower in the list.

When $SYSTEMD_UNIT_PATH is set, the contents overrides the unit load path. If it ends with an empty component (":"), the usual unit load path will be appended to the contents of the variable. Load path when running in system mode (--system).

Path Description
/etc/systemd/system Local configuration
/run/systemd/system Runtime units
/lib/systemd/system Units of installed packages

Load path when running in user mode (--user).
Path Description
$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)
/etc/systemd/user Local configuration
$XDG_RUNTIME_DIR/systemd/user Runtime units (only used when $XDG_RUNTIME_DIR is set)
/run/systemd/user Runtime units
$XDG_DATA_HOME/systemd/user Units of packages installed in the home directory (only used when $XDG_DATA_HOME is set)
$HOME/.local/share/systemd/user Units of packages 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

Unit configuration files provide 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.

Options are configured in [Unit] or [Install] sections. service units may have a type-specific section.
Options not know cause warning log messages. X- options or section names are ignored

Boolean arguments can be written as 1, yes, true, on, 0, no, false and off.

Time can be written as a number suffixed with us, ms, s, min, h, d, w. Default is seconds.
Values are added .
Examples: 50: 50 seconds; 2min 200ms : 2 minutes and 200 milliseconds, i.e. 120200 ms.

Empty lines and lines starting with # or ; are ignored.
Lines ending in a backslash are continued.

To refer to the instance string from within the configuration file use %i.

The unit file may include a [Unit] section, which carries generic information about the unit that is not dependent on the type of unit

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.

Description= A free-form string describing the unit. for use in UIs to show descriptive information along with the unit name.
Documentation=uri [uri …] http://, https://, file:, info:, man:.
Requires=uuu [uuu uuu … ] If this unit gets activated, the units listed will be activated
If one of the listed units gets deactivated or its activation fails,
this unit will be deactivated!
The order in which services are started or stopped. is specified by After= or Before= .
Adding a symlink to a .requires/ accompanying the unit file also acomplishes this.
BindsTo=uuu [uuu uuu … ] When any of the units listed disappears
this unit is stopped. Units disappear if a service terminates on its own, a device is unplugged or a mount point unmounted without involvement of systemd.
Requisite=uuu [uuu uuu … ] If these units not started, they will not be started and the transaction will fail .
Wants=uuu [uuu uuu … ] uuus are started if this unit is.
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.
Dependencies of this type may also be configured by adding symlinks to a .wants/ directory accompanying the unit file.
PartOf=uuu [uuu uuu … ] When systemd stops or restarts the units listed here, the action is propagated to this unit.
This is a one-way dependency i.e. changes to this unit do not affect the listed units.
Conflicts=uuu [uuu uuu … ] negative requirement dependencies.
If a unit has a Conflicts= setting on another unit,
starting this unit will stop the specified unit and vice versa.
If a unit A that conflicts with a unit B is scheduled to be started at the same time as B, the transaction will either fail (in case both are required part of the transaction) or be modified to be fixed (in case one or both jobs are not a required part of the transaction). In the latter case, the job that is not the required will be removed, or in case both are not required, the unit that conflicts will be started and the unit that is conflicted is stopped.
Before=uuu [uuu uuu]
After=uuu [uuu uuu]
ordering dependencies between units.
Independent of and orthogonal to the requirement dependencies configured by Requires=.
It is a common pattern to include a unit name in both the After= and Requires= option, in which case the unit listed will be started before the unit that is configured with these options.

When units with an ordering dependency between them are shut down, the inverse of the start-up order is applied.

OnFailure=uuu [uuu uuu … ] uuu activated when this unit enters the "failed" state.
OnFailureJobMode= fail, replace, replace-irreversibly, flush, ignore-dependencies, ignore-requirements or isolate, . Defaults replace.
How the units listed in OnFailure= will be enqueued. See systemctl's --job-mode= for possible values.
fail, replace, replace-irreversibly,isolate, ignore-dependencies, ignore-requirements or flush.
If isolate", only a single unit may be listed
PropagatesReloadTo=uuu [uuu uuu]
ReloadPropagatedFrom=uuu [uuu uuuj … ]
where reload requests on this unit will be propagated to, or
reload requests on the other unit will be propagated to this unit,
. Issuing a reload request on a unit will automatically also enqueue a reload request on all units that the reload request shall be propagated to via these two settings.
JoinsNamespaceOf=uuu [uuu uuuj … ] For units that start processes (such as service units), units whose network and/or temporary file namespace to join. This only applies to unit types which support the PrivateNetwork= and PrivateTmp= directives (see systemd.exec(5) for details). If a unit that has this setting set is started, its processes will see the same /tmp, /var/tmp and network namespace as one listed unit that is started. If multiple listed units are already started, it is not defined which namespace is joined. Note that this setting only has an effect if PrivateNetwork= and/or PrivateTmp= is enabled for both the unit that joins the namespace and the unit whose namespace is joined.
RequiresMountsFor=fid [fid fidj … ] adds dependencies of type Requires= and After= for all mount units required to access the specified path.
Mount points marked with noauto are not mounted automatically 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).
IgnoreOnIsolate=true|false If true, this unit will not be stopped when isolating another unit. Defaults to false.
StopWhenUnneeded=true|false If true, this unit will be stopped when it is no longer used.
If this option is set, a unit will be automatically cleaned up if no other active unit requires it. Defaults to false.
systemd will not stop units by default unless they are conflicting with other units, or the user explicitly requested their shut down.
RefuseManualStart=true|false
RefuseManualStop=true|false
If true, this unit can only be activated or deactivated indirectly. In this case, explicit start-up or termination requested by the user is denied,
if it is started or stopped as a dependency of another unit, start-up or termination will succeed.
a safety feature to ensure that the user does not accidentally activate units that are not intended to be activated explicitly, and not accidentally deactivate units that are not intended to be deactivated. These options default to false.
AllowIsolate=true|false If true, this unit may be used with the systemctl isolate command.
for target units that shall be used similar to runlevels in SysV init systems, as a precaution to avoid unusable system states. defaults to false.
DefaultDependencies=true|false If true, (the default), a few default dependencies will implicitly be created .
The actual dependencies created depend on the unit type.
For example, for service units, these dependencies ensure that the service is started only after basic system initialization is completed and is properly terminated on system shutdown.
See the respective man pages for details.
Only services involved with early boot or late shutdown should set this option to false. It is highly recommended to leave this option enabled for the majority of common units. If set to false, this option does not disable all implicit dependencies, just non-essential ones.
JobTimeoutSec=t
JobTimeoutAction=
JobTimeoutRebootArgument=
a time-out When a job for this unit is queued, If this time limit is reached, the job will be cancelled, the unit however will not change state or even enter the "failed" mode.
except for device units.
independent from any unit-specific timeout (for example, the timeout set with TimeoutStartSec= in service units) be pending for it.
unit-specific timeouts are useful to abort unit state changes, and revert them.
The job timeout is useful to abort only the job waiting for the unit state to change.
JobTimeoutAction= configures an additional action to take when the time-out is hit. It takes the same values as the per-service StartLimitAction= , see systemd.service(5) for details. Defaults to none.
JobTimeoutRebootArgument= configures an optional reboot string to pass to reboot
Configure unit start rate limiting.
StartLimitIntervalSec=t
StartLimitBurst=n
Default: units which are started more than 5 times within 10 seconds are not permitted to start until the 10 second interval ends.

StartLimitIntervalSec the checking interval Default DefaultStartLimitIntervalSec in manager configuration file, 0 to disables of rate limiting (bad idea).
StartLimitBurst how many starts per interval are allowed (defaults to DefaultStartLimitBurst= in manager configuration file).
These particularly useful in conjunction with the service setting Restart= (see systemd.service(5)); however, they apply to all kinds of starts (including manual), not just those triggered by the Restart= logic.
Units which are configured for Restart= and which reach the start limit are not attempted to be restarted ; they may still be restarted manually later and the restart logic is again activated.

reset-failed will cause the restart rate counter for a service to be flushed, useful if manually starting a unit and the start limit interferes with that.
Rate-limiting is enforced after any unit condition checks are executed, and unit activations with failing conditions are not counted by this rate limiting.

Slice, target, device and scope units do not enforce this setting, as they are unit types whose activation may either never fail, or may succeed only a single time.

StartLimitAction=action Action to take if the rate limit is hit.
action one of:
none: default, hitting the rate limit will trigger no action besides that the start will not be permitted.
reboot: a reboot following the normal shutdown procedure (i.e. equivalent to systemctl reboot).
reboot-force: 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)
reboot-immediate: causes immediate execution of the reboot(2) system call, which might result in data loss. ,
poweroff, poweroff-force, poweroff-immediate have the effect of powering down the system with similar semantics.
RebootArgument= argument for the reboot(2) system call if StartLimitAction or a service's FailureAction is a reboot* action. like the argument to systemctl reboot
ConditionFileIsExecutable= Before starting a unit, verify that the specified condition is true.
If it is not true, the starting of the unit will be (mostly silently) skipped, all ordering dependencies of it are still respected. A failing condition will not result in the unit being moved into a failure state.
The condition is checked at the time the queued start job is to be executed.
Use condition expressions in order to silently skip units that do not apply to the local running system, for example because the kernel or runtime environment doesn't require its functionality.
Use the various AssertArchitecture=, AssertVirtualization=, … options for a similar mechanism that puts the unit in a failure state and logs about the failed check (see below).
ConditionArchitecture=[!]arch check whether the system is running on a specific architecture.
arch: 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).
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.
Negated by prepending an exclamation mark.
ConditionVirtualization=[!] check whether the system is executed in a virtualized environment and optionally test whether it is a specific implementation. Takes either boolean value to check if being executed in any virtualized environment, or one of vm and container to test against a generic type of virtualization solution, or one of qemu, kvm, 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.
Negated by prepending an exclamation mark.
ConditionHost=[!]hostname* 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)).
Negated by prepending an exclamation mark.
ConditionKernelCommandLine= 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=[!] whether the given security module is enabled on the system.
selinux, apparmor, ima, smack and audit.
Negated by prepending an exclamation mark.
ConditionCapability= [!]check whether the given capability exists in the capability bounding set of the service manager (i.e. this does not check whether capability is actually available in the permitted or effective sets, see capabilities(7) for details). Pass a capability name such as "CAP_MKNOD", possibly prefixed with an exclamation mark to negate the check.
ConditionACPower=true|false used to check whether the system has AC power, or is exclusively battery powered at the time of activation of the unit.
If 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.
if 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=[!]/var |/etc 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. Used to implement offline updates of the vendor operating system resources in /usr that require updating of /etc or /var on the next 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=true|false used to conditionalize units on whether the system is booting up with an unpopulated /etc directory. 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=
Similar to the ConditionArchitecture=, ConditionVirtualization=, ..., condition settings described above, these settings add assertion checks to the start-up of the unit. However, unlike the conditions settings, any assertion setting that is not met results in failure of the start job (which means this is logged loudly). Use assertion expressions for units that cannot operate when specific requirements are not met, and when this is something the administrator or user should look into.
SourcePath= A path to a configuration file this unit has been generated from. useful for implementation of generator tools that convert configuration from an external configuration file format into native unit files. This functionality should not be used in normal units. [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).
Alias= A space-separated list of additional names this unit shall be installed under. The names listed here must have the same suffix (i.e. type) as the unit file name. This option may be specified more than once, in which case all listed names are used. At installation time, systemctl enable will create symlinks from these names to the unit filename. Note that not all unit types support such alias names, and this setting is not supported for them. Specifically, mount, slice, swap, and automount units do not support aliasing.
WantedBy=uuu [uuu uuu]
RequiredBy=uuu [uuu uuu … ]
. A symbolic link is created in the .wants/ or .requires/ directory of each of the listed units when this unit is installed by systemctl enable. This has the effect that a dependency of type Wants= or Requires= is added from the listed unit to the current unit. The primary result is that the current unit will be started when the listed unit is started. See the description of Wants= and Requires= in the [Unit] section for details.

WantedBy=foo.service in a service bar.service is mostly equivalent to Alias=foo.service.wants/bar.service in the same file. In case of template units, systemctl enable must be called with an instance name, and this instance will be added to the .wants/ or .requires/ list of the listed unit. E.g. WantedBy=getty.target in a service getty@.service will result in systemctl enable getty@tty2.service creating a getty.target.wants/getty@tty2.service link to getty@.service.

Also= Additional units to install/deinstall when this unit is installed/deinstalled. If the user requests installation/deinstallation of a unit with this option configured, systemctl enable and systemctl disable will automatically install/uninstall units listed in this option as well. This option may be used more than once, or a space-separated list of unit names may be given.
DefaultInstance= In template unit files, this specifies for which instance the unit shall be enabled if the template is enabled without any explicitly set instance. This option has no effect in non-template unit files. The specified string must be usable as instance identifier.

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: 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 the string before the @ character of the unit name.
For non-instantiated units, 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: 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 either the unescaped instance name (if applicable) with / prepended (if applicable), or the unescaped prefix name prepended with /.
%t Runtime directory either /run (for the system manager) or the path $XDG_RUNTIME_DIR resolves to (for user managers).
%u User name name of the user running the service manager instance. In case of the system manager this resolves to root.
%U User UID numeric UID of the user running the service manager instance. In case of the system manager this resolves to 0.
%h User home directory home directory of the user running the service manager instance. In case of the system manager this resolves to /root.
%s User shell shell of the user running the service manager instance. In case of the system manager this resolves to /bin/sh.
%m Machine ID machine ID of the running system, formatted as string. See machine-id(5) for more information.
%b Boot ID boot ID of the running system, formatted as string. See random(4) for more information.
%H Host name hostname of the running system at the point in time the unit configuration is loaded.
%v Kernel release uname -r
%% Single percent sign Use %% 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. To remove dependencies, override the entire 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).

The unit file format is covered by the Interface Stability Promise[2].

SEE ALSO systemd(1), systemctl(1), systemd.special(7), systemd.service(5), systemd.socket(5), systemd.device(5), systemd.mount(5), systemd.automount(5), systemd.swap(5), systemd.target(5), systemd.path(5), systemd.timer(5), systemd.scope(5), systemd.slice(5), systemd.time(7), systemd-analyze(1), capabilities(7), systemd.directives(7), uname(1) NOTES The syntax is inspired by XDG Desktop Entry Specification[1] .desktop files, which are in turn inspired by Microsoft Windows .ini files. 1. XDG Desktop Entry Specification http://standards.freedesktop.org/desktop-entry-spec/latest/ 2. Interface Stability Promise http://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise systemd 232 SYSTEMD.UNIT(5) Manual page systemd.unit(5) line 662/696 (END) (press h for help or q to quit)