Linux Service Unit File Format

References

https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files

[Unit] Section

Description=

• just description

Documentation=

• ususlly a link to the official website

Requires=

• lists any units upon which this unit essentially depends
• the current unit starts when the required units are actived successfully
• required units are started in parallel by default

Wants=

• similar to Requires=, but less strict
• The systemd will attempt to start any units listed by Wants= when the current unit is actived. If wanted units are not found or failed to start, the current unit will continue to function.
• Wanted units are started in parallel unless modified by other directives.

BindsTo=

• similar to Requires=, but also causes the current unit to stop when the associated unit terminates.

Before=

• specifes units that will not be started unitl the current unit is marked as started
• does not imply a dependency relationship

After=

• specifies units that should be started before the current unit

Conflicts=

• lists units that cannot be run at the same time as the current unit

Condition...=

• can be used to provide a generic unit file that will only be run when on appropriate systems

Assert...=

• similar to Condition...
• a negative result causes a failure with this directive

[Service] Section

Type=

• simple: The main process of the service is specified in the start line.
• forking: The service forks a child process and the parent process exits almost immediately. This type tells the systemd the service is sitll running even though the parent process exited.
• onshot: The service will be short-lived and that systemd should wait for the process to exit before continuing on with other units. Used for one-off tasks.
• dbus: The unit will take a name on the D-Bus bus.
• notify: The service will issue a notification when it has finished staring up. The systemd process will wait for this to happen before proceeding to other units.
• idle: The serivce will not be run until all jobs are dispatched.

[Install] Section

WantedBy=

• specifies how a unit should be enabled
• When enabled, a directory will be created within /etc/systemd/system named after the specified unit with .wants appended to the end. A symbolic link to the current unit will be created, creating the dependency.
• For example, if the current unit has WantedBy=multi-user.target, a directory called multi-user.target.wants will be created within /etc/systemd/system and a symbolic link to the current unit will be placed within.
• Disabling the unit removes the link and removes the dependency relationship.

RequiredBy=

• similar to the WantedBy directive, but instead specifies a required dependency that will cause the activation to fail if not met
• When enabled, a unit with this directive will create a directory ending with .requires

Alias=

• allows units to be enabled under another name as well

Also=

• allows units to be enabled or disabled as a set

DefaultInstance=

• For template units (covered later) which can produce unit instances with unpredictable names, this can be used as a fallback value for the name if an appropriate name is not provided.