General question: How does one start a systemd .service after a particular .service has started successfully?
Specific question: How do I start website.service only after mongodb.service has started? In other words website.service should depend on mongodb.service.
In the .service file under the [Unit] section:
[Unit]
Description=My Website
After=syslog.target network.target mongodb.service
The important part is the mongodb.service
The manpage describes it however due to formatting it's not as clear on first sight
After= dependency is only effective when service including After= and service included by After= are both scheduled to start as part of your boot up.
Ex:
a.service
[Unit]
After=b.service
This way, if both a.service and b.service are enabled, then systemd will order b.service after a.service.
If I am not misunderstanding, what you are asking is how to start b.service when a.service starts even though b.service is not enabled.
The directive for this is Wants= or Requires= under [Unit].
website.service
[Unit]
Wants=mongodb.service
After=mongodb.service
The difference between Wants= and Requires= is that with Requires=, a failure to start b.service will cause the startup of a.service to fail, whereas with Wants=, a.service will start even if b.service fails. This is explained in detail on the man page of .unit.
来源：https://stackoverflow.com/questions/21830670/systemd-start-service-after-specific-service