If you want service B to start when service A starts, you can use the Wants parameter to make it happen.

• In the [Unit] section of a systemd .service file, add a line like this:

[Unit]
Wants = b.service

You can also use Before = or After = to specify the order.

[Unit]
Wants = b.service
Before = b.service

Sometimes, you may want service B to wait for service A to complete before starting itself.

• Then you can add the following lines in the [Service] section of the b.service file.

[Service]
ExecStartPre=/bin/sleep 20

Remember to reload systemd for the changes to take effect.

sudo systemctl daemon-reload