create a OS managed service using systemd

systemd(all lowercase, not systemD :) ) is an init system in Linux distros to bootstrap the user space and manage all processes subsequently.
The alternatives are

1. UNIX System V OS/ BSD OS init systems,
2. upstart, 2006, an event-based replacement for the traditional init daemon upstart.
   The recent version of ubuntu uses systemd, as far as I remember previous versions of ubuntu use upstart.
3. runit, 2004
4. launchd in MacOS world.

PS. There's lot of hate against systemd, it being against Unix philosophy that goes as;
Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
Here's a thread which might be useful to dig.

Fun time

I'm creating a streaming service in this example, which will simply write an event to a file continuously. And, if I restart my machine the service will start emitting events to a file right away.
Also I can start or stop service using systemctl start/stop serviceName. I used to use nohup <command> & and to kill it I had to use get the PID and then kill it. I can get rid of all that crap with systemD service.

STEP 1 - create streaming.service at /etc/systemd/system/multi-user.target.wants/streaming.service, also at /etc/systemd/system/streaming.service.

[Unit]                                                                                                                                           
Description=Streaming pipeline                                                                      

[Service] 
Type=forking                                                                         
ExecStart=/usr/local/bin/streaming.sh
TimeoutSec=infinity
Restart=always

[Install]
WantedBy=multi-user.target

STEP 2 - create a bash script at /usr/local/bin/streaming.sh and give current user permission to it. Don't forget #!/bin/bash for the bash scripts.

#!/bin/bash
while true
 do
    echo "Streaming an event at `date`" >> /var/log/streaming.log
 done

chmod 777 /usr/local/bin/streaming.sh

STEP 3 - reload systemd daemon so that it loads streaming.service and enable the service

systemctl daemon-reload
systemctl enable streaming.service

STEP 4 - see the streaming job status

systemctl status streaming.service

I can use journalctl _PID=?? to debug if the service is not working. eg.

journalctl _PID=5871
-- Logs begin at Mon 2016-11-21 16:36:41 PST, end at Tue 2016-12-06 14:34:56 PST. --
Nov 26 04:17:52 y0319t10971 sshd[5871]: Connection closed by 10.16.132.191 [preauth]
Dec 06 14:33:15 y0319t10971 systemd[5871]: Failed at step EXEC spawning /usr/local/bin/streaming.sh: Exec format error

or also can use following command,

root@y0319t10971:~# journalctl -fu streaming.service
-- Logs begin at Mon 2016-11-21 16:36:41 PST. --
Dec 06 14:33:16 y0319t10971 systemd[1]: streaming.service: main process exited, code=exited, status=203/EXEC
Dec 06 14:33:16 y0319t10971 systemd[1]: Unit streaming.service entered failed state.
Dec 06 14:33:16 y0319t10971 systemd[1]: streaming.service failed.
Dec 06 14:34:28 y0319t10971 systemd[1]: Started Streaming pipeline.
Dec 06 14:34:28 y0319t10971 systemd[1]: Starting Streaming pipeline...
Dec 06 14:34:28 y0319t10971 systemd[1]: streaming.service: main process exited, code=exited, status=203/EXEC
Dec 06 14:34:28 y0319t10971 systemd[1]: Unit streaming.service entered failed state.
Dec 06 14:34:28 y0319t10971 systemd[1]: streaming.service failed.
Dec 06 14:44:58 y0319t10971 systemd[1]: Started Streaming pipeline.
Dec 06 14:44:58 y0319t10971 systemd[1]: Starting Streaming pipeline...

STEP 5 - see the events produced by the streaming.service

tail -f /var/log/streaming.log 
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016
Streaming an event at Fri Dec  2 11:27:21 PST 2016

PS. Dont forget to stop the service, otherwise your Storage will be full in few days :)

ll /var/log/streaming.log --block-size=GB
-rw-r--r-- 1 root root 3GB Dec  6 13:51 /var/log/streaming.log

Also, the service would be running right after I reboot the machine.