I set up a service on Raspbian (Jessie) using systemd to make it start after boot. The daemon config looks like this:
ExecStart=/bin/sh -c "exec /home/pi/sources/mydaemon.py >> /home/pi/mydaemon.log 2>&1"
journalctl -u mydaemon.service
Normally you just run a service directly (make sure it's executable has a shebang line):
And use the default redirection of
StandardOutput= to the systemd journal, so you can read the logs with
journalctl -u mydaemon.service.
Systemd nicely controls the file growth and file rotation of the log for you.
It is unrelated that your service runs as root.
If you don't see any log output with the above, also check the entire log for nearby logs that might be attributed to your service:
This is necessary because there's a known issue when some last lines of logging just before a script exists might not be attributed to the script. This would be especially noticeable for a script that just runs a fraction of a second before exiting.