I'd suggest cookie authentication if you'd care to rely on file permissions rather than something you know. That'll work transparently.
I don't think I understand what exactly you are suggesting. Could you provide an example?
The ControlPort supports none, password-based and cookie-based authentication, Damian was suggesting the cookie option:
https://www.torproject.org/docs/tor-manual.html.en#CookieAuthentication https://www.torproject.org/docs/tor-manual.html.en#ControlPort
you can also use the control socket as suggested below
foo@tornode $ sudo -u tor /usr/bin/arm
that is not recommended, Roger tells you why: https://lists.torproject.org/pipermail/tor-relays/2016-May/009259.html
using this way requires even less typing :)