Back in my Motorola days, I adminned a small network of Unix boxes for the development group. One of them developed a hard drive fault, so we sent for a doctor, uh, service guy, and booked a drive replacement visit.
On the appointed day, I sent out an email saying the machine in question would be going offline at 5:30 pm. At 5 pm, I sent around a reminder.
The engineer showed up at 5:30, and we went into the machine room. He watched over my shoulder as I entered
# shutdown -g 0 -i 0 -yand said, "Wow, you're a bit hardcore, aren't you?".
Well, I had given the users plenty warning. Not my fault if anyone was still logged in.
 from memory, since I've not dealt with a SVR3 box in the best part of a decade. That's an immediate shutdown with a [g]race period of 0 seconds (normally there's a minimum sixty second delay between issuing the shutdown command and the actual shutdown), to [i]nit level 0 (power-off condition), answering [y]es to any questions (such as "are you sure you want to shut down the machine?")