How to shutdown a computer instantly (1 to 5 secs) without using a physical switch/socket?

Problem

Is there any command / program via which I may shutdown my system instantly (1 to 5 secs)? Update (2015): Thank you to those who have suggested to turn off using the power socket. :D Update (2016): @technophile suggested that I should add some information to the question describing nature of the pr…