I was prompted to write this post after listening to Jonathan Nadeau talk about accessibility in Ubuntu. He mentioned not knowing how to shutdown a GNU/Linux system from the command line yet. As it happens the command to use is in fact shutdown. Specifically:
shutdown now "The system is going down now"
The command is also available in BSD (Including OSX), Solaris, and even Windows.
‘halt’ also does the trick for me. Cheers!