First of all, this question is not a duplicate of Change computer name of an Ubuntu desktop computer or Change Ubuntu Name.
I know to change hostname, I need to edit the file
/etc. No problem with that. The problem is how to change hostname if the system has already booted into the wrong hostname, without a reboot. I.e., to change the hostname on the fly.
This was used to be done with
/etc/init.d/hostname.sh, but now, under my Ubuntu 13.10 Saucy, that file is no longer there. How to change the hostname on the fly then?