Ubuntu and Debian use the alternatives system to maintain the symbolic links
for programs. For example, when you first run
visudo after a fresh
installation of Ubuntu, it will prompt a list of editors for you to choose
from. Unfortunately, it doesn't tell you how to change that setting again. The
mostly happend situation is: you set it to "nano" at first, and want to change it
to "vi" at a later time.
Here is the command to bring up that list again:
sudo update-alternatives --config editor
Some people suggest using command
EDITOR=vi visudo, or
visudo. They both works, but it's really annoying that you have to remember and
type so many letters everytime you run this command.