alternatives?

Using the Debian alternatives system

update-alternatives --set editor /usr/bin/emacs21

If you wish to change your mind, and use vim in the future you merely need to run:

update-alternatives --set editor /usr/bin/vim

This will cause vim to be executed whenever you run "editor".

For example you can see which "real" program will be executed for a given name:

skx@lappy:~$ update-alternatives --list x-www-browser
/usr/bin/mozilla-firefox

This shows us that executing the command "x-www-browser" will end up executing mozilla-firefox, on your machine something completely different could be executed!