How to turn off alternative Enter with Ctrl+M in Linux

Question

Why is Ctrl+M bound to Enter in Ubuntu Jaunty? How to turn it off?

I'm using Emacs and would like to bind Ctrl+M to some other command.

Answer 1

I think your question is backwards. It is not C-m that is bound to Enter, it is Enter that is bound to C-m. And C-m is the same as RET.

If you run C-h k C-m, you will see something like "RET runs the command ...". C-m sends RET because it is a control code, see http://en.wikipedia.org/wiki/Control_character.

The Enter key is bound to C-m; if you run C-h k Enter, you will see something like "RET (translated from <return>) runs the command ...". See, Enter is being interpreted by emacs as <return> and then that key is getting translated to C-m.

What you want to do is first remove the translation from <return> to RET by binding it directly to what it's currently indirectly bound, e.g. (global-set-key (kbd "<return>") 'newline). Then you're free to bind C-m to whatever you want without affecting Enter.

This assumes you're using the graphical emacs. If you're running it in a terminal, this won't work, because Enter will send C-m, not <return>. You can check that using the window-system variable though.

Answer 2

Note: The issue isn't limited to Linux, it exists on Windows (and presumably Mac) as well. Read the other (non stack-overflow) source of all knowledge: Wikipedia on Carriage Return.

If you want to rebind C-m, be sure to all bind <return> otherwise you run the risk of no longer being able to use the Enter/Return key. Also, in a terminal, Emacs cannot distinguish between the two (C-m and <return>).

In a plain Emacs, the Enter/Return key is bound to <return>, which is (by default) translated to RET (same thing as C-m). If you only rebound the C-m, you'd also be affecting the Enter/Return key.

Try C-h k <return> and you'll see

RET (translated from <return>)

So, rebind both in the appropriate keymap to make sure you get the behavior you want.

It might be instructive to play with the following code:

(defun my-return ()
  (interactive)
  (message "return"))
(defun my-ret ()
  (interactive)
  (message "RET"))
(defun my-c-m ()
  (interactive)
  (message "C-m"))
(global-set-key (kbd "<return>") 'my-return)
(global-set-key (kbd "C-m") 'my-c-m)
(global-set-key (kbd "RET") 'my-ret)

Put that in your *scratch* buffer and press C-j after each line (to evaluate the sexp). Then play with the Enter/Return keys and C-m.