Question

I tried following in the scratch buffer:

(defun r-text-indent ()
  (interactive)
  (indent-line-to 4)) ;; 4 is just for example

(setq indent-line-function 'r-text-indent)

Evaluating those made scratch buffer indent everything 4 spaces. But all other buffers were unaffected.

How do I make this function to be used in all my plain-text buffers?

Answer 1

You can add a hook such that the variable is set for text-mode:

(add-hook 'text-mode-hook
      (lambda ()
         (setq indent-line-function 'r-text-indent)))

See also http://www.emacswiki.org/emacs/ModeHooks and How to change indentation in text-mode for emacs.