Visualise and copy differences between files

To compare the contents of two text files, use M-x ediff-files and open the two files you want to compare. Emacs will then open one above the other in your main emacs window (N.B. a window is called a frame in emacs terminology), and also open a smaller window which says Type ? for help.

Press | to put the two files side by side in your main emacs frame, and then stretch your window nice and wide so you can see both files side by side clearly. Now click back on the small emacs window with the help text.

In your main window, you should see blocks of text that differ between the two buffers highlighted. Press n to move to the next difference and p to move to the previous difference. Press a to copy the text from the left-hand file to the right-hand file, or press b to copy text from the right-hand file to the left-hand file.

Press q to exit ediff and then save your files if you have modified them.

To make the experience a little smoother I add the following to my emacs config file:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; ediff                                                                  ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(require 'ediff)
;; don't start another frame
;; this is done by default in preluse
(setq ediff-window-setup-function 'ediff-setup-windows-plain)
;; put windows side by side
(setq ediff-split-window-function (quote split-window-horizontally))
;;revert windows on exit - needs winner mode
(winner-mode)
(add-hook 'ediff-after-quit-hook-internal 'winner-undo)

Ediff can do more than this, but this is my main use case – see the manual for more.

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