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Given a modified buffer, how can I diff it with the file backing it on disk to see what I've changed?

nosefrog
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2 Answers2

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You want the command M-x diff-buffer-with-file. See the manual:

diff-buffer-with-file is an interactive autoloaded compiled Lisp function in `diff.el'.

(diff-buffer-with-file &optional BUFFER)

View the differences between BUFFER and its associated file. This requires the external program diff to be in your exec-path.

You may also be interested in highlight-changes-mode which automatically and interactively highlights changes made to the buffer after it was enabled.

PythonNut
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  • Along the lines of `highlight-changes-mode` there's also the `diff-hl` package in GNU ELPA. – Stefan Jan 17 '15 at 04:00
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    Yes, but right now it only works to show diffs between the (saved) buffer and a `vc` revision. I worked on diffing the _buffer_ (as opposed to its backing file) in [diff-hl #33](https://github.com/dgutov/diff-hl/issues/33), but it's blocked on a bug in the Emacs core. – PythonNut Jan 17 '15 at 05:27
  • Do you have a bug-report number for that? – Stefan Jan 17 '15 at 14:01
  • @Stefan, I haven't gotten around to it yet. – PythonNut Jan 18 '15 at 05:23
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    Got fixed a (probably) long time ago: works in 2019! :) – mistige Jul 27 '19 at 10:26
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Command ediff-current-file:

ediff-current-file is an interactive autoloaded Lisp function in
`ediff.el'.

(ediff-current-file)

Start ediff between current buffer and its file on disk.
This command can be used instead of `revert-buffer'.  If there is
nothing to revert then this command fails.
Drew
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    `ediff-current-file` has some benefits over `diff-buffer-with-file` if you want interactive review of changes rather than a plain diff. The interactive review even allows to selectively revert some parts, in case you finally don't want to save them. See also http://emacs.stackexchange.com/a/3778/10614 for a more complete answer. – Stéphane Gourichon Dec 29 '15 at 11:20