How can I see the changes made to a modified buffer since the last save?

Question

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

Related/Duplicate: How to view diff when emacs suggests to recover this file? — Kaushal Modi, Jan 17 '15 at 17:11
There has been the same question at unix.stackexchange.com, but with less asnwers. — imz -- Ivan Zakharyaschev, Jan 24 '15 at 10:00

score 43 · Accepted Answer · answered Jan 17 '15 at 00:25

43

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.

answered Jan 17 '15 at 00:25

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
1

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, 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

score 24 · Answer 2 · answered Jan 17 '15 at 02:02

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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.

answered Jan 17 '15 at 02:02

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

score 0 · Answer 3 · edited Dec 12 '23 at 20:40

0

A simple way to achieve this is with the package line-reminder, which shows unsaved and saved changes in current session. You can enable it globally with

(global-line-reminder-mode t)

or on a per-buffer basis with

(line-reminder-mode t)

edited Dec 12 '23 at 20:40

Dan

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answered Dec 10 '23 at 12:59

Talespin

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How can I see the changes made to a modified buffer since the last save?

3 Answers3