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From: John Paul Wallington <jpw@gnu.org>
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Subject: Re: How can I find changes before saving a file?
Date: Wed, 12 Feb 2003 15:35:19 +0000
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Ehud Karni wrote:

> > > > If I have edited a file and want to see what changes I have
> > > > made, how can I do that?
> > > 
> > > You could compare the file with its backup.  But that only works
> > > after saving, of course.
> > 
> > You could temporarily rename the modified buffer, load the file
> > once more into emacs and start an ediff session.
[...]  
>  You could do M-x shell-command cat <file-name> and compare the
>  current buffer with "*Shell Command Output*".

You could use the command `diff-buffer-with-file' from Emacs CVS HEAD.
There is `ibuffer-diff-with-file' in the Ibuffer package too.