Re: Question: Getting 'git diff' to generate /usr/bin/diff output

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jul 17, 2016, 07:46 UTC
Message-ID: <alpine.DEB.2.20.1607170942580.28832@virtualbox>
In-Reply-To: <578a9d5b.xL1egXSTCv+mjhAT%perryh@pluto.rain.com>

Hi,

[Perry, my mailer could not find your MX record, so I am sorry I had to drop you from the explicit To:...]

On Sat, 16 Jul 2016, Perry Hutchison wrote:

Show 11 quoted lines

> norm@dad.org wrote:
> 
> > Would some kind soul be willing to tell me if there is a way to
> > do that, short of making a backup copy of the relevant file, and
> > then doing 'git checkout'. Maybe the '--ext-diff' argument to
> > 'git diff' will do that, but I can't figure out how to use it.
> > ...
> >     git version: 2.5.1.454.g1616360
> 
> 'git cat' would be simpler than making a backup and using 'git checkout',
> but for this use case there's a better way even than that.

$ git cat git: 'cat' is not a git command. See 'git --help'.

;-)

You probably meant `git cat-file blob <SHA-1>`.

Show 13 quoted lines

> Google is your friend when you can't figure something out from (or
> find the right part of) a program's own documentation.  A search for
> 
>     git "external diff program"
> 
> (including the quotes) found several references.
> 
> One possible complication:  It's likely that the command parameters
> which git passes to an external diff program are not exactly what
> /usr/bin/diff requires.  The solution to that sort of problem is to
> write a very short executable script that rearranges the parameters
> as needed, and specify that script (instead of /usr/bin/diff itself)
> as the external diff program.

To spell it out, you probably referred to the --ext-diff option: https://github.com/git/git/blob/v2.9.2/Documentation/diff-options.txt#L528-L531

This option can be used with any diff-producing Git command, but it requires a .gitattribute setting together with the corresponding diff.external setting in the config.

A better method might be to use the `git difftool` command, e.g.

	git difftool -x diff

You can also inspect the diff of a commit, using the ^! suffix, e.g.

	git difftool -x diff origin/master~3^!

If you want to avoid the prompt and get the complete diff for all files in one go, you will have to write a small script as indicated, e.g.

	printf '#!/bin/sh\n\n/usr/bin/diff -r "$@"\n' >my-diff.sh
	chmod a+x my-diff.sh
	git difftool -d -x $PWD/my-diff.sh HEAD^!

I did write all of this in the mail program, so I highly recommend reading the man page of the difftool e.g. to understand the meaning of the -d option, so that you can fix any problems with my examples yourself.

Ciao, Johannes