From: Junio C Hamano <gitster@pobox.com>
Date: Sat, 14 May 2016 18:40:47 GMT
Subject: Re: 'git diff-index' doesn't honor the 'diff.algorithm' variable
Message-ID: <xmqqshxk7aa8.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <9d15b6c8-ed97-7352-3df1-efab1b4ffadb@yandex.ru>

Dmitry Gutov <dgutov@yandex.ru> writes:

> Hi all,
>
> Subj. ...even though it's explicitly mentioned in the subcommand's man
> page. Git version 2.7.4 here.
>
> To elaborate:
>
> - Call 'git config --global diff.algorithm histogram'.

The variable belongs to UI config, meant for Porcelain "git diff",
together with things like "diff.color", "diff.context", etc.

As the point of lower-level plumbing commands in the diff family,
i.e. diff-files, diff-index and diff-tree, are about giving stable
output, which are _not_ affected by random end-user configuration,
for scripted use, it is very much deliberate design decision that
they ignore the UI config variables.

A script that calls diff-index, if it wants to honor end-users'
UI config variables, is allowed to use 'git config' to read them and
turn them into appropriate command line options.  e.g.

    algo=$(git config diff.algorithm)
    case "$algo" in
    minimal|histogram|patience) algo=--$algo ;;
    *) algo= ;;
    esac

    ...
    git diff-index $algo ... other args ...

or something like that.