git-diff-external

#!/usr/bin/env bash

firstprog()
{
    for prog in "$@"; do
        type "$prog" > /dev/null 2>&1 || continue
        echo "$prog"
        return 0
    done
    return 1
}

gnup()
{
    case `"$1" --version` in
        *GNU* ) return 0 ;;
    esac
    return 1
}

main()
{
    if [ $# -ne 7 ]; then
        echo "Usage: ${0##*/} [path] [a_file] [a_sha1] [a_mode] [b_file] [b_sha1] [b_mode]" 1>&2
        exit 1
    fi

    path=$1
    a_file=$2    a_sha1=$3    a_mode=$4
    b_file=$5    b_sha1=$6    b_mode=$7

    : ${EXTDIFF_PROGRAM:=`firstprog gdiff diff`}
    : ${EXTDIFF_OPTIONS:=${GIT_DIFF_OPTS:-'-u'}}

    gdiff_opts=()
    if gnup "$EXTDIFF_PROGRAM"; then
        gdiff_opts=(
            --label "a/$path"
            --label "b/$path"
        )
    fi

    ${EXTDIFF_PROGRAM} "${gdiff_opts[@]}" ${EXTDIFF_OPTIONS} "$a_file" "$b_file"
}

main "$@"

: # Git will complain if external diff returns nonzero

# eof