git-merge-single-file

#!/bin/bash
# Merge one file at a time into HEAD.
# Copyright (c) 2020 D3 Engineering, LLC
# By Christopher White, <cwhite@d3engineering.com>
# SPDX-License-Identifier: CC-BY-SA-3.0

set -Eeuo pipefail

# === Command line ==========================================================

if [[ "$#" -lt 2 ]]; then
    echo "Usage: $0 [-t TOOL] [-g] COMMIT FILE..." 1>&2
    echo "(NOT YET IMPLEMENTED) -t is --tool, and -g is -g, of git-mergetool(1)." 1>&2
    exit 2
fi

isgui=
mergetool=
while getopts "gt:" opt; do
    case "$opt" in
    g)
        isgui='-g'
        ;;
    t)
        mergetool="$OPTARG"
        ;;
    *)
        echo "Unknown option $opt" 1>&2
        exit 2
    esac
done

# === Collect information ===================================================

into="$(git rev-parse HEAD)"
fromname="$1"
from="$(git rev-parse "$fromname")"
base="$(git merge-base "$into" "$from")"

shift   # remove COMMIT
IFS=$'\n' read -r -d '' -a files < <( git ls-files -- "$@" && printf '\0' )

# === Set up tools ==========================================================

# TODO implement this.  Source $gitcore/git-mergetool--lib and use
# those functions.
#gitcore="$(git --exec-path)"
#if [[ "$mergetool" ]]; then
#    # TODO
#fi

# Find an editor.  TODO use a mergetool if specified.
program=
for f in "$(git config core.editor || true)" "${VISUAL:-}" "${EDITOR:-}" 'vi' ; do
    if [[ "$f" ]]; then
        program="$f"
        break
    fi
done

# === Process files =========================================================

tmpdir="$(mktemp -d)"
trap ': set -x ; rm -rf "'"$tmpdir"'" || true' EXIT

for fn in "${files[@]}" ; do
    echo "$fn"

    mkdir -p "$(dirname "${tmpdir}/${fn}")"
    git show "$from":"$fn" > "${tmpdir}/${fn}.THEIRS"
    git show "$base":"$fn" > "${tmpdir}/${fn}.BASE"

    mergeresult=0
    git merge-file -L "ours (HEAD)" -L base -L "theirs ($fromname)" \
        "$fn" "${tmpdir}/${fn}.BASE" "${tmpdir}/${fn}.THEIRS" || \
        mergeresult="$?"

    if (( mergeresult == 0 )) ; then
        :   # nothing to do - clean merge
    elif (( mergeresult > 0 && mergeresult <= 127)); then
        echo "$mergeresult conflicts"
        "$program" "$fn"
    else
        echo 'Merge error' 1>&2
        exit 1
    fi
done