git-auto-fixup

#!/usr/bin/env bash
#
# Automatically create a fixup commit.
#
# This looks at the diff from the index, and then uses git-blame to find a
# suitable commit to fix.
# This is very rudimentary, and will only look for removed lines.

fixup=
while IFS=$'\t' read -r info file; do
  diff_info=($info)
  if [ "${diff_info[4]}" != M ]; then
    echo "Skipping $file (not Modified).." >&2
    continue
  fi

  in_hunk=0
  while read -r line; do
    if [[ "$line" == "@@"* ]]; then
      from=$(echo "$line" | cut -d, -f1 | cut -d- -f2)
      count=$(echo "$line" | cut -d, -f2 | cut -d\  -f1)
      in_hunk=1

    elif [[ "$in_hunk" = 1 ]]; then
      if [[ "${line}" == -* ]]; then
        # typeset -p line
        removed+=("${line#-}")
      fi
    fi
  done < <(HOME=/dev/null git diff --cached "$file")

  # typeset -p removed
  if (( ${#removed[@]} )); then
    while IFS=$'\n' read -r blame; do
      commit=$(echo "$blame" | cut -f1 -d\ )
      line="$(echo "$blame" | cut -f3- -d\ )"

      for l in "${removed[@]}"; do
        if [[ "$line" == "$l" ]]; then
          if [[ -z "$fixup" ]]; then
            fixup="$commit"
          elif [[ "$fixup" != "$commit" ]]; then
            echo "Found multiple fixups: $fixup and $commit.  Aborting."
            exit 1
          fi
        fi
      done
    done < <(HOME=/dev/null git blame -s -L "$from,+$count" HEAD "$file")
  fi
done < <(HOME=/dev/null git diff-index --cached HEAD)

if [[ -z "${diff_info[@]}" ]]; then
  echo "Nothing in the index.  Add files first." >&2
  exit 1
fi

if [[ -z "$fixup" ]]; then
  echo "No fixup found." >&2
  exit 1
fi
git commit --fixup="$fixup"