Bash script to find all single-character commits in a git repository

one_char.sh

first_commit=$(git log --pretty=format:%H | tail -1)

git rev-list --all --no-merges |
while read commit; do
  if [ $commit == $first_commit ]; then break; fi;

  IFS_backup=$IFS
  IFS=$'\n'
  diff_lines=( $(git diff --numstat --minimal -U0 --word-diff=porcelain $commit^ $commit | grep -E '^(\+[^+]|-[^-]).*$') )
  IFS=$IFS_backup

  character_count_diff=$(expr ${#diff_lines[0]} - ${#diff_lines[1]})
  if [ ${#diff_lines[@]} -eq 2 ] && [ ${character_count_diff#-} -lt 2 ]; then
    for (( i=1; i<${#diff_lines[0]}; i++ )); do
      character=${diff_lines[0]:$i:1}
      if [ "$character" = "${diff_lines[1]:$i:1}" ]; then continue; fi;

      next_index=$(expr $i + 1)
      single_subtraction=$(expr "${diff_lines[0]:$i}" = "${diff_lines[1]:$next_index}")
      single_addition=$(expr "${diff_lines[1]:$i}" = "${diff_lines[0]:$next_index}")
      # $single_substitution needs 'x's for cases where (for example) the diff is '-300 +60'
      # and '00' and '000' would otherwise end up being compared as numbers and deemed equal
      single_substitution=$(expr "x${diff_lines[0]:$next_index}" = "x${diff_lines[1]:$next_index}")

      if [ $single_subtraction -eq 1 ] || [ $single_addition -eq 1 ] || [ $single_substitution -eq 1 ]; then
        echo "$(git log --pretty=oneline $commit^...$commit)"
        echo "$(git diff --minimal $commit^ $commit)"
        echo ''
        echo ''
      fi;
      break
    done
  fi;
done