A function to edit author date and committer date of a git commit

git-editdate.sh

# Usage: git-editdate 10 minutes ago
# Usage: git-editdate 3 hours 2 minutes 1 second
# Usage: git-editdate author
#
# Extra Usage:
#   # Set command to edit since HEA0~10
#   env GIT_SEQUENCE_EDITOR='sed --in-place "s/^pick /edit /"' git rebase --interactive HEAD~10
#
#   # do-while
#   while
#     git-editdate author
#     git rebase --continue
#   do true; done
#
function git-editdate {
  local date="$*"

  local format='+%Y/%m/%d %H:%M:%S %z'

  local old_author_date="$(old_author_date:-"$(git show --format=%ai --no-patch HEAD)")"
  local old_author_email="$(old_author_email:-"$(git show --format=%ae --no-patch HEAD)")"
  local old_author_name="$(old_author_name:-"$(git show --format=%an --no-patch HEAD)")"
  local old_committer_date="$(old_committer_date:-"$(git show --format=%ci --no-patch HEAD)")"
  local old_committer_email="$(old_committer_email:-"$(git show --format=%ce --no-patch HEAD)")"
  local old_committer_name="$(old_committer_name:-"$(git show --format=%cn --no-patch HEAD)")"

  local new_author_date
  local new_author_email
  local new_author_name
  local new_committer_date
  local new_committer_email
  local new_committer_name

  if [[ $date = author ]]; then
    new_author_date="$old_author_date"
    new_author_email="$old_author_email"
    new_author_name="$old_author_name"
    new_committer_date="$old_author_date"
    new_committer_email="$old_author_email"
    new_committer_name="$old_author_name"
  elif [[ $date = committer ]]; then
    new_author_date="$old_committer_date"
    new_author_email="$old_committer_email"
    new_author_name="$old_committer_name"
    new_committer_date="$old_committer_date"
    new_committer_email="$old_committer_email"
    new_committer_name="$old_committer_name"
  else
    new_author_date="$(date --date="$old_author_date $date" "$format")"
    new_author_email="$old_author_email"
    new_author_name="$old_author_name"
    new_committer_date="$(date --date="$old_committer_date $date" "$format")"
    new_committer_email="$old_committer_email"
    new_committer_name="$old_committer_name"
  fi

  env \
    GIT_AUTHOR_DATE="$new_author_date" \
    GIT_AUTHOR_EMAIL="$new_author_email" \
    GIT_AUTHOR_NAME="$new_author_name" \
    GIT_COMMITTER_DATE="$new_committer_date" \
    GIT_COMMITTER_EMAIL="$new_committer_email" \
    GIT_COMMITTER_NAME="$new_committer_name" \
    git commit --allow-empty --allow-empty-message --amend --no-edit --reset-author
}