git-autosquash

#!/usr/bin/env bash

set -Eeuo pipefail

COMMIT_SHA="${1:-}"

printerr() { printf "\033[0;31m%s\033[0m\n" "$@" >&2; }

usage() {
  # shellcheck disable=SC2016
  printerr 'usage: git-autosquash $COMMIT_SHA
            COMMIT_SHA defaults to a "fixup!"~1 commit
            use '\''git commit --fixup $COMMIT_SHA'\'' before running'
}

if [ -z "${COMMIT_SHA}" ]; then
  usage
  exit 2
else
  if [ "$(git rev-list --abbrev-commit --max-parents=0 HEAD)" = "${COMMIT_SHA}" ]; then
    GIT_SEQUENCE_EDITOR=: git rebase -i --root --autosquash
  else
    GIT_SEQUENCE_EDITOR=: git rebase -i --autosquash "${COMMIT_SHA}~1"
  fi
fi