tj.git-extras/bin/git-squash

#!/usr/bin/env bash

SQUASH_MSG=
for arg in "$@"; do
    case "$arg" in
        --squash-msg)
            SQUASH_MSG=1
            ;;
        *)
            # set the argument back
            set -- "$@" "$arg"
            ;;
    esac

    shift
done

src="$1"
msg="$2"
if [[ -n "$msg" ]] && [[ -n "$SQUASH_MSG" ]]; then
    >&2 echo "When commit message is given, --squash-msg is not allowed."
    exit 1
fi

is_branch() {
  git show-ref --verify --quiet "refs/heads/$src"
}

is_commit_reference() {
  git rev-parse --verify --quiet "$src" > /dev/null 2>&1
}

is_on_current_branch() {
  local commit_sha
  commit_sha=$(git rev-parse "$src")
  git rev-list HEAD |
    grep -q -- "$commit_sha"
}

commit_if_msg_provided() {
  if test -n "$msg"; then
    git commit -a -m "$msg"
  fi
}

prompt_continuation_if_squashing_master() {
  if [[ $src =~ ^master$ ]]; then
    read -p "Warning: squashing '$src'! Continue [y/N]? " -r
    if ! [[ $REPLY =~ ^[Yy]$ ]]; then
      echo "Exiting"
      exit 1
    fi
  fi
}

squash_branch() {
  prompt_continuation_if_squashing_master
  if [ -n "$SQUASH_MSG" ]; then
      base=$(git merge-base "$src" @)
      msg=$(git log "$base".."$src" --format="%s%n%n%b" --no-merges --reverse)
  fi
  git merge --squash "$src" || exit 1  # quits if `git merge` fails
  commit_if_msg_provided
}

squash_current_branch() {
  if [ -n "$SQUASH_MSG" ]; then
      msg=$(git log "$src"..@ --format="%s%n%n%b" --no-merges --reverse)
  fi
  git reset --soft "$src" || exit 1    # quits if `git reset` fails
  commit_if_msg_provided
}

if is_branch; then
  squash_branch
elif is_commit_reference && is_on_current_branch; then
  squash_current_branch
else
  echo "Source branch or commit reference required." 1>&2 && exit 1
fi