From a branch off master, create a prettified commit with only the prettified files, so that the branch-off-master can be rebased upon it, thus making the prettify-step the first commit.

prettier-prerebase.sh

#!/usr/bin/env bash

# 1) determine which files have been modified from master in the current branch
# 2) create a new, temporary branch from master
# 3) run prettier on the modified files in master
# 4) commit these to the temporary branch
# 5) print some instructions on how to rebase the current branch onto the temporary branch.

set -eo pipefail

MAIN_BRANCH=master
CURRENT_BRANCH=$(git branch --show-current)
ROOT_DIR=$(git rev-parse --show-toplevel)

COMMIT_MESSAGE="run prettier on modified files"

ensure_clean_working_directory() {
  if [ -n "$(git status --porcelain)" ]
  then
     echo -e "*** Error: Git working directory is not clean.\n"
     git status
     exit 1
  fi
}

ensure_working_directory_has_changes() {
  if [ -z "$(git status --porcelain)" ]
  then
     echo -e "*** The changed files have already been prettified---exiting.\n"
     git status
     exit 0
  fi
}

cleanup() {
  git checkout "${CURRENT_BRANCH}"
  popd > /dev/null 2>&1 || exit 0
}

create_branch_name () {
  counter=1
  ORIG_REF="${CURRENT_BRANCH}_prettier_tmp_${counter}"
  NEW_REF=$ORIG_REF
  while git show-ref --quiet "refs/heads/$NEW_REF"
  do
    ((counter++))
    NEW_REF="${CURRENT_BRANCH}_prettier_tmp_${counter}"
  done
  echo "$NEW_REF"
  exit 0
}

checkout_prettier_branch() {
  git checkout -b "${TMP_BRANCH}" "${MAIN_BRANCH}"
}

find_modified_files() {
  git diff --name-only --relative "${MAIN_BRANCH}..${CURRENT_BRANCH}" --diff-filter=M | grep -e '\.ts$'
}

prettify_files () {
  modified_files=$(echo "$1" | xargs)
  # shellcheck disable=SC2086
  npx prettier --write $modified_files
}

ensure_clean_working_directory

trap cleanup EXIT

pushd "${ROOT_DIR}/Chimera" > /dev/null 2>&1

MODIFIED_FILES=$(find_modified_files)

TMP_BRANCH=$(create_branch_name)

echo "...creating temp branch ${TMP_BRANCH}"
checkout_prettier_branch "${TMP_BRANCH}"

echo "...prettifying files: "
echo "${MODIFIED_FILES}"
prettify_files "${MODIFIED_FILES}"

echo "committing changes on branch $(git branch --show-current)"
cd "${ROOT_DIR}"

git add -A .
git commit -m "${COMMIT_MESSAGE}"

echo "*********************************************************"
echo "*** To complete this process, run the following command:"
echo "***"
echo "***       git rebase ${TMP_BRANCH}"
echo "***"
echo "*********************************************************"