Import a repo into a subfolder of another repo preserving commit history

import.sh

#!/bin/bash
set -eux
shopt -s extglob dotglob

DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
TEMP_DIR="${DIR}/import-repos-temp"

# e.g. git@github.com:user/world-domination-frontend.git
SUBPROJECT_REPO="$1"
# e.g. frontend
SUBPROJECT_NAME="$2"
# It must be a local dir that is an existing git project where we want to move the subproject to.
# It needs to have at least one commit and it should not have file that conflict with files in the subproject you want to merge.
DESTINATION_REPO_DIR="$(realpath $3)"

rm -rf "${TEMP_DIR}"
mkdir -p "${TEMP_DIR}"

# We create a local bare repo to host a branch that moves all the content to a subproject folder.
cd "${TEMP_DIR}"
git init --bare "${SUBPROJECT_NAME}.git"

git clone "${SUBPROJECT_REPO}" "${SUBPROJECT_NAME}"
cd "${SUBPROJECT_NAME}"
git remote add -f local file://${TEMP_DIR}/${SUBPROJECT_NAME}.git
TRUNK_BRANCH=$(git rev-parse --abbrev-ref HEAD)

mkdir "${SUBPROJECT_NAME}"
mv !([.]git|${SUBPROJECT_NAME}) "./${SUBPROJECT_NAME}"
git add .
git commit -m "Move everything to the ${SUBPROJECT_NAME} subfolder"
git push local


# Merge the changes we pushed to the local copy of the repo into the destination repo.
cd ${DESTINATION_REPO_DIR}
git remote remove ${SUBPROJECT_NAME} || true
git remote add -f ${SUBPROJECT_NAME} file://${TEMP_DIR}/${SUBPROJECT_NAME}.git
git merge ${SUBPROJECT_NAME}/${TRUNK_BRANCH} --allow-unrelated-histories -m "Merge project ${SUBPROJECT_REPO} into ./${SUBPROJECT_NAME}"

# Cleanup
git remote remove ${SUBPROJECT_NAME}
rm -rf ${TEMP_DIR}