merge_repos.sh

#!/bin/bash

# This script takes a remote repository and merges it into
# the current one as a subdirectory

set -e

# REPOS=('design-system-components' 'design-system-tokens' 'design-system-figma-scripting')
# REPOS=('flight')
REPOS=('design-system-tokens')
 
for repo in "${REPOS[@]}"
do

    REPO_REMOTE="https://github.com/hashicorp/${repo}"
    REPO_DIR_TMP="$(mktemp -d -t "${repo##*/}.XXXX")"
    REPO_NAME=$repo

    echo "REPO REMOTE: $REPO_REMOTE"
    echo "REPO NAME: $REPO_NAME"
    echo "REPO TMP DIR: $REPO_DIR_TMP"
    echo

    # clone other repo
    git clone "$REPO_REMOTE" "$REPO_DIR_TMP"

    # rewrite the entire history into sub-directory
    export REPO_NAME
    (
        cd $REPO_DIR_TMP &&
        git filter-repo --to-subdirectory-filter "packages/"
    )

    # merge the rewritten repo
    git remote add "$REPO_NAME" "$REPO_DIR_TMP" -f
    git fetch "$REPO_NAME"
    # if you're running an older version of git, remove --allow-unrelated-histories
    git merge --allow-unrelated-histories "$REPO_NAME/main" -m "Merge ${REPO_NAME} into design system monorepo"

    # loop through all remote branches and create local copies
    for branch in $(git for-each-ref --format='%(refname:short)' refs/remotes/${REPO_NAME} --no-merged); do
    git checkout -b "${branch}" "${branch}"
    done

    # return to main
    git checkout main

    # delete the rewritten repo
    rm -rf "$REPO_DIR_TMP"
    git remote rm "$REPO_NAME"

done

git filter-repo isn't a git command out of the box. Install brew install git-filter-repo via Homebrew re: https://superuser.com/a/1646987. Another reference: https://github.com/newren/git-filter-repo