merge_repos.sh

#!/bin/bash

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

set -e

if [ -z "$1" ]
then
    echo "Usage:"
    echo "    ./merge_repos.sh <repository> [name]"
    echo "        <repository> remote repository to merge"
    echo "        [name]       sub-directory name (optional)"
    exit
fi

REPO_REMOTE="$1"
REPO_NAME="$2"

# infer a name if one is not provided
if [ -z "$REPO_NAME" ]
then
    REPO_NAME="${REPO_REMOTE##*/}"
#    REPO_NAME="${REPO_NAME%.*}"
fi

#REPO_DIR_TMP="$(mktemp -d -t "${REPO_NAME}.XXXX")"
REPO_DIR_TMP="$(mktemp -d)"

echo "REPO REMOTE: $REPO_REMOTE"
echo "REPO NAME: $REPO_NAME"
echo "REPO TMP DIR: $REPO_DIR_TMP"
echo
# read -p "Press <Enter> to continue"

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

# rewrite the entire history into sub-directory
export REPO_NAME
(
    cd $REPO_DIR_TMP &&
    git filter-branch -f --prune-empty --tree-filter '
        mkdir -p "${REPO_NAME}_tmp"
        git ls-tree --name-only $GIT_COMMIT | xargs -I{} mv {} "${REPO_NAME}_tmp"
        mv "${REPO_NAME}_tmp" "$REPO_NAME"
    '
)

# merge the rewritten repo
git remote add "$REPO_NAME" "$REPO_DIR_TMP"
git fetch "$REPO_NAME"
# if you're running an older version of git, remove --allow-unrelated-histories
git merge --allow-unrelated-histories "$REPO_NAME/master" -m "Merging $REPO_REMOTE into $REPO_NAME"

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

Node script to execute this bash script.

const {execSync} = require('child_process');

let gitReposToMergeInOne = [
    'https://github.com/project/proj-1',
    'https://github.com/project/proj-2',
    // ...more similar repo URLs
]

/**
 * Merge all the repos in one
 * 
 * @param {Array} repos
 * @param {String} branch
 */
const mergeRepos = (repos, branch) => {
    for (let repo of repos) {
        const repoName = repo.split('/').pop()
        execSync(
            `sh merge_repos.sh ${repo} ${repoName} ${branch}`,
            (error) => {
                if (error) {
                    console.error(`exec error: ${error}`);
                    return;
                }
            }
        )
        console.log(`Merging ${repo}`)
    };
}

mergeRepos([...new Set(gitReposToMergeInOne)], 'main')