Creates a new monorepofrom multiple monolit repositories

monorepo_from_monolit.sh

#!/bin/sh
# Creates a new monorepo by fusing multiple repositories

# Child repositories that are going to be fused
CHILDREN="wp/theme/master plugins/gutenblocks/master wp/theme-core/master"

# Name of the created monorepo
MONOREPO="monorepo"

# Exit in case of any error
set -e

# Be verbose
set -x

# create the monorepo
mkdir $MONOREPO
cd $MONOREPO
git init

# Create a first commit. A first commit is needed in order to be able to merge into master afterwards
echo "*~" >.placeholder
git add .placeholder
git commit -m "Initial"
git rm .placeholder
git commit -m "Remove placeholder file"

# Add remotes for all children
for repo in $CHILDREN; do
	repo=$(echo $repo | cut --delimiter / --fields=1,2,3)
	remote=${repo##*/}
	git remote add "${remote}" "git@gitlab.co:user/${repo}.git"
done

# Fetch all child repositories
git fetch --all

# Checkout all the master branches of the child repositories
for repo in $CHILDREN; do
	branch=$(echo $repo|rev| cut -f 1 -d "/" | rev)
	remote=$(echo $repo|rev| cut -f 2 -d "/" | rev)
	git checkout -f -b "${remote}_master" "${remote}/${branch}"
	# Rewrite history to move all repo files into a subdirectory
	export SUBDIRECTORY="${remote}"

 	git filter-branch -f --index-filter '
        git ls-files -s | sed "s|\t|&${SUBDIRECTORY}/|" | GIT_INDEX_FILE=$GIT_INDEX_FILE.new git update-index --index-info && if [ -f "$GIT_INDEX_FILE.new" ]; then mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"; fi' --
done

# Switch back to our master branch
git checkout -f master

# Merge all the repositories in our master branch.
for repo in $CHILDREN; do
	remote=$(echo $repo | rev | cut -f 2 -d "/" | rev)
	git merge --no-commit --allow-unrelated-histories "${remote}_master"

	git commit -a -m "Merge ${remote} in subdir"
done

# remove all child repo branches and remotes
for repo in $CHILDREN; do
	remote=$(echo $repo | rev | cut -f 2 -d "/" | rev)
	git branch -D "${remote}_master"
	git remote remove "${remote}"
done

# prune all history and do an aggressive gc
git reflog expire --expire=now --all && git gc --prune=now --aggressive