Merge history from one Git repository as the history of a subdirectory of another Git repository

merge-to-subdir

#!/bin/bash

# Usage: merge-to-subdir source-repo destination-repo subdir
# 
# Merges the history of source-repo into destination-repo as the
# history of the subdirectory subdir.
# 
# source-repo can be local or remote.
# destination-repo must be local to the machine.
# subdir can be a relative path, in which case intermediate
# directories are created as necessary.
# 
# WARNING: I've only tested it when subdir has not existed within destination-repo.

set -e

function abspath() {
  (cd $(dirname $1) && echo $PWD/$(basename $1))
}


from=${1:?from repo not given}
to=$(abspath ${2:?to repo not given})
subdir=${3:?subdir not given}

scratch=$(abspath $(basename $from .git)-scratch)

# Use the name of the subdirectory as the name of the remote when merging
remote=tmp-$subdir


# Create a copy of $from with contents moved to $subdir

rm -rf $scratch

git clone $from $scratch
cd $scratch

git filter-branch -f --prune-empty --tree-filter "
    mkdir -p .tmp
    mv * .tmp
    if [ -f .gitignore ]; then mv .gitignore .tmp; fi
    mkdir -p $(dirname $subdir)
    mv .tmp $subdir
" -- --all

git gc --aggressive

cd ..


# Merge the copy of $from into $to

cd $to
git remote add $remote $scratch
git fetch $remote
git merge $remote/master
git remote rm $remote

git gc --aggressive

cd ..

# Clean up

rm -rf $scratch

Nice, man! It would be even neater if you put some comments on how to use it, though :-)

Usage: merge-to-subdir source-repo destination-repo subdir

Merges the history of source-repo into destination-repo as the history of the subdirectory subdir.

source-repo can be local or remote.
destination-repo must be local to the machine.
subdir can be a relative path, in which case intermediate directories are created as necessary.

I've only tested it when subdir has not existed within destination-repo.

I cant edit or raise a PR. can you add:
# assumes trunk is named master. change to main as required
and change the merge line to ->
git merge $remote/main --allow-unrelated-histories