Split a SubFolder into a New Repository

So I recently encountered an issue where I needed to extract a number of subfolders out of a Git repository so I could move them into their own repository/project.

I am no git expert and easily and quite frequently end up in a git mess so I took to the Powershell Slack to ask the more knowledgable folk there to see if anyone had already done it. I figured it was possible as most things are possible with git but easily achievable is another thing

git filter-branch

Chris Gardner (@HalbaradKenafin) suggested using git filter-branch so off I went looking for some examples online whilst trying to avoid the official git documentation which is here for the masochists.

Chris linked me to a github article which shows how you can do this and coming from github themselves I was filled with confidence that it was easier than I initially anticipated.

I seemed to get strange results when using Powershell Integrated Console in Visual Studio Code so I tried in Powershell Core console with similar horrible results. Variations of nothing happening at all, to the folder ended up totally empty or errors like below.

Cannot create a new backup.
A previous backup already exists in refs/original/
Force overwriting the backup with -f

Realising this was an exercise in futility I took to Bing (I know) to find another solution.

git subtree

After a little while longer scouring for a solution I stumbled upon this blog post which shows you how to achieve this with git subtree.

So I set about trying to do this with my module.

Create a new clone of the repository

I am well aware of how easy it is to mess up with git so to test this I created a new clone of the remote repository in a scratch area on my laptop

$ cd c:\temp
$ git clone https://github.com/{username}/{repository}
$ cd {repository}

You can then run git subtree

$ git subtree split --prefix {folder}/{subfolder}

# It will enumerate the repository with a visible counter
184/533 (183)

# then it will return a SHA1 hash

Create a new branch at this location

$ git checkout -b {branchname} 85dfee8248b5b68b8fc2a45d3bcf55841ded6eaa

This branch is now available for you to work on. You can verify it worked by running ls and the directory should only contain the contents of the subfolder.

Git history should persist so running git log -1 should show you the last commit message.

$ git log -1
commit 85dfee8248b5b68b8fc2a45d3bcf55841ded6eaa (HEAD -> {branchname})
Author: Brett Miller <brett@millerb.co.uk>
Date:   Sun Feb 3 10:48:52 2019 +0000

    Updated Manifest Tags (#10)

    * Updated Readme with new functionality

Push this folder to another repository

List remote repositories

$ git remote -v
origin  https://github.com/brettmillerb/{repository}.git (fetch)
origin  https://github.com/brettmillerb/{repository}.git (push)

Add a new remote repository

$ git remote add neworigin https://github.com/brettmillerb/{newrepository}.git

List remote repositories

$ git remote -v
origin  https://github.com/brettmillerb/{repository}.git (fetch)
origin  https://github.com/brettmillerb/{repository}.git (push)
neworigin https://github.com/brettmillerb/{newrepository}.git (fetch)
neworigin https://github.com/brettmillerb/{newrepository}.git (push)

Push to the new remote repository from your branch to master

$ git push -u neworigin {branchname}:master

You can then go to your usual git location and clone the repository from master to ensure you have a clean local branch for further development on this repository.