How to split a repository's subdirectories into their own repo

split_repo.md

Okay, so you made a mistake. You thought you should bundle a bunch of your code into a single repository because these disparate apps shared a few libraries and now you're stuck with a swollen monster repo that is a nightmare to deploy from.

(At least... I made this mistake.)

Anyway. You want to split those into multiple repositories and, in the process, regain a shred of your sanity. Let's say you have this repository structure:

monster_repo
│   
└───project1
│   │   file011.txt
|   |
│   └───subfolder1
│       │   file111.txt
│       │   ...
|
└───project2
|   │   file021.txt
|   │   ...
|
└───shared_lib
|   │   ...
│   
└───test
│   │
│   └───project1.tests
|   |    |   ...
|   |
│   └───project2.tests
|       |   ...
|

and you want to separate the two projects. Here's what you're going to do:

1. Clone the repository into whatever you want your subdirectory repo to be called:

    git clone https://github.com/you/monster-repo ./project1-repo
   

2. Forget the old repo's history, you're free now:

    git remote remove origin
    git tag | xargs git tag -d
    git gc --prune=now
   

3. Deploy this ridiculous git incantation that I can't say I fully understand:

    git filter-branch --index-filter \
    'git rm --cached -qr --ignore-unmatch -- . && git reset -q $GIT_COMMIT -- project1 test/project1.tests shared_lib' \
    --prune-empty -- --all
   

4. Profit (by which I mean deal with all the failing tests now that the directory structure is changed)