Git: Move Subdirectory to New Repository

I recently published my Brief emulation mode for Emacs to the MELPA Emacs package repository. This is code I wrote well over twenty years ago and have been using daily since.

I hadn't published it before partly because it was languishing in a sub-directory of my main Emacs repo (and partly because I never had the time before). To become a package in its own right though, it had to have its own repo.

Splitting out a subdirectory to a new repo used to be a complex task in Git (usually involving complicated invocations of git filter-branch), but these days it's reasonably straightforward with the git subtree command (well at least as straightforward as anything else in Git 😀).

The other reason I like the subtree method is it's non-destructive - you can back out at any point and still get back to where you started, plus all the history is retained.

Here's how it works:

Step 1: Create a new branch in the main repo with just the sub-directory commits

$ cd <main-dir>
$ git subtree split -P <sub-dir> -b <new branch>

Step 2: Create new repo in the sub-directory

$ cd <sub-dir>
$ git init

Step3: Pull branch into new repo

$  git pull <main-dir> <branch>

Step 4: Create new remote repo

• Using your upstream repo of choice.

Step 5: Push new repo to remote

$ git remote add origin <remote ref>
$ git push -u origin master

Step 6: Remove sub-directory from original repo

$ cd <main-dir>
$ git rm -r --cached <sub-dir>
$ git commit

(Optional) Step 7: Delete branch from original repo

$ git branch -d <branch>