I have a web application that explores other web applications in a particular way. It contains some web demos in a demos folder and one of the demo should now have it's own repository. I would like to create a separate repository for this demo application and make it a subpackage submodule from main repository without losing its commit history.
Is it possible to keep the commit history from the files in a repository's folder and create a repository from it and use it as a submodule instead?
See the note at the end of this answer (last paragraph) for a quick alternative to git submodules using npm ;)
In the following answer, you will know how to extract a folder from a repository and make a git repository from it and then including it as a submodule instead of a folder.
Inspired from Gerg Bayer's article Moving Files from one Git Repository to Another, Preserving History
At the beginning, we have something like this:
<git repository A> someFolders someFiles someLib <-- we want this to be a new repo and a git submodule! some files In the steps below, I will refer this someLib as <directory 1>.
At the end, we will have something like this:
<git repository A> someFolders someFiles @submodule --> <git repository B> <git repository B> someFolders someFiles Get a fresh copy of the repository to split.
git clone <git repository A url> cd <git repository A directory> The current folder will be the new repository, so remove the current remote.
git remote rm origin Extract history of the desired folder and commit it
git filter-branch --subdirectory-filter <directory 1> -- --all You should now have a git repository with the files from directory 1 in your repo's root with all related commit history.
Create your online repository and push your new repository!
git remote add origin <git repository B url> git push You may need to set the upstream branch for your first push
git push --set-upstream origin master <git repository A> (optional, see comments)We want to delete traces (files and commit history) of <git repository B> from <git repository A> so history for this folder is only there once.
This is based on Removing sensitive data from github.
Go to a new folder and
git clone <git repository A url> cd <git repository A directory> git filter-branch --force --index-filter 'git rm --cached --ignore-unmatch <directory 1> -r' --prune-empty --tag-name-filter cat -- --all Replace <directory 1> by the folder you want to remove. -r will do it recursively inside the specified directory :). Now push to origin/master with --force
git push origin master --force Create a submodule from <git repository B> into <git repository A>
git submodule add <git repository B url> git submodule update git commit Verify if everything worked as expected and push
git push origin master After doing all of this, I realized in my case that it was more appropriate to use npm to manage my own dependencies instead. We can specify git urls and versions, see the package.json git urls as dependencies.
If you do it this way, the repository you want to use as a requirement must be an npm module so it must contain a package.json file or you'll get this error: Error: ENOENT, open 'tmp.tgz-unpack/package.json'.
You may find it easier to use npm and manage dependencies with git urls:
npm init inside both repositoriesnpm install --save git://github.com/user/project.git#commit-ish where you want your dependencies installed
cd someLib before Step 2? You say "The current folder will be the new repository" but actually it will not; the new repository (submodule) is inside that folder.
refs/original/... which is created at step 3.
The solution by @GabLeRoux squashes the branches, and the related commits.
A simple way to clone and keep all those extra branches and commits:
1 - Make sure you have this git alias
git config --global alias.clone-branches '! git branch -a | sed -n "/\/HEAD /d; /\/master$/d; /remotes/p;" | xargs -L1 git checkout -t' 2 - Clone the remote, pull all branches, change the remote, filter your directory, push
git clone [email protected]:user/existing-repo.git new-repo cd new-repo git clone-branches git remote rm origin git remote add origin [email protected]:user/new-repo.git git remote -v git filter-branch --subdirectory-filter my_directory/ -- --all git push --all git push --tags 
GabLeRoux's solution works well except if you use git lfs and have large files under the directory you want to detach. In that case, after step 3 all the large files will remain to be pointer files instead of real files. I guess it's probably due to the .gitattributes file being removed in the filter branch process.
Realizing this, I found the following solution works for me:
cp .gitattributes .git/info/attributes Copying .gitattributes which git lfs uses to track large files to .git/ directory to avoid being deleted.
When filter-branch is done don't forget to put back the .gitattributes if you still want to use git lfs for the new repository:
mv .git/info/attributes .gitattributes git add .gitattributes git commit -m 'added back .gitattributes' filter-branch has been superceded by filter-repo.
The procedure for splitting a subfolder out using filter-repo is documented here:
https://docs.github.com/en/get-started/using-git/splitting-a-subfolder-out-into-a-new-repository
In step 3 of GabLeRoux's answer use:
git filter-repo --path FOLDER-NAME/ 
git help filter-branch, which @GabLeRoux's answer suggests using, specifically says that it "has a plethora of pitfalls that can produce non-obvious manglings of the intended history rewrite [...] Please use an alternative history filtering tool such as git filter-repo[1].". The help page then ends with a SAFETY section that explain 16 different ways that the (albeit general) use of git filter-branch can go wrong.