To add a subtree to a Git repository, first add the remote repository and get its history, then merge it into a subdirectory using the git merge and git read-tree commands. The steps are as follows: 1. Use the git remote add -f command to add a remote repository; 2. Run git merge --s recursive --no-commit to get branch content; 3. Use git read-tree --prefix= to specify the directory to merge the project as a subtree; 4. Submit changes to complete the addition; 5. When updating, first git fetch and repeat the merging and steps to submit the update. This method keeps the external project history complete and easy to maintain.

Adding a subtree to your Git repository is a useful way to include another project as a subdirectory inside your main repo, while still keeping its history intact. It's especially handy when you want to pull in third-party libraries or share code between projects without using submodules. Here's how to do it effectively.

Setting up the remote

Before adding a subtree, you need to have the remote repository set up. If you haven't already added it as a remote, now's the time to do so. This makes it easier to update later.

You can add a new remote like this:

 git remote add -f <remote-name> <repository-url>

For example:

 git remote add -f vendor https://github.com/example/external-project.git

The -f flag fetches the remote right away, which saves you a step. Once that's done, you're ready to actually merge it into your project.

Merging the subtree

Now comes the actual process of merging the external repo as a subtree. You'll use git merge with the --s recursive and --no-commit flags to avoid conflicts and keep things flexible.

Here's the basic command:

 git merge --s recursive --no-commit <remote-name>/<branch-name>

Then tell Git that this is a subtree:

 git read-tree --prefix=<subdir-name>/ -u <remote-name>/<branch-name>

So if you were adding the vendor remote's main branch into a folder called lib/external , it would look like:

 git merge --s recursive --no-commit vendor/main
git read-tree --prefix=lib/external -u vendor/main

Finally, commit the changes:

 git commit -m "Added external project as subtree"

This will bring the entire project into your repo under the specified directory.

Updating the subtree

One of the nice things about subtrees is that you can update them easily. When the external project gets new commits, just pull those changes into your subtree:

1. Fetch the latest changes from the remote:

    git fetch vendor

2. Merge the updates using the same subtree logic:

    git merge --s recursive --no-commit vendor/main
   git read-tree --prefix=lib/external -u vendor/main
   git commit -m "Updated external subtree"

   This keeps your embedded project current without having to manually copy files.

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   That's basically it. Once you've got the hang of it, working with subtrees becomes second nature — it's not complicated, but it's easy to overlook small steps like fetching first or using the correct prefix. Keep a quick reference handy until it clicks.

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