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(Updated in January, 2017.)

The Internet is full of articles on why you shouldn’t use Git submodules. I mostly agree, although I am not so harsh in my evaluation. As I explained in a previous post, submodules are useful for a few use cases but have several drawbacks.

Are there alternatives? The answer is: yes! There are (at least) two tools that can help track the history of software dependencies in your project while allowing you to keep using Git:

  • git subtree
  • Google repo

In this post I will be looking at git subtree and show why it is an improvement – albeit not perfect – over git submodule. As a working example, I run to my usual use case: “How do I easily store and keep up to date the vim plugins used in my dotfiles?”

What is git subtree, and why should I use it?

git subtree lets nest one repository inside another as a sub-directory. It is one of several ways Git projects can manage project dependencies.

Before and after git subtree diagram

There are several reasons git subtree is worth considering:

  • Management of a simple workflow is easy.
  • Older version of Git are supported (even older than v1.5.2).
  • The sub-project’s code is available right after the clone of the super project is done.
  • git subtree does not require users of your repository to learn anything new. They can ignore the fact that you are using git subtree to manage dependencies.
  • git subtree does not add new metadata files like git submodule does (i.e., .gitmodule).
  • Contents of the module can be modified without having a separate repository copy of the dependency somewhere else.

There are also a few drawbacks, but in my opinion they’re are acceptable:

  • You must learn about a new merge strategy (i.e. git subtree).
  • Contributing code back upstream for the sub-projects is slightly more complicated.
  • The responsibility of not mixing super and sub-project code in commits lies with you.

How to use git subtree

git subtree is available in stock version of Git since May 2012 – v1.7.11 and above. The version installed by homebrew on OSX already has subtree properly wired, but on some platforms you might need to follow the installation instructions.

Let me show you the canonical example of tracking a vim plug-in using git subtree.

The quick and dirty: way without remote tracking

If you just want a couple of one-liners to cut and paste, read this paragraph. First add git subtree at a specified prefix folder:

git subtree add --prefix .vim/bundle/tpope-vim-surround https://bitbucket.org/vim-plugins-mirror/vim-surround.git master --squash

(The common practice is to not store the entire history of the subproject in your main repository, but If you want to preserve it just omit the –squash flag.)

The above command produces this output:

git fetch https://bitbucket.org/vim-plugins-mirror/vim-surround.git master
warning: no common commits
remote: Counting objects: 338, done.
remote: Compressing objects: 100% (145/145), done.
remote: Total 338 (delta 101), reused 323 (delta 89)
Receiving objects: 100% (338/338), 71.46 KiB, done.
Resolving deltas: 100% (101/101), done.
From https://bitbucket.org/vim-plugins-mirror/vim-surround.git
* branch master -} FETCH_HEAD
Added dir '.vim/bundle/tpope-vim-surround'

As you can see this records a merge commit by squashing the whole history of the vim-surround repository into a single one:

1bda0bd [3 minutes ago] (HEAD, stree) Merge commit 'ca1f4da9f0b93346bba9a430c889a95f75dc0a83' as '.vim/bundle/tpope-vim-surround' [Nicola Paolucci]
ca1f4da [3 minutes ago] Squashed '.vim/bundle/tpope-vim-surround/' content from commit 02199ea [Nicola Paolucci]

If after a while you want to update the code of the plugin from the upstream repository you can just do a git subtree pull:

git subtree pull --prefix .vim/bundle/tpope-vim-surround https://bitbucket.org/vim-plugins-mirror/vim-surround.git master --squash

This is very quick and painless, but the commands are slightly lengthy and hard to remember. We can make the commands shorter by adding the sub-project as a remote.

Adding the sub-project as a remote

Adding the subtree as a remote allows us to refer to it in shorter form:

git remote add -f tpope-vim-surround https://bitbucket.org/vim-plugins-mirror/vim-surround.git

Now we can add the subtree (as before), but now we can refer to the remote in short form:

git subtree add --prefix .vim/bundle/tpope-vim-surround tpope-vim-surround master --squash

The command to update the sub-project at a later date becomes:

git fetch tpope-vim-surround master
git subtree pull --prefix .vim/bundle/tpope-vim-surround tpope-vim-surround master --squash

Contributing back upstream

We can freely commit our fixes to the sub-project in our local working directory now. When it’s time to contribute back to the upstream project, we need to fork the project and add it as another remote:

git remote add durdn-vim-surround ssh://git@bitbucket.org/durdn/vim-surround.git

Now we can use the subtree push command like the following:

git subtree push --prefix=.vim/bundle/tpope-vim-surround/ durdn-vim-surround master
git push using: durdn-vim-surround master
Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 308 bytes, done.
Total 3 (delta 2), reused 0 (delta 0)
To ssh://git@bitbucket.org/durdn/vim-surround.git
02199ea..dcacd4b dcacd4b21fe51c9b5824370b3b224c440b3470cb -} master

After this we’re ready and we can open a pull-request to the maintainer of the package.

Can I do this without using the git subtree command?

Yes! Yes you can. git subtree is different from the subtree merge strategy. You can still use the merge strategy even if for some reason git subtree is not available. Here is how you would go about it.

Add the dependency as a simple git remote:

git remote add -f tpope-vim-surround https://bitbucket.org/vim-plugins-mirror/vim-surround.git

Before reading the contents of the dependency into the repository, it’s important to record a merge so that we can track the entire tree history of the plug-in up to this point:

git merge -s ours --no-commit tpope-vim-surround/master

Which outputs:

Automatic merge went well; stopped before committing as requested

We then read the content of the latest tree-object into the plugin repository into our working directory ready to be committed:

git read-tree --prefix=.vim/bundle/tpope-vim-surround/ -u tpope-vim-surround/master

Now we can commit (and it will be a merge commit that will preserve the history of the tree we read):

git ci -m"[subtree] adding tpope-vim-surround"
[stree 779b094] [subtree] adding tpope-vim-surround

When we want to update the project we can now pull using the git subtree merge strategy:

git pull -s subtree tpope-vim-surround master

Git subtree is a great alternative

After having used git submoduls for a while I appreciate git subtree much more because it solves lots of the problems with git submodule. As usual, with all things Git, there is a learning curve to make the most of the feature.

Follow me on Twitter @durdn for more things and stuff about Git. And check out Atlassian Bitbucket if you’re looking for a good tool to manage your Git repos.

Update: After publishing this piece, I also wrote an article on the power of Git subtree.

Further reading

Other Git articles you might find interesting:

 


If you’re new to Git, or want to deep-dive on a few commands, head to our free Git resource hub for tutorials, workflows, and articles. Git it? 😉

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