It often happens that while working on one project, you need to use another project from within it. Perhaps it’s a library that a third party developed or that you’re developing separately and using in multiple parent projects. A common issue arises in these scenarios: you want to be able to treat the two projects as separate yet still be able to use one from within the other.

Here’s an example. Suppose you’re developing a website and creating Atom feeds. Instead of writing your own Atom-generating code, you decide to use a library. You’re likely to have to either include this code from a shared library like a CPAN install or Ruby gem, or copy the source code into your own project tree. The issue with including the library is that it’s difficult to customize the library in any way and often more difficult to deploy it, because you need to make sure every client has that library available. The issue with copying the code into your own project is that any custom changes you make are difficult to merge when upstream changes become available.

Git addresses this issue using submodules. Submodules allow you to keep a Git repository as a subdirectory of another Git repository. This lets you clone another repository into your project and keep your commits separate.

Let’s start by adding an existing Git repository as a submodule of the repository that we’re working on. To add a new submodule you use the git submodule add command with the absolute or relative URL of the project you would like to start tracking.

$ git submodule add https://github.com/chaconinc/DbConnector ./connector

By default, submodules will add the subproject into a directory named the same as the repository, in this case “DbConnector”. You can add a different path at the end of the command if you want it to go elsewhere.

If you run git status at this point, you’ll notice a few things.

$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	new file:   .gitmodules
	new file:   DbConnector