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Git Submodule

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From here.

Submodules allow you to include or embed one or more repositories as a sub-folder inside another repository.

Adding a Submodule

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git submodule add https://github.com/<user>/rock rock

Under the hook, git write the submodule information into the “.gitmodules” file of the root repository.

Pull Submodule Content

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# 1. For an existing local working copy which does not have the submodule content.
git submodule update --init --recursive
# 2. Clone a working copy from a remote repository which has the submodule content.
git clone --recursive <project url>

Advice on using submodules (or not)

  • Before you add a repository as a submodule, first check to see if you have a better alternative available. Git submodules work well enough for simple cases, but these days there are often better tools available for managing dependencies than what Git submodules can offer. Modern languages like Go have friendly, Git-aware dependency management systems built-in from the start. Others, like Ruby’s rubygems, Node.js’ npm, or Cocoa’s CocoaPods and Carthage, have been added by the programming community. Even front-end developers have tools like Bower to manage libraries and frameworks for client-side JavaScript and CSS.

  • Remember that Git doesn’t download submodule contents by default. If you’re adding a submodule to an existing project, make sure anyone that works on the project knows they need to run commands like git submodule update and git clone –recursive to ensure they get everything—this includes any automated deployment or testing service that might be involved in the project! We recommend you use something like our “Scripts to Rule Them All” to ensure that all collaborators and services have access to the same repository content everywhere.

  • Submodules require you to carefully balance consistency and convenience. The setup used here strongly prefers consistency, at the cost of a little convenience. It’s generally best to have a project’s submodules locked to a specific SHA, so all collaborators receive the same content. But this setup also makes it difficult for developers in the “parent” repository to contribute changes back to the submodule repository.

  • Remember that collaborators won’t automatically see updates to submodules—if you update a submodule, you may need to remind your colleagues to run git submodule update or they will likely see odd behavior.

  • Managing dynamic, rapidly evolving or heavily co-dependent repositories with submodules can quickly become frustrating. This post was focused on simple, relatively static parent-child repository relationships. A future follow-up post will detail some strategies to help manage more complex submodule workflows.

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