Repo is a repository management tool that we built on top of Git. Repo unifies the many Git repositories when necessary, does the uploads to our revision control system, and automates parts of the Android development workflow. Repo is not meant to replace Git, only to make it easier to work with Git in the context of Android. The repo command is an executable Python script that you can put anywhere in your path. In working with the Android source files, you will use Repo for across-network operations. For example, with a single Repo command you can download files from multiple repositories into your local working directory.
for How to use
https://source.android.com/source/developing.html
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