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Git is a command line version control tool that we use in development so that multiple people can work on one file at a time without stomping all over each other's changes. I would strongly recommend reading though the quick git intro I used in college before continuing down this page; I found it really helpful when I was first learning, and it'll help provide a lot of context for what comes next.
The Ubuntu command line tutorial could also be of interest if you're not familiar with using the command line interface.
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