Introducing Distributed Version Control, and Introducing Yourself to Git

Barbara Biologist and Candace Cosmologist have been hired by Universal Missions (a space services spinoff from Euphoric State University) to figure out where the company should send its next planetary lander. Since their coupled biocosmology simulation software is still being developed, they need to coordinate the changes between Barbara's biology code and Candace's new cosmology simulations. Barbara and Candace are developing more than just a report; their reproducible analysis will include data, code, figures, and text. New online writing collaboration tools such as writeLaTeX, Google Docs, and Microsoft SkyDrive can support their distributed authoring process, but so far they've relied on email and Dropbox to coordinate the rest of their development.

Although they want to be able to work on the report at the same time, they have run into problems doing this in the past. If they take turns, each one will spend a lot of time waiting for the other to finish, but if they work on their own copies and email changes back and forth, things will be lost, overwritten, or duplicated. Barbara and Candace also worry this process of communicating changes to code, data, figures, and text won't scale as they add more scientists to their team. A final concern, is that the text and graphics in their report, might fall out of sync with the code that generated them, and that they might accidentally publish a chart their code can't reproduce and that they can no longer explain.

Barbara and Candace decide to use distributed version control to manage their workflow. Distributed version control is better than mailing files back and forth or cloud-based syncing services because:

• Nothing that is committed to version control is ever lost. This means it can be used like the "undo" feature in an editor, and since all old versions of files are saved it's always possible to go back in time to see exactly who wrote what on a particular day, or what version of a program was used to generate a particular set of results.
• It keeps a record of who made what changes when, so that if people have questions later on, they know who to ask.
• It's hard (but not impossible) to accidentally overlook or overwrite someone's changes, because the version control system highlights them automatically.
• Authors can checkpoint their work, even when they're not connected to the Internet. This means that Barbara can experimentally try out new predator-prey models without worrying about changing the main simulation, and that Candace can checkpoint her work on the report at any time, even when her cable provider's Internet connection mysteriously fails in the middle of the day.

This lesson shows how to use a popular open source distributed version control system called Git. It is more complex than some alternatives, but it is widely used, and is further augmented by a free hosting sites such as BitBucket and GitHub. No matter which distributed version control system you use, the most important thing to learn is not the details of their more obscure commands, but the fundamentals of how they work, and the workflow that they encourage.

Introducing Yourself to Git

The first time you use Git on a new machine, you need to run commands to configure a few things.

First, you need to tell Git your preferred name and email address. We usually prefer a permanent email address so that our authorship is still asserted in the future.

$ git config --global user.name "Barbara Biologist"
$ git config --global user.email "barbara@biologist.net"

Why does Git need your name and email? It turns out that email addresses are an excellent unique identifier and communication mechanism for distributed teams. Every change you make in the version control repository is automatically labeled with your name and email address. This helps others to keep track of your work, when you choose to share it with them.

Next, you need to tell Git that you are using a modern terminal and prefer colored output. (UI stands for user interface).

$ git config --global color.ui "auto"

In newer versions of Git, this option has already been set for you.

Finally, Git wants to know how you like to edit files. For now, we'll use nano, but feel free to change this if you prefer a different editor.

$ git config –global core.editor "nano"

Git commands are written git verb, where verb is a command you want Git to do. Using the config command, you have told Git:

• your name and email address,
• to colorize output,
• what your favorite text editor is, and
• that you want to use these settings globally (i.e., for every project),

The four commands above only need to be run once: Git will remember the settings until you change them. Once Git is configured, you can start using Git.