Co-authored with Arnav Sood
An essential part of modern software engineering is using version control.
We use version control because
In this lecture, we’ll discuss how to use Git and GitHub.
Make sure you create an account on GitHub.com.
git and the GitHub Desktop application.
Optionally (but strongly recommended): On Windows, change the default line-ending by:
Opening a Windows/Powershell console, or the “Git Bash” installed in the previous step.
git config --global core.eol lf git config --global core.autocrlf false
To understand the relationship
Later, you may find yourself using alternatives
Since these lecture notes are intended to provide a minimal path to using the technologies, here we will conflate the workflow of these distinct products.
The fundamental object in GitHub is a repository (or “repo”) – this is the master directory for a project.
One example of a repo is the QuantEcon Expectations.jl package.
On the machine, a repo is a normal directory, along with a subdirectory called
.git which contains the history of changes.
GitHub stores history as a sequence of changes to text, called commits.
Here is an example of a commit, which revises the style guide in a QuantEcon repo.
In particular, commits have the following features
Note: It’s crucial to remember that what’s stored in a commit is only the actual changes you make to text.
This is a key reason why git can store long and complicated histories without consuming massive amounts of memory.
In addition, each GitHub repository typically comes with a few standard text files
.gitignorefile, which lists files/extensions/directories that GitHub shouldn’t try to track (e.g., LaTeX compilation byproducts).
README.mdfile, which is a Markdown file which GitHub puts on the repository website.
LICENSE.txtfile, which describes the terms under which the repository’s contents are made available.
For an example of all three, see the Expectations.jl repo.
Of these, the
README.md is the most important, as GitHub will display it as Markdown when accessing the repository online.
In this section, we’ll describe how to use GitHub to version your own projects.
Much of this will carry over to the collaborative section.
In general, we will always want to repos for new projects using the following dropdown
We can then configure repository options as such
In this case, we’re making a public repo
github.com/quantecon_user/example_repository, which will come with a
README.md, is licensed under the MIT License, and will ignore Julia compilation byproducts.
Note This workflow is for creating projects de novo; the process for turning existing directories into git repos is a bit more complicated.
In particular, in that case we recommend that you create a new repo via this method, then copy in and commit your files (see below), and then delete the old directory.
The next step is to get this to our local machine
This dropdown gives us a few options
git clone https://github.com/quanteconuser/example_repository.git.
Now that we have the repository, we can start working with it.
For example, let’s say that we’ve amended the
README.md (using our editor of choice), and also added a new file
economics.jl which we’re still working on.
Returning to GitHub Desktop, we should see something like
To select individual files for commit, we can use the check boxes to the left of each file.
Let’s say you select only the README to commit. Going to the history tab should show you our change
The Julia file is unchanged.
As of now, this commit lives only on our local machine.
To upload it to the server, you can simply click the “Push Origin” button at the top the screen.
The small “1^” to the right of the text indicates we have one commit to upload.
As mentioned, one of the key features of GitHub is the ability to scan through history.
By clicking the “commits” tab on the repo front page, we see this page (as an example).
Clicking an individual commit gives us the difference view, (e.g., example commit).
Sometimes, however, we want to not only inspect what happened before, but reverse the commit.
Generally, you want to work on the same project but across multiple machines (e.g., a home laptop and a lab workstation).
The key is to push changes from one machine, and then to pull changes from the other machine.
Pushing can be done as above.
To pull, simply click pull under the “repository” dropdown at the top of the screen
First, let’s add a collaborator to the
quanteconuser/example_repository lecture we created earlier.
We can do this by clicking “settings => collaborators,” as follows
GitHub’s website also comes with project management tools to coordinate work between people.
The main one is an issue, which we can create from the issues tab.
You should see something like this
Let’s unpack the different components
For an example of an issue, see here.
You can see open issues at a glance from the general issues tab
The checkboxes are common in GitHub to manage project tasks.
Any project management tool needs to figure out how to reconcile conflicting changes between people.
In GitHub, this event is called a “merge conflict,” and occurs whenever people make conflicting changes to the same line of code.
Note that this means that two people touching the same file is OK, so long as the differences are compatible.
A common use case is when we try to push changes to the server, but someone else has pushed conflicting changes.
GitHub will give us the following window
To fix the conflict, we can go into a text editor (such as Atom)
Let’s say we click the first “use me” (to indicate that my changes should win out), and then save the file.
Returning to GitHub Desktop gives us a pre-formed commit to accept
Clicking “commit to master” will let us push and pull from the server as normal.
One of the defining features of GitHub is that it is the dominant platform for open source code, which anyone can access and use.
However, while anyone can make a copy of the source code, not everyone has access to modify the particular version stored on GitHub.
A maintainer (i.e. someone with “write” access to directly modify a repository) might consider different contributions and “merge” the changes into the main repository if the changes meet their criteria.
A pull request (“PR”) allows any outsiders to suggest changes to open source repositories.
A PR requests the project maintainer to merge (“pull”) changes you’ve worked on into their repository.
There are a few different workflows for creating and handling PRs, which we’ll walk through below.
Note: If the changes are for a Julia Package, you will need to follow a different workflow – described in the testing lecture.
GitHub’s website provides an online editor for quick and dirty changes, such as fixing typos in documentation.
To use it, open a file in GitHub and click the small pencil to the upper right
Here, we’re trying to add the QuantEcon link to the Julia project’s
After making our changes, we can then describe and propose them for review by maintainers.
But what if we want to make more in-depth changes?
A common problem is when we don’t have write access (i.e. we can’t directly modify) the repo in question.
In that case, click the “Fork” button that lives in the top-right of every repo’s main page
This will copy the repo into your own GitHub account.
Clone this fork to our desktop and work with it in exactly the same way as we would a repo we own (as the fork is in your account, you now have write access).
That is, click the “clone” button on our fork
You’ll see a new repo with the same name but different URL in your GitHub Desktop repo list, along with a special icon to indicate that it’s a fork
Commit some changes by selecting the files and writing a commit message
And push by using the dropdown
Below, for example, we’ve committed and pushed some changes to the fork that we want to upstream into the main repo
We should make sure that these changes are on the server (which we can get to by going to the fork and clicking “commits”)
Next, go to the pull requests menu and click “New Pull Request”.
You’ll see something like this
This gives us a quick overview of the commits we want to merge in, as well as the overall differences.
Hit create and then click through the following form.
This opens a page like this on the main repo
The key pieces are
Here’s an example pull request.
To edit a PR, simply push changes to the fork you cloned to your desktop.
For example, let’s say we commit a new change to the README after we create the PR
After pushing to the server, the change is reflected on the PR page
That is, creating a pull request is not like bundling up your changes and delivering them, but rather like opening an ongoing connection between two repositories, that is only severed when the PR is closed or merged.
As you become more familiar with GitHub, and work on larger projects, you will find yourself making PRs even when it isn’t strictly required.
If you are a maintainer of the repo (e.g. you created it or are a collaborator) then you don’t need to create a fork, but will rather work with a git branch.
Branches in git represent parallel development streams (i.e., sequences of commits) that the PR is trying to merge.
First, load the repo in GitHub Desktop and use the branch dropdown
Click “New Branch” and choose an instructive name (make sure there are no spaces or special characters).
This will “check out” a new branch with the same history as the old one (but new commits will be added only to this branch).
We can see the active branch in the top dropdown
For example, let’s say we add some stuff to the Julia code file and commit it
To put this branch (with changes) on the server, we simply need to click “Publish Branch”.
Navigating to the repo page, we will see a suggestion about a new branch
At which point the process of creating a PR is identical to the previous case.
You may want to go beyond the scope of this tutorial when working with GitHub.
For example, perhaps you run into a bug, or you’re working with a setup that doesn’t have GitHub Desktop installed.
Here are some resources to help
Git also comes with a set of command-line tools.
They’re optional, but many people like using them.
Furthermore, in some environments (e.g. JupyterHub installations) you may only have access to the command line.
gitwill have installed a program called
git bash, which installs these tools along with a general Linux-style shell.
To open the terminal in a directory, either right click and hit “open git bash” (in Windows), or use Linux commands like
ls to navigate.
See here for a short introduction to the command line.
As above, you can clone by grabbing the repo URL (say, GitHub’s site-policy repo) and running
git clone https://github.com/github/site-policy.git.
This won’t be connected to your GitHub Desktop, so you’d need to use it manually (
File => Add Local Repository) or drag-and-drop from the file explorer onto the GitHub Desktop
From here, you can get the latest files on the server by
cd-ing into the directory and running
pull from the server, it will never overwrite your modified files, so it is impossible to lose local changes.
Instead, to do a hard reset of all files and overwrite any of your local changes, you can run
git reset --hard origin/master.
Follow the instructions to create a new repository for one of your GitHub accounts. In this repository
Pair-up with another student who has done Exercise 1a and find out their GitHub ID, and each do the following
Pair-wise with the results of Exercise 1b examine a merge-conflict by editing the
README.md file for your repository that you have both setup as collaborators.
Start by ensuring there are multiple lines in the file so that some changes may have conflicts, and some may not.
Just using GitHub’s web interface, submit a Pull Request for a simple change of documentation to a public repository.
The easiest may be to submit a PR for a typo in the source repository for these notes, i.e.
Note: The source for that repository is in
.rst files, but you should be able to find spelling mistakes/etc. without much effort.
Following the instructions for forking and cloning a public repository to your local desktop, submit a Pull Request to a public repository.
Again, you could submit it for a typo in the source repository for these notes, i.e.
https://github.com/QuantEcon/lecture-source-jl, but you are also encouraged to instead look for a small change that could help the documentation in another repository.
If you are ambitious, then go to the Exercise Solutions for one of the Exercises in these lecture notes and submit a PR for your own modified version (if you think it is an improvement!).