Overview

Teaching: 30 min
Exercises: 0 min

Questions

• How do I share my changes with others on the web?

Objectives

• Explain what remote repositories are and why they are useful.

• Push to or pull from a remote repository.

Version control really comes into its own when we begin to collaborate with other people. We already have most of the machinery we need to do this; the only thing missing is to copy changes from one repository to another.

Systems like Git allow us to move work between any two repositories. In practice, though, it’s easiest to use one copy as a central hub, and to keep it on the web rather than on someone’s laptop. Most programmers use hosting services like GitHub, BitBucket or GitLab to hold those master copies; we’ll explore the pros and cons of this in the final section of this lesson.

Let’s start by creating an account at GitHub. The University of Cambridge recently signed up to “GitHub Education”, which gives some additional benefits for staff and students, but we will not be using any of those features. Go to GitHub and create a login, if you don’t have one already. You will need to confirm your email address. Once you have an account, you should be able to create a new repository online. Click on the “+” sign in the top right corner.

This time, I’m calling the repository “picalc”, as it will include some code to calculate the value of “pi”. Let’s make it a “Private” repository for now. The other options can stay as default.

This effectively does the following on GitHub’s servers:

$ mkdir picalc
$ cd picalc
$ git init

Helpfully, GitHub tells you what to do to start a new repository. The first few lines cover what we have already done. You will notice some new commands git remote and git push which we will cover in this section.

Let’s follow some of their suggestions and create some files to put in a new repository. Go back to your home directory and make a new folder:

cd ~
mkdir picalc
cd picalc
git init
gedit pi.py

In the file, let’s put a simple code to calculate an approximate value for “pi” - NB this is not a serious method, it is very slow!

a = 2.0
for n in range(1, 10000):
    a = a * (n*n)/(n*n - 0.25)
print(a)

Try it by running python pi.py. It should print out 3.14151411083.

git add pi.py
git commit -m "Code to calculate pi"

Our local repository contains pi.py, but the remote repository on GitHub doesn’t contain any files yet.

The next step is to connect the two repositories. We do this by making the GitHub repository a remote for the local repository.

We will change the authentication from SSH to HTTPS, as it is easier to work with for now. Use a pattern like this (replacing vlad with your github username) - this should be the same command which GitHub suggests when creating the repository.

git remote add origin https://vlad@github.com/vlad/picalc.git

Make sure to use the URL for your repository rather than Vlad’s: the only difference should be your username instead of vlad.

HTTPS vs. SSH

We use HTTPS here because it does not require additional configuration. You may want to set up SSH access, which is a bit more secure, and does not require a password every time. Follow one of the great tutorials from github, Atlassian/BitBucket and GitLab (this one has a screencast).

If you are feeling confident, it can be done with the following command: ssh-keygen. You then need to upload the file ~/.ssh/id_rsa.pub into your “SSH and GPG settings” on GitHub.

We can check that the command has worked by running git remote -v:

$ git remote -v

origin   https://vlad@github.com/vlad/picalc.git (push)
origin   https://vlad@github.com/vlad/picalc.git (fetch)

The name origin is a local nickname for your remote repository: we could use something else if we wanted to, but origin is the default.

Once the nickname origin is set up, this command will push the changes from our local repository to the repository on GitHub:

$ git push -u origin master

By adding the “-u” option we connect the “upstream” tag “origin” with the repository so you don’t have to always specify “origin” every time you do “git push”. It is now the default.

Counting objects: 9, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (9/9), 821 bytes, done.
Total 9 (delta 2), reused 0 (delta 0)
To https://github.com/vlad/picalc
 * [new branch]      master -> master
Branch master set up to track remote branch master from origin.

Password Managers

If your operating system has a password manager configured, git push will try to use it when it needs your username and password. For example, this is the default behavior for Git Bash on Windows. If you want to type your username and password at the terminal instead of using a password manager, type:

$ unset SSH_ASKPASS

in the terminal, before you run git push. Despite the name, git uses SSH_ASKPASS for all credential entry, so you may want to unset SSH_ASKPASS whether you are using git via SSH or https.

You may also want to add unset SSH_ASKPASS at the end of your ~/.bashrc to make git default to using the terminal for usernames and passwords.

Our local and remote repositories are now in this state:

We can pull changes from the remote repository to the local one as well:

$ git pull origin master

From https://github.com/vlad/picalc
 * branch            master     -> FETCH_HEAD
Already up-to-date.

Pulling has no effect in this case because the two repositories are already synchronized. If someone else had pushed some changes to the repository on GitHub, though, this command would download them to our local repository.

GitHub GUI

Browse to your picalc repository on GitHub.

• Try clicking on the ‘Code’ tab at top left. You can use this to browse all the files in your repository. There are a lot of other advanced tabs, but you probably just want to stick to “Code” for now.

• Now look at the ‘Commits’ button (it probably just says “1 commit” and a clockface). This shows a graphical representation of what you would see with git log. The git commit hashes are shown, and if you click on one, it will give you the git diff between the two neighbouring versions.

Push vs. Commit

In this lesson, we introduced the “git push” command. How is “git push” different from “git commit”?

Adding more commits

Let’s add some more commits to the repository. Edit picalc.py and add some more lines, e.g. change it to:

a = 2.0
nmax = 100000
for n in range(1, nmax):
    a = a * (n*n)/(n*n - 0.25)
print(a)

git add picalc.py
git commit -m "Set nmax parameter"
git push

Now take a look at the web interface again, and see your new commit. Spend a bit of time exploring the web interface.

Fixing Remote Settings

It happens quite often in practice that you made a typo in the remote URL. This exercise is about how to fix this kind of issue. First start by adding a remote with an invalid URL:

git remote add broken https://github.org/this/url/is/invalid

Do you get an error when adding the remote? Can you think of a command that would make it obvious that your remote URL was not valid?

git push broken master

Can you figure out how to fix the URL (tip: use git remote -h)? Don’t forget to clean up and remove this remote once you are done with this exercise.

GitHub README file

In this section we learned about creating a remote repository on GitHub. The ‘Overview’ tab suggests you create a README. Try doing that by clicking on the “Create a README” button. What effect will this have on your repository?

Key Points

• A local Git repository can be connected to one or more remote repositories.

• Use the HTTPS protocol to connect to remote repositories until you have learned how to set up SSH.

• git push copies changes from a local repository to a remote repository.

• git pull copies changes from a remote repository to a local repository.