Git


Git

Written by CS 107E TA Peter McEvoy

What is Git?

Git is a distributed version control system (DVCS), which allows multiple people to work on the same software at the same time. Git is predominantly used by software engineers and developers to coordinate work on software, but the project need not involve software to take advantage of Git. Software is mainly composed of text files, which Git is especially good at managing because it can track small changes made to text files. While Git can also manage more complex data files like images, it lacks the ability to detect the same kind of small changes as in text files.

Why Git?

Suppose that you’re working on a project in C that will compute all powers of an integer x between 0 and a positive integer n. The project consists of only a single file, powers.c, which contains the following function signatures.

// return x^n
int power(int x, int n) {}

// print x^0, ... x^n
void print_powers(int x, int n) {}

// parse argv and delegate to print_powers
int main(int argc, char *argv[]) {}

You implement main first, then print_powers, and then power. You then compile and run your code. It works, but it’s on the slow side, so you decide to optimize power. You finish optimizing and decide to go back through and comment your code thoroughly to justify each of your steps.

Now you recompile and rerun your code. Unfortunately, your optimizations introduced a bug, and so you want to return to your previous working version. If you use standard undo functionality, you’ll revert power back to its slow yet correct form, but you’ll also lose all of your carefully crafted comments. You could try to recreate the correct version of power from memory, but that’s error prone. What you really want is to cherry pick the power code from your editing history and leave everything else as is. With Git, you can do exactly this.

Once you have a working version of a function, you can create a checkpoint in Git (called a commit) so that you can return to it at any time. Git, then, gives you complete peace of mind during the development process since you know that anything that you’ve committed can still be easily accessed.

How does Git work?

We’ve established that Git gives you access to the editing history of your project. But how does it do this? Well, the first step is to create what’s called a repository (repo for short), which is a fancy name for a project managed by Git. As a way of illustrating this, let’s create a project (represented by the directory dummy) and initialize a Git repository inside of it using git init.

$ cd ~
$ mkdir dummy
$ cd dummy
$ ls -al
$ git init
$ ls -al

You’ll notice that the first ls -al shows that the dummy directory is empty aside from the . and .. entries. However, the second ls -al, which is run after git init, shows us the presence of a .git directory. Don’t worry too much about what’s in the .git directory. For now, all you need to know is that Git stores all of the information about your repo within the .git directory.

Now that we’ve set Git up to track the changes in our repo, we can now edit some files.

$ echo "1" > file1.txt
$ echo "2" > file2.txt
$ ls 
$ cat file1.txt
$ cat file2.txt

We now have two files, file1.txt and file2.txt in our repo. Let’s see if Git is tracking our newly created files.

$ git status

Git tells us that we have two untracked files, which is expected since we just created two new files. Git also tells us that we can use git add <file> to include new files in what will be committed. At this point, it’s time to talk about how Git “saves” files.

Instead of saving changes, Git commits them. The idea is similar to saving, but it gives us the ability to choose which subset of the files that were changed to include in the commit. (We can also selectively choose which parts of a given file to commit, but that’s beyond the scope of this guide.) To save/commit a file in Git, we need to first tell Git what it is that we want to save (this is the git add step) and then tell Git that we want to save what we added (this is the git commit step). It’s worth pointing out that each commit requires an associated message, which helps us remember the reason for the commit. If the choice of the word “commit” is unintuitive, then think of git commit as committing the change to Git’s memory.

Let’s go ahead and commit the first file that we created. (As an aside, we can use git commit instead of git commit -m "<some message>". git commit` will simply send us into vim or some other text editor to enter the commit message.)

$ git add file1.txt
$ git commit -m "create file1.txt"

Now we can see our brand new commit using git log.

$ git log

git log shows us that “create file1.txt” was our most recent commit. Success!

Let’s commit the second file and verify that it was in fact committed.

$ git add file2.txt
$ git commit -m "create file2.txt"
$ git log

Now our most recent commit is, as expected, “create file2.txt”. Our two commits comprise the editing history of our project, which we can go back and examine at any time. But what if we want to share our work with a friend? Thankfully, Git (in conjunction with GitHub) can help us with this too.

Sharing work

First, let’s distinguish Git from GitHub. GitHub is a Git server, meaning that it stores Git repositories and serves them upon request to clients. Think of it as a giant USB flash drive that stores your Git repositories and allows you and your friends to access them from any computer in the world. We’ll use GitHub to store the repository that we just created. For demonstration purposes, we’ll assume that your GitHub account already has a repo named dummy created, which can be found at git@github.com:<your-username>/dummy.git.

In Git terminology, the version of the repo living on your computer (the one you created with git init) is called the local copy, and the version living on GitHub’s servers is called the remote copy.

In order to add the work in our local repo to our remote repo, we first need to assign a shorter name to the URL of the remote repo, the one that is hosted by GitHub. We’ll use the name origin, since this remote repo will be the “origin” of your code.

$ git remote add origin git@github.com:<your-username>/dummy.git

Now that we’ve set up a shorter name for our URL, we can create a new branch on our remote repo and push our local changes to it.

$ git push --set-upstream origin master

What we’re doing here is pushing our local commits (the ones that we made with git commit ...) onto the branch master in the remote repo represented by the name origin (git@github.com:<your-username>/dummy.git in this case). In future pushes to the master branch of origin, we will be able to simply do git push instead of git push --set-upstream origin master. The latter, longer version is necessary when the branch that we’re pushing from our local repo doesn’t yet exist on our remote repo.

That was a lot, but fortunately file1.txt and file2.txt should now both show up in GitHub under your dummy repo. We started with just a local copy, and we created a remote copy by specifying the URL of the remote (using git remote add origin). We then pushed all of the work that we’ve done on our local copy to the remote copy using git push.

Getting back to our friend who wants to see our work: she can pull our work from the remote copy of the repo, the one hosted on GitHub. To do so, she’ll use git clone to create a local copy on her computer.

$ git clone git@github.com:<your-username>/dummy.git

This will create a repo named dummy on her computer, which will contain all of the work that has been pushed to the remote copy up to that point.

Now that our friend has a local copy of the repo, she can add a new change and push it to the remote copy.

$ echo "3" > file3.txt
$ cat file3.txt
$ git add file3.txt
$ git commit -m "create file3.txt"
$ git push

She tells us that she just pushed a new change, but when we look at our local copy, we see that we don’t have the new file. To get it, we must pull the change from the remote copy of the repo to our local copy.

$ git pull

Now we have file3.txt. git log will also show that the new latest commit is “create file3.txt”.

What next?

There are many bells and whistles to Git, but the two ideas of recording the editing history and sharing changes with others are at the heart of Git. Git takes a long time to understand; the longer I use it, the more I learn about it. If you’re interested in learning more, here are some useful resources:

Also, please please please ask the teaching staff questions as they come up. It’s often times much easier to explain a Git concept verbally than it is to write down an explanation. Finally, take solace in the fact that the rest of the teaching staff and I have all faced the daunting learning curve of Git before. We’ve managed just fine, and so will you. (This is coming from someone who struggled in CS 106A.)