from https://github.com/kevinzhang96/git-demo Thanks Kevin!
This guide is designed to be used as a quick tutorial on Git for those new to it or anyone who just needs a refresher. We'll cover the basics of committing, branches, and pull requests.
First, if you don't have a GitHub account yet, go to https://github.com and sign up for an account. Once you're done, visit https://github.com/fatmaakcay/CS51-git-demo, and hit "Fork" on the top right. This creates a copy of this repository for your own use; you'll be able to pull updates from this repository while making changes to your own copy. Once you're done, visit your own copy.
If you haven't installed git yet, you can find downloads at git-scm.com/downloads. If you install GitHub's native client at https://desktop.github.com, git will automatically be installed.
Now, we'll set up an SSH key. An SSH key allows you to secure your computer with a secret key, while using a public key to identify yourself to other devices. Open up a terminal (Git Bash on Windows), and run ssh-keygen. Follow the instructions presented; the default options are fine. This creates a private key at ~/.ssh/id_rsa, and a public key at ~/.ssh/id_rsa.pub.
Now, we want to add your public key to GitHub so that GitHub recognizes your computer and allows you to clone repositories easily. Go back to GitHub, and click on your profile photo on the top right, then select "Settings." On the next page, hit "SSH keys" on the left, then "New SSH key" on the right. Back in your Terminal, type cat ~/.ssh/id_rsa.pub | pbcopy. cat ~/.ssh/id_rsa.pub prints your public key and | pbcopy puts it in your clipboard. If Terminal complains about pbcopy, just type cat ~/.ssh/id_rsa.pub and manually copy the printed text. Back in the "New SSH key" page, give your key a name and paste your public key into the text box. Hit "Add SSH key," and you should be all set!
Go back to your copy of this repository, and note the URL listed on the page (looks like git@github.com:fatmaakcay/CS51-..). There's a toggle on the left to pick a clone option; make sure it says SSH, and not HTTPS! Copy this URL. In your Terminal, cd into a directory that you want to store your code in; type git clone into your Terminal then paste the URL. Hit enter. After git's done, you should see a new folder (type ls) called git-demo! cd into it; note that it only contains a README.md file so far. We're going to add more files and submit something called a pull request to the main repository (more on that later).
In your terminal, type touch <your-name>.txt, then open . (replace <your-name> with your name, of course). You should see a folder open up on your computer; this is the git-demo folder! Open up <your-name>.txt, and type in something about yourself (let's say, how much programming experience you've had up until now). Save the file, and close it. Go back into your Terminal, type git add -A, then git commit -am "added name file", and git push. This will push the new file to your copy of this repository!
Now we'll go over a simple example of branching. Branching is mostly useful for when you're working on a feature that you don't want integrated into the main codebase yet - sometimes new code can break functionality in a repository, so it's safer to put that code somewhere else until it's finished.
Go into your terminal and type git checkout -b new_branch (you can replace new_branch with whatever you want). This creates a new local branch called new_branch and switches to it. Here, add a new file and call it whatever you want; you don't have to put anything in it (although that would be preferred), just create a new file. Now, commit it! You can refer to "Getting Started" above; however, you'll notice that git push won't work! This is because GitHub doesn't recognize your new branch yet; it only knows about the main branch (called master). To tell GitHub about the new branch, run git push --set-upstream origin new_branch. You can revisit your copy of the repository; you should now see that a new branch has been created (click on master on the page, you should be able to select your new repository in the dropdown that shows).
You know how to create new branches now; however, you don't know how to merge your changes between branches yet! In your terminal, run git checkout master. If git complains that you have unstashed changes, you made changes that you haven't committed yet; either commit it or run git reset --hard (NOTE: DO NOT RUN THIS COMMAND ARBITRARILY. This drops ALL changes you have made since your last commit, and you will NOT be able to get them back). git checkout master switches you back to the main codebase (the master branch). From here, run git merge new_branch; this pulls the changes from your other branch into the main base. If you forget what your branch was called, you can run git branch to see a list of active branches. Push the updated master branch to your repository!
The last thing we'll cover is pull requests. Pull requests are designed to allow you to contribute to repositories that you don't have write access to, while providing a buffer so that the owners of the original repository can review any changes you make before integrating them into the main codebase.
Revisit https://github.com/fatmaakcay/CS51-git-demo. You should see a "New pull request" button on the screen; hit it. On the new page, hit "compare across forks" - this allows you to integrate changes from separate copies of the original repository into the original. On the right, select your copy of the repository along with the master branch; on the left, select the original copy (fatmaakcay/CS51-git-demo) and the master branch. Once you've done that, hit "Create pull request"; you should be done!
I can now view your changes and integrate them into my repository; if everyone's done everything correctly, the original repository will eventually contain a lot of text files, with one for each member of the class. You can revisit this repository afterwards to see a list of your classmates' names, along with any other files people have created. Congrats; you're now acquainted with almost all features of git you'll ever need!