CS 257: Software Design

Hello, World! with git: a short introduction to version control

Folder name: hello

You will work with one or two partners assigned to you for this assignment. Do the individual work on your own computers, but share a single git repository for your project. Put both/all of your names in the comments at the top of each source file.

Goals

To learn about:

• version control systems in general
• using the git version control system to collaborate with another person
• a few essential git operations: clone, status, add, commit, pull, push

Summary of your tasks

Here's the quick summary of your tasks. Much more detailed instructions are in the numbered sections below.

• Before getting started, make sure you have completed all the steps in the development environment setup assignment.
• Choose one partner's GitHub repository in which to do your team's work for this assignment.
• Create a top-level directory called "hello" in your chosen repository, and a simple hello, world Python program called hello.py in the hello directory.
• Add some items to the .gitignore file in your repository.
• Once your simple hello.py is done, each partner should modify the code in some way. In the process, you will use the git commands that allow multiple people working on different machines to collaborate on shared code. See the adventures of Alice and Bob below.

0. Detailed instructions (featuring Alice and Bob)

The rest of this document contains lots and lots of details. Go slowly, write down your questions as you go, and pay attention to error messages and other information that appears on your screens. Also, talk to your partner about each step: Did you get what happened there? Should we try something different? Should we reread the instructions? What does that sentence mean? Maybe we should bug Jeff or post on Slack about this one...

If you get stuck or just want a little elaboration on a concept, don't hesitate to ask for help, in your discussion group, in office hours, or on Slack.

For the rest of this document, and sometimes in other exercises, I will refer to the partner whose repo you're using for this project as Alice, and each other partner as Bob. This makes writing instructions a lot easier than referring to "the partner whose repo you're using for this project" and "the partner whose repo you're not using for this project". Alice and Bob have been used in computer-related exposition for a long time because they love communicating collaboratively and they're handy to have around.

Have fun!

1. Look at your CS257 repository on GitHub

Go to GitHub, login, and click on your cs257 repository. At your repo's home page, you should see a display showing the files in the repository. Right now, that's probably just README.md, maybe LICENSE if you added a license, and .gitignore.

A repository is essentially a folder in which you can store whatever files and subfolders you want. What makes a repository more than a folder, however, is that the version control system (git, in this case) keeps track of all the changes that get made to the files in the repository. This saving of history makes it possible for you to retrieve old work, track down the time when a bug was introduced (and who introduced it), collaborate on a project with a team of programmers, easily back up your work, etc. Version control systems are a bit of a hassle to learn for the first time, but they're indispensable programming tools and unquestionably worth the effort.

2. Grab a local clone of your repository

Go to your repository's page on GitHub. There should be a big green button labeled "Code". Click it, and copy the URL that appears in the resulting popup window.

Right now, your repository is stored only on the GitHub server. To add files to it, you'll need to clone a copy of the repository onto your working computer. You will then add files to your local copy of the repository and eventually, you'll manually push your changes back up to GitHub.

Create your clone like so:

• Open a Unix terminal
• cd to wherever you want to store your repository's clone
• Execute this command:
  git clone https://github.com/...
  where the "..." is the remainder of the link you just copied from the Code button.

You should now have a folder called "cs257" (or whatever you named your repository). This folder should contain README.md, maybe LICENSE, .gitignore, and a subfolder called .git. You can move cs257 wherever it's convenient for you to have it. (Curious about the contents of the .git folder? Go ahead and poke around to see what's there. It gets more interesting, of course, after you add some files.)

3. Get in touch with your assigned partner

I'll assign you your partner for this assignment. Get in touch promptly. Personal-message me on Slack if your partner is unresponsive after a half-day or so.

Once you're talking, set up a time to do this exercise together. I recommend setting up a Zoom call and planning to make use of screen sharing—I think this will be easier than distanced and masked in the same room. If Alice is doing something, then Alice shares her screen with Bob, and Alice does the typing while Bob watches. Reverse that when it's Bob's turn to do something.

I also recommend planning for a two-hour time slot at first. It might take you more or less time than that, but it's important during this exercise to go slowly and think through every step and the lessons to be learned from every step.

4. Pick one partner's repo, and make sure you both have clones of it

It doesn't matter which partner's repository is the one you use for this assignment, but you need to pick one.

Then, the owner of the chosen repository (who we will call Alice) will need to add Bob(s) as collaborator(s) on Alice's repository. Try to figure out how to do this on github.com. Ask questions if you run into problems.

Once all partners are collaborators on the chosen repo, everyone should "git clone" it to their working machines. You can add an extra argument to the command to name the new repo whatever you want:

5. Alice: add a little bit of code

• (Bob, you're not typing, are you? Bob, you're paying attention to what Alice is doing on her screen-share, right? Stop typing, Bob! It's not your turn yet.)
• OK, Alice: at the top level of your repo, create a folder called hello.
• Using a text editor, create a hello-world Python program hello.py inside the hello folder. Test it to make sure it runs correctly.
• Once you're happy with your hello.py, save it, and then do the following. Go nice and slow, and study and discuss with Bob the output git generates at each step before you move on:
  cd assignments-yourusername/hello git status git add hello.py git status git commit -m "Added the hello.py program" git status

  Note that those three "git status" invocations are there because I want you to see the status messages before and after the "git add". This sort of exploration is part of coming to understand how git works, but it's also something I often do myself to make sure I don't make big changes too fast. Going a little bit slowly like this saves me a lot of hassle in the long run.

• Open up your GitHub repository in a web browser. Take a look at the files, if any, in the repo as stored on the GitHub server.
• Now, execute:
  git push
• Reload your GitHub web page. What, if anything, has changed?
• Talk with Bob about what each of those commands did. What's the meaning of "git add" and "git commit"? What does "git push" do? Ask around, search the internet, etc. We'll talk more about these commands as time goes on, but start now to try to understand these operations—don't just memorize them.

6. Bob: can you see Alice's changes?

(Alice? Hands off the keyboard. Bob is sharing his screen now. Fair is fair.)

Bob, at your terminal on your computer, cd to your clone of Alice's repository, and do

Does the hello.py file appear in your copy of the repo? It should. If it doesn't, try to figure out what went wrong, then ask for help if you're stuck.

7. Bob: make a change; Alice: get Bob's changes

Bob: Change something in hello.py (add a comment, add a print statement, whatever). Then do the git status/add/status/commit/push sequence from section 5 above to get your changes included in the master copy on github.com.

Alice: do a "git pull" and see if you ended up with Bob's version of the code.

8. Alice: add a Makefile

• Create a file called Makefile in the hello directory. In fact, just download this Makefile and save it in hello.
• Test it out by:
  cd hello make
• Say to Bob: "There must be some reason Jeff wants us to use a Makefile, but it can't possibly be just to save our having to type 'python3 hello.py' every time." Shake your head at the inexplicable ways of CS professors.
• Ask Jeff about Makefiles, tab characters, and why programmers fight over the weirdest things.
• git add Makefile, git commit, git pull (just in case you don't have all of Bob's stuff yet), and then git push. Intersperse with "git status" to make sure you know what you're doing.
• Bug Bob to pull the changes so you're in sync.

9. Bob: edit the .gitignore file

While we work on a project, it's common for the project directory to accumulate files that aren't really part of the project. For example, if you write program.py and module.py and program.py includes an "import module" statement, you may discover a module.pyc or a folder called __pycache__ in your working directory. This isn't source code, and it doesn't matter if you delete it—it'll just get recreated next time you "import module". So you don't want that litter included in your repository.

There are lots of other examples of files that shouldn't be saved in a repo. Like the .DS_Store files the macOS Finder generates whenever you look at a folder in a window or the .class files you get when you compile a .java file. Accidentally adding these files to your repository is all too easy. And since these files typically change every time you run the Python program or recompile the Java program or look at the folder in a window, all those changes get added to the repository every time you commit, which makes every clone of your repo take up a lot of unnecessary disk space. It's a mess, and we hate messes.

To prevent you from accidentally adding these extraneous files to your repository, you can tell git to ignore them completely via a .gitignore file. If Alice followed the setup instructions, she already has a .gitignore file full of weird stuff. Note that one line in .gitignore says "*.py[cod]" and another says "__pycache__/", which will cause git to ignore __pycache__ folders and .pyc, .pyo, and .pyd files.

To save macOS users a little pain, Bob is going to add the line ".DS_Store" to the .gitignore. .DS_Store files get created whenever you open a folder in the macOS Finder. You can delete them with no negative consequences, and you definitely do not want to include them in Alice's git repository.

• (It's Bob's turn. So Alice, you know what to do. Bob, carry on.)
• Use a text editor to open .gitignore, and add one line to it:
  .DS_Store
• Add, commit and push your changes up to GitHub.
• Remind Alice to pull the changes so you're in sync.

IMPORTANT ODDITY: by default, macOS, Windows, and Unix command shells don't display files whose names start with a period. So if you open your repo's top-level folder in a macOS Finder window, you probably won't see your .gitignore file. Similarly, a simple:

command at a Unix command line won't show you .gitignore. But this command does:

(the "-a" stands for "all").

10. Alice: tag the repo

• Do whatever you need to do to be satisfied with your hello.py. If you feel like entertaining the grader, go nuts--this is just a trivial Python program, and you can do whatever you want with it. But make sure that your project runs with a simple "make" command in the hello directory.
• status, add, status, commit, push, partner pull, etc. etc. Get it so that your completed program is the same in Alice's clone, in Bob's clone, and on github.com.
• Alice: "tag" your code.
  git tag -a hello-world -m "tagged for assignment due 8 Jan 2021" git push origin hello-world
  Please use that exact tag name ("hello-world") so it's easy for the grader to access everybody's code.
  NOTE: a regular "git push" does not push tags. If you want a tag to be shared with your collaborators (and your professor and grader, which may be more important in this context), you need to do that special "git push origin tag-name" command shown above. We can discuss the meaning of "origin" later, or you can research it yourself.
• Did you succeed in tagging? Let's find out:
  git tag --list
  Also, check github.com to see whether the tag has appeared there.
• Bob:
  git pull git tag --list
  Did you get the new tag?
• Alice & Bob: any guess what tags are good for?

11. Yipee!

All done.