Using git

git is version control software. It is useful for managing code projects, particularly projects with partners.

This guide provides instructions for using git for CS lab and project work with partners. It includes setup instructions for using git repos hosted on Swarthmore's GitHub server, available to all Swarthmore students and faculty. It also covers git commands and tools, troubleshooting errors, some advanced features, and links to other git resources.

Contents:

Git Overview
You will either use an existing remote master git repository (repo) or create a remote master repository on our system for your project. You and your partner(s) each checkout (clone) a private local copy of the remote master repo. You then each add and commit changes to your local private cloned copies. Local changes can be shared with your partner by pushing them to the master repo. Your partner will then pull your pushed changes from the remote master into their local repo. Git automatically merges changes pulled from the master into the local repo's files. Any changes committed to a git repo can be recovered, and a version of files from any commit in the commit history can be recovered. Commit often as you work on a project (and push fairly often too).
Setup for using repos on Swarthmore GitHub Enterprise
Many CS courses numbered 31 or higher use Swarthmore's GitHub Enterprise server. This is very similar to the public GitHub.com, but is only visible to Swarthmore students, faculty and staff. Students in CS courses should follow these steps to clone their lab git repos.

Accessing Swarthmore GitHub

The web address is https://github.swarthmore.edu. You should be able to log in with your typical Swarthmore credentials (the same username / password you use for email). Upon logging in, you should see a welcome page that lists the repositories you currently own.

First Time Set-up and Generating ssh keys

If you have never used Swarthmore's GitHub server or if you forgot your ssh-key password, you will need to complete the one time config steps for using Swarthmore GitHub before proceeding. You can create and add public ssh keys for multiple accounts (e.g. one for your CS account and another for your laptop account).

Accessing Course Repos

For repositories created by your professor for CS labs, we'll be using a feature of GitHub called "organizations", which allows us to control everything for the course. The organization to use will have a name something like CSnum-semester, for example use the one named CS31-F17 for the Fall 2017 semester of CS31. From the main GitHub page, select the organization you want to use. You can also jump directly to the organization url, which looks something like https://github.swarthmore.edu/CS31-F17 (your instructor may have a link to it off the course or lab assignment webpages).

On the organization page, you'll see a list of repositories that you can access. The repositories will be named according to what they contain and which users can access them. For example, suppose two teammates (Grace Hopper, ghopper1, and Alan Turing, aturing1) are working together. When Grace accesses the organization, she would expect to see:

Cloning a local copy of the repository

You and your partner will both create a private local copy, or clone, of your shared remote master repo (ex. Lab2-ghopper1-aturing1). You each will edit and commit changes locally and push and pull changes back to the remote repo on GitHub to share them with each other.
  1. On GitHub, click on your and your partner's lab repository, and it'll take you to a page that lists its contents (and many other useful things). On the lower right hand side, you'll see some text that says "You can clone with HTTPS, SSH, or Subversion". Click the "SSH" link to list the link you will use to clone.
  2. On the CS machine, first cd into your cs31/labs subdirectory: 
    [~]$ cd
[~]$ cd cs31/labs
    then run git clone [URL] to locally clone the repository, where [URL] is the SSH URL given to you by GitHub. For example, Grace might run the following command to clone her and Alan's lab2 repo: 
    [labs]$ git clone git@github.swarthmore.edu:CS31-F17/Lab2-ghopper1-aturing1.git  ./lab2
  3. Then cd into your local repo directory and access the files there 
    [labs]$ cd lab2
[lab2]$ ls
CMakeLists.txt  README.md lab2.cpp

If you get authentication errors, make sure you've uploaded an SSH key to your GitHub account. (See: one-time configuration steps)

Using your shared repo

You now have a private local copy of a git repo. All changes, additions, deletions, to this project can be done with git commands that will push changes to and pull changes from the github server.

Commands for using a git repo

Git basics: the add,commit,push loop

Git has a lot of commands and a lot of features. But to get started, we'll learn three basic commands for recording changes and submitting your work to the github server for grading.

git add

Let's suppose you have a file program.cpp that you edit to complete part of your assignment. If you would like to submit this file to the github server the first step is to run the command: 
git add program.cpp
This git command informs git that you wish for changes to this file to be included in the next step, called commit. You will need to use git add to submit newly created files as well as files that you previously submitted but then modified later.

git commit

The git commit command records all the changes to recently added files to a local copy of your git history along with a message of your choosing describing the changes. 
git commit -m "finished lab assignment"
The -m "message" can contain any message you want, but a short descriptive message is best. Note that in the git commit command, there is no mention of what files to commit. Git will only commit files that have been added to be part of the next commit using the git add, which is why it is important to run git add prior to running git commit.

git push

The git commit command only records your changes locally, but to share your code with your partner, instructor, or grader, you must send your changes to the github server. This is done with the command: 
git push
The push command will transfer all local commits that are not already on the github server. You must run git add, git commit, and git push at least once per lab assignment, otherwise the instructor(s) and grader(s) will not have access to your work.

If you make changes to a file later and wish to publish those changes, repeat the add, commit, push loop again to publish another commit to the github server. In general, we will only look at the most recently pushed commit, but git maintains a history of all commits, making it possible to review past versions of the code.

General workflow

After cloning a remote master repo, you and your partner will each work in your own local private copies of the repo. You can add and commit changes to your private copy as you go, and choose when to push your changes to the remote master repo. Your partner can then pull your changes from the remote master into their copy. To get changes from your partner, your partner should first push the changes, and then you should pull. If you are not working with a partner, you will not need to use the pull command.

An example of a sequence of common git commands: 

 # from within your local private git repo
 cd ~/cs31/labs/lab01/

 # check the status of your repo
 $ git status

 # see the changes to the file (compared to your last local commit):
 $ git diff blah.cpp

 # add a new file to the repo on the next commit:
 $ touch foo.cpp .
 $ git add foo.cpp
 # or add new changes to an existing repo file to the next commit:
 $ vim blah.cpp      # edit with your favorite editor
 $ git add blah.cpp

 # commit your changes (adds and deletes) to your local repository:
 $ git commit

 # push committed changes from your local to the remote master repository
 #  note: the first time you push, you may need to do:
 #        git push -u origin master
 $ git push

 # your partner can now pull your changes from the remote into their local:
 $ git pull

Some notes about these commands:

.gitignore

It is useful to add a .gitignore file to your repo. It tells git to automatically ignore adding certain files or file types to the repo, such as .o files, executable files, and editor backup files, like vim .swp files. This way, if you do a git add git doesn't try to add all kinds of kooky files to the repo. Here is an example .gitignore file for a C program that builds an executable file named myprog: 
*.o
myprog
*.swp

# OSX, vim, emacs goofiness
.DS_STORE
.DS_STORE?
*~
*#
.#*
.nfs*
.*.sw?
You add a .gitignore file to a git repo just like you add any other file: 
git add .gitignore
git commit -m "ignore .o files"
git push

Common git commands and git tools

Common commands:

git add     # add changes or new files to the next commit
git commit  # commit your changes to your local repository
git push    # push your committed changes to the remote master repository
            # (note: the first time you may have to do: push origin master
git status  # see what has changed between your copy and the master version
git pull    # pull changes pushed to remote master into your local copy
            # (git will try to merge changes into your copy)
git help    # list common git commands  git help --all lists all commands
git rm      # remove a file at the next commit
git mv      # move a repo file
git diff    # see a diff between a file(s) and latest commit (local repo)
git diff master origin/master  # diff between your copy and master's
git branch -a # see all the branches (likely only master)

git visualization tools:


Troubleshooting


Advanced git Features

git Branches

For CS course projects it is likely sufficient for you and your partner(s) to clone versions of a single master repository and then push and pull changes to it. For larger and longer existing projects, you may want to use git's support for creating separate branches of your repository. For example, different branches could be associated with different users or with different features, and users can push, pull, and merge changes from branch to branch.

See the Git Branch guide for more details.

git tags

You likely won't need to use tags for course projects, but they are handy for long-lived projects, particularly for code that you may want to release. A tag is a way to name a snapshot of the repository. It is often useful for code releases and for tagging big version changes to code. A tag is like a static branch (you cannot update the tagged version, but you can check it out just like any branch).

See the Git Tag guide for more details.