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Git Walkthrough

Objective

Learn the basics of Git and understand what role Github plays in the ecosystem of Ruby and Ruby-on-Rails. Learn concepts, such as snapshot, repository, pull, push and topical branches.

Git

Git is a fast, open source, distributed version control system. It's unlike CVS, SourceSafe, ClearCase, Subversion or Perforce, and is somewhat like Mercurial (Hg). Distributed means it works offline. Git is an essential enabler of the ecosystem of Ruby and Ruby-on-Rails.

Git Snapshots

A git commit is a snapshot of all files and meta information at a given point in time, not a file diff.

A New Git Repository

Create a GIT repostory offline. We'll call this one red.

$ mkdir cs5356
$ cd cs5356
cs5356$ mkdir red
cs5356$ cd red

cs5356/red$ git init
Initialized empty Git repository in /home/dblock/cs5356/red/.git/

cs5356/red$ touch README.md

cs5356/red$ git add README.md

cs5356/red$ git status

# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
# new file:   README.md
#

cs5356/red$ git commit -m "Added README."
[master (root-commit) 3923738] Added README.
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 README.md

cs5356/red$

Our Local Repository

Our previous commit placed our changes into our local repository, a concept unique to distributed version control systems like Git and Hg.

Examine the history of this local repository.

cs5356/red$ git log

commit 392373835e0f7702251d45a088664918e9125c97
Author: dblock
Date:   Wed Aug 26 09:34:08 2015 -0500

Added README.

Pushing to a Remote Repository: Github

Create a new repository on Github called red. That's an empty repository with no files. Github ran the same git commands as we just did on their servers.

We can connect our local red repository with Github's by adding a remote location called origin. This name is just a convention.

cs5356/red$ git remote add origin git@github.com:<username>/red.git

One can now see the remote branch listed:

cs5356/red$ git remote
origin

Push all our changes to Github.

cs5356/red$ git push -u origin master

Next time you can push all pending changes on all branches.

cs5356/red$ git push

The opposite of pushing is pulling. The following updates the local copy to the remote changes on the master branch.

cs5356/red$ git pull origin master

Cloning

Use git clone to get a copy of an existing Github repository to your local machine.

cs5356$ git clone git@github.com:<username>/red.git green

This creates a new folder called green with the contents of the red repository, a local repository from the remote repository on github.

Topic Branches

Unlike other source controls, branches are cheap. In Git it's considered a good practice to never work on the master branch and create topic branches instead. A topic branch is created off master.

First, list your branches. You will see that you are on the master branch.

cs5356/red$ git branch

Ensure the master branch is up-to-date.

cs5356/red$ git pull origin master

Create a new branch readme-changes.

cs5356/red$ git checkout -b readme-changes
Switched to a new branch 'readme-changes'

cs5356/red$ git status
# On branch readme-changes
nothing to commit (working directory clean)

Make some changes on the branch, commit them and push them to Github.

cs5356/red$ git push origin readme-changes

When the feature on a branch is ready, merge it to master.

cs5356/red$ git checkout master
cs5356/red$ git merge readme-changes

Visualize the Branches

Commits add to the log inside Git, allowing one to visualize the changes to each particular branch.

cs5356/red$ git log --graph

Use the built-in GUI GitK to see this graph

cs5356/red$ gitk

Forking

To contribute to someone else's work, create a new fork via Github, then clone your fork to your local environment. To submit changes, make pull requests.

To pull changes made to the original work, one would need to set up an upstream remote repository.

cs5356/red$ git remote add upstream git@github.com:<original_author>/<upstream_source>.git

cs5356/red$ git remote
origin
upstream

To pull any changes made to upstream one would run the following command:

cs5356/red$ git pull upstream master

Homework

Fork https://github.com/Cornell-CS5356-Fall2015/cs5356, clone your fork, add a page for yourself under people, including a photo and some personal links. Add a link to this page to a Students section under People. Commit, push and make a pull request.