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Version Control Collaboratively

Back To Local Version Control

Presented By : Anthony Scopatz Developed By : Katy Huff


GitHub is a site where many people store their open (and closed) source code repositories. It provides tools for browsing, collaborating on and documenting code. Your home institution may have a repository hosting system of it's own. To find out, ask your system administrator. GitHub, much like other forge hosting services ( launchpad, bitbucket, googlecode, sourceforge etc.) provides :

  • landing page support my research is at cyclus.github.com
  • wiki support
  • network graphs and time histories of commits
  • code browser with syntax highlighting
  • issue (ticket) tracking
  • user downloads
  • varying permissions for various groups of users
  • commit triggered mailing lists
  • other service hooks (twitter, etc.)

github pasword

Setting up github at first requires a github user name and password. Please take a moment to create a free one (if you want to start paying, you can add that to your account some other day).

github ssh keys

It will help you to set up automatic authorization, so that github can handshake with your computer (in this case, your virtual machine). There are some setup instructions
on the website, but I'll do this along with you at the front of the room as well.

$ cd ~/.ssh

It will likely say "no such file or directory."

$ ssh-keygen -t rsa -C "your_email@youremail.com"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/thw/.ssh/id_rsa):  <press enter>

The path that it provides will be to this home directory. This is okay. Press enter. You may enter a passphrase. You'll see something like this :

Created directory '/home/thw/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/thw/.ssh/id_rsa.
Your public key has been saved in /home/thw/.ssh/id_rsa.pub.
The key fingerprint is:
09:06:c6:0f:24:b7:84:ef:22:74:de:95:f0:99:64:5d your_email@youremail.com
The key's randomart image is:
+--[ RSA 2048]----+
|  .+*   . .E     |
|  .=o+ o .       |
|   ..oB +        |
| . ....B .       |
|. o.. . S        |
|. ....           |
| . .             |
|                 |
|                 |

git config : Configuring your git environment

Once you've set up your rsa keys, you need to tell github who you are. Crack open a terminal.

$ git config --global user.name "Firstname Lastname"
$ git config --global user.email "your_email@youremail.com"

Unless your name is Firstname Lastname, please don't copy the above lines verbatim. Make the appropriate substitutions.

If you did this properly, you'll have a file in your home (~) directory that's called .gitconfig . It's contents should look like :

      name = Katy Huff
      email = southside@gmail.com

This configuration step allows github to properly credit the authorship of changes you make in your repository. For projects with numerous authors, this is essential.

Another configuration step for some will be to set their favorite text editor as git's text editor of choice. This is optional, since vi is usually the default, but can be done with the following command (if you like nano for example):

$ git config --global core.editor nano

git remote : Steps for Forking a Repository

A key step to interacting with an online repository that you have forked is adding the original as a remote repository. By adding the remote repository, you inform git of a new option for fetching updates and pushing commits.

The git remote command allows you to add, name, rename, list, and delete repositories such as the original one upstream from your fork, others that may be parallel to your fork, and so on.

Exercise : Fork Our GitHub Repository

While you probably already have a copy of the UofCSCBC2012 repository, GitHub doesn't know about it yet. You'll need to tell github you want to have an official fork of this repository.

Step 1 : Go to our repository from your browser, and click on the Fork button. Choose to fork it to your username rather than any organizations.

Step 2 : Clone it. From your terminal :

$ git clone git@github.com:username/UofCSCBC2012.git
$ cd UofCSCBC2012

Step 3 :

$ git remote add upstream git://github.com/thehackerwithin/UofCSCBC2012.git
$ git remote -v
origin  git@github.com:username/UofCSCBC2012.git (fetch)
origin  git@github.com:username/UofCSCBC2012.git (push)
upstream        git://github.com/thehackerwithin/UofCSCBC2012.git (fetch)
upstream        git://github.com/thehackerwithin/UofCSCBC2012.git (push)

All repositories that are clones begin with a remote called origin.

git fetch : Fetching the contents of a remote

Now that you have alerted your repository to the presence of others, it is able to pull in updates from those repositories. In this case, if you want your master branch to track updates in the original UofCSCBC2012 repository, you simply git fetch that repository into the master branch of your current repository.

The fetch command alone merely pulls down information recent changes from the original master (upstream) repository. By itself, the fetch command does not change your local working copy. To update your local working copy to include recent changes in the original (upstream) repository, it is necessary to also merge.

git merge : Merging the contents of a remote

To incorporate upstream changes from the original master repository (in this case thehackerwithin/UofCSCBC2012) into your local working copy, you must both fetch and merge. The process of merging may result in conflicts, so pay attention. This is where version control is both at its most powerful and its most complicated.

Exercise : Fetch and Merge the Contents of Our GitHub Repository

Step 1 : Fetch the recent remote repository history

$ git fetch upstream

Step 2 : Make certain you are in the master branch and merge the upstreeam master branch into your master branch

$ git checkout master
$ git merge upstream/master

Step 3 : Check out what happened by browsing the directory.

git pull : Pull = Fetch + Merge

The command git pull is the same as executing git fetch followed by git merge. Though it is not recommened for cases in which there are many branches to consider, the pull command is shorter and simpler than fetching and merging as it automates the branch matching. Specificially, to perform the same task as we did in the previous exercise, the pull command would be :

$ git pull upstream
Already up-to-date.

When there have been remote changes, the pull will apply those changes to your local branch, unless there are conflicts with your local changes.

git push : Sending Your Commits to Remote Repositories

The git push command pushes commits in a local working copy to a remote repository. The syntax is git push [remote] [local branch]. Before pushing, a developer should always pull (or fetch + merge), so that there is an opportunity to resolve conflicts before pushing to the remote.

We'll talk about conflicts later, but first, since we have no conflicts and are up to date, we can make a minor change and send our changes to your fork, the "origin."

$ git push origin master

If you have permission to push to the upstream repository, sending commits to that remote is exactly analagous.

$ git push upstream master

In the case of the UofCSCBC2012 code, new developer accounts will not allow this push to succeed. You're welcome to try it though.

git merge : Conflicts

This is the trickiest part of version control, so let's take it very carefully.

In the UofCSCBC2012 code, you'll find a file called Readme.md. This is a standard documentation file that appears rendered on the landing page for the repository in github. To see the rendered version, visit your fork on github, https://github.com/username/UofCSCBC2012/ .

For illustration, let's imagine that, suddenly, each of the developers on the UofCSCBC2012 code would like to welcome visitors in a language other than English. Since we're all from so many different places and speak so many languages, there will certainly be disagreements about what to say instead of "Welcome."

I, for example, am from Texas, so I'll push (to the upstream repository) my own version of Welcome on line 2 of Readme.md.

You may speak another language, however, and may want to replace the english word Welcome with an equivalent word that you prefer (willkommen, bienvenido, benvenuti, etc.).

You'll want to start a new branch for development. It's a good convention to think of your master branch as the "production branch," typically by keeping that branch clean of your local edits until they are ready for release. Developers typically use the master branch of their local fork to track other developers changes in the remote repository until their own local development branch changes are ready for production.

Exercise : Experience a Conflict

Step 1 : Make a new branch, edit the readme file in that branch, and commit your changes.

$ git branch development
$ git checkout development
Switched to branch 'development'
$ gedit Readme.md &
<edit the readme file and exit gedit>
$ git commit -am "Changed the welcome message to ... "

Step 2 : Mirror the remote upstream repository in your master branch by pulling down my changes

$ git checkout master
Switched to branch 'master'
$ git fetch upstream
$ git merge upstream/master
Updating 43844ea..3b36a87
 README.rst |   2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

Step 3 : You want to push it to the internet eventually, so you pull updates from the upstream repository, but will experience a conflict.

$ git merge development
Auto-merging Readme.md
CONFLICT (content): Merge conflict in Readme.md
Automatic merge failed; fix conflicts and then commit the result.

git resolve : Resolving Conflicts

Now what?

Git has paused the merge. You can see this with the git status command.

# On branch master
# Unmerged paths:
#   (use "git add/rm <file>..." as appropriate to mark resolution)
#       unmerged:      Readme.md
no changes added to commit (use "git add" and/or "git commit -a")

The only thing that has changed is the Readme.md file. Opening it, you'll see something like this at the beginning of the file.

<<<<<<< HEAD
>>>>>>> development

The intent is for you to edit the file, knowing now that I wanted the Welcome to say Howdy. If you want it to say Willkommen, you should delete the other lines. However, if you want to be inclusive, you may want to change it to read Howdy and Willkommen. Decisions such as this one must be made by a human, and why conflict resolution is not handled more automatically by the version control system.

Howdy and Willkommen

This results in a status To alert git that you have made appropriate alterations,

$ git add Readme.md
$ git commit
Merge branch 'development'

# It looks like you may be committing a MERGE.
# If this is not correct, please remove the file
# and try again.
$ git push origin master
Counting objects: 10, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 762 bytes, done.
Total 6 (delta 2), reused 0 (delta 0)
To git@github.com:username/UofCSCBC2012.git