Version Control With Git: Using Remote Repositories

github.com?

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
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.)

NOTE Public repos have public licences by default. If you don't want to share (in the most liberal sense) your stuff with the world, pay github money for private repos, or host your own.

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).

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 SWC-bootcamp 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 https://github.com/YOU/boot-camps.git
$ cd boot-camps

Step 3 :

$ git remote add upstream https://github.com/USERNAME/boot-camps.git
$ git remote -v
origin  https://github.com/YOU/boot-camps.git (fetch)
origin  https://github.com/YOU/boot-camps.git (push)
upstream        https://github.com/USERNAME/boot-camps.git (fetch)
upstream        https://github.com/USERNAME/boot-camps.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 SWC-bootcamp 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 USERNAME/boot-camps) 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 YYYY-MM-PLACE branch and merge the upstream YYYY-MM-PLACE branch into your YYYY-MM-PLACE branch

$ git checkout YYYY-MM-PLACE
$ git merge upstream/YYYY-MM-PLACE

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.

Exercise : Push a change to github

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 YYYY-MM-PLACE

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

$ git push upstream YYYY-MM-PLACE

In the case of the YYYY-MM-PLACE 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 YYYY-MM-PLACE 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/YOU/boot-camps/tree/YYYY-MM-PLACE/README.md).

For illustration, let's imagine that, suddenly, each of the developers on the YYYY-MM-PLACE 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 Tamil Nadu, India, so I'll push (to the upstream repository) my own version of Welcome on line 5 of Readme.md.

You may speak another language, perhaps even English, however, and may want to replace the Tamil word 'vanakkam' with an equivalent word that you prefer (welcome, 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 (in this case your YYYY-MM-PLACE 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'
$ kate Readme.md &
<edit the readme file and exit kate>
$ git commit -am "Changed the welcome message to ... "

Step 2 : Mirror the remote upstream repository in your master branch (in this case your YYYY-MM-PLACE branch) by pulling down my changes

$ git checkout YYYY-MM-PLACE
Switched to branch 'YYYY-MM-PLACE'
$ git fetch upstream
$ git merge upstream/YYYY-MM-PLACE
Updating 43844ea..3b36a87
Fast-forward
 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 YYYY-MM-PLACE
# 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
Vanakkam
=======
Willkommen
>>>>>>> development
=====================

The intent is for you to edit the file, knowing now that I wanted the Welcome to say Vanakkam. 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 Vanakkam 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.

Vanakkam 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'

Conflicts:
  Readme.md
#
# It looks like you may be committing a MERGE.
# If this is not correct, please remove the file
# .git/MERGE_HEAD
# and try again.
#
$ git push origin YYYY-MM-PLACE
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/boot-camps.git

gitolite

Gitolite is a way for you to host your own multi-user git repositories. I'm not going to go into details here, but all you need is a machine with some drive space and network access. You can install minimal ubuntu, then sudo apt-get install gitolite will pull in everything you need. At that point, your collaborators will only need to send you their public SSH keys for you to configure pull and push access to the repos.