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Oct 29 2013

Git!

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This post was first published at ScraperWiki.

As software company, use of some sort of software source control system is inevitable, indeed our CEO wrote TortoiseCVS – a file system overlay for the early CVS source control system. For those uninitiated in the joys of software engineering: source control is a system for recording the history of file revisions allowing programmers to edit their code, safe in the knowledge that they can always revert to a previous good state of code if it all goes horribly wrong. We use Git for source control, hosted either on Github or on Bitbucket. The differing needs of our platform and data services teams fit the payment plans of the two different sites.

Git is a distributed source control system created by Linus Torvalds, to support the development of Linux. Git is an incredibly flexible system which allows you to do pretty much anything. But what should you do? What should be your strategy for collective code development? It’s easy to look up a particular command to do a particular thing, but less is written on how you should string your git commands together. Here we hope to address this lack.

We use the “No Switch Yard” methodology, this involves creating branches from the master branch on which to develop new features and regularly rebasing against the master branch so that when the time comes the feature branch can be merged into the master branch via a pull request with little fuss. We should not be producing a byzantine system by branching feature branches from other feature branches. The aim of “No Switch Yard” is to make the history as simple as possible and make merging branches back onto master as easy as possible.

How do I start?

Assuming that you already have some code in a repository, create a local clone of that repository:

git clone git@github.com:scraperwiki/myproject.git

Create a branch:

git checkout -b my-new-stuff

Start coding…adding files and committing changes as you go:

git add -u
git commit -m "everything is great"

The -u switch to git add simply checks in all the tracked, uncommitted files. Depending on your levels of paranoia you can push your branch back to the remote repository:

git push

How do I understand what’s going on?

For me the key revelation for workflow was to be able to find out my current state and feel pleasure when it was good! To do this, fetch any changes that may have been made on your repository:

git fetch

and then run:

git log --oneline --graph --decorate --all

To see an ASCII art history diagram for your repository. What you are looking for here is a relatively simple branching structure without too many parallel tracks and with the tips of each branch lined up between your local and the remote copy.
You can make an alias to simplify this inspection:

git config --global alias.lg 'log --oneline --graph --decorate'

Then you can just do:

git lg --all

I know someone else has pushed to the master branch from which I branched – what should I do?

If stuff is going on on your master branch, perhaps because your changes are taking a while to complete, you should rebase. You should also do this just before submitting a pull request to merge your work with the master branch.

git rebase -i

Allows you to rebase interactively, this means you can combine multiple commits into a single larger commit. You might want to do this if you made lots of little commits whilst achieving a single goal. Rebasing brings you up to date with another branch, without actually merging your changes into that branch.

I’m done, how do I give my colleagues the opportunity to work on my great new features?

You need to rebase against the remote branch onto which you wish to merge your code and then submit a pull request for your changes. You can submit a pull request from the web interface at Github or Bitbucket. Or you can use a command line tool such as hub.  The idea of using a pull request is that it makes your changes visible to your colleagues, and keeps a clear record of those changes. If you’ve been rebasing regularly you should be able to merge your code automatically.

An important principle here is “ownership”, in social terms you own your local branch on which you are developing a feature, so you can do what you like with it. The master branch from which you started work is in collective ownership so you should only merge changes onto it with the permission of your colleagues and ideally you want others to look at your changes and approve the pull themselves.

I started doing some fiddling around with my code and now I realise it’s serious and I want to put it on a branch, what did I do?

You need to stash your code, using:

git stash

Then create a branch, as described above, and then retrieve the contents of the stash:

git stash pop

That’s how we use git – what do you do?