Thursday, 19 August 2010

The power of git - splitting a commit


Every blog has to have a customary first post - or at least, almost every blog seems to. Rather than mine just saying that I'm starting a new blog, and I'll try and keep it up to date and blah blah blah, I thought I'd actually post some content.

For the past 9 months or so, I've been fairly heavily involved in an open source project called Mahara. Mahara is an ePortfolio system and I'm sure that I'll post about it more in the near future so I won't go into lots of detail now. Needless to say though, the work I've been doing on it has involved a variety of customisations and any sane developer stores, manages and tracks their work using a version control system or VCS. The mahara project uses git.

Git is probably the most powerful version control system I've come across and enables you to manage your source and commits with minimal effort and confusion - it's probably the only VCS I've ever used which works for you rather than making you work around it. That said, some of the really powerful features of git take a bit of understanding.

Earlier today I was telling someone how you can use git in some really cool and interesting ways. One of the things I do frequently is to take a commit and split it into a series of logical commits and I thought that I'd share this.

Setting up a quick git repository
If you want to try and follow with my examples, I've pushed my sample repository to gitorious.org. You can skip the first part by cloning it with:
git clone git://gitorious.org/thamblings/git-split-commits.git

Say that you've created a quick git repository with a few files in it:
# Create a new git repository
cd /tmp
mkdir git-example
cd git-example/
git init

# Commit the first file
echo "First File" >> one 
git add one 
git commit -m "First file committed"

# Now add a few more commits/files
echo "Second File" >> two 
echo "Third File" >> three
git add .
git reset
git add two 
git commit -m "Second File"
git add three
git commit -m "Third File"

# Now add two files in the same commit
echo "Fourth File" >> four
echo "Fifth File" >> five
git add .
git commit -m "Fourth and Fifth Files"

# And another file
echo "Sixth File" >> six 
git add .
git commit -m "Sixth File"

# Let's see what we've done
git log

But we made a mistake...
But wait a second - files four and five should have been in different files. Let's split them out.

Let's rewrite history
First we'll check out a new branch, and then we'll do an interactive rebase to edit the commit:
git checkout -b fixcommits
git rebase -i HEAD~2

We should get something like:
pick 0d3abd7 Fourth and Fifth Files
pick feaeea8 Sixth File

We want to edit commit 0d3abd7, so change that to an edit - you can use 'e' instead:
edit 0d3abd7 Fourth and Fifth Files
pick feaeea8 Sixth File

That takes us to the point after the commit 'Fourth and Fifth Files' and before 'Sixth File'.
We can then use git reset the state of our current HEAD.
git reset HEAD~1

We can then re-add and re-commit each file:
git add four
git commit -m "Fourth File"
git add five
git commit -m "Fifth File"

We can then continue our rebase and we'll be left with our finished article:
git rebase --continue

Let's get things back to master
And if we're happy with what we have, we can check it back in to our master branch with another rebase
git checkout master
git rebase fixcommits

The same also works for other git additions. So you could add parts of a file with an interactive git add (git add -pi) for example.