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Saturday, July 18, 2009

The Journey to Git, Part IV--Branching

Git post 4: Branching. Before you read this post, you should be familiar enough with Git to be comfortable creating a repository and adding and committing files to it. The previous posts in this series will bring you up to that point if you aren't.

Note: This post assumes that you followed along with the commands given in the previous post and have a version-controlled directory with a couple of commits in it. If you don't have that, then create a new git repository in an empty directory, and create a file named "foo" with one line of text in it and commit it. That will get you to where you need to be to start this walkthrough.


Articles in this series:

I'm going to assume that you're already familiar with the concept of a branch in a VCS. What you need to know is that Git branches are first-class citizens. They're very lightweight and easy to create and work with. With Git, it's not a question of if you'll branch, but of how many branches you'll have going at once.

Creating Branches

Let's create a branch for some concurrent development:

git branch mybranch

That produces no output if it's successful, but it creates a new branch named "mybranch" starting from your current location: the most recent commit of your current branch. View your current branches with:

git branch

It should show the following, indicating you have two branches and are currently on branch master:

* master

Now switch to the branch you just created:

git checkout mybranch

Note: You can both create and move to a branch at the same time with: "git checkout -b mybranch"

In a previous post, you used "git checkout" to pick out a specific commit and make that commit your working tree. It does essentially the same thing when you give it a branch name as an argument. In another post, I'll cover checkout a bit more thoroughly. For now, let's create a new file on this branch:

echo "Branch file" >> bar
git add bar
git commit -m "Added bar on mybranch"

Now "git status" should show that you're on branch mybranch and have nothing to commit, while "git log" will show all three of the commits you've made so far. Move back to master with:

git checkout master

Now bar is gone because it only exists on mybranch, and you'll see that "git log" doesn't show the commit that added it. That's because it shows the commit history of the current branch.

Renaming Branches

This will be a short section:

git branch -m mybranch myotherbranch

That's it. The branch is renamed.

Deleting Branches

Deleting branches is almost as simple. Try this:

git branch -d myotherbranch

It should give you a message like:

error: The branch 'myotherbranch' is not an ancestor of your current HEAD.
If you are sure you want to delete it, run 'git branch -D myotherbranch'.

The keyword "HEAD" refers to the most recent commit of the branch you've checked out. What it's telling you is that you have commits in myotherbranch that aren't in your current branch. If that weren't the case, the delete would have succeeded. Git is very flexible, but it tries to protect you from losing things accidentally. Do what the error said and use the flag that forces a delete:

git branch -D myotherbranch

You'll see a confirmation message. That branch--along with the changes on it--are gone. Make a habit of using the -d form, and you're less likely to accidentally lose things by deleting branches that have changes that don't exist elsewhere.

That's really all there is to branching in Git. Like I said, it's very easy to work with branches in Git. Stick around for more branch goodness in the form of merging in my next post: Part V--Merging

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