Now that we’ve covered the mechanics behind Git branches, we can discuss the practical impact that they have on the software development process. Instead of introducing new commands, this module covers how the typical Git user applies this workflow to real projects, as well as some of the problems that arise in a branched environment.
To Git, a branch is a branch, but it’s often useful to assign special meaning to different branches. For example, we’ve been using master as the stable branch for our example project, and we’ve also used a temporary branch to add some CSS formatting. Temporary branches like the latter are called topic branches because they exist to develop a certain topic, then they are deleted. We’ll work with two types of topic branches later in this module.
Amid our exploration of Git branches, we’ll also discover that some merges cannot be “fast-forwarded.” When the history of two branches diverges, a dedicated commit is required to combine the branches. This situation may also give rise to a merge conflict, which must be manually resolved before anything can be committed to the repository.
DOWNLOAD THE REPOSITORY FOR THIS MODULE
If you’ve been following along from the previous module, you already have everything you need. Otherwise, download the zipped Git repository from the above link, uncompress it, and you’re good to go.
Let’s start by checking out the crazy branch.
git branch git checkout crazy git log --oneline
The crazy branch is a longer-running type of topic branch called a feature branch. This is fitting, as it was created with the intention of developing a specific feature. It’s also a term that makes Git’s contribution to the development workflow readily apparent: branches enable you to focus on developing one clearly defined feature at a time.
This brings us to my rule-of-thumb for using Git branches:
Following these simple guidelines will have a dramatic impact on your programming efficiency.
Merge the CSS Updates
Note that the CSS formatting we merged into master is nowhere to be found. This presents a bit of a problem if we want our experiment to reflect these updates. Conveniently, Git lets us merge changes into any branch (not just the master branch). So, we can pull the updates in with the familiar git merge command. Remember that merging only affects the checked-out branch.
git merge master git log --oneline
As of Git 1.7.10, this will open your editor and prompt you for a message explaining why the commit was necessary. You can use the default Merge branch ‘master’ into crazy. When you save and close the file, you’ll notice an extra commit in your project history. Recall that our first merge didn’t add any new commits; it just “fast-forwarded” the tip of the master branch. This was not the case for our new merge, which is shown below.
Take a moment to examine why the current merge couldn’t be a fast-forward one. How could Git have walked the crazy pointer over to the tip of the master branch? It’s not possible without backtracking, which kind of defeats the idea of “fast-forwarding.” We’re left with a new way to combine branches: the 3-way merge.
A 3-way merge occurs when you try to merge two branches whose history has diverged. It creates an extra merge commit to function as a link between the two branches. As a result, it has two parent commits. The above figure visualizes this with two arrows originating from the tip of crazy. It’s like saying “this commit comes from both the crazy branch and from master.” After the merge, the crazy branch has access to both its history and the master history.
The name comes from the internal method used to create the merge commit. Git looks at three commits (numbered in the above figure) to generate the final state of the merge.
This kind of branch interaction is a big part of what makes Git such a powerful development tool. We can not only create independent lines of development, but we can also share information between them by tying together their histories with a 3-way merge.
Now that we have access to the CSS updates from master, we can continue developing our crazy experiment. Link the CSS stylesheet to rainbow.html by adding the following HTML on the line after the element.
Stage and commit the update, then check that it’s reflected in the history.
git status git commit -a -m "Add CSS stylesheet to rainbow.html" git log --oneline
Notice that we skipped the staging step this time around. Instead of using git add, we passed the -a flag to git commit. This convenient parameter tells Git to automatically include all tracked files in the staged snapshot. Combined with the -m flag, we can stage and commit snapshots with a single command. However, be careful not to include unintended files when using the -a flag.
We still need to add a navigation link to the home page. Change the “Navigation” section of index.html to the following.
As usual, stage and commit the snapshot.
git commit -a -m "Link index.html to rainbow.html" git log --online
Next, we’re going to brainstorm an alternative to the current rainbow.html page. This is a perfect time to create another topic branch:
git branch crazy-alt git checkout crazy-alt
Remember, we can do whatever we want here without worrying about either crazy or master. When git branch creates a branch, it uses the current HEAD as the starting point for the new branch. This means that we begin with the same files as crazy (if we called git branch from master, we would have had to re-create rainbow.html). After creating the new branch, our repository’s history looks like:
Our boss called in with some breaking news! He needs us to update the site immediately, but what do we do with our rainbow.html developments? Well, the beauty of Git branches is that we can just leave them where they are and add the breaking news to master.
We’ll use what’s called a hotfix branch to create and test the news updates. In contrast to our relatively long-running feature branch (crazy), hotfix branches are used to quickly patch a production release. For example, you’d use a hotfix branch to fix a time-sensitive bug in a public software project. This distinction is useful for demonstrating when it’s appropriate to create a new branch, but it is purely conceptual—a branch is a branch according to Git.
git checkout master git branch news-hotfix git checkout news-hotfix
Change the “News” list in index.html
And, create a new HTML page called news-1.html
Blue Is The New Hue
European designers have just announced that Blue will be this year's hot color.
Return to home page (index.html)
We can’t use git commit -a to automatically stage news-1.html because it’s an untracked file (as shown in git status). So, let’s use an explicit git add:
git add index.html news-1.html git status git commit -m "Add 1st news item"
Test these additions in a browser to make sure that the links work, it’s typo free, etc. If everything looks good, we can “publish” the changes by merging them into the stable master branch. Isolating this in a separate branch isn’t really necessary for our trivial example, but in the real world, this would give you the opportunity to run build tests without touching your stable project.
Remember that to merge into the master branch, we first need to check it out.
git checkout master git merge news-hotfix
Since master now contains the news update, we can delete the hotfix branch:
git branch -d news-hotfix git branch
The following diagram reflects our repository’s history before and after the merge. Can you figure out why was this a fast-forward merge instead of a 3-way merge?
Also notice that we have another fork in our history (the commit before master branches in two directions), which means we should expect to see another merge commit in the near future.
Ok, let’s finish up our crazy experiment with one more commit.
git checkout crazy
Note that the news article is nowhere to be found, as should be expected (this branch is a completely isolated development environment).
We’ll finish up our crazy experiment by adding a news item for it on the home page. Change the news list in index.html.
Astute readers have probably observed that this directly conflicts with what we changed in the news-hotfix branch. We should not manually add in the other news item because it has no relationship with the current branch. In addition, there would be no way to make sure the link works because news-1.html doesn’t exist in this branch. This may seem trivial, but imagine the errors that could be introduced had news-hotfix made dozens of different changes.
We’ll simply stage and commit the snapshot as if there were no conflicts:
git commit -a -m "Add news item for rainbow" git log --oneline
Look at all those experimental commits (marked with asterisks below)!
*42fa173 Add news item for rainbow *7147cc5 Link index.html to rainbow.html *6aa4b3b Add CSS stylesheet to rainbow.html b9ae1bc Merge branch 'master' into crazy ae4e756 Link HTML pages to stylesheet 98cd46d Add CSS stylesheet *33e25c9 Rename crazy.html to rainbow.html *677e0e0 Add a rainbow to crazy.html 506bb9b Revert "Add a crazzzy experiment" *514fbe7 Add a crazzzy experiment 1c310d2 Add navigation links 54650a3 Create blue and orange pages b650e4b Create index page
We’re finally ready to merge our crazy branch back into master.
git checkout master git merge crazy
You should get a message that reads:
Auto-merging index.html CONFLICT (content): Merge conflict in index.html Automatic merge failed; fix conflicts and then commit the result.
This is our first merge conflict. Conflicts occur when we try to merge branches that have edited the same content. Git doesn’t know how to combine the two changes, so it stops to ask us what to do. We can see exactly what went wrong with the familiar git status command:
# On branch master # Changes to be committed: # # new file: rainbow.html # # Unmerged paths: # (use "git add/rm ..." as appropriate to mark resolution) # # both modified: index.html #
We’re looking at the staged snapshot of a merge commit. We never saw this with the first 3-way merge because we didn’t have any conflicts to resolve. But now, Git stopped to let us modify files and resolve the conflict before committing the snapshot. The “Unmerged paths” section contains files that have a conflict.
Open up index.html and find the section that looks like:
Git went ahead and modified the conflicted file to show us exactly which lines are afflicted. The format of the above text shows us the difference between the two versions of the file. The section labeled <<<<<<< HEAD shows us the version in the current branch, while the part after the ======= shows the version in the crazy branch.
We can change the affected lines to whatever we want in order to resolve the conflict. Edit the news section of index.html to keep changes from both versions:
The <<<<<<<, =======, and >>>>>>> markers are only used to show us the conflict and should be deleted. Next, we need to tell Git that we’re done resolving the conflict:
git add index.html git status
That’s right, all you have to do is add index.html to the staged snapshot to mark it as resolved. Finally, complete the 3-way merge:
We didn’t use the -m flag to specify a message because Git already gives us a default message for merge commits. It also gives us a “Conflicts” list, which can be particularly handy when trying to figure out where something went wrong in a project. Save and close the file to create the merge commit.
The final state of our project looks like the following.
Since our crazy experiment has been successfully merged, we can get rid of our feature branches.
git branch -d crazy git branch -d crazy-alt
As noted in the last module, the git branch -d command won’t let you delete a branch that contains unmerged changes. But, we really do want to scrap the alternative experiment, so we’ll follow the error message’s instructions for overriding this behavior:
git branch -D crazy-alt
Because we never merged crazy-alt into master, it is lost forever. However, the crazy branch is still accessible through its commits, which are now reachable via the master branch. That is to say, it is still part of the structure of the repository’s history, even though we deleted our reference to it.
Fast-forward merges are not reflected in the project history. This is the tangible distinction between fast-forward merges and 3-way merges. The next module will discuss the appropriate usage of both and the potential complications of a non-linear history.