GitNoon Lesson 3

In this tutorial, you will learn how to:

1. Work on a separate branch
2. Merge your branch into main
3. Resolve a merge conflict

Viewing a repo as a graph

Before we get started, we’re going to create a git alias to conveniently run git log with options that will display a succinct graphical view of all branches in our repo:

git config --global alias.graph "log --all --graph --oneline --decorate"

Open your terminal to your blog repo (cd blog), and try running it:

git graph

• (press q to exit git graph)

Understanding the graph

• You should see a commit on each line
• You should see labels for your main branch and the origin/main remote branch.
  • The key idea to understand branches in Git is that a branch is effectively a pointer to a specific commit.
• You should also see a HEAD label pointing to your main branch.
  • HEAD indicates the branch that our repo has checked-out, which has two key implications:
    1. HEAD represents the current version of the files in our directory - any file changes will be compared to HEAD
    2. When you run git commit, the new commit will have the current HEAD commit as its parent, and the HEAD branch will be updated to point to the new commit
       • You can already see these parent/child links between commits by the chain/history of commits in the log.
  • Think of HEAD like the head on a spinning hard disk - it points to the data that is currently being read or written.
• You may also see origin/HEAD, which just indicates the default branch for new clones of the origin remote.

Working on a separate branch

We can keep our work isolated from the main branch while it is still a work-in-progress by committing to a separate branch.

To do so, let’s create a new branch for some additions to the README:

git branch readme-additions

We can see our newly created branch in the graph; it points to the same commit as whatever branch HEAD was pointing to:

git graph

Now let’s check-out our new branch:

git switch readme-additions

We can confirm our current branch by again checking the graph and also by checking git status:

git graph
git status

Now let’s make some changes on our new branch, open up the README:

nano README.md

And add more information at the bottom of the file:

## Built with

* Markdown

Save and exit nano (Ctrl + x, y, Enter), then add and commit the README:

git status
git add README.md
git status
# We'll use a shortcut to write the message inline
git commit -m "Add tool info to README"
git status

Now let’s see what that’s done to the graph:

git graph

Success! The new commit has been added to the readme-additions branch, but not main.

Now let’s switch back to the main branch, again confirming the change to HEAD in the graph and status:

git switch main
git graph
git status

When we look at the log from main, we don’t see that commit:

git log

And when we look at the README, we don’t see our latest additions:

cat README.md

Merge your branch into main

Now that we’ve finished the work on our feature branch let’s merge it back into main.

This is as simple as running:

git merge readme-additions

• TIP: To remember which branch will be updated by a merge, always remember that almost all git commands will only change the branch you are checked-out on.
  • In this case, we are checked-out on main, so we will be updating the main branch by merging in readme-additions

Now let’s see what that’s done to the graph:

git graph

We can see that main has been updated to the same commit as readme-additions, and the README on main now has the additions:

cat README.md

Fast-forward merges

Notice that Git told us this was a fast-forward merge:

• A fast-forward merge is only possible when the current branch has no commits that aren’t already in the branch we are merging in
  • i.e. Our current branch is linearly/directly behind the branch being merged in.
• A fast-forward merge just updates the current branch to point to the same commit as the named branch.
• We also saw a fast-forward merge in the last lesson when updating our local main branch with the latest commits fetched from origin/main.

Non-fast-forward merges

Let’s see what happens when a fast-forward merge is not possible.

First off, let’s undo that previous merge by resetting the main branch to point to the same commit as origin/main again:

git reset --hard origin/main
git graph

IMPORANT: git reset is a dangerous command because it can delete both commits and uncommitted changes. We’ll learn more about it in the next lesson.

Now let’s add a commit directly to the main branch, as if someone else had merged in some changes while we were still working on readme-additions:

nano README.md

Change the README’s title to something else:

# My Awesome Blog

Save and exit nano (Ctrl + x, y, Enter), then add and commit the README:

git status
git add README.md
git status
# We'll use a shortcut to write the message inline
git commit -m "Update README title"
git status

Now let’s look at the graph:

git graph

• The branches have diverged!
• There is now a commit in main that doesn’t exist in readme-additions
• This means that main cannot be fast-forwarded to readme-additions
• Aside: git log can be used to check which commits exist in one branch but not another with the “two dots” syntax:

# Commits in readme-additions but not main
git log main..readme-additions
# Commits in main but not readme-additions
git log readme-additions..main

Now what happens when we merge :

git merge readme-additions

• Git opens a text editor for us to edit a commit message for a merge commit
  • Just save and exit to confirm the message (Ctrl + x, y, Enter)

Now check the graph:

git graph

• Git has added a merge commit that combines the commits from both branches.
• The main branch has been updated to point to the new merge commit.

So what does the README look like now on the main branch?

cat README.md

Because each branch had made changes to different parts of the same file, Git was able to combine both sets of changes into a single file without us having to do anything!

Resolve a merge conflict

But what if both branches had changed the same part of the same file?

Let’s find out!

Again, let’s undo the previous merge and the commit we made on main by resetting the main branch to point to the same commit as origin/main again:

git reset --hard origin/main
git graph

Now let’s make a commit directly on main again, but this time editing the same part of the file as our commit on readme-additions:

nano README.md

Add some different content to the bottom of the file:

## Thanks

* Git

Save and exit nano (Ctrl + x, y, Enter), then add and commit the README:

git status
git add README.md
git status
# We'll use a shortcut to write the message inline
git commit -m "Add thanks to README"
git status

Look at the graph, and you should see the branches diverging again:

git graph

Now let’s see what happens when we try to merge readme-additions into main:

git merge readme-additions

• Aha! Git tells us that there is a merge conflict in the README
• A merge conflict is not a cause for panic!
• Let’s see more about the conflict by checking the status:

git status

• We can see README.md is an unmerged path where both branches made modifications
• Let’s see what’s going on in the README:

git diff README.md

• We see two alternative hunks at the bottom of the file
  • One from our current branch (i.e. HEAD, which is main)
  • One from the branch we are merging in (i.e. readme-additions)
• We need to manually edit the file to have the correct content that resolves this conflict
  • This should often involve a conversation with whomever wrote the other code
  • Don’t just blindly choose to keep one or the other
  • If you don’t want to resolve it now, you could run git merge --abort
• (press q to exit git diff)

nano README.md

To resolve this conflict, let’s remove the conflict markers and combine the two changes into one:

## Thanks

* Git
* Markdown

• Save and exit nano (Ctrl + x, y, Enter)
• Check git status:

git status

The README is still unmerged, let’s check git diff again:

git diff README.md

• The two columns of pluses and minuses can see the results of our manual conflict resolution:
  • The changes from readme-additions will effectively lose Built with but gain Thanks and Git
  • The changes from main will effectively gain Markdown

We can now add our resolution:

git add README.md
git status

The resolution is now staged and ready to be committed, so let’s complete the merge with a merge commit:

git commit

• Git opens a text editor for us to edit a commit message for a merge commit
  • Just save and exit to confirm the message (Ctrl + x, y, Enter)
• The merge is now complete!
• Let’s check what that’s done to the repo:

git status
git graph

• We see a merge commit just like we did for the previous merge
• This was a relatively straightforward conflict
• Sometimes Git is able to automatically merge changes that may still be incompatible with each other
  • For example, maybe one branch renamed a variable, while the other branch added a new reference to the old variable name.
  • So you should validate and test after any merge!

To wrap-up, let’s undo the last merge and commit on master by resetting again:

git reset --hard origin/main
git graph

Conclusion

• You’ve learned a lot about branching and merging in this lesson, including three different kinds of merging:
  • Fast-forward merges
  • Non-fast-forward merges with merge commits
  • Merges with conflicts
• In the next lesson, you’ll learn how to:
  1. Make a pull request for your feature branch
  2. Rebase a feature branch that is conflicting with main in order to avoid unnecessary merge commits.