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Git from zero to hero: branches, merging and conflicts

Part 4 of the Git guide: what a branch really is, fast-forward and merge commits, how a three-way merge decides, and how to resolve a conflict calmly.

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A branch is the label from part 2 that moves forward with each commit. This part uses one to try a pizza recipe in the recipe book without touching main.

Branches

git switch -c pizza

You’re now on pizza. Commits go there, and main stays put:

echo "# Pizza" > pizza.md
git add pizza.md
git commit -m "Add pizza recipe"
echo "Bake 20 minutes." >> pizza.md
git commit -am "Add baking time"

main has commits A, B and C. A second branch called feature starts at B and has its own commits D and E. HEAD points at feature, so the next commit goes there.

A branch costs almost nothing, so make one per idea.

git branch            # list branches, * marks the current one
git switch main       # go back; pizza.md disappears from the folder
git switch pizza      # and it's back

What a branch really is

A branch isn’t a copy of your project. It’s a tiny file with one commit hash in it:

cat .git/refs/heads/main
cat .git/HEAD

9c1d4e2a7b8c9d0e1f2a3b4c5d6e7f8091a2b3c4
ref: refs/heads/pizza

The first file is the main branch: a hash, nothing else. The second is HEAD, saying “I’m on pizza”.

  • Creating a branch writes one small file.
  • Switching changes what HEAD says and swaps your working files for that snapshot.
  • Deleting a branch removes the label. The commits stay, and git reflog can find them until Git cleans up unreachable commits after a grace period.

Merging

The pizza is done. Bring it into main:

git switch main
git merge pizza

What happens depends on whether main moved while you were away:

Two scenarios. Top, fast-forward: main had nothing new, so Git just moves the main label forward to the last commit of feature. Bottom, merge commit: both branches have new commits, so Git creates a merge commit M with two parents.

  • If main hasn’t changed, Git does a fast-forward: it moves the main label ahead. No new commit.
  • If both branches have new commits, Git makes a merge commit with two parents, tying the histories together.

Then clean up:

git branch -d pizza    # safe: refuses if it wasn't merged

How Git merges

Git finds the merge base, the last commit both branches share, and compares each branch against it:

Commits A and B are shared history. main continues with C and feature continues with D. Both C and D point back to B, the merge base. The merge commit M points to both C and D.

git merge-base main pizza    # prints the hash of B

Git combines what changed from B to C and from B to D:

  • A line changed on one side only is taken as is.
  • A line changed on both sides, to the same thing, is taken once.
  • A line changed on both sides, differently, is a conflict. Git can’t know which you meant.

Most of the time the two branches touch different places, so merges usually just work.

Conflicts

Auto-merging pizza.md
CONFLICT (content): Merge conflict in pizza.md
Automatic merge failed; fix conflicts and then commit the result.

Open the file. Git marked both versions:

<<<<<<< HEAD
Bake 20 minutes.
=======
Bake 25 minutes.
>>>>>>> pizza

Between <<<<<<< and ======= is yours (main), and the rest is theirs (pizza). Decide, delete the markers, and keep what you want:

Bake 25 minutes.

git add pizza.md
git commit            # finishes the merge

Changed your mind halfway? git merge --abort puts everything back. A conflict is not an error, it’s Git asking a question only you can answer.

  • Merge main into your branch often (or rebase onto it), so differences never pile up. Keep branches short-lived and focused.
  • git status during a conflict lists Unmerged paths, the files still waiting for you. git add marks one resolved.
  • To take one side wholesale, use git checkout --ours <file> or --theirs, then git add it. During a rebase the meaning flips: “ours” is the branch you’re rebasing onto, “theirs” is your own commit being replayed.
  • Editors (VS Code, JetBrains) offer one-click “accept current / incoming”, and git mergetool opens a visual tool.

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