Engineering Craft › Version Control (Git)

Merge

Combining the histories of two branches.

Also known as: git merge, merge commit, merging branches

Merging combines the work from one branch into another. You switch to the branch that should receive the changes and merge the other one in.

git switch main
git merge feature/login

What Git does

• Fast-forward: if main hasn’t moved since you branched, Git just slides main forward to your latest commit. No new commit is made (fast-forward merge).

Before:  A ─ B (main)            After:  A ─ B ─ C ─ D (main, feature)
                \ C ─ D (feature)

• True merge: if both branches have new commits, Git creates a merge commit with two parents, tying the histories together.

A ─ B ─ E ─────── M (main)
     \           /
      C ─── D ───  (feature)

When changes overlap

If both sides changed the same lines, Git stops and asks you to resolve the merge conflict, then finish with a commit. git merge --abort returns to how things were.

In practice

Merges usually happen when a pull request is accepted, and the host offers different styles: a merge commit, a squash into one commit, or a rebase (merge strategies).

To bring the latest main into your feature branch while you work, you can merge main in, or rebase onto it (merge vs rebase).

Tips

• Commit or stash first, so your uncommitted work isn’t mixed into a conflict.
• Pull before merging so you merge the latest main.
• Test after merging, since clean merges can still produce code that doesn’t work together.
• Merging doesn’t delete the branch. Remove it afterwards if it’s done.