Branches are just names

People picture a branch as a whole separate copy of the project. It isn’t. A branch is a name attached to one commit: a tiny file holding a hash. Making a branch costs nothing and copies nothing, which is why git users make them constantly.

Every commit also remembers its parent, the commit that came before it. So a branch name plus the parent links gives you a whole history: start at the commit the name points to and follow the parents back.

When you commit on a branch, git makes the new commit and moves that branch’s name forward to it. The other branches don’t move. That’s the whole trick.

Branch, change, merge

You start with one commit on main. Make a branch, commit on it, then go back to main and watch your change vanish from the file. It isn’t lost; main simply points at an older snapshot. Then merge.

Fast-forward vs. a real merge

In this mission, main didn’t move while you were away. Merging only had to slide the main label forward to where idea was. Git calls this a fast-forward, and it creates no new commit.

If main had moved, both branches would hold work the other lacks. Then git does a three-way merge. It compares both branch tips with their last common ancestor, combines the changes, and records a merge commit with two parents. Try it in the playground: commit on both branches before you merge.

git switch -c idea
echo "bamboo can grow 90cm in a day" >> notes.md
git commit -am "add a fact"
git switch main
git merge idea
git log --oneline --graph --all