Fast-forward merges

A branch lets you work on something without touching main. When the work is done, you merge it: git merge <branch> brings that branch's commits into the branch you're on.

Merge into the branch you're on

git merge always changes the branch you're on, never the one you name. To bring contact into main, switch to main first:

git switch main
git merge contact

The other way round, git merge main while you're on contact, would bring main into contact instead, and leave main where it was.

Fast-forward

Say main hasn't moved since contact split off from it. Then contact is simply ahead: its history already has every commit on main, plus some more. There's nothing to combine, so git moves the main label forward to contact's commit. That's a fast-forward. It makes no new commit, and the history stays a straight line.

$ git merge contact
Updating a4b8d97..0e81585
Fast-forward
 contact.html | 1 +
 index.html   | 1 +
 2 files changed, 2 insertions(+)
 create mode 100644 contact.html

Updating names the commit main was on and the one it moved to. Below that, git sums up what changed in your files.

When there's nothing to bring in, because every commit on the other branch is already on yours, git merge says Already up to date. and changes nothing.

A merge brings every commit

A merge brings in every commit the other branch has and yours doesn't, not only its newest one. If you started colours from heading, then colours has heading's commit too, and merging colours brings in both. The Live Graph shows it: heading's commit is in colours' line of history.

Deleting a merged branch

Once a branch is merged, its commits are on main too, so git branch -d deletes the branch without complaint. Only the label goes: the commits stay where they are. git branch -d checks the branch you're on: it deletes a branch whose commits are all in your branch's history, and refuses otherwise.

Keeping a straight line

A fast-forward is only possible while the branch you're on hasn't moved. If you commit on docs and then merge main into it, both have moved on, and git has to join them with a merge commit instead. That's the next Lesson. To bring a branch up to date and keep its history a straight line, merge first, then commit.

Command	What it does
git merge <branch>	Brings <branch>'s commits into the branch you're on. If your branch hasn't moved since <branch> split off, it fast-forwards: your branch's label moves forward, and no new commit is made.