Merge commits

A fast-forward only works while your branch hasn't moved since the other one split off. Often both have: you committed on main while contact was still in progress. The branches have diverged: each has commits the other doesn't.

A merge commit joins two branches

git merge contact can't fast-forward then. Instead, git makes a merge commit: a new commit on the branch you're on, with two parents, your branch's commit and the other branch's. Its files combine the changes made on both sides since they split.

$ git merge contact
Merge made by the 'ort' strategy.
 contact.html | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 contact.html

ort is the name of the method git uses to combine the changes; you can ignore it. Below it, git sums up what the merge brought into your files.

A merge commit gets a ready-made message, Merge branch 'contact'. On your own computer, git first opens your text editor with that message, so you can change it; saving it as it is is fine.

Only the branch you're on moves. contact still points at its own commit, which is now in main's history too, so git branch -d contact can delete it.

When both branches changed the same lines of the same file, git can't combine them by itself. That's a merge conflict, and it's the next Lesson. In this one, the branches always change different files.

Reading a merge in the history

git log --oneline --graph draws the two lines of history and where they join:

*   1c0c498 (HEAD -> main) Merge branch 'contact'
|\
| * 5b2a470 (contact) Add a contact page
* | a6b0f1b Make the heading purple
|/
* a4b8d97 Style the heading
* ab89288 Add the home page
* 37d7cf7 Add a README

A merge commit on purpose: --no-ff

When a merge can fast-forward, git merge does, and afterwards nothing in the history shows that those commits were ever on a branch of their own. git merge --no-ff <branch> ("no fast-forward") makes a merge commit anyway. Many teams merge finished work this way: the work's commits stay grouped behind one merge commit, which is easy to find later.

When the branches have diverged, --no-ff changes nothing: git makes a merge commit either way.

Command	What it does
git merge <branch>	Fast-forwards when it can. When the branches have diverged, makes a merge commit that joins them.
git merge --no-ff <branch>	Always makes a merge commit, even when git could fast-forward.