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git merge
is a tricky little command because—like too many other Git commands, in my opinion—it can do one of several different things. One of these is a true merge, in which Git creates a new commit that has two (or more but let's not go there) parents. The other is what Git calls, variously, a fast-forward or, especially when done viagit merge
, a fast-forward merge (kind of a misnomer as there is no actual merging occurring!). Fast-forward is actually a property of label movement. Note that when we usegit branch -f
orgit reset
, we move some branch name ... 1/git push
andgit fetch
favor fast-forward operations: they will happily adjust branch names, and for fetch, remote-tracking names, that result in fast-forwards. They object to non-fast-forward updates, requiring--force
or equivalent. This guarantees that the change, whatever it is, preserves all the existing commits and merely adds some more (or in the degenerate case, changes nothing at all—the label already points to the new commit). Anyway, that's great for fetch and push, but what about merge? 3/git merge <commit-hash-or-name>
operation, the merge code inspects the current commit (usingHEAD
to find its hash ID) and the target commit (whatever you named on the command line), and does one of these ancestor tests. If the current commit is an ancestor of the target commit, the merge operation is deemed unnecessary: a fast-forward will do. This is where--ff-only
and--no-ff
enter the picture: the former requires it and the latter forbids it, forcing Git to make a merge commit instead. 4/