How does Git's fast-forward mode work?

Merging a branch is one of the most common operations when working with Git. Depending on your team and projects you've been a part of, you might have heard of or even used Git's fast-forward mode when merging. Fast-forward mode is the default in Git, however GitHub will essentially override this by default and create a merge commit instead.

Fast-forward merge

As stated above, Git's default is to use fast-forward merge. It will take the commits from the branch being merged and place them at the tip of the branch you're merging into. This creates a linear history, which is also the main advantage of using fast-forward merge. If you want to emulate fast-forward merge on GitHub, you can use the "Rebase and merge" option.

Non fast-forward merge

GitHub, on the other hand, uses non fast-forward merge by default. It will create a merge commit at the tip of the branch you're merging into, optionally referencing the branch being merged in the commit message. This has the advantage of keeping track of branches more explicitly than fast-forward merge. If you want to get the same behavior in a Git terminal, you can use the --no-ff flag.

As a side note, you can configure the default Git merge behavior, using git config. To learn how to do so, you can take a look at the relevant snippet.