Local branch vs remote branch on "git fetch"

Question

I have a remote branch that my friend created, this branch does not exist in my local repo (not checked out).

I run git fetch,then git checkout <myFriendBranch>, now i'm up to date with my friend branch, and I do not need to run git merge or git pull.

I wonder why?

For example, if there is a branch that we both working on, and exist on my local device,git fetch alone won't update my working directory and I will need to run git merge or git pull.

Answer 1

Let's start by explaining ow git store its information internally.

git store its data inside the .git folder. This folder contains information about local and remote branches.

Both local and remote branches are simply a pointer to a SHA1, and are stored inside the .git folder as u can see below.

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What is git fetch?

git fetch is a porcelain command which sync the remote repository to your local repository. It simply downloading all the hunks, the delta of content which does not exists yet on your machine into your .git folder without making any modifications to your working directory, simply download and store the ocntent for later use.

git fetch does not delete deteled remote branches so if you want to "clean" your local repository and remove the deleted branches use the --prune flag.
adding --all will fetch all the reomtes if you have more than a single one.

git fetch --all --prune

At this point the latest commits are only stored inside the .git folder. If you wish to update your local workind dir you need to merge it. keep on understand to how and why.

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What is git checkout?

git checkout <name> is switching between branches. First git check to see if the branch exixts locally - if it was already checked out previously, if not if will search for it under the remotes and than will checkout it as a new local branch.

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What is git pull?

git pull is simply an alises for git fetch + git merge.
git merge will add the desired branch content into your current branch.
git pull origin master for exampel will fetch the remote and will download the missing content as explaind above and than will merge (update) it into your local working directory.

Here is a detailed output of the git pull command, you can see that git first fetch the content and than merge it back to your branch

Answer 2

It seems your question is about when you do have to merge or pull, vs. when you don't (and can instead just fetch and checkout). Since pull is basically "fetch and merge", this boils down to: when do I have to merge, and why?

You merge to incorporate changes (which, for example, might be changes fetched from a remote) into an existing branch.

In your first example, your friend created a branch, and you're just copying your friend's branch as it is; you're not incorporating their changes into any existing branch, but rather you're creating a new branch from their changes. So you don't need to merge anything. Instead, after fetch you create a new branch. (You happen to be using a shortcut to cause the branch to be created, because git recognizes your intent when you try to checkout a non-existing branch but exactly one remote tracking ref matches the given name.)

Answer 3

git fetch downloads the changes from your friend's repository and creates a branch named origin/friends-branch in your local repository. When you do git checkout friends-branch, Git recognizes the remote tracking branch and automatically creates a new local branch with the name friends-branch.