What's the use of `-u` in `git push -u origin master`? [duplicate]

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What exactly does the "u" do? "git push -u origin master" vs "git push origin master"

In Github, when you created an empty repository, the instructions ask you to execute

git push -u origin master

So my question is, what's the use of -u option?

After reading the manpage I still didn't get it.

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git can set a particular branch in a remote repository to be the default "upstream" branch for that particular branch. For example, if you clone an existing repository, git will, by default, associate your master branch with the master branch in the origin repository, i.e. the one you're cloning from. This means that git can provide helpful defaults, such as being able to just use git pull while on master rather than having to specify a repository and a branch to fetch and merge from. It's also this association that lets git produce its helpful "Your branch is ahead of origin/master by 10 commits" messages...

However, if you haven't cloned from an exisiting repository, but you're wanting to set up a new origin remote that represents your newly created GitHub repository, you have to manually tell git to associate your master with master in the new origin repository. The -u to git push means "as well as pushing, associate my master branch with the one I'm pushing to". You only need to do this once for that association to be recorded in .git/config.