Git: difference "git rebase origin/branch" VS "git rebase origin branch"

git rebase <upstream> <branch>

is equal to

git checkout <branch>
git rebase <upstream>

By default <branch> is HEAD.

[1] https://www.kernel.org/pub/software/scm/git/docs/git-rebase.html

@Mar's answer is right and perfectly solved this question, just add one comment.

if you want to rebase a branch based on remote master branch, git rebase origin/master is not enough, it will not get new commits directly from origin/master. You need to git fetch before git rebase origin/master.

or you can use another way to rebase a branch.

1. switch to master git checkout master
2. git pull origin master
3. switch back to your own branch git checkout {your branch}
4. git rebase origin/master

then, your branch is updated to newest commits.