Make an existing Git branch track a remote branch?

Given a branch foo and a remote upstream:

As of Git 1.8.0:

git branch -u upstream/foo

Or, if local branch foo is not the current branch:

git branch -u upstream/foo foo

Or, if you like to type longer commands, these are equivalent to the above two:

git branch --set-upstream-to=upstream/foo

git branch --set-upstream-to=upstream/foo foo

As of Git 1.7.0 (before 1.8.0):

git branch --set-upstream foo upstream/foo

Notes:

• All of the above commands will cause local branch foo to track remote branch foo from remote upstream.
• The old (1.7.x) syntax is deprecated in favor of the new (1.8+) syntax. The new syntax is intended to be more intuitive and easier to remember.
• Defining an upstream branch will fail when run against newly-created remotes that have not already been fetched. In that case, run git fetch upstream beforehand.

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See also: Why do I need to do `–set-upstream` all the time?