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.