How to fast-forward a branch to head?

In your situation, git rebase would also do the trick. Since you have no changes that master doesn't have, git will just fast-forward. If you are working with a rebase workflow, that might be more advisable, as you wouldn't end up with a merge commit if you mess up.

username@workstation:~/work$ git status
# On branch master
# Your branch is behind 'origin/master' by 1 commit, and can be fast-forwarded.
#   (use "git pull" to update your local branch)
#
nothing to commit, working directory clean
username@workstation:~/work$ git rebase
First, rewinding head to replay your work on top of it...
Fast-forwarded master to refs/remotes/origin/master.
# On branch master
nothing to commit, working directory clean

git checkout master
git pull

should do the job.

You will get the "Your branch is behind" message every time when you work on a branch different than master, someone does changes to master and you git pull.

(branch) $ //hack hack hack, while someone push the changes to origin/master
(branch) $ git pull   

now the origin/master reference is pulled, but your master is not merged with it

(branch) $ git checkout master
(master) $ 

now master is behind origin/master and can be fast forwarded

this will pull and merge (so merge also newer commits to origin/master)
(master) $ git pull 

this will just merge what you have already pulled
(master) $ git merge origin/master

now your master and origin/master are in sync

Doing:

git checkout master
git pull origin

will fetch and merge the origin/master branch (you may just say git pull as origin is the default).

Try git merge origin/master. If you want to be sure that it only does a fast-forward, you can say git merge --ff-only origin/master.