sudo apt-get install alien, there is an error:
Unable lock administrator directory (/var/lib/dpkg/), is another process using it?
How do I fix this?
karel's answer should be correct in nearly every case... however, there are some cases where dpkg forgets to clean its lock after its done (it happened to me a couple of times), so then you have to remove it:
sudo rm -f /var/lib/dpkg/lock
But don't remove it unless you are certain dpkg isn't running:
ps -eo command | grep dpkg | grep -v 'grep' # If there is output, then it means # dpkg is running