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:
But don't remove it unless you are certain dpkg isn't running:
It could be that some other package or package update is already downloading, and it hasn't finished downloading yet, so the /var/lib/dpkg/ directory is locked until it finishes. Try to install alien again in a couple of hours.