Hi, here you can download a minecraft launcher installer for Ubuntu and some Debian based distros (right now I've only tested Linux Mint and Ubuntu, but I think it can work on most debian based distros) this is not only a minecraft launcher installer, since minecraft requires java, this will install it for you. This also creates a launcher, which means you can place it in your dock, search it in a menu (like the mint menu, or the unity's dash) + it'll have the minecraft icon.
Q1: How do I update?
A1: Minecraft updates itself, so you'll only need to download it once.
Q2: Is this legal?
A2: yes it is legal, this is only the launcher, if you don't own the game you cant play, the game is downloaded when you log in.