If you have a older PC and have learned Linux you can have a closer Look at the new OS of PcBSD or DektopBSD
Its a easy way to learn „real Unix“ and understand the structure of a Unix OS, you can setup all kinds of BSD Unix on a very high count of different hardware. The most popular programs of Linux can be used here too, many commands are like linux commands and you can compile always the newest programs onto your System for the highest performance instead of pre-compiled software like redhat-debian-packages (.deb/.rpm)