Hmm...!!! Dude is right but a bit explanation is needed here. Linux comes from Linus, who introduce a Unix like OS free under GPL license. But Unix is developed in order to port different application from PDP11. It was born in Bell lab. It was a copyright product of AT&T. Later different flavor comes to the market such as Sco unix, Berkely Unix. Those are not free, in order to use that you have to purchase it. Basically, there are lot of similarity between linux and unix console based command. For further information please read the following link:
http://www.tchpc.tcd.ie/Support/Training/User_Course/x208.html
-DK
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...we too are stardust...