Unix, GNU and bash

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http://ubuntuforums.org – Hi all, I seem to be getting myself confused with all the media out there talking about unix, gnu etc etc. Can anyone tell me what the real difference between unix and gnu? What I gather is that gnu is a unix-like operating system and that bash is the command shell so why then when I type bash commands on a mac it doesn't understand the command despite it being a bash shell. But is OS X a unix OS compared to Linux being gnu? Are all Linux distros gnu? If all Linux distros are gnu and use the same command shell, why if I was to run the command to unzip a file on say kali I'd type -zxvf where a (Hardware)