never has been a Linux kernel in Mac OS X
well... as near to having one as makes little difference.
A user isn't likely to notice much difference when using one or the other from the command-line. Only when you start looking at structs for things like volume information do things differ, but then only slightly.
Once you know how to do something on one, it's not a great leap to do it on the other - 99% of the time ;-)
Pre OS X was GUI only and therefore much less fun, I think :-)