UNIX question

I've been looking at ads for quant positions at hedge funds to get a general idea of the skills they are looking for and have come across UNIX a few times.

I've played around with Linux in the past, and my question is does anyone have an idea of what exactly these funds are running? Also, what kind of UNIX skills are they looking for (or do they mean programming on a UNIX environment)?

Would knowing Linux suffice or something like FreeBSD?

Thanks in advance.
Anybody asking specifically for Unix skills probably wants you to be comfortable with:

sed, awk, grep etc.
bash/csh scripting
the whole standard *nix directory structure, installers, makefiles etc

Playing around for a couple of hours with a Linux box won't cut it if they're going to ask you any questions at all

It's not about "knowing" a bunch of different *nixes; they're all close enough to each other that until you get to expert level they're identical. It's about actual experience using the standard tools.
I'd like just to jump in with some reading recommendations for brushing up Unix skills. As for work with Unix shell and commands, I'd recommend good old A Practical Guide to the Unix System by Mark Sobell. Note also that there exist more recent, Linux based, versions of this book, but in any case - digest this one, and you'll be an expert Unix user.

As far as Unix programming tools (gcc, make, etc.) concerned, I'd recommend another good old one: Programming with GNU Software by Oram and Loukides. Both this one, and Sobell book mentioned above, are very readable, and would get you up to the speed very quickly.

Another good one, that is freely available, for Unix programming tools is Linux Development Platform by Rehman and Paul. Finally, as far as Unix system, and especially network programming concerned (that I guess would be of most interest for these doing algo trading and other stuff related with low-latency), best books by far are Advanced Programming in the UNIX Environment and UNIX Network Programming by Stevens.