Doesn't that depend on the shell which you're running? BASH and KORN are $?, C Shell is something like the variable status
; I just don't know C Shell well enough to know how to call out a variable, is that same as BASH $<variable-name>?
Either case, you're either running BASH or something else. In Linux your environment sometimes contains your shell, so if you type:
env | grep -i shell
at a terminal or command prompt it should print out your shell. Or if you do:
ls -l <result of the which command>
$ which sh
$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 2010-11-29 15:35 /bin/sh -> bash
This tells me that my shell is using bash.