$SHELL should typically be set to reflect the configured login shell, I.E. the shell specified in /etc/passwd. Or, as POSIX[0] puts it: "This variable shall represent a pathname of the user's preferred command language interpreter." The currently running shell is not necessarily the "preferred" shell, for example if you're doing "xterm -e zsh" or running a specific ksh script when you normally prefer (log in to) bash.
Being vaguely defined, it is of course open to interpretation and might vary from system to system, thus being a prime example of the kind of frustrating Unix gotchas that spawned the original article.
Edit: To tie in to my earlier comment - where should I look for info on this variable in Linux? I know it exists, but how do I find out more about the behavior? 'apropos "\$SHELL"' gives me nothing. 'apropos SHELL' gives me a list of commands which doesn't include much of interest. Digging deeper, there's login(1), which briefly touches the subject, and environ(7), which gives a good description of it but of course dives in head-first and starts off by describing an array of pointers. Not overly helpful for the novice.
No, It tells you what program image file is going to be used when a program "shells out" or spawns a shell to run a shell command (e.g. via the ! command in mail or the :shell command in nvi).
It is a way for applications to know where the shell to invoke is, not a way for a user to find out what program xe is using to run commands. echo $0 is more informative on that score.
