Yes, and /bin/sh is often not bash. A sufficiently hosed system may give you only /bin/sh until you can repair things, and knowing these tricks in those cases can be a real lifesaver.
A sufficiently hosed system may not even boot. Under normal circumstances, however, you have a full set of shells, depending on how... feature-rich is the OS you are using.
Is there any modern serious OS that does not provide bash?
By default? Windows (heh). And most Unix servers don't come with bash by default. Though I think you can install it on most anything, it is not a good idea to change the default root shell on Unix servers.
That being said, OSX and Linux have Bash as the default shell. If that's all you deal with then go ahead and learn Bash (and the other GNU utilities) instead of POSIX sh. Unless you're an admin there's no point in being so pedantic.
This is neither serious nor modern, nor particularly operating :-P
> And most Unix servers don't come with bash by default
Most Unix systems are either Mac, Linux or BSD boxes, all of them provide bash (it may be a tiny little bit more complicated under BSDs). I will risk saying the list of recent Unixes (as in "with versions published in the last 5 years") that don't provide bash is limited to 3 items.
Perhaps four. IRIX 6.5.30 was launched in late 2006.
edit: maybe five. Tru64 had a launch (patches) in 2010. Guess I should have said "major version"...
