Not really. It is in the Linux world, but outside that, there are lots of
Unices that don't come with Bash. In fact, one of the things that are most
annoying to Unix people who don't use Linux is the widespread hidden
assumption that /bin/sh points to bash. The fact that bash doesn't act
like a POSIX shell, even in POSIX compat mode doesn't help.
Please don't assume bash is on a system if you don't have to. Most
shellscripts can be easily rewritten to be POSIXly correct, with just
a little bit of work. If you really have to use some more advanced
features, please consider using /bin/ksh, which is also standardized
by POSIX. Again, many kshs deviate from the standard, so be careful
here too.