APPENDIX AThe Shell Command Language
The POSIX shell language spec is full of things you probably never knew about the shell. Unfortunately, many of them are not true of historic shells; if they were, this would have been a very short book.
There are a number of features formally specified by POSIX, which this book has described as unportable; not every shell complies with POSIX. You may be more disturbed to discover that there are a number of features that this book has described as portable, but which POSIX does not specify. The most noticeable example is the
#! script header. Don't worry—while such features may not be portable to every POSIX machine, they are quite consistently available on anything that looks like, or even smells a little ...