Nearly all of the shell scripts ever written don't need to run on Linux, and BSD, and Mac, and/or whatever esoteric Unixes that are still out there. If you are

Nearly all of the shell scripts ever written don't need to run on Linux, and BSD, and Mac, and/or whatever esoteric Unixes that are still out there. If you are writing a shell script with the intention of running in many places, fine. But if you do that, the POSIX compatibility will be the least of your worries.