I can't think of any time where I have needed the same script to run on multiple systems, because that's not the sort of thing I use shell scripts for. But ther

I can't think of any time where I have needed the same script to run on multiple systems, because that's not the sort of thing I use shell scripts for. But there are many times where I have needed to write a script for a specific system that was running busybox or dash or ksh, and it is worth knowing what features I can use in those cases.