support for POSIX shell

[MaxPeal]

it really helps if it runs also just in a plain POSIX shell,
so it will run in busybox systems or alpine linux, (or ksh ;-) ) without a bash.

[OneOfTheInfiniteMonkeys]

I agree, good suggestion.

It has been rather a long time since I wrote POSIX compliant scripts. Lookig at the internet circa 1995, exciting!