good check written in bash

Nice plugin, one of the few that is written in bash and not perl (which always seems to require the Nagios::Plugin perl module).

There is a typo at the end, the OK statement on line 103 has “$available” spelled incorrectly so it won’t return the mem usage on an OK result. Easy fix though.

I also changed the exit code on “usage” stuff to exit 4 which is the Unknown state.

Thanks for a nice check.