Modify to make it accept and generate files containing the zpool command...

... outputs

This is primarily to make it work with FreeNAS:
You can't really install nrpe onto FreeNAS directly, but you can install it into a FreeBSH Jail, however Jails can't access the underlying ZFS system.
The workaround, after these modifcations is to run in cron with a -o on the main FreeNAS into a directory shared with the Jail, then run via nrpe with a -i in the Jail.