shellcheck pass

note: which is non-standard. Use builtin 'command -v' instead. [SC2230]
note: Use -n instead of ! -z. [SC2236]

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #8367