echo -a $rand(0,9) $+ $rand(0,9) $+ $rand(a,z) $+ $rand(A,Z) $+ $rand(0,9) $+ $rand(0,9) $+ $rand(A,Z) $+ $rand(a,z)
Could be replaced and possibly shortened, with
$+($R(0,9),$r(0,9),$R(0,9),$r(a,z),$R(A,Z),$r(A,z),$R(0,9))
and so on if you get my point