# 1
(rand(900000000)+10 ** 9).to_s
# 2
([(0..9).to_a]*9).map{|i|i.sample}.join
# 3
sprintf("%09d", rand(10 ** 9))
# 4
Array.new(9){ Array(0..9).sample }.join
# 5
9.times.map{rand 10}.join
# 6
"#{rand(10 ** 9)}".rjust(9, '0')