Simple Ruby script for launching instance

#!/usr/bin/ruby
require 'aws-sdk-core'
def run_inst(size,ami,num)
  Aws.config[:region] = 'eu-west-1'
  ec2 = Aws::EC2::Client.new
  ec2.run_instances(image_id:"#{ami}",min_count:"1",max_count:"#{num}",key_name:"KEY",security_group_ids:["SG-ID"],instance_type:"#{size}",subnet_id:"SUB-ID",instance_initiated_shutdown_behavior:"stop")
end
if ARGV.count == 0
  puts "usage: run-instances.rb INSTANCE_SIZE AMI INSTANCE_NUM "
else
  run_inst(ARGV[0],ARGV[1],ARGV[2])
end