My sample ruby script to create an EC2 instance

ec2.rb

#!/usr/bin/env ruby

# You will need the following enviromnet variables contiang the your AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY
# $ env AWS_ACCESS_KEY_ID=<ACCESS_ID> AWS_SECRET_ACCESS_KEY=<SECRET_KEY> ruby ec2.rb

# This creates an instance on AWS using the t2.nano and it will perform the following steps:
# 1. *WARNING* It will remove all existing internet gateways, subnets, vpcs and assiocated resources
# 2. Because t2.nano/micro must be created in a VPC, the internet gateway and security group must be configured:
#    http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Scenario1.html
# 3. Wait for the instance to be made available


require 'aws-sdk'

# MODIFY THIS!
dry_run = true

# =========================================
Aws.config.update({
  region: 'ap-southeast-1'  
})

ec2 = Aws::EC2::Resource.new()

# Naive way to check for AWS credentials
begin 
	ec2.instances().count()
rescue Aws::Errors::MissingCredentialsError
	puts "Missing AWS credentials"
	exit
end

ec2.vpcs.each do |vpc|
 vpc.internet_gateways.each do |gw|
  vpc.detach_internet_gateway({internet_gateway_id:gw.id})
 end
end

# Delete all gateways
ec2.internet_gateways.map(&:delete)

# Delete all subnets
ec2.subnets.map(&:delete)

# Delete all vpcs
ec2.vpcs.map(&:delete)

# Create a VPC with cidr 192.168.1.0/24
puts "Creating VPC"

begin
	vpc = ec2.create_vpc({cidr_block:'192.168.1.0/24'})
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to create VPC. Error: #{error}"
	exit
end

vpc.modify_attribute({
  enable_dns_hostnames: {
    value: true,
  },
})

puts "Creating subnet"
begin
	subnet = vpc.create_subnet({cidr_block:'192.168.1.0/24'})
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to create Subnet. Error: #{error}"
	exit
end


# Allow all
security_group = vpc.security_groups.first()
security_group.authorize_ingress({
  ip_protocol: "-1",
  from_port: -1,
  cidr_ip: "0.0.0.0/0",
 })
  
puts "Creating internet gateway"
begin
	gw = ec2.create_internet_gateway()
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to create Internet Gateway. Error: #{error}"
	exit
end

begin
	vpc.attach_internet_gateway({internet_gateway_id:gw.id})
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to attach Internet Gateway to VPC. Error: #{error}"
	exit
end

# Assiocate route with subnet
route_table = ec2.route_tables.first()

begin
	route_table.associate_with_subnet({
	  subnet_id: subnet.id,
	})
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to associate subnet with route table. Error: #{error}"
	exit
end

# Add route through the internet gateway
begin
	route_table.create_route({
	  destination_cidr_block: "0.0.0.0/0",
	  gateway_id: gw.id  
	})
rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to create Route. Error: #{error}"
	exit
end

# Using Amazon Linux AMI (HVM / 64-bit) 
begin
	puts "Creating EC2 instance"
	instances = ec2.create_instances({dry_run:dry_run, image_id:"ami-c9b572aa", instance_type:"t2.nano",
	                             min_count:1, max_count:1, key_name:"default",
	                             network_interfaces: [
	                              {
	                                subnet_id:subnet.id,
	                                groups:[security_group.id],                
	                                device_index: 0,
	                                associate_public_ip_address: true,
	                                private_ip_address: "192.168.1.100"                             
	                              }
	                             ],
	                            })

	inst = instances.first()

	# Wait until the instance is running using the Ruby waiter. 
	# http://docs.aws.amazon.com/sdkforruby/api/Aws/EC2/Client.html#waiter_names-instance_method
	# http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-lifecycle.html

	# It is not clear to me whether all the previous creations are sync / async.
	ec2.client.wait_until(:instance_running, instance_ids:[inst.instance_id]) do |w|
	 
	  # seconds between each attempt
	  w.interval = 3
	 
	  # maximum number of polling attempts before giving up
	  #w.max_attempts = 240
	 
	  #w.before_attempt do |n|
	  	#   # n - the number of attempts made    
	  #end
	 
	  w.before_wait do |n, resp|
	  #   # n - the number of attempts made
	  #   # resp -the client response from the previous attempt
	  	puts "Waiting for instance to be running..."
	  end
	end

	# Public DNS/IP may not be available yet when instance is running
	# I could also wait till :instance-status-ok and it should give me the dns/ip:
	# https://gist.github.com/samukasmk/8425826#file-boto_puppet_bootstrap_instances-py-L68
	puts "Instance #{inst.instance_id} is ready."

rescue Aws::EC2::Errors::ServiceError => error
	puts "Failed to create instance. Error: #{error}"
end

run.sh

#!/bin/bash

env AWS_ACCESS_KEY_ID=<ACCESS_ID> AWS_SECRET_ACCESS_KEY=<SECRET_KEY> ruby ec2.rb