creds_test.rb

#!/usr/bin/env ruby
require 'aws-sdk'

$stdout.print 'Enter your MFA serial: '
mfa_serial = $stdin.gets.chomp

$stdout.print 'Enter your MFA code: '
mfa_code = $stdin.gets.chomp

$stdout.print 'Enter your creditials profile name: '
profile = $stdin.gets.chomp

sts = Aws::STS::Client.new(profile: profile, region: 'us-east-1')
sts_creds = sts.get_session_token(serial_number: mfa_serial, token_code: mfa_code, duration_seconds: 28800).credentials

creds_obj = Aws::Credentials.new(sts_creds.access_key_id, sts_creds.secret_access_key, sts_creds.session_token)

ec2 = Aws::EC2::Client.new(region: 'us-east-1', credentials: creds_obj)

puts ec2.describe_instances.reservations.first.instances.map { |i| i.instance_id }