access S3 using ruby fog with IAM roles

fog_s3_iam.rb

require 'rubygems'
require 'fog'

# assuming your IAM role is named 'my-role' and has S3 access:
uri = URI('http://169.254.169.254/latest/meta-data/iam/security-credentials/my-role')

res = Net::HTTP.get_response(uri)
aws_creds = JSON.parse(res.body)

# use the credentials manually:
con = Fog::Storage.new({
  :provider                 => 'AWS',
  :aws_access_key_id        => aws_creds['AccessKeyId'],
  :aws_secret_access_key    => aws_creds['SecretAccessKey'],
  :aws_session_token        => aws_creds['Token']
})

# or simply use the IAM Role name:
con = Fog::Storage.new({
  :provider                 => 'AWS',
  :use_iam_profile          => 'my-role'
})

directory = con.directories.get('my-s3-dir')

file = directory.files.get('my-s3-sub-dir/my-file.txt')

# load S3 file in memory:
file_contents = file.body