List the S3 buckets for your AWS account/region. Can also list the contents of a bucket or the contents of a file, depending on how many command-line arguments you supply. ;) Uses the AWS SDK for Ruby.

list_buckets_contents_s3.rb

# List the S3 buckets that are available for the user identified by either
# `aws-config.txt` or in the environment.
require 'aws-sdk'
require 'yaml'

CONFIG_FILE_NAME = 'aws-config.txt'

# Load aws-config.txt to see if there are AWS credentials there.
#
# We're expecting a YAML-formatted file like this:
#
#     ---
#     access_key_id:     ACCESSKEYID
#     secret_access_key: SECRETACCESKEY
#
begin
  config_file = File.open(CONFIG_FILE_NAME) { |f| f.read }
  options = YAML.load(config_file)
  AWS.config(options)
rescue
  puts "** Hmmm, no config file (#{CONFIG_FILE_NAME})!"
  # Without setting AWS credentials specifically, the AWS SDK will try to load
  # them from the environment variables. Check the environment variables, just
  # to make sure...
  if ENV['AWS_ACCESS_KEY_ID'].nil? or ENV['AWS_SECRET_ACCESS_KEY'].nil?
    puts "** No AWS access information in environment, either. Aborting!"
    exit
  end
end

# Create a new S3 object
s3 = AWS::S3.new

# If the user supplied arguments, the first is the bucket name, the second is
# the object name.
bucket_name, object_name = ARGV

if bucket_name.nil?
  # No bucket name given, so list all of the buckets.
  s3.buckets.each { |b| puts b.name }
else
  # The user provided at least a bucket name...
  b = s3.buckets[bucket_name]

  # If the user specified a bucket but it's not there, alert the user.
  unless b.exists?
    puts "The bucket \"#{bucket_name}\" doesn't exist!"
    exit
  end

  # Did the user also provide an object name?
  if object_name.nil?
    # List the contents of the bucket.
    b.objects.each { |o| puts o.key }
  else
    # Get the contents of the object and send to stdout. We can use this to
    # download objects from S3.
    o = b.objects[object_name]
    if o.exists?
      # It is better to read the file in *chunks* rather than all at once. This
      # is not so important for small objects, but much more important for large
      # ones... Without a block, read will return the entire object, even if
      # it's HUUUGE.
      o.read { |chunk| $stdout.write(chunk) }
    else
      puts "object \"#{object_name}\" doesn't exist in bucket \"#{bucket_name}\""
    end
  end
end