Comparing two files via MD5 hash on Amazon S3 using Ruby

compare.rb

require 'digest/md5'
require 'aws/s3'

#set your AWS credentials
AWS::S3::Base.establish_connection!(
  :access_key_id     => 'XXX',
  :secret_access_key => 'XXX'
)

#get the S3 file (object)
object = AWS::S3::S3Object.find('02185773dcb5a468df6b.pdf', 'your_bucket')
#separate the etag object, and remove the extra quotations
etag = object.about['etag'].gsub('"', '')

#get the local file
f = '/Users/matt/Desktop/02185773dcb5a468df6b.pdf'
digest = Digest::MD5.hexdigest(File.read(f))

#lets see them both
puts digest + ' vs ' + etag

#a string comparison to finish it off
if digest.eql? etag
  puts 'same file!'
else
  puts 'different files.'
end

etag doesn't appear to always use the md5:

- (String) etag

Returns the object's ETag.

Generally the ETAG is the MD5 of the object. If the object was uploaded using multipart upload then this is the MD5 all of the upload-part-md5s.