Computing SHA-* hashes of files in ruby is in principle very easy.

You can either use the OpenSSL module or the digest one and the sytax is almost interchangeable. I am using the digest module just because it seems to be slightly faster (I timed it and on my machine it takes few milliseconds less).

My first code was

    require 'digest'

filename = '/path/to/the/file'
Digest::SHA1.hexdigest(File.read(filename))

Then I made the hashes for a folder containing files of different sizes. For some reasones I checked them with shasum and with my great surprise I’ve discovered that some hashes (namely the ones related to files bigger than few hundred megabytes) were wrong! Remarkably, the same happened with different SHA functions, and using openssl.

I don’t really know why the problem happened, but apparently the solution is fairly easy. I am now using a variation of the script proposed in the previous link that seems to run pretty smoothly and got rid of the error:

    require 'digest/sha1'

filename = '/path/to/the/file'
buf = ''
d = Digest::SHA1.new
f = File.open(filename)

while not f.eof
  # reads the file in chunks of 65536 bytes
  f.read(65536, buf)
  d.update(buf)
end

d.hexdigest

I am using this long code just because it requires some milliseconds less than Digest::SHA1.file(filename).hexdigest to run (and this last is slightly faster than the openssl equivalent).

I hope it could be of help if you happen to have the same problem.