I needed to generate crypt-style SHA512 passwords in ruby for an /etc/shadow file. After a bunch of Googling and messing around with the OpenSSL library I finally found a very simple built-in way to handle this.

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require 'securerandom'

'password'.crypt('$6$' + SecureRandom.random_number(36 ** 8).to_s(36))

You'll get a string that looks like:

$6$4dksjo1b$Lt194Dwy7r/7WbM8MezYZysmGcxjaiisgTrTBbHkyBZFXeqQTG0J5hep4wLM/AmYxlGNLRy0OWATLDZCqjwCk.

If you don't want to use the SecureRandom module you can replace the random call with simply rand(36 ** 8) though this isn't recommended.

Enjoy!