dicey

https://github.com/trinistr/dicey

Dicey provides a CLI executable and a Ruby API for fast calculation of distribution of weights or probabilities of dice rolls, with support for all kinds of numeric dice, and non-numeric ones too! Results can be exported as JSON, YAML or a gnuplot data file. It can also be used to roll dice. While not the primary focus, rolling is well supported, including ability to seed random source for reproducible results.