I often need to convert currencies. This can be done from various search engines, but it's faster (for me) to just type a command in a term, and get results immediately. HTH.

change.rb

#!/usr/bin/env ruby
#
# change.rb -- currency conversions on the CLI
#
# Requirements:
#
#   gem install nas-yahoo_stock moneta slop
#
# Getting started:
#
#   1. Run change.rb --help
#   2. Format currencies using ISO 4217.
#   3. Stock are cached in ~/.change.cache for 24 hours

require 'pathname'
require 'fileutils'

require 'yahoo_stock' # nas-yahoo_stock
require 'moneta'
require 'slop'

module Currency

  module Utils
    module_function

    # key/value store to cache stock values.
    def store
      return @store if @store
      store_path = Pathname.new(ENV['HOME']) + '.change.cache'
      @store = Moneta.new(:PStore, file: store_path.to_s, expires: expire)
    end

    # Rounding
    def precision
      3
    end

    # Cache expiration
    def expire
      86400
    end
  end

  # Get currency conversions from Yahoo Finance API
  class Stock
    def initialize(options = {})
      @cache = options.fetch(:cache) { {} }
    end
    
    def get(name)
      @cache.fetch(name) { fetch(name) }
      @cache[name]
    end
    alias :[] :get

    private

    def fetch(name)
      query = YahooStock::Quote.new(stock_symbols: [name])
      query.results(:to_array).output.each_with_index do |quote, i|
        @cache[ name ] = quote[1].to_f
      end
    end
  end

  # Convert amounts between currencies.
  Converter = Struct.new(:amount, :from, :to) do
    include Utils

    # @return [Float]
    def convert
      change = Stock.new(cache: store).get(stock_symbol)
      (amount.to_f * change.to_f).round(precision)
    end

    def stock_symbol
      "#{ from }#{ to }=X".upcase
    end
  end
end

opts = Slop.parse do
  banner "Usage: change.rb -f <currency> -t <currency> -a <amount>"

  on :f, :from=,   'Origin currency', argument: :optional
  on :t, :to=,     'Target currency', argument: :optional
  on :a, :amount=, 'Amount',          argument: :optional
  on :h, :help, 'Show help'
end

if opts.help? || !opts.from? || !opts.to? || !opts.amount?
  puts opts
  exit(1)
end

value = Currency::Converter.new(opts[:amount], opts[:from], opts[:to]).convert
puts "#{ opts[:amount] } #{ opts[:from] } = #{ value } #{ opts[:to] }"