When in doubt, make a CLI


I just made a gem to convert between crypto and fiat currencies.

❯ bit
  bit convert AMOUNT FROM TO  # Converts between fiat and cypto currencies
  bit help [COMMAND]          # Describe available commands or one specific command
  bit ping                    # pong!
  bit rate FROM TO            # Get exchange rate between fiat and crypto currencies

❯ bit convert 1 btc usd

❯ bit convert 3142 btc doge

❯ bit rate BTC JPY


The perk of being an engineer is that when some problem arises to pain your butt cheek, you just built a tool to solve it.
me, 2021

I have been working at (probably) the largest crypto trading platform in my country, so, naturally, I care about the market. However, it’s a real hassle to google every once in a while. Typing too much can ironically end my career. Also, the web is slowed down by ads and it makes my potato of a mac burn hotly. A CLI should be a convenient solution since (a) I basically live in my iterm2 10 hours every day, (b) it’s fast, and (c) it’s a worthwhile engineer challenge for me.


The calculation part turns out to be pretty easy. Coinbase provides an endpoint so convenient. I just need to get the rate of a currency in USD only.

def convert(from_currency, to_currenty)
  usd_rate(to_currenty) / usd_rate(from_currency)

The fun part is to create a cli out of it. You just need:

  • thor: for the interface
  • bundler: to publish the package
  • aruba: heck you can even cucumber test the cli, just like this:

    Feature: Ping
      As a CLI
      I want to test if the program works
      Scenario: It works
        When I run `bit ping`
        Then the output should contain "pong!"

This guide has everything you need to build your gem.

lessons learned

When in doubt, make a CLI
me, probably

I can believe there are already 1000 people who downloaded my gems.


Never before had I felt so much responsible for my code. I did put a great effort into writing tests but those gems were experimentally lack of features. Sorry for the clickbaity description, but thanks for the motivation to make better software.