Writing a Command Line Tool in Ruby

Yesterday I tried to do a post on creating your first Ruby Gem as a command line tool. It didn’t go very well. In an effort to better understand what goes on with command line tools I’m going to start from the most basic, a ruby script that is in our PATH.

I’ve previously added the folder ~/bin to my PATH so I know that if I drop an executable script in there I should be able to run it by just typing its name. Let’s give that a shot. First we’ll make a new file just called sherp without any file extension. Make sure to chmod 755 sherpa so that it’s executable. Then we’ll add the following:

#!/usr/bin/env ruby

puts 'I am the sherpa!!!'

If I now type sherpa into the command line, it should fire back I am the sherpa!!!

Ok cool so we’ve got that part working. Now let’s see if we can get some arguments in there. We’ll iterate the ARGV object to see what comes in.

#!/usr/bin/env ruby

ARGV.each do |arg|
    puts arg
end

With that little bit of code we should be able to pass just about anything to our command and have it echoed back.

sherpa foo bar baz

 => foo
 => bar
 => baz

Ok cool. Now let’s step this up a notch or two. Let’s say we want to send in commands and options. For that we’ll use the built-in OptionParser. Here’s a link to the article I’ve been following which details how to use the OptionParser. In my case, I’m going to tell sherpa to either say_hello or say_goodbye. When I pass in the -n flag it should accept a name, otherwise it will use the name ‘Master’. So the line sherpa say_hello -n Rob should produce Hello Rob and likewise if I left off the option and just used sherpa say_hello it should produce Hello Master.

Here’s the code to accomplish this:

#!/usr/bin/env ruby

require 'optparse'

options = {}

opt_parser = OptionParser.new do |opt|
  opt.banner = "Usage: opt_parser COMMAND [OPTIONS]"
  opt.separator  ""
  opt.separator  "Commands"
  opt.separator  "     name: the name the sherpa should use when addressing you"
  opt.separator  ""
  opt.separator  "Options"

  opt.on("-n","--name NAME","tell the sherpa what to call you") do |name|
    options[:name] = name
  end

  opt.on("-h","--help","help") do
    puts opt_parser
  end
end

opt_parser.parse!
name = options[:name] || 'Master'

case ARGV[0]
when "say_hello"
  puts "Hello #{name}"
when "say_goodbye"
  puts "Goodbye #{name}"
else
  puts opt_parser
end

And there we go, our first command line Ruby tool. I’ll pick it up tomorrow to try to improve upon it. We’re starting small but at least we have something that works!

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