Thor is hard.

cli.rb

# my_gem/cli.rb

require "thor"

require "my_gem/cli/foo_bar"
require "my_gem/cli/foo_baz"
require "my_gem/cli/install"

module MyGem
  class CLI < Thor
    desc "foo:bar", "Do bar to foo"
    define_method "foo:bar" do
      # MyGem::CLI::FooBar is my own independently unit-test bit of code.
      FooBar.run(options)
    end

    desc "baz", "Do baz to foo"
    method_option "force",
      aliases: ["-f"],
      desc: "Force the issue"
    define_method "foo:baz" do
      # MyGem::CLI::FooBaz is my own independently unit-test bit of code.
      FooBaz.run(options)
    end

    desc "install", "Install MyGem"
    method_option "dry-run",
      desc: "Fake it"
    def install
      # MyGem::CLI::Install is a Thor::Group including Thor::Actions.
      # It's used very much like a Rails generator.
      #
      # The problem is that the "install" task shows up twice when
      # calling the my_gem executable, once as "install" and once
      # more as "my_gem:c_l_i:install".
      #
      # I also have to duplicate options between the method options
      # here and the class options in the Install class.
      #
      # How can I have a command that acts as a generator without
      # accidentally creating two commands?
      Install.start
    end
  end
end

install.rb

# my_gem/cli/install.rb

module MyGem
  class CLI < Thor
    class Install < Thor::Group
      include Thor::Actions
      
      desc "install", "Install MyGem"
      class_option "dry-run",
        desc: "Fake it"

      source_root File.expand_path("../templates", __FILE__)

      def do_first_thing
        # TODO: Create some file.
      end

      def do_next_thing
        # TODO: Update some other file.
      end
    end
  end
end

my_gem

#!/usr/bin/env ruby

# bin/my_gem

require "my_gem/cli"

MyGem::CLI.start