A `which` command for Ruby

which.rb

# frozen_string_literal: true

require 'fileutils'

module FileUtils
  # Returns the full path to an executable command if it exists in the PATH.
  #
  #   FileUtils.which('ruby') # => "/usr/bin/ruby"
  #   FileUtils.which('matz') # => nil
  #
  # Keyword arguments:
  #
  # - <tt>env: 'PATH'</tt> - the PATH environment variable.
  # - <tt>separator: File::PATH_SEPARATOR</tt> - the PATH separator.
  #
  def which(command, env: 'PATH', separator: File::PATH_SEPARATOR)
    ENV.fetch(env) { break }.split(separator).each do |dir|
      next unless Dir.exist?(dir)

      path = File.join(dir, command)
      next unless File.file?(path) && File.executable?(path)

      return path
    end

    nil
  end
  module_function :which
end

path = FileUtils.which(ARGV.unshift)
abort unless path

puts path