def command(command, options)
      options = {
          'print_command' => false,
          'failure_message' => "required command '#{command}' is not available."
      }.merge(string_keys(options))
      system_exit_on_error("which #{command}", options)
    end
def command(command, options)
      options = {
          'print_command' => false,
          'failure_message' => "required command '#{command}' is not available."
      }.merge(string_keys(options))
      system_exit_on_error("which #{command}", options)
    end
Determine if the command exists on the current system (uses which). If it does
 not hard exit with a message to stdout.

 command - the string of the command to find
 options - see system_exit_on_error

 return only if it finishes successfully