Simplest Drupal 7 drush command

x.drush.inc

<?php

// This file is belonged to a module called x.
// It's all devoted to the drush commands related to the module.

/**
 * Define the drush commands' interface
 */
function x_drush_command()
{
  $items['great-action'] = [
    'description' => 'Description of the new perfect command!',
  ];

  return $items;
}

/*
 * Implement the great-action logic
 */
function drush_x_great_action()
{
  drupal_set_message(t('Hello World from the newly defined perfect action!'));
}

After adding the file, you should...

drush cc all
drush great-action

And we expect the output:

Hello World from the newly defined perfect action!