How to add a custom Action in Drupal 7 Rules and handle the variable from inside the module

Using the hook_rules_action_info() you can add your own Action which you will be able to select from inside Rules UI.

/**
 * Implementation of hook_rules_action_info().
 */
function devlubo_rules_action_info() {
  return array(
    'my_custom_action' => array(
      'label' => t('DevLubo - Custom Action'),
      'arguments' => array(
        'node_id' => array('type' => 'integer', 'label' => t('Node ID')),
      ),
      'provides' => array(
        'devlubo_custom_variable_output' => array(
            'type' => 'decimal',
            'label' => t('This will provide custom variable output'),
          ),
        ),
      'base' => 'devlubo_rules_alter_the_custom_variable',
      'module' => 'devlubo'
    ),
  );
}

Explaining the code in line by line fashion you can see the following:

Defining your custom action:

'my_custom_action' => array(

How will your action be listed in a select list in Rules UI:

'label' => t('DevLubo - Custom Action'),

Which arguments does your custom action take (in this example Node ID)

'arguments' => array(
    'node_id' => array('type' => 'integer', 'label' => t('Node ID')),
),

Which variables will the custom action provide to work with later in Rule execution

'provides' => array(
    'devlubo_custom_variable_output' => array(
       'type' => 'decimal',
       'label' => t('This will provide custom variable output'),
    ),
),

Custom function to work with the variable

'base' => 'devlubo_rules_alter_the_custom_variable',

As you can see, we have defined in hook_rules_action_info() which function will handle the variable. So we can as an example set the value to custom number as a return value.

/**
 * Action: Set a variable to custom value.
 */
function devlubo_custom_variable_output($args, $element) {
  return array('devlubo_custom_variable_output' => "500");
}