Drupal batch example

mymodule.admin.inc

<?php

/**
 * @file
 * Administrative tasks for My module.
 */

/**
 * Page callback for Batch example form.
 */
function mymodule_batch_example_form($form, &$form_state) {
  $nodes_query = new EntityFieldQuery();
  $nodes_query ->entityCondition('entity_type', 'node')
    ->entityCondition('bundle', 'recording')
    ->count();

  $nodes_count = $nodes_query->execute();

  if (!$nodes_count) {
    drupal_set_message('You cannot submit this form because there are no article nodes available.', 'error');
  }

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Start batch'),
    '#disabled' => $nodes_count ? FALSE : TRUE,
  );

  return $form;
}

/**
 * Submit function for batch form.
 */
function mymodule_batch_example_form_submit($form, &$form_state) {
  batch_set(mymodule_override_node_titles_batch());
}

function mymodule_override_node_titles_batch() {
  $batch = array(
    'title' => t('Loading nodes'),
    'operations' => array(
      array('mymodule_override_node_titles_batch_op', array()),
    ),
    'finished' => 'mymodule_override_node_titles_batch_finished',
    // 'file' is optional and can be useful for outsourcing batch code.
    'file' => drupal_get_path('module', 'mymodule') . '/mymodule.admin.inc',
  );

  return $batch;
}

/**
 * Batch operation for overriding node titles.
 */
function mymodule_override_node_titles_batch_op(&$context) {
  if (empty($context['sandbox'])) {
    $context['sandbox'] = array();
    $context['sandbox']['progress'] = 0;
    $context['sandbox']['current_node'] = 0;

    $nodes_query = new EntityFieldQuery();
    $nodes_query ->entityCondition('entity_type', 'node')
      ->entityCondition('bundle', 'article');

    $nodes = $nodes_query->execute();
    $max = count($nodes['node']);

    // Save node count for the termination message.
    $context['sandbox']['max'] = $max;
  }

  // Process nodes by groups of 5 (arbitrary value).
  // When a group of five is processed, the batch update engine determines
  // whether it should continue processing in the same request or provide
  // progress feedback to the user and wait for the next request.
  // That way even though we're already processing at the operation level
  // the operation itself is interruptible.
  $limit = 5;

  // Retrieve the next group of nids.
  $next_query = new EntityFieldQuery();
  $next_query->entityCondition('entity_type', 'node')
    ->entityCondition('bundle', 'article')
    ->propertyCondition('nid', $context['sandbox']['current_node'], '>')
    ->pager($limit);

  $next = $next_query->execute();
  $next = node_load_multiple(array_keys($next['node']));

  foreach ($next as $node) {
    // Here we can do the actual operation like setting a field value
    // and saving the node.

    // Set a random node title.
    $node->title = user_password();

    node_save($node);

    $context['results'][] = $node->title;

    // Update our progress information.
    $context['sandbox']['progress']++;
    $context['sandbox']['current_node'] = $node->nid;
    $context['message'] = t('Fetching information for @remaining of @total songs.', array(
      '@remaining' => $context['sandbox']['max'] - $context['sandbox']['progress'],
      '@total' => $context['sandbox']['max'],
    ));
  }

  // Inform the batch engine that we are not finished,
  // and provide an estimation of the completion level we reached.
  if ($context['sandbox']['progress'] != $context['sandbox']['max']) {
    $context['finished'] = ($context['sandbox']['progress'] >= $context['sandbox']['max']);

  }
}

/**
 * Function which is called when batch is finished.
 */
function mymodule_override_node_titles_batch_finished($success, $results, $operations) {
  if ($success) {
    $message = format_plural(count($results), 'One title has been overriden.', '@count titles have been overridden.');
  }
  else {
    $message = t('Finished with an error.');
  }
  drupal_set_message($message);
}

mymodule.module

<?php

/**
 * Implements hook_menu().
 */
function mymodule_dev_menu() {
  $items = array();

  $items['admin/config/development/mymodule'] = array(
    'title' => 'My module',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('mymodule_batch_example_form'),
    'access arguments' => array('administer site configuration'),
    'file' => 'mymodule.admin.inc',
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}