example.install.php

/**
 * Explanation of what this update does.
 */
function example_update_7100(&$sandbox) {
  $limit = 1;

  if (!isset($sandbox['processed'])) {
    $nids = db_select('node', 'n')
      ->fields('n', array('nid'))
      ->condition('type', 'page', '=')
      ->execute()
      ->fetchCol();

    if (empty($nids)) {
      watchdog('example', 'There are no items to process.', array(), WATCHDOG_NOTICE);
      return;
    }

    $sandbox['processed'] = 0;
    $sandbox['total'] = count($nids);
    $sandbox['nids'] = $nids;
  }

  for ($i = 0; $i < $limit; $i++) {
    if ($nid = array_shift($sandbox['nids'])) {
      // Do processing here.
    }

    $sandbox['processed']++;
  }

  $sandbox['#finished'] = ($sandbox['processed'] >= $sandbox['total']) ? 1 : ($sandbox['processed'] / $sandbox['total']);

  $args = array(
    '!processed' => $sandbox['processed'],
    '!total' => $sandbox['total'],
    '!pct' => round(($sandbox['processed'] / $sandbox['total']) * 100, 2),
  );

  return t('Completed !processed/!total (!pct%)', $args);
}