Code snippet for field creating update hook

field_update_hook.php

<?php

/**
 *  Code snippet for field create update hook.
 */
function module_update_7001() {
  $t = get_t();

  $fields = array();

  if (!field_info_field('field_name')) {
    $fields['field_name'] = array(
      'field_name' => 'field_name',
      'type' => 'image',
      'cardinality' => 1,
    );
  }

  foreach ($fields as $field) {
    field_create_field($field);
  }

  $instances = array();

  if (!field_info_field('entity', 'field_name', 'bundle')) {
    $instances['field_name'] = array(
      'entity_type' => 'entity',
      'bundle' => 'bundle',
      'field_name' => 'field_name',
      'label' => $t('Field Name'),
      'description' => $t('Field description'),
      'settings' => array(
        'file_directory' => 'images',
      ),
      'widget' => array(
        'type' => 'media_generic',
      ),
    );
  }

  foreach ($instances as $instance) {
    field_create_instance($instance);
  }
}