Magento: Snippet to update all products (put it into the shell folder)

products.php

<?php
require_once 'abstract.php';

class Mage_Shell_Products extends Mage_Shell_Abstract
{
  private function execution_time($start, $current)
  {
    return date("H:i:s", $current - $start);
  }

  public function run()
  {
    echo "Product updater\n";
    $progress = array('-', '\\', '|', '/');
    $execution_start = microtime(true);
    $products = Mage::getModel('catalog/product')->getCollection()
      ->addAttributeToSelect('*')
      ->addAttributeToFilter('type_id', Mage_Catalog_Model_Product_Type::TYPE_SIMPLE); // Simple products only
    $i = 0;
    foreach($products as $product) {
      // Change weight of all simple products to 0.1 (replace it with your own code)
      $product->setWeight(0.1);
      $product->save();
      echo $progress[$i%4] . ' ' . ($i+1) . ' products updated (' . $this->execution_time($execution_start, microtime(true)) . ")\r";
      $i++;
    }
    echo "\n";
  }
}

$shell = new Mage_Shell_Products();
$shell->run();

// Contants:
// Mage_Catalog_Model_Product_Type::TYPE_SIMPLE
// Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE
// Mage_Catalog_Model_Product_Type::TYPE_GROUPED