Magento – Magento log is not writing in log folder

From what I understand, the Mage logger will not function so early in the application spin up.

If you look at the Mage::log function itself, you'll find:

if (!self::getConfig()) { return; }

As Mage::getConfig() will return null until the application's config is initialised later on inside the Mage::run() function, the Mage::log function will exit without doing anything when called from index.php.

To test the logging is actually working, try placing your test later on, perhaps in Mage_Core_Controller_Varien_Front::dispatch which works for me.

With the latest patch installed, file modes for newly created logs changed to 0640. See SUPEE-6285 broken down by Phillip Jackson.

So this is intended behaviour that you can overcome by e.g. changing modes, groups, or setting up ACL.