Commissioning of the ATLAS Trigger and Data Acquisition System with Single-Beam and Cosmic Rays

ATLAS is one of the two general purpose detectors at the Large Hadron Collider (LHC). It's three-level Trigger and DAQ system (TDAQ) has to investigate a huge rate of events (up to 1 GHz at nominal operating conditions) and retain only the potentially interesting ones (budgeted to ~200 Hz). This paper will give an overview of the system and its innovative features. The early experience on LHC single-beam in 2008 will be shown and then focus on the valuable experience gained in running the DAQ and the trigger reconstruction and event selection in the fast-changing environment of the detector commissioning with cosmic data. Results on system functionality and performance based on preselected simulated events will be also presented. Finally, the readiness of the system for LHC collisions, expected later this year, will be discussed.