code: def GetClientOs(client_id, token=None):
    
    if data_store.RelationalDBEnabled():
      kb = data_store.REL_DB.ReadClientSnapshot(client_id).knowledge_base
    else:
      with aff4.FACTORY.Open(client_id, token=token) as client:
        kb = client.Get(client.Schema.KNOWLEDGE_BASE)
  
    return kb.os
docs: Returns last known operating system name that the client used.