A command to complete all at once? I'm afraid it doesn't exist, but you could try the console command setstage (desired quest ID) (Desired quest stage). For instance,
With friends like these ID IS
DB02 while stage
200 stands for its final stage, therefore typing
setstage db02 200 will get the quest complete. Keep in mind that some DB quests ofter you choices (IE: sparing or killing Cicero) so you must make your choice via a command with an itermediary stage number before completing it. I suggest taking a look
at this link. Just scroll it down and you'll find all the info of the quest stages.
EDIT:
i know the process, my issue is i cant figure out what the last mission is in the dark brotherhood quest line
Simple. There you go:
http://www.uesp.net/wiki/Skyrim:Dark_Brotherhood