This morning, I sat down at my Mac (which is never powered off) to find that it rebooted itself this morning at 3:30am.
In trying to figure out why, I ran across this wonderful article:
It describes use of the log show command to determine why the system had shut down, which I believe is already well-known (although I hadn’t heard of it until today). More useful, it includes a list of shutdown cause codes (updated for Big Sur).
In my case, the shutdown/reboot was due to cause -20: “BridgeOS T2-initiated shutdown”. So something running on the T2 chip caused the shutdown. I don’t know if the T2 crashed or if it decided to send a shutdown command to the rest of the system for some other reason.
Unfortunately, I don’t know if there is any kind of log file from the T2 (or if I would even have access to it if there is) that might tell me why it rebooted my system at 3:30 this morning. There was nothing significant in /var/log/sytem.log at that time - just the usual set of messages that get generated when the system reboots.
I did notice quite a lot of log messages in /var/log/install.log at that time. I’m wondering if Apple pushed an update that caused a reboot (maybe an update to the T2’s BridgeOS), but I’ve never heard of that happening before, especially since I have automatic updates disabled on my system.
Anyone here have a clue?