c - How to detect pending system shutdown on Linux?

Question

I am working on an application where I need to detect a system shutdown. However, I have not found any reliable way get a notification on this event.

I know that on shutdown, my app will receive a SIGTERM signal followed by a SIGKILL. I want to know if there is any way to query if a SIGTERM is part of a shutdown sequence?

Does any one know if there is a way to query that programmatically (C API)?

As far as I know, the system does not provide any other method to query for an impending shutdown. If it does, that would solve my problem as well. I have been trying out runlevels as well, but change in runlevels seem to be instantaneous and without any prior warnings.

Answer 1

Maybe a little bit late. Yes, you can determine if a SIGTERM is in a shutting down process by invoking the runlevel command. Example:

#!/bin/bash
trap "runlevel >$HOME/run-level; exit 1" term
read line
echo "Input: $line"

save it as, say, term.sh and run it. By executing killall term.sh, you should able to see and investigate the run-level file in your home directory. By executing any of the following:

sudo reboot
sudo halt -p
sudo shutdown -P

and compare the difference in the file. Then you should have the idea on how to do it.