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.

Answer 2

From man shutdown:

If the time argument is used, 5 minutes before the system goes down the /etc/nologin file is created to ensure that further logins shall not be allowed.

So you can test existence of /etc/nologin. It is not optimal, but probably best you can get.

Answer 3

There is no way to determine if a SIGTERM is a part of a shutdown sequence. To detect a shutdown sequence you can either use use rc.d scripts like ereOn and Eric Sepanson suggested or use mechanisms like DBus.

However, from a design point of view it makes no sense to ignore SIGTERM even if it is not part of a shutdown. SIGTERM's primary purpose is to politely ask apps to exit cleanly and it is not likely that someone with enough privileges will issue a SIGTERM if he/she does not want the app to exit.

Answer 4

Its a little bit of a hack but if the server is running systemd if you can run

/bin/systemctl list-jobs shutdown.target

... it will report ...

JOB UNIT            TYPE  STATE
755 shutdown.target start waiting     <---- existence means shutting down

1 jobs listed.

... if the server is shutting down or rebooting ( hint: there's a reboot.target if you want to look specifically for that )

You will get No jobs running. if its not being shutdown.

You have to parse the output which is a bit messy as the systemctl doesnt return a different exit code for the two results. But it does seem reasonably reliable. You will need to watch out for a format change in the messages if you update the system however.