Trap Computer Shutdown Signal

[Penguin Guy]

Hi! I've got a shell script that downloads and displays a sunmap (http://www.opentopia.com/images/cams/world_sunlight_map_rectangular.jpg) as the background every 60 seconds. The program reverts the background to the original when it ends by using a trap statement - everything works fine apart from when the script is ended by a computer shutdown, in which case the background is not reverted. This is part of a small Launchpad project and this issue is putting on hold a merge, so any help would be greatly appreciated.

View File (http://bazaar.launchpad.net/~codemonkeydevs/codemonkey/trunk/annotate/head:/earthwallpaper.sh_new) - Download File (http://bazaar.launchpad.net/~codemonkeydevs/codemonkey/trunk/download/head:/earthwallpaper.sh_ne-20100603132600-z2020xi4jj5slqog-1/earthwallpaper.sh_new) - View Bug (https://bugs.launchpad.net/codemonkey/+bug/589355)

Thanks!


SOLVED
Turns out trap doesn't work with loops, use the wait statement to fix this.

[trent.josephsen]

I must be misunderstanding you. If you're about to shut down the computer, why do you care whether it gets reverted or not?

[Penguin Guy]

If you're about to shut down the computer, why do you care whether it gets reverted or not?
Because when you next start the computer you'll have no wallpaper.

[trent.josephsen]

Because when you next start the computer you'll have no wallpaper.

Oh! I set my wallpaper in .xinitrc, so that never even occurred to me.

Maybe SIGKILL? But I would have expected SIGTERM to work.

[Penguin Guy]

Maybe SIGKILL? But I would have expected SIGTERM to work.
I'm pretty sure you can't trap SIGKILL, but I'll give it a go anyway.

EDIT: Nope, that doesn't work either.

[PaulM1985]

From searching the net it seems that SIGTERM should work.

However if it doesn't when the system shuts down rc.d scripts are called and maybe you could add something in there to notify your program.

Below is the link I plagiarised this information from :-)

http://stackoverflow.com/questions/2832376/how-to-detect-pending-system-shutdown-on-linux

Hope it is of some help.

Paul

[Penguin Guy]

From searching the net it seems that SIGTERM should work.

However if it doesn't when the system shuts down rc.d scripts are called and maybe you could add something in there to notify your program.

Below is the link I plagiarised this information from :-)

http://stackoverflow.com/questions/2832376/how-to-detect-pending-system-shutdown-on-linux

Hope it is of some help.

Paul
I'm already using SIGTERM, and rc.d scripts are really a bit advanced for this program. Are there any other possible suggestions?

[trent.josephsen]

I'm pretty sure you can't trap SIGKILL, but I'll give it a go anyway.

No, of course not. I meant that perhaps your script is being sent KILL instead of TERM, even though TERM should end the script. Sorry for miscommunicating.

[Penguin Guy]

I meant that perhaps your script is being sent KILL instead of TERM, even though TERM should end the script.
Looks like it is, but why?

[ov3rcl0ck]

Shell scripts are too high level to catch SIGTERM or SIGKILL. Maybe theres a way to do it, but I don't think trapping a signal will do it, seeing as the signal is sent to BASH/shell rather than your script. Unless BASH will relay this information to a shell script, which I don't think it does.

[trent.josephsen]

And kill -TERM works as you expect? Does it behave the same way when you log out without shutting down, or does it continue running while you're logged out?

[dwhitney67]

Shell scripts are too high level to catch SIGTERM or SIGKILL. Maybe theres a way to do it, but I don't think trapping a signal will do it, seeing as the signal is sent to BASH/shell rather than your script. Unless BASH will relay this information to a shell script, which I don't think it does.

Wrong!

Here's an example script where SIGTERM is caught:


#!/bin/bash


trap dothis SIGTERM


function dothis()
{
echo "Revert to original wallpaper here."
exit 0
}

while [ true ]
do
sleep 2
done

Run this script, then after referencing the PID of the script, issue a SIGTERM signal to it with something like:


kill -15 <pid>

# or

kill -s SIGTERM <pid>

To get a list of all of the signals that are recognized by Bash, run

trap -l

Obviously, as mentioned earlier, SIGKILL cannot be intercepted.

[Penguin Guy]

On a suspicion I googled traps and loops and found this page: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_12_02.html
It seems you need to add a wait statement if you want traps to work with loops, now I know what exactly the problem is it should be easy to fix it, so I'll mark this thread [SOLVED].

Thanks for all the help!