Automatic shutdown of Pi based on timer

Question

I'm looking to have my pi shutdown automatically based on a time interval (rather than the time of day). This is for an art installation and the reason I am turning off the pi is because it will connected to a projector, and to get the projector to shutoff, it needs to not be receiving a signal. I've tried editing cron with the follow command to no success:

@reboot shutdown -h +5

This was the test sample, my goal is have it shutdown after 3 hours. This is to save lamp life as the sculpture will run continuously, 24/7. I have a timer switch connected to the pi and projector that will turn it back on automatically.

Is there another way to do this?

Answer 1

You've made a couple of mistakes, all par for the course. Let's step through this, get it working, and hopefully learn one other trick to help you help yourself in the future:

First, the "other trick": When you run a command from the terminal, your error messages stderr go to the terminal, and you see them immediately. When you run a command as a cron user, those error messages go "somewhere else", and you don't see them when the script execution is attempted. However, you can "redirect" the stderr to a file for post-op review later. I've modified your command to add the stderr redirection as follows:

@reboot shutdown -h +5  > /home/pi/cronjoblog 2>&1  

Copy that over your current crontab entry. Then save your crontab file, and reboot:

$ sudo reboot   

NOTE: The $ is not an input, it's part of the command line in the shell.

Five minutes later, you'll note that your RPi has not halted, but a file containing an error message is now available here: /home/pi/cronjoblog

Inspecting this file, you'll see the following:

$ cat cronjoblog
/bin/sh: 1: shutdown: not found  
$  

This message simply means that the shell couldn't find shutdown. It couldn't find it because the cron user has a different $PATH than you do as user pi. The solution is easy enough - simply give the full file spec for shutdown:

@reboot /sbin/shutdown -h +5  > /home/pi/cronjoblog 2>&1  

If you modify your crontab file, then reboot again to run this, the file /home/pi/cronjoblog will inform you of the other mistake you've made: shutdown requires root privileges, i.e. sudo. I'll skip the intervening trial-and-error process (but you shouldn't), and show a functional crontab entry here:

@reboot /usr/bin/sudo /sbin/shutdown -h +5  > /home/pi/cronjoblog 2>&1  

That should do it... let us know if you have other issues, and we'll address those.