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I'm trying to run a simple bash script that turns off my notebook if it is not pluged on AC with a cron job.

My script is as follows:

#!/bin/bash

if ! on_ac_power; then 
    poweroff

fi

And I've configured cron to run it every minute like so:

*/1 * * * * /home/user/Documents/script.sh

The script works just fine if I run it manually, but otherwise, it seems to have no effect under cron.

What am I missing here?

Rogerio Schmitt
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    Cron is not the correct tool for this. What you want is a service. what is on_ac_power? it gives nothing when on ac and when not on ac so seems useless for this. And poweroff I would add a directory to. – Rinzwind Aug 23 '21 at 15:31
  • @Rinzwind on_ac_power tests whether the computer is running on line power. Add a directory do poweroff, how so? As I said the script works just fine if I run it manually, but cron seems to not be running it. – Rogerio Schmitt Aug 23 '21 at 15:43
  • Apart from the previous comment, poweroff should require root privileges; sudo poweroff might work if the user in question is not required to enter a password. Better yet, put the cron entry into /etc/crontab and specify that it's to be executed as root (of course, the script should be moved to a sensible location as well in that case). – Markus Ueberall Aug 23 '21 at 15:45
  • @MarkusUeberall never use sudo in scripys and never ever in thingd that have ti be run in cron !! Instead run the script in root's cron if needed. – Soren A Aug 23 '21 at 16:05
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    @MarkusUeberall by the way, poweroff (/usr/sbin/poweroff) liks to /bin/systemctl wgich can be executed by anyone so sudo isn't needed anyway. – Soren A Aug 23 '21 at 16:11
  • @Soren A: /bin/systemctl can be executed by anyone, but executing poweroff as a non-root user will not necessarily work if other users are logged in (see poweroff -i, which I actually forgot to mention above); sudo poweroff always works immediately. – Markus Ueberall Aug 23 '21 at 16:23
  • @MarkusUeberall .. but sudo doesn't worke in crontab ... the script must be run in root's cron. And for logged in users, there are seldom more than one user logged in on home systems . but off course a point to be aware of on larger systems. – Soren A Aug 23 '21 at 16:46
  • @soren A sudo does work in crontab as long as you don't need to enter a password (see e.g. https://askubuntu.com/questions/796617/how-to-avoid-password-request-for-sudo-for-crontab-scripts); whether it's a good idea to use it for the OP's use case is another question (see previous comments pointing to a service and /etc/crontab addressing this). – Markus Ueberall Aug 24 '21 at 07:07
  • Please clarify, in your question above, the full command you execute when you run the command manually, as well as output from ls showing the filesystem permissions of the script. – James S. Aug 25 '21 at 14:42

1 Answers1

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First of all I was not running my cron under root.

Apparently for a cron to run under root you have to add it with sudo crontab -e, anything added just with crontab -e will not run as sudo.

Second as pointed in the reference:

cron runs in a very limited environment by default so a lot of commands that run via command name from a users terminal need to the full path in a crontab or a declaration at the beginning of the crontab to expand the path.

So I ended up replacing poweroff by /sbin/shutdown in my script, and now it runs as expected.

#!/bin/bash

if ! on_ac_power; 
then 
    /sbin/shutdown

fi

Reference

Rogerio Schmitt
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