Shutdown a linux computer depending on the battery level

Question

Is there a way to shutdown a linux-based computer when the battery charge drops below a certain level? I have searched through the shutdown and poweroff manuals but found nothing relevant.

Answer 1

All right! I did some more thorough research on the topic, and it appears that this can be done with a script. The source of inspiration for my script below, is the following topic in the arch linux forums:

https://bbs.archlinux.org/viewtopic.php?id=162900 ,and more specifically the first post by vacant.

#!/bin/bash
BATTINFO=`acpi -b`
CRITICAL_PERCENTAGE=21
FILE_LOCATION="/home/username/LOW_BATTERY" #change this to correct username!

if [[ `echo $BATTINFO | grep Discharging` && `echo $BATTINFO | cut -c 25-26 ` -lt $CRITICAL_PERCENTAGE ]]
then
echo `date` >> $FILE_LOCATION 
echo "Was forced to hibernate, due to low battery status">>$FILE_LOCATION       
echo $BATTINFO >> $FILE_LOCATION
sudo pm-hibernate 
fi

The job can be automated using a cron job, by editing the root user's crontab file (e.g using su in a terminal, and then crontab -e), for the script to be executed once in every regular interval (e.g 2 minutes). As mentioned by davidgo above, there is a (very) similar question answered in http://unix.stackexchange.com, that could be used as a reference for a more advanced approaching.

Answer 2

Here you can find useful UDEV rules for this: https://wiki.archlinux.org/title/Laptop#Hibernate_on_low_battery_level

just replace "hibernate" with "poweroff" in the suggested UDEV rule file content and it should switch off your computer instead of hibernating it.

Here is the code snippet you should save in /etc/udev/rules.d/99-lowbat.rules:

# Suspend the system when battery level drops to 5% or lower
SUBSYSTEM=="power_supply", ATTR{status}=="Discharging", ATTR{capacity}=="[0-5]", RUN+="/usr/bin/systemctl poweroff"

Before doing that, make sure your battery is sending UDEV messages as the discharge proceeds, otherwise the aforementioned code will not work. You can watch for new UDEV messages with:

udevadm monitor --property

If this does not show any message while the battery discharges, you have to use another method like adding this script to cron, also taken from the same link above (but it requires you to install acpiclient, that seems is not packaged for Debian):

#!/bin/sh
acpi -b | awk -F'[,:%]' '{print $2, $3}' | {
    read -r status capacity

    if [ "$status" = Discharging -a "$capacity" -lt 5 ]; then
        logger "Critical battery threshold"
        systemctl poweroff
    fi
}

In case you do not have the acpi command available, you can use this small script (please let me know it it does not work for you. In case there is more than one battery, it will shutdown the computer if any of these is discharging below 10%):

#!/bin/sh
for battery in /sys/class/power_supply/*bat*
do
        status=`cat $battery/status`
        capacity=`cat $battery/capacity`

        if [ "$status" = Discharging -a "$capacity" -lt 10 ]; then
                logger "Critical battery threshold"
                systemctl poweroff
        fi
done