How to put a daemon on your laptop to hibernate when battery low
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[I don't use systemd, but I think it should work with it too.]

Create /opt/battery/battery-watch containing:

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#!/bin/bash

while true; do
  cabled=`cat /sys/class/power_supply/ADP1/online`
  if [ $cabled -eq 0 ]; then
    status=`cat /sys/class/power_supply/BAT1/capacity`
    if [ $status -lt 10 ]; then
      swapoff -a
      swapon -a
      pm-hibernate
    fi
  fi
  sleep 60
done
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The first test (the 'cabled' thing) is to check if the computer
is plugged into the wall, in which case we don't care about the
battery level and so we don't sleep if too low. You can adapt or
remove it.

Adapt the path of "capacity" to your own case. Adapt sleep time
(here 60 seconds). Adapt charge below which you hibernate (here
10 for 10%).

Create /etc/init.d/battery containing:

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#!/bin/sh

set -e

BWATCH="/opt/battery/battery-watch"
BWATCH_NAME="battery-watch"

# Check for daemon presence
[ -x "$BWATCH" ] || exit 0

# Get lsb functions
. /lib/lsb/init-functions

set +e

case "$1" in
  start)
    log_daemon_msg "Starting battery-watch service" "battery-watch"
    start-stop-daemon --background --start --quiet --exec "$BWATCH" --name $BWATCH_NAME
    log_end_msg $?
    ;;
  stop)
    log_daemon_msg "Stopping battery-watch service" "battery-watch"
    start-stop-daemon --stop --quiet --retry 5 --name $BWATCH_NAME
    log_end_msg $?
    ;;
  restart)
    $0 stop
    sleep 1
    $0 start
    ;;
  reload|force-reload) 
    log_daemon_msg "Reloading battery-watch services" "battery-watch"
    start-stop-daemon --stop --signal 1 --exec "$BWATCH"
    log_end_msg $?
    ;;
  status)
    status_of_proc "$BWATCH_NAME" "$BWATCH" "system-wide $BWATCH_NAME" && exit 0 || exit $?
    ;;
  *)
    log_success_msg "Usage: /etc/init.d/battery {start|stop|restart|reload|force-reload|status}"
    exit 1
esac
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Maybe not perfect. If you have a better program, share, thanks.

Then run: update-rc.d battery defaults
That will create links in /etc/rc*.d

Here I have:

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/home/sed> ls -al /etc/rc*/*battery*
lrwxrwxrwx 1 root root 17 Jan 29 11:41 /etc/rc2.d/S03battery -> ../init.d/battery
lrwxrwxrwx 1 root root 17 Jan 29 11:41 /etc/rc3.d/S03battery -> ../init.d/battery
lrwxrwxrwx 1 root root 17 Jan 29 11:41 /etc/rc4.d/S03battery -> ../init.d/battery
lrwxrwxrwx 1 root root 17 Jan 29 11:41 /etc/rc5.d/S03battery -> ../init.d/battery
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