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
  status=`cat /sys/class/power_supply/BAT1/capacity`
  if [ $status -lt 10 ]; then
    swapoff -a
    swapon -a
    pm-hibernate
  fi
  sleep 60
done
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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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