#!/bin/bash
default_capacity_threshold=50

if [ -e /usr/lib/bash/sleep ]
then
    enable -f /usr/lib/bash/sleep sleep
else
    echo 'Warning: not found: /usr/lib/bash/sleep (try: "sudo apt install bash-builtins")' >&2
fi

main()
{
    capacity_threshold=${1:-$default_capacity_threshold}
    while true
    do
        check_batteries /sys/class/power_supply/BAT*
        sleep 1
    done
}

check_batteries()
{
    for dir in "$@"
    do
        [ -d "$dir" ] || continue
        check_battery "$dir"
    done
}

check_battery()
{
    battery_dir=$1

    read status < "$battery_dir"/status || return
    [ "$status" = Discharging ] || return 0

    read capacity < "$battery_dir"/capacity || return
    case "$capacity" in
        '' | *[^0-9]*)
            echo "Error: unexpected value for battery capacity: $capacity" >&2
            return 1
            ;;
    esac

    if [ "$capacity" -lt "$capacity_threshold" ]
    then
        systemctl suspend
    fi
}

main "$@"