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#!/bin/bash
capacity_threshold=50

battery_dir=/sys/class/power_supply/BAT0
status_file=$battery_dir/status
capacity_file=$battery_dir/capacity

UNIT=powerloss-handler
CMD=$0

self_install()
{
    if systemctl --user is-active "$UNIT" >/dev/null
    then
        systemctl --user stop "$UNIT"
    fi
    systemctl --user reset-failed "$UNIT" 2>/dev/null
    systemd-run --user \
                --unit "$UNIT" \
                --property Restart="on-failure" \
                -- "$CMD"
}

go()
{

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

    read status < "$status_file" || return

    if [ -t 2 ]
    then
        printf '\r  %20s   %.2d%%\r' "$status" "$capacity" >&2
    fi

    [ "$status" = Discharging ] || return 0
    if [ "$capacity" -lt "$capacity_threshold" ]
    then
        systemctl suspend
    fi
}

main()
{
    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

    while true
    do
        go
        sleep 1
    done
}

if [ "$1" = install ]
then
    self_install
else
    main
fi