gistfile1.sh

#!/bin/bash
if [ $# -ne 1 ]; then
    battery_id='BAT0'
else
    battery_id=$1
fi
battery_max=`cat /proc/acpi/battery/$battery_id/info | head -3 | tail -1 | awk '{print $4}'`
if [ "$(cat /proc/acpi/battery/$battery_id/state | grep -o discharging)" == "discharging" ]; then
    prepend="-"
else
    prepend="+"
fi

battery_current=`cat /proc/acpi/battery/$battery_id/state | head -5 | tail -1 | awk '{print $3}'`
battery_level=$((100*$battery_current/$battery_max))
echo "$prepend$battery_level"