#!/usr/bin/env sh
battery=$1

print_battery_status() {
  battery_capacity=$(cat "/sys/class/power_supply/${battery}/capacity") \
    || exit

  if   [ "$battery_capacity" -lt 25  ]; then echo critical
  elif [ "$battery_capacity" -lt 50  ]; then echo low
  elif [ "$battery_capacity" -lt 75  ]; then echo medium
  elif [ "$battery_capacity" -lt 100 ]; then echo high
  else                                       echo full
  fi
}

while :; do
  print_battery_status
  sleep 20
done