Bash script that checks a laptop battery status via sysfs and performs automatic shutdown if capacity is below a user-specified threshold (30% by default)

monit_laptop_battery.sh

#!/usr/bin/env bash

##################
# Battery watchdog
##################

DEFAULT_BAT="BAT0"
DEFAULT_CAPACITY_LOW='30'

capacity_now () {
  if [[ $(cat "/sys/class/power_supply/${BAT:-$DEFAULT_BAT}/uevent") =~ POWER_SUPPLY_CAPACITY=([0-9]*) ]]; then
    echo "${BASH_REMATCH[1]}"; return 0
  else
    echo "NA"; return 1
  fi
}

check_bat () {
  if cat "/sys/class/power_supply/$1/uevent" > /dev/null 2>&1; then return 0; else return 1; fi
}

msg () {
  echo "[Battery Watchdog] [$(date +%T)] [$1]: $2"
}


# main logic
if [[ "$(id -u)" -ne 0 ]]; then
  msg 'ERROR' 'User is not root. This script needs root permission.'; exit 1
fi

trap "msg 'INFO' 'Received a signal to stop. Bye!'; exit 0" INT HUP TERM

while getopts 'b:c:' OPT; do
  case $OPT in
    b) BAT="$OPTARG";;
    c) CAPACITY_LOW="$OPTARG";;
    \?) msg 'ERROR' 'Invalid option. Use -b for BAT dir name and -c lowest capacity before shutting down.'; exit 1;;
  esac
done

if ! check_bat "${BAT:-$DEFAULT_BAT}"; then
  msg 'ERROR' "The battery ${BAT:-$DEFAULT_BAT} does not exist at '/sys/class/power_supply/'"; exit 1
fi

msg 'INFO' 'Checking battery status...'

if [[ $(cat "/sys/class/power_supply/${BAT:-$DEFAULT_BAT}/uevent" 2> /dev/null) =~ "POWER_SUPPLY_STATUS=Discharging" ]]; then
  msg 'INFO' 'Battery is discharging. Checking level...'
  if capacity_now > /dev/null 2>&1; then
    if [[ $(capacity_now) -lt "${CAPACITY_LOW:-$DEFAULT_CAPACITY_LOW}" ]]; then
      msg 'WARNING' "Battery level is below threshold (${CAPACITY_LOW:-$DEFAULT_CAPACITY_LOW}): $(capacity_now)"
      msg 'INFO' 'Starting shutdown procedure in 5 minutes. Bye!'; sleep 300
      shutdown now
    else
      msg 'INFO' "Battery level is above threshold (${CAPACITY_LOW:-$DEFAULT_CAPACITY_LOW}): $(capacity_now)."
      msg 'INFO' 'Nothing to be done. Bye!'; exit 1
    fi
  else
    msg 'ERROR' 'Unable to retrieve the capacity level.'; exit 1
  fi
else
  msg 'INFO' 'Battery is not discharging. Bye!'; exit 0
fi