Read battery percentage and charge/discharge state. Useful as a tmux status plugin. Uses shell builtins heavily to be as fast as possible.

batstat.sh

#!/bin/sh

set -o nounset
set -o errexit

BAT=/sys/class/power_supply/BAT0

if [[ ! -d $BAT ]]; then
  echo 'No battery'
  exit 0
fi

cd $BAT

read BAT_FULL < charge_full
read BAT_NOW  < charge_now
BAT_PCT=$(( 100 * $BAT_NOW / $BAT_FULL ))

BAT_STAT=""
read STATUS < status
case $STATUS in
	Full)
		BAT_STAT="="
		;;
	Discharging)
		BAT_STAT="<"
		;;
	Charging)
		BAT_STAT=">"
		;;
esac

echo "${BAT_STAT}${BAT_PCT}%"