So this just reads /sys/class/power_supply/BAT0/energy_now (or charge_now) and /sys/class/power_supply/BAT0/charge_full and outputs it nicely. While the asm is cool, how large would the bash script to do it?
This project started as a rewrite of a Bash script (for learning purposes), so I can easily answer this question. It's indeed small. Here's the full Bash script I used to use (including comments):
#!/usr/bin/env bash
# Print the remaining amount of energy (or charge)
# stored in the battery identified by `BAT0`. This
# is what works on my ThinkPad X220 and may not be
# portable to other laptops without changes.
cd /sys/class/power_supply/BAT0 || exit $?
# Sometimes there are `energy_now` and `energy_full`
# pseudofiles, and sometimes there are `charge_now`
# and `charge_full` pseudofiles instead.
if [[ -e energy_now ]]; then
now=$(<energy_now)
full=$(<energy_full)
unit=Wh
elif [[ -e charge_now ]]; then
now=$(<charge_now)
full=$(<charge_full)
unit=Ah
fi
percent=$((100 * now / full))
# Convert from microwatt-hours (or microampere-hours)
# to watt-hours (or ampere-hours).
now=$(bc <<< "scale=1; $now / 1000000")
full=$(bc <<< "scale=1; $full / 1000000")
echo "$now $unit / $full $unit (${percent}%)"
