Script to print size and available space on a subvol quota

volstat.sh

#!/bin/sh

# this script is called as `volstat.sh <path/to/subvol>`
set -e

vol=$1
id=$(btrfs subvol show $vol | grep 'Subvolume ID:'| cut -f 4)
output=$(btrfs qgroup show --raw -r $vol| grep "^0/$id")

size=$(echo $output | cut -d ' ' -f 4)
used=$(echo $output | cut -d ' ' -f 3)

# this should output `<size> <available>` space on that subvolume in bytes

echo $size $(( $size - $used ))