How To check LXD container BTRFS disk usage on Linux

#!/usr/bin/env bash
# Usage: 
# Find LXD container disk size and how much space they are using when storage back end set to BTRFS.
# Tested on Ubuntu Linux 20.04 LTS only
# Syntax:
# /path/to/lxd-btrfs-df 
# /path/to/lxd-btrfs-df | more
# /path/to/lxd-btrfs-df | grep "container-name"
# /path/to/lxd-btrfs-df  /dev/sda2
# ----------------------------------------------------------------------------
# Written by Vivek Gite <https://www.cyberciti.biz/>
# (c) 2020 Vivek Gite under GNU GPL v2.0+
# ----------------------------------------------------------------------------
# Last updated: 23/Oct/2020
# ----------------------------------------------------------------------------
set -eu -o pipefail
 
# Default set to aws /dev/xvdf but override using the cli arg
DEV="${1:-/dev/xvdf}"
 
# BTRFS mount point 
MNT="/mnt/btrfs"
 
# Look for btrfs binary
_BTRFS="$(command -v btrfs)"
 
 
# Am i root user? if not die.
[ "$(id -u)" -ne "0" ] && { echo "This script must be run as root."; exit 1; }
 
# Failsafe stuff
[ "$_BTRFS" == "" ] && { echo "btrfs command not found."; exit 2; }
[ ! -d "$MNT" ] && mkdir -p "$MNT"
[ ! -b "$DEV" ] && { echo "$DEV not found. Try '$0 /dev/BTRFS_DEVICE' command."; exit 3; }
 
# Is $DEV mounted?
if ! mount | grep -q "^${DEV}" 
then
	mount "$DEV" "$MNT" 
else
	MNT="$(mount | grep ^"${DEV}" | awk '{print $3}')"
fi
 
# My quick df for lxd containers 
echo -e "\n\t* Disk usage for LXD containers with BTRFS fs\n"
for d in "${MNT}"/containers/*
do 
	$_BTRFS filesystem du -s "$d"
done
 
# Run df on mount fs too to get total disk usage 
echo -e "\n\t* Total (df) for $MNT BTRFS fs\n"
$_BTRFS filesystem df "$MNT"
 
umount "$MNT"