New and updated version of backup.sh. The old script had many limitations and was often unreliable. This script, while requiring slightly more user input, is much more robust.

backup.sh

#!/bin/sh

#Find a date in file name:
#ls | grep -E $datematch

increment() {
    # Creates a backup version of a btrfs subvolume and sends it to to an
    # external drive. Assumes there are drives mounted in ./hd and ./bu
    # $1 subvolume to backup
    # $2 date of parent
    # $3 date of child
    local old=$1-$2
    local new=$1-$3
    echo Backing up $1...
    btrfs sub snap -r ./hd/$1 ./hd/$new
    echo Sending $new to external drive...
    sync
    btrfs send -p ./hd/$old ./hd/$new | btrfs receive ./bu/
    echo -----------------------------------------------------------------------
}

if [ "$(whoami)" != "root" ]; then
    echo Please run this script as root.
    exit
fi

cd /mnt
#Mount drives
mount /dev/sda3 ./hd
mount /dev/sdb1 ./bu

echo /dev/sda3:
ls ./hd
echo -----------------------------------------------------------------------
echo /dev/sdb1:
ls ./bu
echo -----------------------------------------------------------------------

#User Prompts
read -p 'Backup @ [y/n]? ' bu_root
read -p 'Backup @home [y/n]? ' bu_home
read -p 'Backup @Dropbox [y/n]? ' bu_dropbox
read -p 'Date of previous version: ' old_ver

today=$(date -I)

#Backup function calls
if ([ $bu_root = "y" ] || [ $bu_hd = "Y" ]); then
    increment @ $old_ver $today
fi

if ([ $bu_home = "y" ] || [ $bu_home = "Y" ]); then
    increment @home $old_ver $today
fi

if ([ $bu_dropbox = "y" ] || [ $bu_dropbox = "Y" ]); then
    increment @Dropbox $old_ver $today
fi