snaps=($root_dir/btrbk/$subvol_dir.20*) # Assumes we are in the 21st century.
if [[ ! ${snaps[*]} ]]; then
# no snapshots yet
+ # TODO: make this an error and override with a cli flag
echo "$0: warning: no snapshots found at $root_dir/btrbk/$subvol_dir.20*. this is expected for a brand new volume"
continue
fi