fat32_img_size.sh

function fat32_img_size {
	declare dir="$1"
  
	declare -i unit_sector=512
  
	declare -i reserved_sectors=32
	declare -i num_fats=2
	declare -i num_paths=$(find "$dir" -mindepth 1 | wc -l)
	declare -i fat_sectors=$((num_paths * unit_sector / num_fats - 1))
  
	declare -i user_data_sectors=0
	while read -r path; do
		declare -i path_size=$(du -b "$path" | cut -f 1)
    
		declare -i sectors=$((path_size / unit_sector))
		[[ $((path_size % unit_sector)) -eq 0 ]] || ((sectors++))
    
		((user_data_sectors += sectors))
	done < <(find "$dir" -mindepth 1 -type f)
  
	declare -i fat32_img_size=$(((reserved_sectors + fat_sectors * num_fats + num_paths + user_data_sectors) * unit_sector))
	declare -i sectors_per_cluster=8
	declare -i cluster_size=$((sectors_per_cluster * unit_sector))
	while ((fat32_img_size % cluster_size != 0)); do
		((fat32_img_size += unit_sector))
	done
  
	echo "$fat32_img_size"
}