dus.sh

#!/bin/bash
dus () {
  size=0
  # checking whether the dir argument provided
  if [ "$1" != "" ]; then
    sub_dir="/$1"
  else
    sub_dir=""
  fi
  # iterating over all files in all subdirectories
  for f in $PWD$sub_dir/**/*
  do
    # checking whether that's the path to the file
    if [[ -f $f ]]; then
      # calculating size in bytes
      tmp=`wc -c "$f" | awk '{print $1}'`
      ((size+=tmp))
    fi
  done
  echo "$size"
}