#! /bin/bash
#pss <file>   return the size of a physical sector of the devise where the file is.
file=$1
function usedSpace() {
  df -k --output=used $1 | tail -n 1 
}
init=`usedSpace $file`
echo "Some text to the file" > $file$$
last=`usedSpace $file` 
echo `expr $last - $init` Kbytes
rm  $file$$