#! /bin/bash
#pss <file>   return the size of a physical sector of the devise where the file is.
file=$1;
IFS='K'   # inter field separator
set `df -BK --output=used $file | tail -n1`
init=$1
echo "Some text to the file" > $file$$
set `df -BK --output=used $file | tail -n1`
last=$1 
echo `expr $last - $init` Kbytes
rm  $file$$