#! /bin/bash
#pss <file>   return the size of a physical sector of the devise where the file is.
file=$1;
init=`df -BK --output=used $file | tail -n 1 | awk -FK '{print $1}'`
echo "Some text to the file" > $file$$
last=`df -BK --output=used $file | tail -n 1 | awk -FK '{print $1}'`
echo `expr $last - $init` Kbytes
rm  $file$$