Thanks for help. I have another problem. If I use
sed 's:$old_string:$new_string:g' oldfile > newfile
where $old_string and $new_string are variables containing the string, it does not work. The both variables contain the right string, I checked it using
echo $old_string $new_string.
. I think sed does not deal with $old_string and $new_string as being variables, bacause it worked if I replaced the variables with the real strings.
Thanks for help