A bash script for replacing a string in every file of a directory

It replaces all occurrences of the search string with the replacement string in every file of a specified directory:

#!/bin/bash

string1='abc';
string2='123';
directory='test_directory';
for file in $(grep -lr $string1 $directory); do
  echo $file;
  cp $file /tmp/temporal_file;
  sed s/${string1}/${string2}/ /tmp/temporal_file > $file;
  rm -rf /tmp/temporal_file;
done;