Multiple search and replace in bash

msr.sh

#!/bin/bash
# $1 : original file
# $2 : destination file
# $3 : patterns file

if [ ! $# -eq 3 ];
then
  echo Usage : $0 original destination patterns
  exit 1
fi

for i in $1 $3
do
  if [ ! -f $i ];
  then
    echo File $i does not exist
    exit 1
  fi
done

tmp_file=$(mktemp)
cp $1 $tmp_file

OIFS=$IFS
IFS=","
while read old new
do
  sed -i.bak "s/$old/$new/g" $tmp_file
done < $3
IFS=$OIFS

mv $tmp_file $2