Shell Programming and Scripting

I have a current code working(named subst1) having a user be able to type this line to substitute words using the sed command:

Code:

subst1 old-pattern new-pattern filename

Here is my shell script:

Code:

#!/bin/bash
# subst1

ARGS=3
E_BADARGS=65

if [ $# -ne "$ARGS" ]
then
  echo "Usage: `basename $0` old-pattern new-pattern filename"
  exit $E_BADARGS
fi

old_pattern=$1
new_pattern=$2

if [ -f "$3" ]
then
    file_name=$3
else
    echo "File \"$3\" does not exist."
    exit $E_BADARGS
fi

mv $file_name $file_name.bak
sed -e "s/$old_pattern/$new_pattern/g" $file_name.bak > $file_name

exit 0

The problem with this script is that it changes the date and creates a bak file. I need to alter this script so that when a user inputs the command line, the script checks if the given old-pattern is in the file and if it is, go ahead and do the substitution. If the old-pattern is not found, the script should do nothing. Any help would be appreciated!