s/search/replace for all files in a directory

Here's a quick and dirty shell script I wrote to rename all the files in a directory based upon a search and replace string:

#!/bin/sh

S=$1
R=$2

if [ -z "$R" ]; then
  echo "Usage: rename.sh <search> <replace>"
  exit
fi

for i in `ls *$S*`; do mv $i $(echo $i | sed s/$S/$R/); done

Be careful, virtually no error checking is done.