Recursive searching via grep and sed

echo "Enter string to be placed"
read NEWSTRING
echo "==================================="
echo "string to be replaced is $OLDSTRING"
echo "string to be placed is $NEWSTRING  "
echo "==================================="
oldstr=$OLDSTRING #string to be replaced
newstr=$NEWSTRING #new string to be placed
echo "Enter folder path where we will find the string"
read FOLDERPATH

### grep oldstring and output it in grep_output.txt    STEP1
grep -rl $oldstr $FOLDERPATH > grep_output.txt

### since there might be spaces or special characters on filenames, use sed to enclose them with quote and output in  sed_output.txt  STEP2
#for i in `cat grep_output.txt`
#do sed -e "s/'/'\\\\''/g;s/\(.*\)/'\1'/" grep_output.txt  > sed_output.txt
#done

for i in `cat grep_output.txt`
do sed -i "s/$oldstr/$newstr/g" $i > sed_output_new_old_string.txt
done


## NOTE ##
# 2013 latest
## STEP 2 Seems not working just edit the grep_output and remove the spaces

# If this doesnt work, on the first time, do again then comment STEP 1 and STEP 2
# Also try to remove the spaces on the grep_output