recursive change sed grep and sed - recursion

Question

I'm still working in this little script but still having problems. Getting error
sed: no input files
I think the problem is on:
for i in `cat sed_output.txt` do sed 's/$oldstr/$newstr/g' > sed_output_new_old_string.txt done
echo "Enter string to be replaced"
read OLDSTRING
echo "Enter string to be placed"
read NEWSTRING
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
grep -rl $oldstr $FOLDERPATH > grep_output.txt
### spaces or special characters on filenames, use sed to enclose them with quote
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 sed_output.txt`
do sed 's/$oldstr/$newstr/g' > sed_output_new_old_string.txt
done

Answer 1

If
You know the file on which you need to do the substitution
You know what needs to be substituted
You know the replacement
then;
sed 's/substitution/replacement/g' filename
will scan file line by line looking for substitution text and replace it with replacement text.
For recursive substitution, you can do;
for file in path/to/folder; do
sed -i.bak 's/substitute/replacement/g' "$file"
done
The -i option will do in file changes. I added .bak to it so you have the backup of original files which will be renamed to file.bak.
If you are using variables in your sed then I would suggest you to use " instead of ' so that you variables gets interpolated correctly.