Error messages in basic UNIX shell scripts

Ok, so I wrote the following program just to learn how to do this, the error messages and questions follow:

#1/usr/local/bin/tcsh -f

if ( $USER == "blayney") then
   echo "hey there jeff"
   umask 777
endif
echo "what file to replace strings in current working directory?"
set userinput = $<
echo "if file exists and is executable, what string to replace?
set string = $<
echo "replace with what
set rstring = $<
echo "this may take a few moments"
sed "s/$string/$rstring/g"  $userinput  > $userinput\.temp
echo "successfully replaced all occurences of $string with $rstring in file $userinput, would you like to save changes? (y/n)"
set save = <$
if ( $save == "y" )
then
mv $userinput\.temp $userinput
echo "all changes have been saved"
if ( $save == "n" )
then echo "changes were not saved"
endif
endif
rm $userinput\.temp



so I will get to the point where the program will say: successfully replaced all occurences of jeff with man in file.txt, would you like to save changes? (y/n)
then right under that it says the following: $: No such file or directory.

Also, how do you get the program to check if the file exists, is executable, and not write protected?

if any of you out there actually want to edit this and make it an acceptable program by your standards that is fine too. As long as I can ask questions about what you did. I would like to keep it relatively simplistic though.