File edit script exceptions how to pass to script

i wrote script for if file exists script do file edit problem with the script get stuck not existing as for exit i mentioned exit 0 , and how to give the exception for script it should add

./script -- add  hi
./script --add "hi how are you"

now below script with case it is working for ./script -- add hi
how to add exception it should not take any string example

./script --add "hi how are you"

and how to send my arguments should accept and add the string to file ./script --add "hi how are you"

function fileread()  {
filename=$1
echo $filename
if [ $2 == add ]
    then
        echo $3 >> $filename
        echo "add opeartion completed"
        # cat $filename
        shift 
        else
        echo " add not done"
fi

if [ ! -z "${4/ //}" ]
                            then
                            echo "adding"
                               fileread $fileName $3 $4
                            else
                            echo "arguments or values not proper"
                            helpFunc --arg2;
                            exit 1                             
                            fi                                
                            


exit 0