How to check if grep has no output?

Question

So I need to check if the recipient username is in /etc/passwd which contains all the users in my class, but I have tried a few different combinations of if statements and g

Answer 1

Just do a simple if like this:

if grep -q $address  /etc/passwd
then 
   echo "OK";
else
   echo "NOT OK";
fi

The -q option is used here just to make grep quiet (don't output...)