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

The -z check is for variable strings, which your grep isn't giving. To give a value from your grep command, enclose it in $():

if [ -z $(grep $address /etc/passwd) ]