Help with shell script - creating users

Shell Programming and Scripting
1 
echo -e "Enter in a username : \c"
read username
grep "^$username:" /etc/passwd

What I'm trying to do is take in a username from my script and I need to be able to check if that username already exists. If it does the script should display a message saying that the user already exists and exit.

It's probably easy, but I am having a lot of trouble with it. I don't know how to structure my conditional properly.

grep "^$username:" /etc/passwd

This works fine with searching through the passwd file for the user, but how do I put this in the form of a conditional?

If anyone can help I would really appreciate it. Thanks.

2 
status=$(grep "^$username:" /etc/passwd)

if [[ "$status" == "" ]]; then
    # useradd command
else
   echo " the user $username already exists and exit."
   exit
fi
3

Hmmmm, does the originial poster happen to attend BCC?!?!?!? lol