Homework & Coursework Questions

Hey I'm new here, and I really need some help asap! This assignment is due at midnight tonight, and I'm really having a hard time with it. Basically here's the assignment:
Task
Write a shell script to add individual users to a Linux system. The script should prompt for the username to create and ensure the username is unique on the system. (Be sure your check for uniqueness actually works-bo, bob, and bobby should all be creatable, if they do not already exist on a system.)
The script should also obtain the user's full name and obtain the preferred user shell. (Be sure to display a valid list of shells prior to shell selection.)

It requires me to create a script that creates a new user, with their full name, and their default bash shell. It also requires me to have an error system which will say something like "Sorry that username already exists please try again" and then loops back to the beginning "Enter your username: "
The same thing is required for the bash shell message. If you enter an invalid shell it will relist the available shells until you enter a correct one.

Here's the code I've come up with so far, sorry if it seems botched up, I've been hacking at it trying to get it right for about 4 hours now.

Code:

#! /bin/bash
echo ""
echo "Enter your username: "
read userName
cut -d: -f1 /etc/passwd | grep $userName > /dev/null
OUT=$?
if [ $OUT -eq 0 ];
then
	echo "$userName already exists!"
else
	echo "Enter your full name: "
	read fullName
fi
echo "Available Shells"
echo "----------------"
echo "/bin/sh"
echo "/bin/bash"
echo "/sbin/nologin"
echo "/bin/tcsh"
echo "/bin/csh"
echo "/bin/ksh"
echo "Enter the user's shell: " 
read regShell
if [ $regShell = "/bin/sh" ] || [ $regShell = "/bin/bash" ] || [ $regShell = "/sbin/nologin" ] || [ $regShell = "/bin/tcsh" ] || [ $regShell = "/bin/csh" ] || [ $regShell = "/bin/ksh" ]
then
useradd -c $fullName -d /home/$userName -s $regShell $userName
else
echo "Invalid Shell entered."
fi

Any help would be GREATLY appreciated.

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