How to check if a user exists and create the user if the name is free?

I’m trying to create a user creator script, and I keep getting stuck. I just started learning bash shell scripting, so I’m not very good.
Here’s my script:

#!/bin/bash
clear
echo "Welcome to"
echo "USER CREATOR"
echo "How many user accounts would you like to create?"
read num
echo "What would you like the username prefix to be? (e.g. 'Student' (Student01, Student02, Student03...) or 'User' (User01, User02, User03...)"
read prefix

declare -i maxaccounts=0
declare -i accountnum=1
zero="0"

while [ $num -gt $maxaccounts ]
do
    if [ $accountnum -lt 10 ]
    then
        username=${prefix}${zero}${accountnum}
    else
        username=${prefix}${accountnum}
    fi

    if test -d /home/$username; then
        accountnum=$accountnum+1
        maxaccounts=$maxaccounts+1
    else
        useradd $username
        accountnum=$accountnum+1
        maxaccounts=$maxaccounts+1
    fi
done

Now, it CREATES the user accounts just fine… but for some reason I can’t figure out, it isn’t actually checking to see if the username exists AND skipping it if it does. What can I do?
(ALSO I’m using a Centos Virtual Machine for reference)

Here’s some of the other forums and sites I’ve looked at to try to figure out how to write this code:
superuser: find-out-if-user-name-exists

stackoverflow: check-whether-a-user-exists?

stackoverflow: bash-script-to-validate-the-existence-of-user-name-in-etc-passwd

stackoverflow: how-would-i-check-if-user-exists-in-a-bash-script-and-re-ask-for-new-user-if-use

sslhow: check-user-shell-in-linux

unix.stackexchange: check-if-user-exist-in-etc-passwd-if-exist-create-new-one-with-prefix

baeldung: user-exists-check

>Solution :

Like this:

if getent passwd username &>/dev/null; then
   echo 'username exists'
else
   do_something
fi

Or just:

getent passwd username &>/dev/null || do_something