Something

#!/bin/bash

# declare Variables
CREATOR="Simon Nickau"
YN="Yes No"
UND=0

echo
echo Hello to $CREATOR"'"s new server setup script!
echo
echo Please enter the username for the new server!

function username {
  echo username:
  read UN
}
while [ $UND -ne 1 ]; do
  username
  echo is $UN correct?
  select opt in $YN; do
    if [ "$opt" = "Yes" ]; then
      echo done
      let UND=1
    elif [ "$opt" = "No" ]; then
      echo Please re-enter:
      let UND=0
    else
      echo Bad option, Please retry.
    fi
  done
done