#!/bin/sh
queryFQDN()
{
  printf "What is the Fully Qualified Domain Name?\n"
  read FQDN
  ! [ -e "/etc/httpd/sites-enabled/$FQDN" ]
}
until queryFQDN; do
  printf "FQDN already exists.\nYou will have to choose another value.\n"
done

#!/bin/sh
while : ; do
  printf "What is the Fully Qualified Domain Name?\n"
  read FQDN
  [ -e "/etc/httpd/sites-enabled/$FQDN" ] || exit 0
  printf "FQDN already exists.\nYou will have to choose another value.\n"
done