#!/bin/bash
whitelist='/var/www/html/fw/whitelist.txt'
PATH=/bin:/usr/bin:/sbin:/usr/sbin

#failure if it doesn't exist
[[ -f $whitelist ]] || {
    printf "'%s' is missing.\n' "$whitelist" >&2
    exit 1
}

#actual work
while read -r ip; do
    iptables -A INPUT -s "$ip" -j ACCEPT
done < "$whitelist"

#empty it, but don't remove it (preserves ownership/permissions)
>"$whitelist"