Check for open ports in a firewall

listen.bash

#!/usr/bin/env bash

main() {
        local first=$1
        local last=$2
        local pids=()

        [[ -z "${first}" || -z "${last}" ]] && { >&2 echo "listen.sh <first> <last>"; return 1; }

        for (( port=first; port <= last; port++ )); do
                if netstat -an | grep -qE ":${port} .*LISTEN"; then
                        echo "skipping ${port}"
                        continue
                fi
                nc -l $port &
                local pid=$!
                pids+=("${pid}")
        done

        echo "press any key to stop"
        read -r done

        for pid in "${pids[@]}"; do
                kill -9 $pid &> /dev/null
        done

        return 0
}

main "$@"

scan.bash

#!/usr/bin/env bash

main() {
    local host=$1
    local first=$2
    local last=$3

    for (( port=first; port <= last; port++ )); do
        ncat "${host}" "${port}" -c "echo ${port} open"
        sleep $((RANDOM%5+1))
    done

    return 0
}

main "$@"