bash script to find a free port to listen to

findport.sh

#!/bin/bash
#
# Please run as root.
# Usage: bash findport.sh 3000 100
#


if [[ -z "$1" || -z "$2" ]]; then
  echo "Usage: $0 <base_port> <increment>"
  exit 1
fi


BASE=$1
INCREMENT=$2

port=$BASE
isfree=$(netstat -tapln | grep $port)

while [[ -n "$isfree" ]]; do
  port=$[port+INCREMENT]
  isfree=$(netstat -tapln | grep $port)
done

echo "$port"
exit 0

netstat: ln: unknown or uninstrumented protocol I'm getting this. Any clue?

while :; do PORT="`shuf -i $LOWERPORT-$UPPERPORT -n 1`"; ss -lpn | grep -q ":$PORT " || break; done

read LOWERPORT UPPERPORT < /proc/sys/net/ipv4/ip_local_port_range
while :; do shuffPort="`shuf -i $LOWERPORT-$UPPERPORT -n 1`"; netstat -tapln | grep $shuffPort || break; done