What's the easiest way to find an unused local port?

Question

What's the easiest way to find an unused local port?

Currently I'm using something similar to this:

port=$RANDOM
quit=0

while [ "$quit" -ne 1 ]; do
  netstat -a | grep $port >> /dev/null
  if [ $? -gt 0 ]; then
    quit=1
  else
    port=`expr $port + 1`
  fi
done

It feels awfully roundabout, so I'm wondering if there's a more simple path such as a builtin that I've missed.

Answer 1

The script in your answer has a race condition, the only way to avoid it is to atomically check if it is open by trying to open it. If the port is in use, the program should quit with a failure to open the port.

For example, say you're trying to listen with netcat.

#!/bin/bash
read lowerPort upperPort < /proc/sys/net/ipv4/ip_local_port_range
while :; do
    for (( port = lowerPort ; port <= upperPort ; port++ )); do
        nc -l -p "$port" 2>/dev/null && break 2
    done
done

Answer 2

On Linux, you could do something like:

ss -tln | 
  awk 'NR > 1{gsub(/.*:/,"",$4); print $4}' |
  sort -un |
  awk -v n=1080 '$0 < n {next}; $0 == n {n++; next}; {exit}; END {print n}'

To find the first free port above 1080. Note that ss -D would bind on the loopback interface, so in theory you could reuse port 1080 if a socket has it bound on another address. Another way would be to actually try and bind it:

perl -MSocket -le 'socket S, PF_INET, SOCK_STREAM,getprotobyname("tcp");
  $port = 1080;
  ++$port until bind S, sockaddr_in($port,inet_aton("127.1"));
  print $port'

Answer 3

This is part of a function I have in my .bashrc, which dynamically creates SSH tunnels and tries to use any port in a range:

   lps=( 7002 7003 7004 7005 7006 7007 7008 7009 7010 7011 )
   lp=null

   # find a free listening port
   for port in ${lps[@]}; do
      lsof -i -n -P |grep LISTEN |grep -q ":${port}"
      [ $? -eq 1 ] && { lp=$port; break; }
   done
   [ "$lp" = "null" ] && { echo "no free local ports available"; return 2; }
   return $port

YMMV