#!/usr/bin/env sh
#set -x

name=$(basename "$0")
used_tcp_ports=$(sed 1d /proc/net/tcp \
    | awk '($4 == "0A") { print $2 }' \
    | awk -F : '($1 == "00000000" || $1 == "0100007F") { print $2 }' \
    | perl -e 'foreach my $line (<>) { print hex($line) . "\n"; }'
)

perror() {
  >&2 printf "%s: %s\n" "$name" "$*"
}

port_is_free() {
  for used_port in $used_tcp_ports; do
    [ "$port" = "$used_port" ] && return 1
  done

  :
}

port=$1

if ! { [ "$port" -gt 0 ] && [ "$port" -lt 65536 ]; } 2> /dev/null; then
  perror "you must specify a valid TCP port"
  exit 1
fi

while ! port_is_free; do
  port=$((port + 1))
done

printf %s\\n "$port"