A small Go program to find a free / unused TCP port

freeport.go

package main

import (
	"fmt"
	"net"
	"os"
)

func main() {
	ln, err := net.Listen("tcp", "")
	if err != nil {
		fmt.Printf("%v", err)
		os.Exit(1)
	} else {
		fmt.Printf("%d", ln.Addr().(*net.TCPAddr).Port)
		ln.Close()
	}
}