http-redirect.go

package main

import (
	"flag"
	"fmt"
	"log"
	"net/http"
	"net/url"
	"os"
	"strings"
)

type trace struct {
	URL        string
	StatusCode int
	Location   string
}

var maxRedirects int

func init() {
	flag.IntVar(&maxRedirects, "m", 10, "max redirects")
	flag.Usage = usage
}

func usage() {
	fmt.Fprintf(os.Stderr, "Usage: %s [OPTIONS] URL\n\n", os.Args[0])
	fmt.Fprintln(os.Stderr, "OPTIONS:")
	flag.PrintDefaults()
}

func main() {
	flag.Parse()
	args := flag.Args()
	if len(args) != 1 {
		flag.Usage()
		os.Exit(2)
	}

	nextURL := parseURL(args[0]).String()
	client := &http.Client{
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			return http.ErrUseLastResponse
		},
	}

	for i := 0; i < maxRedirects; i++ {
		t, err := visit(client, nextURL)
		if err != nil {
			log.Println(err)
			continue
		}
		printRedirect(t)

		if !isRedirect(t.StatusCode) || t.Location == "" {
			break
		}
		nextURL = t.Location
	}
}

func visit(client *http.Client, url string) (*trace, error) {
	resp, err := client.Get(url)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	t := &trace{URL: url, StatusCode: resp.StatusCode}

	if url, err := resp.Location(); err == nil {
		t.Location = url.String()
	}
	return t, nil
}

func parseURL(uri string) *url.URL {
	if !strings.Contains(uri, "://") && !strings.HasPrefix(uri, "//") {
		uri = "//" + uri
	}

	url, err := url.Parse(uri)
	if err != nil {
		log.Fatalf("could not parse url %q: %v", uri, err)
	}

	if url.Scheme == "" {
		url.Scheme = "http"
		if !strings.HasSuffix(url.Host, ":80") {
			url.Scheme += "s"
		}
	}
	return url
}

func isRedirect(status int) bool {
	return status > 299 && status < 400
}

func printRedirect(t *trace) {
	fmt.Println()
	fmt.Printf("Get: %q\n", t.URL)
	fmt.Printf("StatusCode: %d\n", t.StatusCode)
}