package main

/**
 * @website http://albulescu.ro
 * @author Cosmin Albulescu <cosmin@albulescu.ro>
 */

import (
	"bytes"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"path"
	"strconv"
	"time"
)

func PrintDownloadPercent(done chan int64, path string, total int64) {

	var stop bool = false

	for {
		select {
		case <-done:
			stop = true
		default:

			file, err := os.Open(path)
			if err != nil {
				log.Fatal(err)
			}

			fi, err := file.Stat()
			if err != nil {
				log.Fatal(err)
			}

			size := fi.Size()

			if size == 0 {
				size = 1
			}

			var percent float64 = float64(size) / float64(total) * 100

			fmt.Printf("%.0f", percent)
			fmt.Println("%")
		}

		if stop {
			break
		}

		time.Sleep(time.Second / 5)
	}
}

func DownloadFile(url string, dest string) {

	file := path.Base(url)

	log.Printf("Downloading file %s from %s\n", file, url)

	var path bytes.Buffer
	path.WriteString(dest)
	path.WriteString("/")
	path.WriteString(file)

	start := time.Now()

	out, err := os.Create(path.String())

	if err != nil {
		fmt.Println(path.String())
		panic(err)
	}

	defer out.Close()

	headResp, err := http.Head(url)

	if err != nil {
		panic(err)
	}

	defer headResp.Body.Close()

	size, err := strconv.Atoi(headResp.Header.Get("Content-Length"))

	if err != nil {
		panic(err)
	}

	done := make(chan int64)

	go PrintDownloadPercent(done, path.String(), int64(size))

	resp, err := http.Get(url)

	if err != nil {
		panic(err)
	}

	defer resp.Body.Close()

	n, err := io.Copy(out, resp.Body)

	if err != nil {
		panic(err)
	}

	done <- n

	elapsed := time.Since(start)
	log.Printf("Download completed in %s", elapsed)
}

func main() {
	DownloadFile("https://wordpress.org/wordpress-4.4.2.zip", "./")
}


p.s
간단하면서 여러가지 응용할 수 있는 소스입니다.
진행바 부분은 디자인 하기 나름이고... 
go루틴에 채널...그러고 보면 golang 이 작업하기 참 편한 언어입니다. ^^


출처 : https://gist.github.com/albulescu/e61979cc852e4ee8f49