go_download_file.go

package main

import (
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	for i := 1; i < 20000; i++ {
		fileName := fmt.Sprintf("app_%v.png", i)
		fileURL := fmt.Sprintf("https://stcstg.zalopay.com.vn/img/history/3x/%v", fileName)
		err := DownloadFile(fileName, fileURL)
		if err != nil {
			fmt.Println(err)
		} else {
			fmt.Println("Downloaded: " + fileURL)
		}
	}
}

// DownloadFile will download a url to a local file. It's efficient because it will
// write as it downloads and not load the whole file into memory.
func DownloadFile(filepath string, url string) error {

	// Get the data
	resp, err := http.Get(url)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.ContentLength == -1 || resp.ContentLength == 305 {
		return fmt.Errorf("content-length error: %v", resp.ContentLength)
	}

	// Create the file
	out, err := os.Create(filepath)
	if err != nil {
		return err
	}
	defer out.Close()

	// Write the body to file
	_, err = io.Copy(out, resp.Body)
	return err
}