Golang : Get download file size

Part of a good user experience is to let your users know in advance what are the next expected actions and the information related to the actions. For example, information such as the number of bytes or file size to be allocated in their hard disk before hitting the download button. Knowing the file size before downloading is also useful in creating gauge bar or progress bar.

For this tutorial, we will learn how to get a file name and the size from a given URL. First, by extracting the filename from the given URL and then get the download file size information from response header.

Here we go!

 package main

 import (
 "fmt"
 "net/http"
 "os"
 "strconv"
 "strings"
 )

 func main() {
 url := "https://d1ohg4ss876yi2.cloudfront.net/preview/golang.png"

 // we are interested in getting the file or object name
 // so take the last item from the slice
 subStringsSlice := strings.Split(url, "/")
 fileName := subStringsSlice[len(subStringsSlice)-1]

 resp, err := http.Head(url)
 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 // Is our request ok?

 if resp.StatusCode != http.StatusOK {
 fmt.Println(resp.Status)
 os.Exit(1)
 // exit if not ok
 }

 // the Header "Content-Length" will let us know
 // the total file size to download
 size, _ := strconv.Atoi(resp.Header.Get("Content-Length"))
 downloadSize := int64(size)

 fmt.Println("Will be downloading ", fileName, " of ", downloadSize, " bytes.")

 }

Sample output :

Will be downloading golang.png of 113857 bytes.