Golang : How to tell if a file is compressed either gzip or zip ?

Sometimes it is good to check if a file is really in compressed form or not by examining the actual data bytes and not relying on the file extension alone. This tutorial will show you how to use http.DetectContentType() function to determine if a file is really compressed or not.

Here are the codes:

 package main

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

 func main() {

 flag.Parse() // get the arguments from command line

 sourcefile := flag.Arg(0)

 // open a file that you wish to check
 // if it is compressed or not

 if sourcefile == "" {
 fmt.Println("Usage : detectcompress sourcefile")
 os.Exit(1)
 }

 file, err := os.Open(sourcefile)

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 defer file.Close()

 buff := make([]byte, 512)

 // why 512 bytes ? see http://golang.org/pkg/net/http/#DetectContentType

 _, err = file.Read(buff)

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 filetype := http.DetectContentType(buff)

 fmt.Println(filetype) // for info

 switch filetype {
 case "application/x-gzip", "application/zip":
 fmt.Println("File is compressed with gzip or zip")
 default:
 fmt.Println("File is not compressed")

 }
 }

Sample output :

./detectzip testfile.sql

text/plain; charset=utf-8

File is not compressed

---

./detectzip testfile.unknown

application/zip

File is compressed with gzip or zip

Reference :

http://golang.org/pkg/net/http/#DetectContentType