From: Tim Hunter <cyclists@...>
Date: 2006-03-22T08:58:50+09:00
Subject: Re: Newbie: Better way to check a magic number on a file?

Wes Gamble wrote:
> All,
> 
> I am trying to determine if I am dealing with an Excel file that I 
> upload to a Web server.
> 
> Here is the first test - to verify the magic number of the file that I 
> have:
> 
> #Check the magic number first to see if it's even an Office file
> 
>     magic_number = nil
>     File.open(path_to_file, "r") do |f|
>       magic_number = f.read(6)
>     end
>     return false unless 
> magic_number.unpack('H*').to_s().eql?('d0cf11e0a1b1')
> 
> This seems a little bit less than optimal.  I have to unpack my string 
> into an array and then reconvert it back to a string to compare it?
> 
> Surely there is a simpler way?
> 
> Should magic_number.eql?('d0cf11e0a1b1') work?
> 
> Wes
> 

I'm guessing the magic number is constant? Why not construct a constant 
string that is the magic number and then compare the bytes you get from 
the file to that string? Simple, fast, and ez-to-read.

MAGIC_STRING = [0xd0,0xcf,0x11,0xe0,0xa1,0xb1].pack('c*').freeze

if magic_number == MAGIC_STRING ...