Newbie: Better way to check a magic number on a file?


#1

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


#2

magic_number.eql?(‘d0cf11e0a1b1’) doesn’t work which kind of makes
sense.

Anyhow, is this the best way to do it? It feels very hokey to me :).

Thanks for the patience as well - I rarely inspect magic numbers…

WG

Wes G. 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


#3

Wes G. wrote:

magic_number = nil

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 …


#4

On 21-Mar-06, at 6:46 PM, Wes G. wrote:

magic_number = nil

Should magic_number.eql?(‘d0cf11e0a1b1’) work?

There are a number of simpler ways, one might be

EXCEL_FILE_MAGIC_NUMBER = [‘d0cf11e0a1b1’].pack(‘H*’)

later

magic_number == EXCEL_FILE_MAGIC_NUMBER # => true or false

or you can use octal or hex escapes to have a string literal to
compare against e.g.

irb(main):006:0> “\320\317\021\340\241\261” == “\xd0\xcf\x11\xe0\xa1
\xb1”
=> true

Maybe there are good bits of the Pickaxe (Programming Ruby) or some
on-line tutorials which can help you see some of the more ruby-ish
ways to do things.

Hope this helps,

Mike

Mike S. removed_email_address@domain.invalid
http://www.stok.ca/~mike/

The “`Stok’ disclaimers” apply.


#5

An even better way would be to take advantage of the fact that I’m
uploading through a browser and just look at the MIME type

:slight_smile: :slight_smile: :slight_smile:

Man, I’m dumb :).

But on the other hand, now I now about the IO module, and modules in
general and all kinds of things.

Thanks, everybody.

Wes

Mike S. wrote:

On 21-Mar-06, at 6:46 PM, Wes G. wrote:

magic_number = nil

Should magic_number.eql?(‘d0cf11e0a1b1’) work?

There are a number of simpler ways, one might be

EXCEL_FILE_MAGIC_NUMBER = [‘d0cf11e0a1b1’].pack(‘H*’)

later

magic_number == EXCEL_FILE_MAGIC_NUMBER # => true or false

or you can use octal or hex escapes to have a string literal to
compare against e.g.

irb(main):006:0> “\320\317\021\340\241\261” == “\xd0\xcf\x11\xe0\xa1
\xb1”
=> true

Maybe there are good bits of the Pickaxe (Programming Ruby) or some
on-line tutorials which can help you see some of the more ruby-ish
ways to do things.

Hope this helps,

Mike

Mike S. removed_email_address@domain.invalid
http://www.stok.ca/~mike/

The “`Stok’ disclaimers” apply.