Write binary file from text string that represents hex bytes in Ruby

Question

I have the following text string that represents hexadecimal bytes that should appear in file to create.

str = "001104059419632801001B237100300381010A"

I want to create a file that contains the above string in order when I open the created file with a Hex editor I see the same bytes

When I run this script

File.open("out.dat", 'w') {|f| f.write(str.unpack('H*')) }

it creates the file out.dat and when I open this file in a Hex editor contains this

5B2233303330333133313330333433303335333933343331333933363333333233383330333133303330333134323332333333373331333033303333333033303333333833313330333133303431225D

and I would like the content when I open the file in Hex editor be the same a text string

 001104059419632801001B237100300381010A

How can I do this?

I hope make sense. Thanks

Aleksei Matiushkin · Accepted Answer · 2019-01-06 07:19:25Z

1

You have to split the string by aligned bytes in the first place.

str.
  each_char.     # enumerator
  each_slice(2). # bytes
  map { |h, l| (h.to_i(16) * 16 + l.to_i(16)) }.
  pack('C*')

 #⇒ "\x00\x11\x04\x05\x94\x19c(\x01\x00\e#q\x000\x03\x81\x01\n"

or, even better:

str.
  scan(/../).
  map { |b| b.to_i(16) }.
  pack('C*')

Now you might dump this to the file using e.g. IO#binwrite.

edited Jan 6, 2019 at 7:19

answered Jan 6, 2019 at 7:12

Aleksei Matiushkin

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3 Comments

Ger Cas Over a year ago

Aleksei, thanks for helping me with this solution. Would your second solution with scan(/../) change if there are spaces between bytes in the hex string? I ask because the original string has spaces between bytes and I apply pre processing to remove them first and maybe with your solution could handle those spaces in the same step.

Aleksei Matiushkin Over a year ago

scan(/..(?=\s)/) should work out of the box (positive lookahead for two symbols followed by a space.) Also, if there are spaces, just use split(' ') instead of scan, it’s simpler and faster.

Ger Cas Over a year ago

Excellent. Thanks so much again

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