How to change file header in terminal

Question

I have a file that should be a png but its signature is incorrect and therefore won't open. It's signature should be 8950 4e47 0d0a 1a0a but it is instead 8965 4e34 0d0a b0aa which makes it a.eN4 file. Is there any way to correct this in terminal?

Answer 1

You can use xxd to do this.

I have a Vim mapping for it, but you can also purely do it with xxd and your favourite editor:

xxd <bad_file >hexdump

Now edit hexdump with your favourite editor. It is important that you edit the hexadecimal part, not the ASCII column on the right, which is only FYI. After you're done, save the file and do:

xxd -r <hexdump >good_file

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I guess you could do it in one go:

xxd <bad_file | sed '1 s/8950 4e47 0d0a 1a0a/8965 4e34 0d0a b0aa/` | xxd -r >good_file