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I'm trying to understand how ID3 tags work, so, after reading some documentation, I started to look at some mp3's raw data. Vim is usually my editor of choice, so, after some googling, I found out I could use xxd to see an hex representation of my files by invoking 

:%!xxd

Everything worked fine, but when I put everything back in order with 

:%!xxd -r

and quit, I found out that the file was modified; vlc could no longer play it, and diff told me the files differed. I thought I modified something by accident, but further experiments showed me that even opening the file and using xxd and then xxd -r changes somehow the file.

Why is that? How can I prevent it from happening? Am I doing something wrong?

gcali
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  • According to the example in [xxd manpage](https://linux.die.net/man/1/xxd), to dump all edited bytes to another file (and avoid messing up original file), you need to specify file path in the vim command, e.g. `:%!xxd -r > /path/to/new/file` – Ham Dec 23 '22 at 04:29

3 Answers3

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Obviously if you do not intend to change anything to a file you could quit vim using :q!.

As @RunHolt points out, vim and xxd can change a binary file. e.g. changing LF to CRLF or appending an LF character at the end of file.

You can prevent this by setting the binary option:

Either start vim as: vim -b filename or type :set binary before loading the file into the buffer.

heijp06
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  • By using the binary option everything worked fine. And yes, I know I could have used :q! to quit, but the main idea was to be able to modify the file via vim, so I had to find a way to save the file if necessary. Thank you very much! – gcali Mar 27 '12 at 20:06
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    @Odexios: You are welcome. I probably should not have mentioned the `:q!` command, but you never know, sometimes the most obvious things are the hardest to find. – heijp06 Mar 27 '12 at 20:18
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On windows binary files (not sure about other platforms), :%!xxd puts the end-of-file marker in the last two bytes (0x0d, 0x0a). For some reason %!xxd -r doesn't remove them.

I often remove them manually (just delete both characters than run %!xxd -r)

Might be something that could be fixed directly with xxd.

RunHolt
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2

You probably did not load the file as a binary file with vim -b. I.e. the damage was already done.

xxd is a red herring here; xxd followed by xxd -r is transparent. It is intended for editing binary files. xxd does not add any bytes; it produces an accurate hexdump, which is accurately reversed by xxd -r (unless you corrupt it).

For viewing only, you could just run xxd from the shell:

$ xxd binaryfile | vim -     # just use vim as a reader

I've edited executables with vim -b and filtering through xxd and back through xxd -r. They ran fine.

Also, xxd is a Vim-specific program that comes with the Vim distribution. It might be useful to you to know od, e.g

od -tx1 file
Kaz
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