Vim modifying a file if used with xxd

Question

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?

Delorsedelos

According to the example in xxd manpage, 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 – Tito

Answer 1

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.

Singe

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! – Delorsedelos

@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. – Singe

Answer 2

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.

Schoenberg

Answer 3

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

Nutritive

Didn't know! Also, you can set bin after the file has already been opened (before xxd). – Schoenberg