Vim modifying file if used with xxd
I am trying to understand how ID3 tags work, so after reading some documentation I started looking at some raw mp3 data. Vim is usually my editor of choice, so after some googling I found that I could use xxd to view the hex representation of my files by calling
:%!xxd
Everything worked fine, but when I put everything ok with
:%!xxd -r
and exit, I found out that the file was changed; vlc couldn't play anymore and diff told me the files were different. I thought I changed something by accident, but further experiments showed me that even opening the file and using xxd and then xxd -r modifies the file in some way.
Why? How can I prevent this? Am I doing something wrong?
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Obviously, if you're not going to change anything to the file, you can exit vim
with :q!
.
As @RunHolt points out, vim
and xxd
can modify the binary. for example changing LF to CRLF or adding an LF character at the end of the file.
You can prevent this by setting the option binary
:
Either run vim
as: vim -b filename
or type :set binary
before loading the file into the buffer.
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You probably didn't download the file as a binary with vim -b
. That is, the damage has already been done.
xxd
- red herring; xxd
It followed by xxd -r
, transparently. It is designed for editing binary files. xxd
does not add bytes; it produces an exact hexdump that changes to exactly xxd -r
(if you don't damage it).
For viewing only, you can simply run xxd
from the shell:
$ xxd binaryfile | vim - # just use vim as a reader
I edited executables from vim -b
and filtered through xxd
and back through xxd -r
. They did a great job.
In addition, xxd is a Vim-specific program that comes with the Vim distribution. It may be helpful od
for you to know for example
od -tx1 file
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In Windows binaries (not sure about other platforms) :%!xxd
puts an end-of-file marker in the last two bytes (0x0d, 0x0a). %!xxd -r
Doesn't delete them for some reason .
I often delete them manually (just remove both characters than the mileage %!xxd -r
)
Perhaps this can be fixed with xxd
.
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