CMD does not append WSL output

Question

In cmd.exe, I try to append WSL2 output to some file, like this:

C:\somepath>echo Some contents> somefile

C:\somepath>bash.exe --noprofile --norc -c "echo APPEND">> somefile
Processing fstab with mount -a failed.


C:\somepath>type somefile
APPEND
ntents

C:\somepath>

As you can see, somehow the output is not appended to the file but overwrites the start of the file. I cannot imagine this is by design somehow.

It only happens with WSL or Bash, not with regular commands of course. But I would expect cmd to be responsible for writing to the correct place. What is so special about WSL output?

The "Processing fstab with mount -a failed." is a line I see with every WSL invocation. I don't think it's related...?

Answer 1

The boundaries between Windows and WSL/Linux are not well-defined, so sometimes there would be strange side-effects.

In your case, probably bash didn't know that somefile had some data already in it, so it just overwrote the beginning of the file's contents.

You could formulate the command differently, so as to avoid crossing too many boundaries :

bash.exe --noprofile --norc -c "echo APPEND >> somefile"