I like to avoid using "fake" directories like this. 

- It requires assumptions about how people's file systems are organized (which is a matter of intense personal preference). 
- you have to create directories for commands like this to work as you expect. 

I think the solution is either to:

- have it output text to the command line as before and let people copy that into a keyfile themselves, 
- create and write the file to a reasonable location on the file system, like `/etc/mongodb/`...