Is there a way to interact with file descriptors without having to open a file on disk?
This won't work without a file first being assigned to fd3:
echo a >&3
Depends on what you want to do. Suppose you want to write strings at your leisure and print them reversed. set up a file descriptor redirecting into a process substitution waiting for stdin:
$ exec 3> >(rev) $ echo hello world >&3 dlrow olleh
When you're done with it, you can close the file descriptor
$ exec 3>&- $ echo hello world >&3 bash: 3: Bad file descriptor