In fish shell, this blocks the input:
$ mkfifo fifo
$ ls > fifo &
However this works in zsh and bash.
So how can I start this kind of background process which stdout is redirected to a fifo?
As written, this will probably never work in fish shell. fish always opens redirections before fork, which will deadlock in this case, as the fifo has no reader.
I think the best workaround is to make someone else open the file, for example tee:
> mkfifo fifo
> ls | tee fifo >/dev/null &