If you're sending a message to some process for it to consume, like X11 or like standard input to your shell, sure, you could technically interoperate via disk,

If you're sending a message to some process for it to consume, like X11 or like standard input to your shell, sure, you could technically interoperate via disk, but - UNIX itself has no inotify/kqueue/etc. feature, blocking reads are via pipes or sockets, not regular files (plain UNIX tail -f polls the file every fraction of a second) - you're appending to the end of the file for a message that's going to get read once, and the file model of UNIX doesn't let you truncate a file from the beginning, so it will just grow indefinitely for no reason - one benefit of the open socket/pipe approach is you get notified if the other side exits/crashes, you can't detect that from a disk file