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Say I have
node foo.js | node bar.js
is there a way to pass a handle on foo's stdin to bar?
I have a rare case where I'd like to communicate backwards in the pipeline.
At the least I know that I could send node bar.js the pid of node foo.js. Given that pid, on *nix, I should be able to write to foo's stdin using:
/proc/<pid>/fd/0
but is there a way to do the same on MacOS?
In the general case no, because the write handle of the stdin of foo is only in the hands of its parent process (foo only sees the read handle)... You have to set up a specific pipe (anonymous or FIFO) between the two; and this is better anyway because you don't know what would happen if bar wrote to that handle as the same time as foo's parent.
/proc/<pid>/fd/0would get you a handle on pid's stdin but what would you do with that? What if it's a terminal device, a regular file, a socket, a pipe? Here, it looks like you wantfoo.jsstdin to be a pipe (or other IPC channel like a socketpair) andbar.jsto have a file descriptor to the other end of that pipe.