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On a Linux machine, if I have the pid of a process, I can write to the stdin of that process using something like:

/proc/<pid>/fd/0

but on MacOS, this doesn't appear possible. I am told that I should use mkfifo / named pipes.

However using named pipes is making the problem harder to solve. Is there a way on MacOS to store a reference to a fd on the filesystem if you control the start of the process?

For example, if I do:

#!/usr/bin/env node
node script.js &
NODEJS_PID=$!

I can easily get the pid of this process, represented above by the NODEJS_PID var. However, is there a way to capture a reference to the file descriptor of the stdin to this nodejs process?

If I can get the path to the fd that would be cool, because I need to write to that fd from another process altogether.

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Since you know the PID on OS X, lsof might work for you:

lsof -p 12345 | awk '$4=="0u"{print $NF}'
/dev/ttys039
# then you can write to the FD
echo "hi" > /dev/ttys039
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  • thanks can you comment on the lsof line and let people know what it does? Commented Jul 7, 2017 at 23:06
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    lsof lists the open files on a system. The -p option lists open files for a particular process id. Commented Jul 7, 2017 at 23:09
  • just curious, how do you know this is the fd for stdin? Commented Jul 7, 2017 at 23:16
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    @AlexanderMills But note that the example shown doesn't actually do what you want. If you write to a terminal, that displays the text on the terminal, it doesn't feed input to a program that's reading from the terminal. This goes whether you refer to the terminal directly through its /dev entry or via a Linux magic link under /proc/$pid/fd. It works with a pipe because pipes aren't bidirectional. Commented Jul 7, 2017 at 23:46
  • I think I follow what you are saying Gilles, so mkfifo is really the best way to do this? Commented Jul 8, 2017 at 0:58

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