How to write data to existing process's STDIN from external process?

Your code will not work.
/proc/pid/fd/0 is a link to the /dev/pts/6 file.

$ echo 'foobar' > /dev/pts/6
$ echo 'foobar' > /proc/pid/fd/0

Since both the commands write to the terminal. This input goes to terminal and not to the process.

It will work if stdin intially is a pipe.
For example, test.py is :

#!/usr/bin/pythonimport os, sysif __name__ == "__main__":    print("Try commands below")    print("$ echo 'foobar' > /proc/{0}/fd/0".format(os.getpid()))    while True:        print("read :: [" + sys.stdin.readline() + "]")        pass

Run this as:

$ (while [ 1 ]; do sleep 1; done) | python test.py

Now from another terminal write something to /proc/pid/fd/0 and it will come to test.py

I want to leave here an example I found useful. It's a slight modification of the while true trick above that failed intermittently on my machine.

# pipe cat to your long running process( cat ) | ./your_server &server_pid=$!# send an echo to your cat process that will close cat and in my hypothetical case the server tooecho "quit\n" > "/proc/$server_pid/fd/0"

It was helpful to me because for particular reasons I couldn't use mkfifo, which is perfect for this scenario.