What exactly is the difference between pipes and streams

1. If you need to save the intermediate file after the processing is done, then inter-process communication (such as through a pipe or socket) is not particularly valuable.  Similarly, if you need to run the two programs at vastly different times, you should just do it the way you're doing it now.

2. Back when Unix was created, disks were very small, and it was common for a rather benign command to consume all the free space in a file system. For example,

   some_command_that_produces_a_lot_of_output | grep some_very_obscure_string

   produces output that's much smaller than the size of the output of the first command (i.e., the size of the intermediate file that would be created if you ran the commands the way you're running your programs).

3. Data flowing through pipes and sockets is (probably) not written to disk at all.  Therefore, these IPC solutions may be

   • more efficient (faster) than disk-based solutions.
   • more secure than disk-based solutions, if the intermediate data are more sensitive than the end result.

If you get rid of the killing and shutdown stuff (which is unsafe and you may, in an extreme, but not unfathomable case when child.py dies before the (head -n 1 shutdown; kill -9 $parent) & subshell does end up kill -9ing some innocent process), then child.py won't be terminating because your parent.py isn't behaving like a good UNIX citizen.

The cat std_out & subprocess will have finished by the time you send the quit message, because the writer to std_out is child_original.py, which finishes upon receiving quit at which moment it closes its stdout, which is the std_out pipe and that close will make the cat subprocess finish.

The cat > std_in isn't finishing because it's reading from a pipe originating in the parent.py process and the parent.py process didn't bother to close that pipe. If it did, cat > stdin_in and consequently the whole child.py would finish by itself and you wouldn't need the shutdown pipe or the killing part (killing a process that isn't your child on UNIX is always a potential security hole if a race condition caused due to rapid PID recycling should occur).

Processes at the right end of a pipeline generally only finish once they're done reading their stdin, but since you're not closing that (child.stdin), you're implicitly telling the child process "wait, I have more input for you" and then you go kill it because it does wait for more input from you as it should.

In short, make parent.py behave reasonably:

from __future__ import print_function
from subprocess import Popen, PIPE
import os

child = Popen('./child.py', stdin=PIPE, stdout=PIPE)

for letter in 'abcde':
    print('Parent writes to child: ', letter)
    child.stdin.write(letter+'\n')
    child.stdin.flush()
    response = child.stdout.readline()
    print('Response from the child:', response)
    assert response.rstrip() == letter.upper(), 'Wrong response'

child.stdin.write('quit\n')
child.stdin.flush()
child.stdin.close()
print('Waiting for the child to terminate...')
child.wait()
print('Done!')

And your child.py can be as simple as

#!/bin/sh
cat std_out &
cat > std_in
wait #basically to assert that cat std_out has finished at this point

(Note that I got rid of that fd dup calls because otherwise you'd need to close both child.stdin and the child_stdin duplicate).

Since parent.py operates in line-oriented fashion, gnu cat is unbuffered (as mikeserv pointed out) and child_original.py operates in a line oriented fashion, you've effectively got the whole thing line-buffered.

Note on Cat: Unbufferred might not be the luckiest term, as gnu cat does use a buffer. What it doesn't do is try to get the whole buffer full before writing things out (unlike stdio). Basically it makes read requests to the os for a specific size (its buffer size), and writes whatever it receives without waiting to get a whole line or the whole buffer. (read(2) can be lazy and give you only what it can give you at the moment rather than the whole buffer you've asked for.)

(You can inspect the source code at http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat.c ; safe_read (used instead of plain read) is in the gnulib submodule and it's a very simple wrapper around read(2) that abstracts away EINTR (see the man page)).