Example usage of the Process class from GDScript. Check below for example applications

process.gd

extends Node

const BLOCKING = false
const PRINTRAW = true

onready var te = get_node("TextEdit")

var process = Process.new()
var timer = Timer.new()

func _ready():
	add_child(timer)
	
	process.set_cwd("/home/whatever")
	process.set_environment({ "HMM": "okthx" }, false)
	process.set_redirect_flags(Process.REDIRECT_ALL) # | Process.REDIRECT_STDERR_TO_STDOUT
	
	if not process.start("/home/whatever/testprocess", [ "a", "--b", "hello there!" ]):
		printerr("Failed to start process")
		return
	
	prints("Process PID: ", str(process.get_pid()))
	
	var input = [
		"whatever\n",
		"<3\n",
		"a\n",
		"quit\n"
	]
	
	for text in input:
		var data = text.to_utf8()
		if process.can_write(data.size()):
			process.write(data)
		else:
			print("Cannot write :(") # not running or buffer is full
	
	if BLOCKING:
		block()
	else:
		timer.set_wait_time(0.030)
		timer.connect("timeout", self, "_poll_process")
		timer.start()

func print_raw_utf8(data):
	if te != null:
		te.set_text(te.get_text() + data.get_string_from_utf8())
	elif PRINTRAW:
		# accurate output, but need to check terminal
		printraw(data.get_string_from_utf8())
	else:
		# not accurate output since it adds new line
		print(data.get_string_from_utf8())

func read_all(channel):
	process.set_read_channel(channel)
	if process.get_available_bytes() > 0:
		return process.read_all()
	return PoolByteArray()

func on_process_exit():
	prints("Process exited.", OS.get_ticks_msec())
	prints("Exit code:", process.get_exit_code())
	
	if process.get_exit_status() == Process.EXIT_STATUS_CRASH:
		print("Process crashed")

func block():
	if process.get_state() != Process.STATE_NOT_RUNNING:
		process.wait_for_started(-1)
		assert(process.get_state() != Process.STATE_STARTING) # shouldn't timeout with -1
		
		if process.get_state() == Process.STATE_RUNNING:
			prints("Process started.", OS.get_ticks_msec())
			
			process.wait_for_finished(-1)
			assert(process.get_state() == Process.STATE_NOT_RUNNING) # shouldn't timeout with -1
			on_process_exit()
		else:
			prints("exec failed", OS.get_ticks_msec()) # unix
	
	print("Output:")
	
	print_raw_utf8(read_all(Process.CHANNEL_STDOUT))
	print_raw_utf8(read_all(Process.CHANNEL_STDERR))

func _poll_process():
	process.poll()
	if process.get_state() == Process.STATE_RUNNING:
		print_raw_utf8(read_all(Process.CHANNEL_STDOUT))
		print_raw_utf8(read_all(Process.CHANNEL_STDERR))
	elif process.get_state() == Process.STATE_NOT_RUNNING:
		timer.stop()
		on_process_exit()
	
	# remaining output
	
	print_raw_utf8(read_all(Process.CHANNEL_STDOUT))
	print_raw_utf8(read_all(Process.CHANNEL_STDERR))

testprocess.d

import std.stdio;
import std.file;
import std.algorithm;
import core.thread;
import std.process;

int main(string[] args)
{
    writeln("Working directory: " ~ getcwd());
    //stdout.flush();
    writeln("Arguments:");
    //stdout.flush();

    string[string] env = environment.toAA();
    writeln(env);
    //stdout.flush();

    foreach (string arg; args) {
        writeln(arg);
        //stdout.flush();
    }

    //Thread.sleep(dur!"seconds"(5));

    writeln("Infinite input loop (until you type q or quit):");
    //stdout.flush();

    while (true) {
        string line = readln();
        if (line is null)
            continue;
        if (line.endsWith("\n")) {
            line = line[0..$-1];
        }
        if (line == "quit" || line == "q") {
            //writeln("Quit!");
            //stdout.flush();
            break;
        }

        foreach (char elem; line) {
            writef("%x\n", elem);
            //stdout.flush();
        }

        stderr.writeln("Input: " ~ line);
        stderr.writeln("Input: " ~ line[1..$]);
    }

    //Thread.sleep(dur!"seconds"(5));

    writeln("Finished!");
    //stdout.flush();

    return 0;
}

testprocess.py

import sys
import os
import time

print 'Working directory: ' + os.getcwd()
#sys.stdout.flush()
print 'Arguments:'
#sys.stdout.flush()

print(os.environ)
#sys.stdout.flush()

for arg in sys.argv:
    print arg
    #sys.stdout.flush()

print 'Infinite input loop (until you type q or quit):'
sys.stdout.flush()

#time.sleep(5)

while True:
    line = raw_input()
    if line.endswith('\n'):
        line = line[:-1]
    if line == 'quit' or line == 'q':
        #print 'Quit!'
        #sys.stdout.flush()
        break
    if line:
        for elem in line:
            print elem.encode("hex")
        print "Input: " + line
        print "Input: " + line[1:]
        sys.stdout.flush()

#time.sleep(5)

print 'Finished!'
sys.stdout.flush()