dup2.c

#include <unistd.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv) {
	printf("argv[1]: %s\n", argv[1]);

	// this is this function, not the subproc
	int outOfThis[2], intoThis[2];
	if((pipe(outOfThis) < 0) || (pipe(intoThis) < 0))
		return -2;

	int p = fork();
	if(p < 0)
		return -1;

	// read | write
	if(p == 0) {
		// dup read end of "out" onto stdin
		dup2(outOfThis[0], 0);
		// close not needed outOfThis
		close(outOfThis[0]);
		close(outOfThis[1]);

		// dup write end of "in" onto stdout
		dup2(intoThis[1], 1);
		// close not needed intoThis
		close(intoThis[0]);
		close(intoThis[1]);

		char *args[] = { argv[1], "arg", NULL };

		execv(argv[1], args);
		perror("oh shit");
	} else {
		// we shouldn't read our output or write into our input
		close(outOfThis[0]);
		close(intoThis[1]);

		char *out = "1\n";
		for(int i = 0; i < 5; ++i) {
			if(write(outOfThis[1], out, 3) < 3) {
				fprintf(stderr, "write failed in main\n");
				return -4;
			}
			printf("wrote \"%s\" in main\n", out);
			char buf[4096] = { 0 };
			if(read(intoThis[0], buf, 4096) < 1) {
				fprintf(stderr, "read failed in main\n");
				return -5;
			}
			printf("read \"%s\" in main\n", out);
		}
	}
}

test.py

#!/usr/bin/env python2
import sys

while not sys.stdin.closed:
	line = sys.stdin.readline()
	print line

test.sh

#!/bin/bash

while read line; do
	echo $line
done