How to capture the stdout of a child process using fork/exec/pipe/dup2 only?

Question

so I'm trying to capture the output of a child process using fork/exec/pipe/dup2 only but the output keeps printing to terminal instead. This is my code right now:

import os
import sys

pipeIn, pipeOut = os.pipe()
processid = os.fork()
if processid == 0:
    try:
        os.close(pipeIn)
        os.dup2(pipeOut, 0)
        os.execv(sys.executable, [sys.executable, 'helloWorld.py'])
        sys.exit(0)
    except FileNotFoundError:
        print("ERROR: File not found.")
        sys.exit(1)
elif processid == -1:
    print("ERROR: Child was unable to run.")
    sys.exit(1)
else:
    wait = os.wait()
    if wait[1] == 0:
        os.close(pipeOut)
        output = os.read(pipeIn, 100)
    else:
        output = "ERROR"

Can anyone tell me what's going wrong? And how I can capture the output of the exec command instead of printing it to terminal. (No temp files are allowed either).

Answer 1

Standard output is FD 1, but you're dup 'ing your pipe over FD 0 (standard input) instead. Change os.dup2(pipeOut, 0) to os.dup2(pipeOut, 1) .