Suppress output of os.execv()

Question

I'm building a program similar to strace which shows some basic information about syscalls used by an executable.
It accepts an executable name and some arguments, and then to run it I create a child process using os.fork and within it I run the process using os.execvp. The problem is that I want to suppress the output of the child process, but I can't get it to work for all cases.
Currently I use os.dup2(os.open('/dev/null', os.O_WRONLY), 1) to suppress the output, which works for most cases, but for example if I try running python3 with os.execvp, the output is not suppressed.

This is how I currently run the process:

import os
import shutil

def run_process(name, *args, show_output='0'):
    """
    Start a process with arguments.
    - `name` - the name of the process.
    - `args` - the arguments to pass to the process.
    - `show_output` - whether to show the output of the process or not.
    """
    if (shutil.which(name) is None) and (not os.path.isfile(name)) and (not os.path.exists(name)):
        os.kill(os.getppid(), 9)
        print('Command/File `{}` not found.'.format(name))

    ptrace(PTRACE_TRACEME, 0, 0, 0)
    if show_output.lower() not in ['1', 'true', 'yes', 'y']:
        # redirect output to /dev/null
        os.dup2(os.open('/dev/null', os.O_WRONLY), 1)

    try:
        os.execvp(name, [name] + list(args))
    except Exception as e:
        # redirect output back to stdout:
        os.dup2(os.open('/dev/tty', os.O_WRONLY), 1)
        os.kill(os.getppid(), 9)
        print(f'An error occured when running the process: {e}')

pid = os.fork()
if pid == 0:
    run_process('python3', show_output='0') # output is shown
    # output is not shown:
    # run_process('ls', show_output='0')
else:
    # ptrace stuff...

Any idea on how can I get the output to fully disappear?

You redirected one standard writable file descriptor and not the other. — Davis Herring, Feb 19 '22 at 17:46
So you are saying that the problem might be that I haven't redirected `stderr`? — Yuval.R, Feb 19 '22 at 17:47
Precisely. Some programs use it (in some circumstances) and others don’t, so it matches your description of the symptoms. — Davis Herring, Feb 19 '22 at 17:49
Do you know how can I do that? (I'm not so familiar with `os.dup2`) — Yuval.R, Feb 19 '22 at 17:50
It’s [**2**](https://en.m.wikipedia.org/wiki/Standard_error_stream)! — Davis Herring, Feb 19 '22 at 17:52
Does this answer your question? [What are file descriptors, explained in simple terms?](https://stackoverflow.com/questions/5256599/what-are-file-descriptors-explained-in-simple-terms) — Davis Herring, Feb 19 '22 at 18:21
Not really, as I didn't know that that was the problem to begin with, but adding another dup2 line except with 2 instead of 1, did answer my question (I know it's related to file descriptors, after you mentioned it here) — Yuval.R, Feb 19 '22 at 18:48

Suppress output of os.execv()

0 Answers0