Python execv and pipe output

Question

I'd like to use Python's os.execv to replace my current process, but I also want the new process to send stdout to another process (to collect logs and ship them over the network). The process collecting logs also needs to be started by the original Python process.

I'm guessing I need to do some fork, dup2, execv stuff, but I need some help.

In bash, it might look something like this

#!/bin/bash
exec ./foo ∣ ./bar

the subprocess module can do this for you: gist.github.com/JacobIRR/dced5ce0f19e4f376aaef19d0e80d9d7 — JacobIRR
– JacobIRR, Commented May 19, 2017 at 17:52
@JacobIRR That runs the command in a subprocess, it doesn't replace the current process. — Barmar
– Barmar, Commented May 19, 2017 at 18:12
Minor note, that doesn't quite do what you think it does in Bash - the main process follows the last stage of the pipeline, so the exec has no effect. — ephemient
– ephemient, Commented May 19, 2017 at 19:44
@ephemient thanks for the clarification. I guess that means I'm unsure how to do this in both Python and Bash! — Jesse Shieh
– Jesse Shieh, Commented May 19, 2017 at 19:46
The closest analogue in Bash would be with process substitution such as exec ./foo > >(./bar). — ephemient
– ephemient, Commented May 19, 2017 at 22:54

ephemient · Accepted Answer · 2017-05-19 21:03:27Z

4

You can set up the pipe and processes this way.

import os

(read_fd, write_fd) = os.pipe()
pid = os.fork()
if pid == 0:
    os.dup2(read_fd, 0)
    os.close(read_fd)
    os.close(write_fd)
    os.execlp('./bar', './bar')
    os._exit(-1)  # in case exec fails
os.close(read_fd)
os.dup2(write_fd, 1)
os.close(write_fd)
os.execlp('./foo', './foo')

It's still convenient to use subprocess though, at least for the first part.

import os
import subprocess

p = subprocess.Popen(['./bar'], stdin=subprocess.PIPE)
os.dup2(p.stdin.fileno(), 1)
p.stdin.close()
os.execlp('./foo', './foo')

answered May 19, 2017 at 21:03

ephemient

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