Python: fork, pipe and exec

Question

I want to execute a program in a python application, it will run in the background but eventually come to the foreground.

A GUI is used to interact with it. But controls are offered via a console on stdin and stdout. I want to be able to control it using my application's GUI, so my first idea was:

• Fork
• in the parent, dup2 stdin and stdout in order to access them
• exec the child

Is this easily implementable in python and how? Are there alternative ways to achieve what I want, what would that be?

Mae

The answers are "yes" and "yes". Those answers to the questions you wrote don't seem helpful. Please update your question to clarify what you want to know. – Bolt

Answer 1

This is reasonably easy using the standard Python subprocess module:

http://docs.python.org/py3k/library/subprocess.html

Cari

Indeed, I'll go this way then. I was looking for 2.X modules, fortunately it has existed for a long time (docs.python.org/library/subprocess.html). – Mae

The accepted answer should be https://mcmap.net/q/1278176/-python-fork-pipe-and-exec -- a mere link to the Python docs does not add sufficient value. – Coachwhip

Answer 2

First, the python subprocess module is the correct answer.

As an subprocess example:

import subprocess
x = subprocess.check_output(["echo","one","two","three"])

Where x will be the output (python3 bytes class: x.decode('utf-8') for string)

Note that this will NOT duplicate stderr. If you need stderr as well, you can do something like:

x = subprocess.check_output(["bash","-c", 'echo foo; echo bar >&2'],stderr=subprocess.STDOUT)

Of course, there are many other ways of capturing stderr, including to a different output variable.

Using direct control

However, if you are doing something tricky and need to have direct control, examine the code below:

import os
rside, wside = os.pipe()
if not os.fork():
    # Child

    os.close(rside)
    # Make stdout go to parent
    os.dup2(wside, 1)
    # Make stderr go to parent
    os.dup2(wside, 2)
    # Optionally make stdin come from nowhere
    devnull = os.open("/dev/null", os.O_RDONLY)
    os.dup2(devnull, 0)
    # Execute the desired program
    os.execve("/bin/bash",["/bin/bash","-c","echo stdout; echo stderr >&2"],os.environ)
    print("Failed to exec program!")
    sys.exit(1)

# Parent
os.close(wside)
pyrside = os.fdopen(rside)

for line in pyrside:
   print("Child (stdout or stderr) said: <%s>"%line)

# Prevent zombies!  Reap the child after exit
pid, status = os.waitpid(-1, 0)
print("Child exited: pid %d returned %d"%(pid,status))

---

Note: @Beginner's answer is flawed in a few ways: os._exit(0) was included which immediately causes the child to exit, rendering everything else pointless. No os.execve() rendering the primary goal of the question pointless. No way to access the child's stdout/stderr as another question goal.

Flounder

Answer 3

That is not much complex in structure to build !

Check this example

if os.fork():
    os._exit(0)
    os.setsid()
    os.chdir("/")
    fd = os.open("/dev/null", os.O_RDWR)
    os.dup2(fd, 0)
    os.dup2(fd, 1)
    os.dup2(fd, 2)
if fd 2:
    os.close(fd)

This python code sets an id, changes the dir, opens a file and process and close !

Hodgkinson

that's possibly how I would do it in C, but I hope python has some module that can help me achieve this simply and in a portable way. – Mae