Storing stdout from an exec process into a variable

Question

I currently have the following code whereby I am using os.execv() to execute some sort of process. For example:

process_param = [exec_path, f]
pid = os.fork(                      
try:
   if (pid > 0):
      #parent 
      time_lim = 55
      for _ in range(time_lim):
         child_pid, status = os.waitpid(-1, os.WNOHANG)

         if (child_pid != 0):
            break

         time.sleep(1)
        
   elif (pid == 0):
      os.execv(exec_path, process_param)
   else:
      status = 1
except ChildProcessError:
   status = 1

Before you tell me to use subprocess, please note that (for assignment purposes), I should stick to this particular method of running bash commands through Python.

I would like to do something similar to this:

var_a=`cat some_file`

Does anyone know how I can capture the output derived from the os.execv() command (located in the child process) and store it in a variable (perhaps returning it to the parent process)?

You will need to connect the standard output filehandle of the new process to something in the parent process which reads it. As you note, `subprocess` already does this for you - perhaps look at its implementation for an example. — tripleee, Nov 08 '20 at 13:53
I was recommended that I use os.pipe(). However, I'm not sure I understand its implementations. — knuckles the man, Nov 08 '20 at 13:58
It oughtn't be hard to find an exposition of how this is done in C on Unix-like systems; mapping the constructs to Python `os.*` calls should then be straightforward. — tripleee, Nov 08 '20 at 14:03

score 1 · Answer 1 · answered Nov 08 '20 at 14:53

Using os.pipe and os.dup2. You can learn more from https://docs.python.org/3/library/os.html#os.dup2.

import os
import sys

r, w = os.pipe()
pid = os.fork()
if pid:
    # parent
    os.close(w)
    r = os.fdopen(r)
    print(r.read())
    sys.exit(0)
else:
    # child
    os.close(r)
    # duplicate stdout and stderr to parent
    os.dup2(w, 1)
    os.dup2(w, 2)
    os.execv('/usr/bin/ls', ['-l'])
    sys.exit(0)

Storing stdout from an exec process into a variable

1 Answers1