Run interactive Bash with popen and a dedicated TTY Python

All we need is an easy explanation of the problem, so here it is.

I need to run an interactive Bash instance in a separated process in Python with it’s own dedicated TTY (I can’t use pexpect).
I used this code snippet I commonly see used in similar programs:

master, slave = pty.openpty()

p = subprocess.Popen(["/bin/bash", "-i"], stdin=slave, stdout=slave, stderr=slave)

os.close(slave)

x = os.read(master, 1026)

print x

subprocess.Popen.kill(p)
os.close(master)

But when I run it I get the following output:

$ ./pty_try.py
bash: cannot set terminal process group (10790): Inappropriate ioctl for device
bash: no job control in this shell

Strace of the run shows some errors:

...
readlink("/usr/bin/python2.7", 0x7ffc8db02510, 4096) = -1 EINVAL (Invalid argument)
...
ioctl(3, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7ffc8db03590) = -1 ENOTTY (Inappropriate ioctl for device)
...
readlink("./pty_try.py", 0x7ffc8db00610, 4096) = -1 EINVAL (Invalid argument)

The code snippet seems pretty straightforward, is Bash not getting something it needs? what could be the problem here?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

This is a solution to run an interactive command in subprocess. It uses pseudo-terminal to make stdout non-blocking(also some command needs a tty device, eg. bash). it uses select to handle input and ouput to the subprocess.

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import os
import sys
import select
import termios
import tty
import pty
from subprocess import Popen

command = 'bash'
# command = 'docker run -it --rm centos /bin/bash'.split()

# save original tty setting then set it to raw mode
old_tty = termios.tcgetattr(sys.stdin)
tty.setraw(sys.stdin.fileno())

# open pseudo-terminal to interact with subprocess
master_fd, slave_fd = pty.openpty()


try:
    # use os.setsid() make it run in a new process group, or bash job control will not be enabled
    p = Popen(command,
              preexec_fn=os.setsid,
              stdin=slave_fd,
              stdout=slave_fd,
              stderr=slave_fd,
              universal_newlines=True)

    while p.poll() is None:
        r, w, e = select.select([sys.stdin, master_fd], [], [])
        if sys.stdin in r:
            d = os.read(sys.stdin.fileno(), 10240)
            os.write(master_fd, d)
        elif master_fd in r:
            o = os.read(master_fd, 10240)
            if o:
                os.write(sys.stdout.fileno(), o)
finally:
    # restore tty settings back
    termios.tcsetattr(sys.stdin, termios.TCSADRAIN, old_tty)

Method 2

This is the solution that worked for me at the end (as suggested by qarma) :

libc = ctypes.CDLL('libc.so.6')

master, slave = pty.openpty()
p = subprocess.Popen(["/bin/bash", "-i"], preexec_fn=libc.setsid, stdin=slave, stdout=slave, stderr=slave)
os.close(slave)

... do stuff here ...

x = os.read(master, 1026)
print x

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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