How to open new pseudo-terminal pair using Python?

Pseudo-terminals (pty) are an advanced and powerful technique in Python when dealing with terminal-based processes or simulating interactive sessions programmatically. A pseudo-terminal allows a process to connect to a real terminal, enabling control over terminal input and output.

Python provides a built-in pty module that helps in creating and managing these terminal pairs. In this article, we will learn how to open a new pseudo-terminal pair using Python.

Understanding Pseudo-terminals

A pseudo-terminal consists of two parts: a master and a slave. The master end is controlled by your Python program, while the slave end behaves like a regular terminal that can be used by other processes.

Using pty.openpty()

The pty module provides the openpty() function that returns a tuple containing file descriptors for the master and slave ends of a new pseudo-terminal pair.

Syntax

master_fd, slave_fd = pty.openpty()

Example

Here's how to create a new pty pair using pty.openpty() ?

import os
import pty

# Open a new pseudo-terminal pair
master_fd, slave_fd = pty.openpty()

# Get the name of the slave device
slave_name = os.ttyname(slave_fd)

print("Master FD:", master_fd)
print("Slave FD:", slave_fd)
print("Slave device path:", slave_name)

# Clean up
os.close(master_fd)
os.close(slave_fd)

The output of the above code is ?

Master FD: 3
Slave FD: 4
Slave device path: /dev/pts/3

Using pty to Spawn a Shell

You can spawn a shell process connected to the slave end of the pseudo-terminal. This is useful for terminal emulation or controlling shell processes programmatically.

Example

Here's how to spawn a shell and interact with it through the master end ?

import os
import pty
import subprocess
import time

master_fd, slave_fd = pty.openpty()

try:
    # Start a shell connected to the slave
    proc = subprocess.Popen(
        ['/bin/bash'],
        stdin=slave_fd,
        stdout=slave_fd,
        stderr=slave_fd,
        close_fds=True
    )
    
    # Close slave fd in parent process
    os.close(slave_fd)
    
    # Send command to shell
    os.write(master_fd, b'echo "Hello from pty"\n')
    time.sleep(0.1)  # Give time for command to execute
    
    # Read output
    output = os.read(master_fd, 1024)
    print("Shell Output:", output.decode().strip())
    
    # Terminate the shell
    os.write(master_fd, b'exit\n')
    proc.wait()
    
finally:
    os.close(master_fd)

When you run the above program, a new bash shell is started, and it prints the message from our Python script ?

Shell Output: Hello from pty

Key Points

• Use pty.openpty() to create a low-level terminal emulation channel
• Use os.ttyname() to identify the slave terminal path
• Always close file descriptors to prevent resource leaks
• Connect subprocesses to the slave end to simulate terminal sessions
• The master end is used by your Python program for reading/writing

Platform Compatibility

Note: The pty module is available only on Unix-like systems (Linux, macOS) and is not supported on Windows. For Windows pseudo-terminal functionality, you can use the pywinpty library on Windows 10+.

Conclusion

Opening a pseudo-terminal pair in Python provides powerful capabilities for terminal emulation and process control. Use pty.openpty() for basic pty creation and combine it with subprocess management for advanced terminal-based applications.