To set the stage: what is `forkpty()`, and why should we care? Well, let’s say you have a program that needs to run another program in its own shell. You could use subprocesses or pipes, but those methods can be messy and error-prone. Thats where `forkpty()` comes in it allows us to create a new process with its own terminal window (or pseudo-terminal), which we can then control like any other shell.
Here’s how you use it:
# Import necessary libraries
import os # Importing the os library to access operating system functionalities
import subprocess # Importing the subprocess library to create and manage subprocesses
# Create a new pseudo-terminal and fork the child process into it
pty_fd = os.open(os.devnull, os.O_RDWR) # Open a pseudo-terminal file descriptor for reading and writing
master_fd, slave_fd = os.pipe() # Create a pipe and return two file descriptors, one for reading and one for writing
pid = os.forkpty(slave_fd) # Fork the child process into the pseudo-terminal, returning the process ID
if pid == 0: # If the process ID is 0, it is the child process
subprocess.call(['bash', '-i'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT) # Execute the bash shell with interactive mode, using the pipe as the input and output streams
else: # If the process ID is not 0, it is the parent process
os.close(slave_fd) # Close the file descriptor for the slave end of the pipe
os.dup2(master_fd, 0) # Duplicate the master file descriptor to the standard input (0)
os.dup2(master_fd, 1) # Duplicate the master file descriptor to the standard output (1)
os.dup2(master_fd, 2) # Duplicate the master file descriptor to the standard error (2)
os.close(master_fd) # Close the file descriptor for the master end of the pipe
Wow! That’s a lot of code for something that seems so simple, right? Don’t Worry lets break it down piece by piece:
1. We start by opening the null device (which is basically an empty file that we can use to create our pseudo-terminal) and getting its file descriptor (`pty_fd`) using `os.open()`.
2. Next, we call `os.pipe()` to create a pipe between two ends one for reading (`master_fd`) and the other for writing (`slave_fd`). This will allow us to communicate with our child process through this pipe.
3. We then use `os.forkpty(slave_fd)` to fork a new process into our pseudo-terminal, which we created in step 1. The `pid` variable now contains the PID of our child process.
4. If we’re running as the child process (i.e., if `pid == 0`), then we call `subprocess.call()` to run a new shell with input and output piped through our pipe. This allows us to control the shell like any other terminal window.
5. In the parent process, we close the slave file descriptor (since we’re not using it anymore) and duplicate the master file descriptor into standard input/output/error streams using `os.dup2()`. We then close the master file descriptor since we no longer need it either.
And thats it! You now have a new shell running in its own pseudo-terminal, which you can control like any other terminal window.
But wait there’s more! Did you know that `forkpty()` also allows us to pass environment variables and working directories to our child process? Heres an example:
# This script creates a new pseudo-terminal and forks a child process into it, allowing for control like any other terminal window.
import os, subprocess
# Open a new pseudo-terminal and assign it to pty_fd
pty_fd = os.open(os.devnull, os.O_RDWR)
# Create a pipe and assign the read and write ends to master_fd and slave_fd respectively
master_fd, slave_fd = os.pipe()
# Fork the child process into the slave_fd
pid = os.forkpty(slave_fd)
# Check if the process is the child process
if pid == 0:
# If it is, call the bash shell with the -i flag for interactive mode
# Also pass in the stdin, stdout, and stderr arguments for the subprocess
# Lastly, set the environment variable HOME to the specified directory
subprocess.call(['bash', '-i'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env={'HOME': '/home/myuser'})
else:
# If it is not the child process, close the slave_fd
os.close(slave_fd)
# Duplicate the master_fd to the standard input, output, and error
os.dup2(master_fd, 0)
os.dup2(master_fd, 1)
os.dup2(master_fd, 2)
# Close the master_fd
os.close(master_fd)
# The script now creates a new shell running in its own pseudo-terminal, with the ability to pass environment variables and working directories to the child process.
In this example, we’re passing the `HOME` environment variable to our child process using the `env` parameter of `subprocess.call()`. This allows us to set up a custom working directory for our shell (or any other program that we run).
It may seem like overkill at first glance, but trust me once you start using it, you’ll wonder how you ever lived without it.