Hw to use subprocess Popen in Python and how redirects work with both options?

General Discussions
1

What’s the difference between subprocess.Popen and subprocess.call in Python, and how can I use them?

I want to call an external program from Python, and I’ve used both Popen() and call() for this purpose.

What distinguishes the two?

For example, I want to run the following command from Python:

./my_script.sh > output

I’ve read the documentation, and it mentions that call() is a convenience or shortcut function. Does using call() instead of Popen() cause us to lose any control or functionality?

Could you explain how to use Python subprocess Popen and how redirects work with both options?

2

subprocess.Popen is a more flexible option for running external programs as it allows you to interact with the process (e.g., read from/write to stdin, stdout, stderr). It gives you full control over the execution and management of the process.

Here’s how you can use Popen to run a command and redirect the output: import subprocess

# Using Popen for full control
with open('output', 'w') as outfile:
    process = subprocess.Popen(['./my_script.sh'], stdout=outfile, stderr=subprocess.PIPE)
    process.wait()  # Wait for the process to complete

Popen starts the process, and we redirect stdout to the output file. The stderr is captured separately, but you can handle it as needed. process.wait() ensures the script runs to completion before continuing.

3

subprocess.call is a simpler function for running external commands. It is more concise but less flexible compared to Popen. call() is typically used when you don’t need to interact with the process directly and just want to run a command and wait for its completion.

Here’s how you can use call() for redirection:

import subprocess

# Using call as a shortcut
subprocess.call(['./my_script.sh'], stdout=open('output', 'w'))

Explanation: call() runs the command and redirects stdout to the output file. This is a simpler approach if you don’t need to handle additional process details.

4

If you need more advanced control, such as redirecting both stdout and stderr or interacting with the process in real-time, Popen can be used with pipes. You can redirect both standard output and error, or interact with the process during execution.

import subprocess

# Using Popen with pipes to capture both stdout and stderr
with open('output', 'w') as outfile:
    process = subprocess.Popen(['./my_script.sh'], stdout=outfile, stderr=subprocess.PIPE)
    stderr_output = process.stderr.read()  # Capture stderr if needed
    process.wait()
    print(f"Error output: {stderr_output.decode()}")

Popen is used with pipes to capture both stdout and stderr. Here, we are writing the standard output to the output file and capturing any error output for further handling.