How can I retrieve the output of a process run using Python subprocess.call?

General Discussions
1

How can I retrieve the output of a process run using Python subprocess.call?

I want to capture the output of a process executed using subprocess.call(). When I attempt to pass a StringIO.StringIO object to the stdout parameter, I encounter the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 444, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 588, in __init__
    errread, errwrite) = self._get_handles(stdin, stdout, stderr)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 945, in _get_handles
    c2pwrite = stdout.fileno()
AttributeError: StringIO instance has no attribute 'fileno'

How can I properly retrieve the output of a process while using the python subprocess.call method?

2

The subprocess.run function allows you to capture the output using the capture_output=True parameter. This method is preferred in modern Python versions.

import subprocess

result = subprocess.run(['ls', '-l'], capture_output=True, text=True)
output = result.stdout
print("Captured Output:\n", output)
3

If you need more control or are using Python 2.x, subprocess.Popen can capture the output through stdout.

import subprocess

process = subprocess.Popen(['ls', '-l'], stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
stdout, stderr = process.communicate()
print("Captured Output:\n", stdout)
4

If you want to capture output without using pipes, you can redirect stdout to a file and then read its contents.

import subprocess

with open('output.txt', 'w') as f:
    subprocess.call(['ls', '-l'], stdout=f)

with open('output.txt', 'r') as f:
    output = f.read()
print("Captured Output:\n", output)