no output from subprocess.check_output()

This is supposed to work, but it's possible that the actual command you're running (presumably not ls) is buggy and sending all of its output to stderr instead of stdout. Try

output = subprocess.check_output(commandList, stderr = subprocess.STDOUT)

Suggestion : 2

The standard input and output channels for the process started by call() are bound to the parent's input and output. That means the calling program cannot capture the output of the command. To capture the output, we can use check_output() for later processing.,Here is another output after we specified stdout=subprocess.PIPE and stderr=subprocess.PIPE just as before to set up the pipe.,Writing to a process can be done in a very similar way. If we want to send data to the process's stdin, we need to create the Popen object with stdin=subprocess.PIPE., Because a subprocess is independent, it executes concurrently with the original process. That is, the process that created the subprocess can go on to work on other things while the subprocess carries out its own work behind the scenes.

Popen

Popen(['/bin/sh', '-c', args[0], args[1], ...])

Suggestion : 3

Subclass of SubprocessError, raised when a process run by check_call(), check_output(), or run() (with check=True) returns a non-zero exit status.,check_call() and check_output() will raise CalledProcessError if the called process returns a non-zero return code.,Subclass of SubprocessError, raised when a timeout expires while waiting for a child process.,Output of the child process if it was captured by run() or check_output(). Otherwise, None.

os.system
os.spawn *

>>> subprocess.run(["ls", "-l"]) # doesn 't capture output
CompletedProcess(args = ['ls', '-l'], returncode = 0)

   >>>
   subprocess.run("exit 1", shell = True, check = True)
Traceback(most recent call last):
   ...
   subprocess.CalledProcessError: Command 'exit 1'
returned non - zero exit status 1

   >>>
   subprocess.run(["ls", "-l", "/dev/null"], capture_output = True)
CompletedProcess(args = ['ls', '-l', '/dev/null'], returncode = 0,
   stdout = b 'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr = b '')

Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])

>>>
import shlex, subprocess
   >>>
   command_line = input() /
   bin / vikings - input eggs.txt - output "spam spam.txt" - cmd "echo '$MONEY'" >>>
   args = shlex.split(command_line) >>>
   print(args)['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"] >>>
   p = subprocess.Popen(args) # Success!

Popen(['/bin/sh', '-c', args[0], args[1], ...])

with Popen(["ifconfig"], stdout = PIPE) as proc:
   log.write(proc.stdout.read())

Suggestion : 4

A CompletedProcess object has attributes like args, returncode, etc. subprocess.CompletedProcess,This causes the python program to block until the subprocess returns.,run() returns a CompletedProcess object instead of the process return code. A CompletedProcess object has attributes like args, returncode, etc. subprocess.CompletedProcess ,subprocess.call() does not raise an exception if the underlying process errors!

import subprocess

subprocess.call(["ls", "-lha"])
# >>> 0(the
   return code)

import subprocess

# no Python Exception is thrown!
   subprocess.call(["./bash-script-with-bad-syntax"])
# >>> 127

subprocess.call("ls -lha", shell = True)
# returns 0(the
   return code)

import subprocess
import sys

# create two files to hold the output and errors, respectively
with open('out.txt', 'w+') as fout:
   with open('err.txt', 'w+') as ferr:
   out = subprocess.call(["ls", '-lha'], stdout = fout, stderr = ferr)
# reset file to read from it
fout.seek(0)
# save output(
   if any) in variable
output = fout.read())

# reset file to read from it
ferr.seek(0)
# save errors(
   if any) in variable
errors = ferr.read()

output
# total 20 K
# drwxrwxr - x 3 felipe felipe 4, 0 K Nov 4 15: 28 .
# drwxrwxr - x 39 felipe felipe 4, 0 K Nov 3 18: 31. .
# drwxrwxr - x 2 felipe felipe 4, 0 K Nov 3 19: 32 .ipynb_checkpoints
# - rw - rw - r--1 felipe felipe 5, 5 K Nov 4 15: 28 main.ipynb

errors
# ''
empty string

import subprocess

# unlike subprocess.call, this throws a CalledProcessError
#
if the underlying process errors out
subprocess.check_call(["./bash-script-with-bad-syntax"])

import subprocess

# errors in the created process are raised here too
output = subprocess.check_output(["ls", "-lha"], universal_newlines = True)

output
# total 20 K
# drwxrwxr - x 3 felipe felipe 4, 0 K Nov 4 15: 28 .
# drwxrwxr - x 39 felipe felipe 4, 0 K Nov 3 18: 31. .
# drwxrwxr - x 2 felipe felipe 4, 0 K Nov 3 19: 32 .ipynb_checkpoints
# - rw - rw - r--1 felipe felipe 5, 5 K Nov 4 15: 28 main.ipynb

Suggestion : 5

For instance, we write

import os
import subprocess

c = subprocess.call(['echo', 'foo'],
   stdout = subprocess.DEVNULL,
   stderr = subprocess.STDOUT)