How to suppress or capture the output of subprocess.run()?

Question

From the examples in docs on subprocess.run() it seems like there shouldn’t be any output from

subprocess.run(["ls", "-l"])  # doesn't capture output

However, when I try it in a python shell the listing gets printed. I wonder if this is the default behaviour and how to suppress the output of run().

Asked By: planetp

||

Source

Answer 1

Here is how to suppress output, in order of decreasing levels of cleanliness. They assume you are on Python 3.

You can redirect to the special subprocess.DEVNULL target.

import subprocess

# To redirect stdout (only):
subprocess.run(['ls', '-l'], stdout=subprocess.DEVNULL)

# to redirect stderr to /dev/null as well:
subprocess.run(['ls', '-l'], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

# Alternatively, you can merge stderr and stdout streams and redirect
# the one stream to /dev/null
subprocess.run(['ls', '-l'], stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT)

If you want a fully manual method, can redirect to /dev/null by opening the file handle yourself. Everything else would be identical to method #1.

import os
import subprocess

with open(os.devnull, 'w') as devnull:
    subprocess.run(['ls', '-l'], stdout=devnull)

Here is how to capture output (to use later or parse), in order of decreasing levels of cleanliness. They assume you are on Python 3.

NOTE: The below examples use text=True.

This causes the STDOUT and STDERR to be captured as str instead of bytes.

Omit text=True to get bytes data

text=True is Python >= 3.7 only, use universal_newlines=True on Python <= 3.6

universal_newlines=True is identical to text=True but more verbose to type but should exist on all Python versions

If you simply want to capture both STDOUT and STDERR independently, AND you are on Python >= 3.7, use capture_output=True.

import subprocess

result = subprocess.run(['ls', '-l'], capture_output=True, text=True)
print(result.stdout)
print(result.stderr)

You can use subprocess.PIPE to capture STDOUT and STDERR independently. This works on any version of Python that supports subprocess.run.

import subprocess

result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE, text=True)
print(result.stdout)

# To also capture stderr...
result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
print(result.stdout)
print(result.stderr)

# To mix stdout and stderr into a single string
result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
print(result.stdout)

Answered By: SethMMorton

Answer 2

ex: to capture the output of ls -a

import subprocess
ls = subprocess.run(['ls', '-a'], capture_output=True, text=True).stdout.strip("n")
print(ls)

Answered By: Shaeinst

How to suppress or capture the output of subprocess.run()?

Question:

Answers: