CODEWITHSUNDEEP
pythonpython-3.xsubprocess
planetp
planetp

Reputation: 16065

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

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().

Upvotes: 246

Views: 323744

Answers (2)

SethMMorton
SethMMorton

Reputation: 48735

Suppressing

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

  1. 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
)
  1. 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
    )

Capturing

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 universal_newlines=True (Python <= 3.6).

  • This causes the STDOUT and STDERR to be captured as str instead of bytes.
    • Omit universal_newlines=True to get bytes data
  • Python >= 3.7 accepts text=True as a short form for universal_newlines=True
  1. 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, # Python >= 3.7 only
    text = True # Python >= 3.7 only
)
print(result.stdout)
print(result.stderr)
  1. 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,
    universal_newlines = True # Python >= 3.7 also accepts "text=True"
)
print(result.stdout)

# To also capture stderr...
result = subprocess.run(
    ['ls', '-l'],
    stdout = subprocess.PIPE,
    stderr = subprocess.PIPE,
    universal_newlines = True # Python >= 3.7 also accepts "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,
    universal_newlines = True # Python >= 3.7 also accepts "text=True"
)
print(result.stdout)

Upvotes: 414

Shaeinst
Shaeinst

Reputation: 417

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)

Upvotes: 30

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