How to pipe the output of a command to `/dev/null` in python

Question

I am trying to pipe the output of the following command to /dev/null so that it doesn't get printed for the user that is running the program.

import os
os.system("ping -c 1 192.168.unknown.host > /dev/null")

But I get the error:

ping: unknown host  192.168.unknown.host

The problem is that this message is produced by something other than stdout, probably stderr and I can easily redirect it to /dev/null using

ping -c 1 192.168.unknown.host >& /dev/null

However, when I use that in python, I get the following error:

sh: 1: Syntax error: Bad fd number

Is it possible to solve this? (I just don't want that message to be printed).

`&>` instead of `>&`. Or is this just a typo? This works only with bash 4, IIRC. Otherwise use `> /dev/null 2>&1`. — pfnuesel, Dec 03 '14 at 13:55
It's not a type, that command works in shell but not in python (either case); But your suggestion worked well. — orezvani, Dec 03 '14 at 13:58
`os.system` is using `/bin/sh` which does not support the `>&` redirection. Try `/bin/sh -c ''` from the shell and it'll fail too. — Etan Reisner, Dec 03 '14 at 14:00
It works in terminal, but python returns that strange error. — orezvani, Dec 03 '14 at 14:00

Bartosz Marcinkowski · Answer 1 · 2014-12-03T14:08:20.780

10

import subprocess

subprocess.Popen(
    ['ping', "-c", "192.168.unknown.host"],
    stdout=subprocess.DEVNULL,
    stderr=subprocess.DEVNULL,
)

As pointed out by Ashwini Chaudhary, for Python below 3.3 you have to manually open os.devnull:

import os
import subprocess

with open(os.devnull, 'w') as DEVNULL:
    subprocess.Popen(
        ['ping', "-c", "192.168.unknown.host"],
        stdout=DEVNULL,
        stderr=DEVNULL,
    )

edited Dec 03 '14 at 14:08

answered Dec 03 '14 at 13:58

Bartosz Marcinkowski

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2

Note that `subprocess.DEVNULL` is only for Python 3.3+, for earlier versions you'll have to use object returned by `open(os.devnull)`. – Ashwini Chaudhary Dec 03 '14 at 14:04

score 2 · Answer 2 · answered Dec 03 '14 at 13:58

You should take a look at the Python subprocess module. It offers several methods that can handle standard input, output and error.

Something like this:

import subprocess as sub
import shlex

cmd = "ping -c 1 myhost"
cmdarg = shlex.split(cmd)
p = sub.Popen(cmdarg,stdout=sub.PIPE,stderr=sub.PIPE)
output, errors = p.communicate()

How to pipe the output of a command to `/dev/null` in python

2 Answers2