Execute shell command using Python subprocess

Question

I want to execute the following command via a python script:

sudo cat </dev/tcp/time.nist.gov/13

I can execute this command via the command line completely fine. However, when I execute it using subprocess, I get an error:

Command ['sudo','cat','</dev/tcp/time.nist.gov/13'] returned non-zero exit status 1

My code is as follows

import subprocess
subprocess.check_output(['sudo','cat','</dev/tcp/time.nist.gov/13'])

As I mentioned above, executing the command via the command line gives the desired output without any error. I am using the Raspbian Jessie OS. Can someone point me in the right direction?

The redirection operator < is part of shell functionality. Your call doesn't use the shell so cat processes the parameter directly (and doesn't understand it) — Michael Butscher
– Michael Butscher, Commented Sep 8, 2018 at 0:36
@MichaelButscher Thanks for the reply. How do I execute this command using subprocess then? — Umar Dastgir
– Umar Dastgir, Commented Sep 8, 2018 at 0:39

David Maze · Accepted Answer · 2018-09-08 00:55:33Z

2

You don't want to use subprocess for this at all.

What does this command really do? It uses a bash extension to open a network socket, feeds it through cat(1) to reroute it to standard output, and decides to run cat as root. You don't really need the bash extension, or /bin/cat, or root privileges to do any of this in Python; you're looking for the socket library.

Here's an all-Python equivalent:

#!/usr/bin/env python3
import socket
s = socket.create_connection(('time.nist.gov', 13))
try:
    print(s.recv(4096))
finally:
    s.close()

(Note that all of my experimentation suggests that this connection works but the daytime server responds by closing immediately. For instance, the simpler shell invocation nc time.nist.gov 13 also returns empty string.)

answered Sep 8, 2018 at 0:55

David Maze

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3 Comments

Umar Dastgir Over a year ago

Works. Thanks :)

jadki Over a year ago

Can you explain why subprocess is the wrong way to approach this? Besides the fact it isn't working for Umar haha

David Maze Over a year ago

In general it's both easier and faster to try to do things natively in a single language if you can. If you're using subprocess for basic file I/O (ls cat grep sed ...) (or in this case, network I/O) you should just use your local language primitives for it instead.

jadki · Accepted Answer · 2018-09-08 00:42:05Z

1

Give this a try:

import subprocess

com = "sudo cat </dev/tcp/time.nist.gov/13"

subprocess.Popen(com, stdout = subprocess.PIPE, shell = True)

answered Sep 8, 2018 at 0:42

jadki

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2 Comments

Umar Dastgir Over a year ago

Doesnt work, error: cannot open /dev/tcp/time.nist.gov/13L No such file or directory

jadki Over a year ago

Turn it into the full path. I don't know what that is on your pc. Ie, C:/User/.../dev/tcp/time.nist.gov/13. Or put the script into the same folder as the application you are trying to call.

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