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I can execute a terminal command using os.system() but I want to capture the output of this command. How can I do this?

Marcello B.
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AssemblerGuy
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4 Answers4

51
>>> import subprocess
>>> cmd = [ 'echo', 'arg1', 'arg2' ]
>>> output = subprocess.Popen( cmd, stdout=subprocess.PIPE ).communicate()[0]
>>> print(output)
arg1 arg2

There is a bug in using of the subprocess.PIPE. For the huge output use this:

import subprocess
import tempfile

with tempfile.TemporaryFile() as tempf:
    proc = subprocess.Popen(['echo', 'a', 'b'], stdout=tempf)
    proc.wait()
    tempf.seek(0)
    print(tempf.read())
Ajeet Verma
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Jiří Polcar
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33

The recommended way in Python 3.5 and above is to use subprocess.run():

from subprocess import run
output = run("pwd", capture_output=True).stdout
Sven Marnach
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  • PIPE is not defined – Cherona Apr 29 '20 at 02:48
  • @Cherona It's deifned in the `subprocess` module, so you need to import it. – Sven Marnach Apr 29 '20 at 19:25
  • I also updated the answer with the most recent API to do this. – Sven Marnach Apr 29 '20 at 19:32
  • How can I explicitly open read from a terminal? (If stdin has already been redirected to reading from a shell pipe?) – Helen Craigman Oct 07 '20 at 10:14
  • @HelenCraigman I don't think that's possible. If stdin is not connected to a terminal, what terminal would you like to read from? – Sven Marnach Oct 07 '20 at 12:22
  • @sven When I pipe to a python program (for example, in ubuntu: `cat file.txt | python3 program.py`, I would like to have the python program read lines from "file.txt", and interact with the user at the terminal – Helen Craigman Oct 08 '20 at 16:36
  • @sven (Or, actually in my case: `script.sh | python3 program.py`). stdin reads lines from the pipe, and I need the user to interact with the Python program. – Helen Craigman Oct 08 '20 at 16:43
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    @HelenCraigman I see. On Unix, you can still read from the "controlling terminal" by opening `/dev/tty` for reading and writing. I'm not sure that pattern is a good idea, though. – Sven Marnach Oct 08 '20 at 19:44
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    FileNotFoundError: [WinError 2] The system cannot find the file specified – fourk Feb 21 '21 at 09:54
  • On Win10 and Python 3.7.9. this now results in the line `output = run("pwd", capture_output=True).stdout` failing with error: `FileNotFoundError: [WinError 2] The system cannot find the file specified`. – lazarea Jan 17 '22 at 21:30
  • @lazarea On Windows, there is no `pwd` executable. The name of the executable is just an example. – Sven Marnach Jan 18 '22 at 14:18
15

You can use Popen in subprocess as they suggest.

with os, which is not recomment, it's like below: 

import os
a  = os.popen('pwd').readlines()
gerry
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0

The easiest way is to use the library commands

import commands
print commands.getstatusoutput('echo "test" | wc')
Mahmoud
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    Where do you get the commands module? It doesn't appear to be on pip for Python3. – Brōtsyorfuzthrāx May 04 '14 at 00:20
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    @Shule commands is an older module. It is replaced by the subprocess module. https://docs.python.org/2/library/subprocess.html#subprocess.check_output – aaroh Jun 25 '18 at 06:21