Store output of subprocess.Popen call in a string

0 votes
asked Mar 23, 2010 by mark

I'm trying to make a system call in Python and store the output to a string that I can manipulate in the Python program.

#!/usr/bin/python
import subprocess
p2 = subprocess.Popen("ntpq -p")

I've tried a few things including some of the suggestions here:

Retrieving the output of subprocess.call()

but without any luck.

9 Answers

0 votes
answered Jan 23, 2010 by alex
 import os   
 list = os.popen('pwd').read()

In this case you will only have one element in the list.

0 votes
answered Jan 23, 2010 by mark-byers

Assuming that pwd is just an example, this is how you can do it:

import subprocess

p = subprocess.Popen("pwd", stdout=subprocess.PIPE)
result = p.communicate()[0]
print result

See the subprocess documentation for another example and more information.

0 votes
answered Mar 23, 2010 by eli-bendersky

This worked for me for redirecting stdout (stderr can be handled similarly):

from subprocess import Popen, PIPE
pipe = Popen(path, stdout=PIPE)
text = pipe.communicate()[0]

If it doesn't work for you, please specify exactly the problem you're having.

0 votes
answered Mar 23, 2010 by mike-graham

In Python 2.7 or Python 3

Instead of making a Popen object directly, you can use the subprocess.check_output() function to store output of a command in a string:

from subprocess import check_output
out = check_output(["ntpq", "-p"])

In Python 2.4-2.6

Use the communicate method.

import subprocess
p = subprocess.Popen(["ntpq", "-p"], stdout=subprocess.PIPE)
out, err = p.communicate()

out is what you want.

Important note about the other answers

Note how I passed in the command. The "ntpq -p" example brings up another matter. Since Popen does not invoke the shell, you would use a list of the command and options—["ntpq", "-p"].

0 votes
answered Jan 8, 2013 by mpen

I wrote a little function based on the other answers here:

def pexec(*args):
    return subprocess.Popen(args, stdout=subprocess.PIPE).communicate()[0].rstrip()

Usage:

changeset = pexec('hg','id','--id')
branch = pexec('hg','id','--branch')
revnum = pexec('hg','id','--num')
print('%s : %s (%s)' % (revnum, changeset, branch))
0 votes
answered Jan 28, 2013 by paolo-rovelli

subprocess.Popen: http://docs.python.org/2/library/subprocess.html#subprocess.Popen

import subprocess

command = "ntpq -p"  # the shell command
process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=None, shell=True)

#Launch the shell command:
output = process.communicate()

print output[0]

In the Popen constructor, if shell is True, you should pass the command as a string rather than as a sequence. Otherwise, just split the command into a list:

command = ["ntpq", "-p"]  # the shell command
process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=None)

If you need to read also the standard error, into the Popen initialization, you can set stderr to subprocess.PIPE or to subprocess.STDOUT:

import subprocess

command = "ntpq -p"  # the shell command
process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)

#Launch the shell command:
output, error = process.communicate()
0 votes
answered Jan 11, 2014 by patrick-wolf

This works perfectly for me:

import subprocess
try:
    #prints results and merges stdout and std
    result = subprocess.check_output("echo %USERNAME%", stderr=subprocess.STDOUT, shell=True)
    print result
    #causes error and merges stdout and stderr
    result = subprocess.check_output("copy testfds", stderr=subprocess.STDOUT, shell=True)
except subprocess.CalledProcessError, ex: # error code <> 0 
    print "--------error------"
    print ex.cmd
    print ex.message
    print ex.returncode
    print ex.output # contains stdout and stderr together 
0 votes
answered Jan 5, 2015 by gravmatt

This was perfect for me. You will get the return code, stdout and stderr in a tuple.

from subprocess import Popen, PIPE

def console(cmd):
    p = Popen(cmd, shell=True, stdout=PIPE)
    out, err = p.communicate()
    return (p.returncode, out, err)

For Example:

result = console('ls -l')
print 'returncode: %s' % result[0]
print 'output: %s' % result[1]
print 'error: %s' % result[2]
0 votes
answered Jan 8, 2016 by corey-goldberg

for Python 2.7+ the idiomatic answer is to use subprocess.check_output()

You should also note the handling of arguments when invoking a subprocess, as it can be a little confusing....

If args is just single command with no args of its own (or you have shell=True set), it can be a string. Otherwise it must be a list.

for example... to invoke the ls command, this is fine:

from subprocess import check_call
check_call('ls')

so is this:

from subprocess import check_call
check_call(['ls',])

however, if you want to pass some args to the shell command, you can't do this:

from subprocess import check_call
check_call('ls -al')

instead, you must pass it as a list:

from subprocess import check_call
check_call(['ls', '-al'])

the shlex.split() function can sometimes be useful to split a string into shell-like syntax before creating a subprocesses... like this:

from subprocess import check_call
import shlex
check_call(shlex.split('ls -al'))
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