Can't retrieve output from subprocess check_output

Question

I'm wondering how to save the output of the program in a file.

In particular, I'm trying to save all output from vowpal_wabbit application to a file, when running it from the Python like that:

rez1 = subprocess.check_output([parameters], shell=True, universal_newlines=True) print(rez1)

However it printed out nothing, while the program itself executes well. It's strange because when run from the Terminal with the very same parameters it provides me some useful information.

Can anyone suggest a solution?

P.S. Python 3.4.1 (IPython via Anaconda), Mac OS X

don't use a list argument with shell=True it is an error in most cases. — jfs
– jfs, Commented Sep 21, 2014 at 14:36

johntellsall · Accepted Answer · 2014-07-17 17:44:33Z

3

Some programs write to stdout and also stderr. To build on @Tichodroma's code, the following prints 'stdout' to stdout, 'stderr' to stderr, merges the two streams, and captures both outputs correctly:

source

import subprocess res = subprocess.check_output( "echo stdout; /bin/echo stderr 1>&2", shell=True, stderr=subprocess.STDOUT, ) print 'DATA:', res.replace('\n', '.') print 'END'

output

DATA: stdout.stderr. END

answered Jul 17, 2014 at 17:44

johntellsall

15.2k4 gold badges48 silver badges40 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

night_bat Over a year ago

Works just fine. Thank a lot! By the way, it appears subprocess.Popen(...) also could handle this. What method is better?

johntellsall Over a year ago

I like the check_xx functions because they raise exceptions if the program exits with an error. They're obvious, and I like obvious :)

jfs Over a year ago

@user3058525: A program may also print to a terminal directly. You might need screen utility to capture such output.

jfs Over a year ago

nitpick: use universal_newlines=True on Python 3 to enable the text mode. And print s -> print(s).

user1907906 · Accepted Answer · 2014-07-10 07:50:30Z

If your program prints to STDOUT, you are using subprocess.check_output correctly. A working example:

res = subprocess.check_output("date", shell=True, universal_newlines=True) print(res)

Output:

Thu Jul 10 09:49:31 CEST 2014\n

Collectives™ on Stack Overflow

Can't retrieve output from subprocess check_output

2 Answers 2

source

output

4 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

source

output

4 Comments

Comments

Linked

Related