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Python: How to get stdout after running os.system?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I want to get the stdout in a variable after running the os.system call.

Lets take this line as an example:

batcmd="dir"
result = os.system(batcmd)

result will contain the error code (stderr 0 under Windows or 1 under some linux for the above example).

How can I get the stdout for the above command without using redirection in the executed command?

Answers:

If all you need is the stdout output, then take a look at subprocess.check_output() (added in Python 2.7):

import subprocess

batcmd="dir"
result = subprocess.check_output(batcmd, shell=True)

Because you were using os.system(), you’d have to set shell=True to get the same behaviour. You do want to heed the big red warning message about passing untrusted arguments to your shell.

If you need to capture stderr as well, simply add stderr=subprocess.STDOUT to the call:

result = subprocess.check_output([batcmd], stderr=subprocess.STDOUT)

to redirect the error output to the default output stream.

Questions:
Answers:

These answers didn’t work for me. I had to use the following:

import subprocess
p = subprocess.Popen(["pwd"], stdout=subprocess.PIPE)
out = p.stdout.read()
print out

Or as a function (using shell=True was required for me on Python 2.6.7 and check_output was not added until 2.7, making it unusable here):

def system_call(command):
    p = subprocess.Popen([command], stdout=subprocess.PIPE, shell=True)
    return p.stdout.read()

Questions:
Answers:

I would like to expand on the Windows solution. Using IDLE with Python 2.7.5, When I run this code from file Expts.py:

import subprocess
r = subprocess.check_output('cmd.exe dir',shell=False) 
print r

…in the Python Shell, I ONLY get the output corresponding to “cmd.exe”; the “dir” part is ignored. HOWEVER, when I add a switch such as /K or /C …

import subprocess
r = subprocess.check_output('cmd.exe /K dir',shell=False) 
print r

…then in the Python Shell, I get all that I expect including the directory listing. Woohoo !

Now, if I try any of those same things in DOS Python command window, without the switch, or with the /K switch, it appears to make the window hang because it is running a subprocess cmd.exe and it awaiting further input – type ‘exit’ then hit [enter] to release. But with the /K switch it works perfectly and returns you to the python prompt. Allrightee then.

Went a step further…I thought this was cool…When I instead do this in Expts.py:

import subprocess
r = subprocess.call("cmd.exe dir",shell=False) 
print r

…a new DOS window pops open and remains there displaying only the results of “cmd.exe” not of “dir”. When I add the /C switch, the DOS window opens and closes very fast before I can see anything (as expected, because /C terminates when done). When I instead add the /K switch, the DOS window pops open and remain, AND I get all the output I expect including the directory listing.

If I try the same thing (subprocess.call instead of subprocess.check_output) from a DOS Python command window; all output is within the same window, there are no popup windows. Without the switch, again the “dir” part is ignored, AND the prompt changes from the python prompt to the DOS prompt (since a cmd.exe subprocess is running in python; again type ‘exit’ and you will revert to the python prompt). Adding the /K switch prints out the directory listing and changes the prompt from python to DOS since /K does not terminate the subprocess. Changing the switch to /C gives us all the output expected AND returns to the python prompt since the subprocess terminates in accordance with /C.

Sorry for the long-winded response, but I am frustrated on this board with the many terse ‘answers’ which at best don’t work (seems because they are not tested – like Eduard F’s response above mine which is missing the switch) or worse, are so terse that they don’t help much at all (e.g., ‘try subprocess instead of os.system’ … yeah, OK, now what ??). In contrast, I have provided solutions which I tested, and showed how there are subtle differences between them. Took a lot of time but…
Hope this helps.

Questions:
Answers:

commands also works.

import commands
batcmd = "dir"
result = commands.getoutput(batcmd)
print result

It works on linux, python 2.7.

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