python subprocess missing arguments

Question

Have been trying to get something like this to work for a while, the below doesn't seem to be sending the correct arg to the c program arg_count, which outputs argc = 1. When I'm pretty sure I would like it to be 2. ./arg_count -arg from the shell outputs 2...

I have tried with another arg (so it would output 3 in the shell) and it still outputs 1 when calling via subprocess.

import subprocess
pipe = subprocess.Popen(["./args/Release/arg_count", "-arg"], shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
out, err = pipe.communicate()
result = out.decode()
print "Result : ",result
print "Error : ",err

Any idea where im falling over? I'm running linux btw.

user707650 · Accepted Answer · 2013-10-15 09:50:55Z

5

From the documentation:

The shell argument (which defaults to False) specifies whether to use the shell as the program to execute. If shell is True, it is recommended to pass args as a string rather than as a sequence.

Thus,

pipe = subprocess.Popen("./args/Release/arg_count -arg", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

should give you what you want.

answered Oct 15, 2013 at 9:50

user707650

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4 Comments

jayjay Over a year ago

Thanks, this does seem to fix the argument issue, but now I get no output whatsoever...

jayjay Over a year ago

for whatever reason, the script has to be in the same directory as the executable to get a valid result? or i dont know how to write a filepath...

user707650 Over a year ago

That shouldn't have to be the case; I can easily put my version of arg_count (in this case a 2-line Python script) in a different directory and get the correct result. Unless your arg_count itself has some awkward problem.

Eiad Samman Over a year ago

Wasn't getting errors with arguments with out options used as a sequence, but when options paired with args (-arg option) the targeted process couldn't assign the option to the corresponding arguments (in my case 'Missing argument for option x') Used your solution and passed args as a string fixed the issue

jfs · Accepted Answer · 2013-10-15 10:05:40Z

2

If shell=True then your call is equivalent to:

from subprocess import Popen, PIPE

proc = Popen(['/bin/sh', '-c', "./args/Release/arg_count", "-arg"],
             stdout=PIPE, stderr=PIPE)

i.e., -arg is passed to the shell itself and not your program. Drop shell=True to pass -arg to the program:

proc = Popen(["./args/Release/arg_count", "-arg"],
             stdout=PIPE, stderr=PIPE)

If you don't need to capture stderr separately from stdout then you could use check_output():

from subprocess import check_output, STDOUT

output = check_output(["./args/Release/arg_count", "-arg"]) # or
output_and_errors = check_output(["./args/Release/arg_count", "-arg"],
                                 stderr=STDOUT)

answered Oct 15, 2013 at 10:05

jfs

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