Question

I’m trying to run some external executable code from within Python and then make use of the output.The code I’m using takes an external file and returns a single number (the number of images encoded in that file). When I run from the command line, I see the following:

me@ubuntu:~/nist/hsfsys/bin$ ./nummis  /usr/local/hsfsys/data/by_class/4a/train_4a.mis 
3962

Where 3962 is a correct output as near as I can tell

However, when I try to use subprocess from within Python, I get the following error:

me@ubuntu:~/nist/hsfsys/bin$ python
Python 2.7.3 (default, Apr 20 2012, 22:39:59) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> subprocess.check_output(["./nummis","/usr/local/hsfsys/data/by_class/4a/train_4a.mis"])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/subprocess.py", line 544, in check_output
raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['./nummis', '/usr/local/hsfsys/data/by_class/4a/train_4a.mis']' returned non-zero exit status 32
>>> subprocess.call(["./nummis","/usr/local/hsfsys/data/by_class/4a/train_4a.mis"])
3962
32

How should I interpret this “non-zero exit status 32” ? If something is wrong, why don’t I see it on the command line? If nothing is wrong, why is Python complaining & how can I get it to stop complaining?

Answer 1

The command line only reports the exit status when explicitly asked for it.

After calling your program from the command line, try

echo $?

in order to show the exit status. If it shows 32 as well, it is the called program which is guilty. It doesn’t properly return 0; or return EXIT_SUCCESS; in its main().