python subprocess.Popen error in linux

Question

I am now using Python to run a command-line executable program that was developed by myself:

import subprocess
cmd = '../../run_demo'
print cmd
subprocess.Popen(cmd)

This script runs very well in windows. However, it runs on linux, the following errors are given:

Traceback:
  File "script.py", line 6, in <module>
     subprocess.Popen(cmd)
  File "/user/lib/python2.5/subprocess.py", line 623, in _init_
     erread, errwrite)
  File "/user/lib/python2.6/subprocess.py", line 1141, in _execute_child
     raise child_exception
  OSError: [Errno 2] No such file or directory

As the executable command is printed in the script print cmd, if I copy the content of cmd, and then run in the command line, then the executable program can run. Any ideas? Thanks.

Is run_demo a script file ?? in which case the command would be cmd = '../.././run_demo' — DOOM
– DOOM, Commented Mar 13, 2014 at 11:58
What's your working directory when you run this? Does ../../run_demo work from the shell when you're in that working directory? — Wooble
– Wooble, Commented Mar 13, 2014 at 12:02
@DOOM Thanks for comments. run_demo is an executable program (written in c++ and compiled with g++) — feelfree
– feelfree, Commented Mar 13, 2014 at 12:02

zmo · Accepted Answer · 2014-03-13 12:10:49Z

well, as the error says:

OSError: [Errno 2] No such file or directory

there's no file such as '../../run_demo' in the path you're giving. My bet is that you're trying to call a script relative to the script's path whereas, it's relative to the path you're running it from.

So first thing, print what you've got at ../..:

import os
print os.listdir('../../')

there you'll see if there's a run_demo within.

Then, print the path of the current script:

pwd = os.path.abspath(os.path.join(os.path.dirname(__file__), os.path.pardir))

and now try to play with the relative path to get from pwd to the run_demo path, for example:

rundemo_exec = os.path.join(pwd,'..','..','run_demo')

and finally, once you've verified all that, you want to call properly Popen:

subprocess.Popen([rundemo_exec])

or

subprocess.Popen(rundemo_exec, shell=True)

depending on whether you want to embed it in a shell or not.

N.B.: however the script is indeed or not in the path you're giving, you say you're making a "portable" application between linux and windows, so definitely you need to use os.path.join().

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