Debugging a subprocess.Popen call

Question

I have been using `subprocess.Popen` successfully in the past, when wrapping binaries with a python script to format arguments / customize etc... Developing a nth wrapper, I did as usual... but nothing happens. Here is the little code: ``` print command p = subprocess.Popen(command, shell = True) result = p.communicate()[0] print vars(p) return result ``` And here is the output: ``` /usr/bin/sh /tmp/run/launch.sh {'_child_created': True, 'returncode': 0, 'stdout': None, 'stdin': None, 'pid': 21650, 'stderr': None, 'universal_newlines': False} ``` As you can see, the goal is to create a shell script setting up everything I need, and then executing it. I would prefer to use real python code, but unfortunately `launch.sh` call 3rd party shell scripts that I have no wish to try and replicate (though I've been insisting for a python api for over a year now). The problem is that: - the shell script is not executed (it should spawn process and output some little things) - no python exception…

Accepted Answer

I've finally found the answer to my question, thanks to `badp` and his suggestions for debugging.

From the python page on the subprocess module:

The executable argument specifies the program to execute. It is very seldom needed: Usually, the program to execute is defined by the args argument. If `shell=True`, the executable argument specifies which shell to use. On Unix, the default shell is `/bin/sh`. On Windows, the default shell is specified by the `COMSPEC` environment variable. The only reason you would need to specify `shell=True` on Windows is where the command you wish to execute is actually built in to the shell, eg `dir`, `copy`. You don’t need `shell=True` to run a batch file, nor to run a console-based executable.

Since I am on Linux and using `shell=True`, my command is in fact a list of arguments to be executed by executable, which defaults to `/bin/sh`. Thus the full command executed was: `/bin/sh /usr/bin/sh /tmp/run/launch.sh`... which did not work so well.

And I should have used either:

```
subprocess.Popen('/tmp/run/launch.sh', shell=True)
```

or

```
subprocess.Popen('/tmp/run/launch.sh', executable = '/usr/bin/sh', shell=True)
```

It's tricky that `shell=True` would actually modify the default executable value on Linux only...

Debugging a subprocess.Popen call

Solution

Problem