Debugging a subprocess.Popen call

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 is raised
• there is nothing in the p object that indicates that an error occurred

I have tried check_call without any success either...

I am at a loss regarding what I should do, and would be very glad if someone could either point my mistake or direct me toward resolution...

EDIT:

• Trying to run this on Linux (sh)
• shell is necessary for variable substitution in the scripts invoked

EDIT 2:

Following badp suggestion, I tweaked the code and added

subprocess.Popen('ps', shell = True).communicate()

Righ开发者_如何学编程t after p = ... line that creates the process, here is the output:

/usr/bin/sh /tmp/run/launch.sh
  PID TTY          TIME CMD
29978 pts/0    00:00:01 zsh
 1178 pts/0    00:00:01 python
 1180 pts/0    00:00:00 sh <defunct>
 1181 pts/0    00:00:00 ps
None

Apparently the process is launched (even though <defunct>) and one should also note that I have a little problem passing the parameters in...

Thanks.

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...

Try this:

p = subprocess.Popen(command,
                     shell = True, #is this even needed?
                     stdin = subprocess.PIPE,
                     stdout = subprocess.PIPE,
                   # stderr = subprocess.STDOUT #uncomment if reqd
                    )

Tested working on Windows with the ping command. This lets you communicate, which might help you find out why the script isn't launched in the first place :)