os.system vs subprocess

Nate walton.nathaniel at gmail.com
Mon Jun 22 00:42:51 CEST 2009

On Jun 21, 3:49 pm, Christian Heimes <li... at cheimes.de> wrote:
> Nate wrote:
> > gmapcreator = subprocess.Popen("java -Xms128M -Xmx512M -jar
> > gmapcreator.jar -dfile=censettings.xml", stdin=subprocess.PIPE,
> > stdout=subprocess.PIPE, stderr=subprocess.PIPE)
> Try this:
> gmapcreator = subprocess.Popen(
>     ["java", "-Xms128M", "-Xmx512M", "-jar", "gmapcreator.jar",
>     "-dfile=censettings.xml"], stdin=subprocess.PIPE,
>     stdout=subprocess.PIPE, stderr=subprocess.PIPE)
> out, err = gmapcreator.communicate("stdin input")
> The subprocess doesn't use the shell so you have to apply the command as
> a list of strings.
> Do you really need stdin, stdout and stderr as pipes? If you aren't
> carefully with the API your program can block.
> Christian


Thanks for your response. Related to this talk about shells, maybe you
could point me towards a resource where I could read about how windows
commands are processed w/w/o shells?  I guess I assumed all subprocess
commands were intepreted by the same thing, cmd.exe., or perhaps the

I guess this also ties in with me wondering what the whole subprocess
Popen instantiation encompassed (the __init__ of the subprocess.Popen

I don't think I need stdin, stdout, stderr as pipes, I just was faced
with several options for how to capture these things and I chose
subprocess.PIPE.  I could also os.pipe my own pipes, or create a file
descriptor in one of a couple of ways, right?  I'm just unclear on
which method is preferable for me.  All I want to do is pull the first
2 lines from the output before gmapcreator.jar finishes.  The first 2
lines of output are always the same except a few numbers.

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