What's the problem with executing commands in Windows CMD from Python?

Question

I'm having huge trouble with passing commands to CMD from Python.

First, I open a CMD process:

    cmdprocess = subprocess.Popen("cmd",
                                  stdin  = subprocess.PIPE,
                                  stdout = subprocess.PIPE,
                                  stderr = subprocess.PIPE)

Then, I do something, for example:

for i in range(500):
     #time.sleep(1)
     command = ("dir > " + os.path.join("C:\\", str(i)) + "\r\n").encode("utf-8")
     print(command)
     cmdprocess.stdin.write(command)

So this is supposed to create 500 small text files in a folder. I tested it in Python 3.2 x64 and 3.2 x86 and the result for both is: it counts up to about 250-350 in the Python shell, and then just stops. No error, nothing. There are then the files 1-80 in the specified folder.

Now, I thought that maybe Python is too fast and so had it sleep(1) for 1 second between the commands. Now, it counts up to about 200 before the first file appears in the folder! and then stops at about 270.

What happens here and how can I force CMD to execute a command immediately?

Is there a reason you're using subprocess rather than the standard Python tools for creating files? — Velociraptors
– Velociraptors, Commented Mar 10, 2011 at 4:45
yes, of course: I'm actually using the mklink command to create symlinks to files. I just tried to reduce the problem to a smaller, more general command which is easier to write and doesn't need any prerequisites. The problem is identical in both commands. — Felix Dombek
– Felix Dombek, Commented Mar 10, 2011 at 5:32

Bo Buchanan · Accepted Answer · 2011-03-10 03:09:12Z

3

Are you handling the output in the PIPES? They might be filling. If you fill up the stdout or stderror buffers from the process, it will stop execution.

answered Mar 10, 2011 at 3:09

Bo Buchanan

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16 Comments

Felix Dombek Over a year ago

No, I'm not ... but if I just run the commands one by one by hand, it doesn't generate any output either. The files just don't appear. The problem with reading from the pipes is that it interrupts the program if I want to read more characters than have been written. I don't get a '' at the end of file, read(1) just blocks ...

Felix Dombek Over a year ago

I just tried peek() and it works, but I only see that there is absolutely no output ... which hinted me at a possible solution: I might try flush on the input stream ...

Bo Buchanan Over a year ago

Try using cmdprocess = subprocess.Popen("cmd", stdin = subprocess.PIPE, shell = True)

Felix Dombek Over a year ago

@Bo: I tried that, it now says cmdprocess.stdin.write(command) IOError: [Errno 22] Invalid argument before it stops. Any idea what's going wrong there? Does this mean that stdin as the implicit first argument to write somehow suddenly ceased to exist? I think I'm trying the debugger on this ...

Bo Buchanan Over a year ago

From what I have it runs all the way through 499 I just get access denied errors before it won't let me write files directly at c:\ here is the full code I have

import subprocess import os cmdprocess = subprocess.Popen("cmd", stdin  = subprocess.PIPE, shell = True)  for i in range(500):      command = ("dir > " + os.path.join("C:\\", str(i)) + "\r\n").encode("utf-8")      print(command)      cmdprocess.stdin.write(command)

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JungJik Lee · Accepted Answer · 2011-03-10 13:56:22Z

1

I think you'd better to use pywin32 package. there are win32pipe and win32process modules. I also had same issue but I could not resolve it without pywin32-site-package... So now I am using them... If you need sample code and you're using windows, I will attach it. if you mean linux... it's same but you need another one like IO select.

edited Mar 10, 2011 at 13:56

answered Mar 10, 2011 at 4:46

JungJik Lee

1488 bronze badges

1 Comment

JungJik Lee Over a year ago

it's quite long... plz.check flankton.tistory.com/entry/Python-Command-Process out

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