Why subprocess.Popen doesn't work when args is sequence?

Question

I'm having a problem with subprocess.Popen when args parameter is given as sequence.

For example:

import subprocess
maildir = "/home/support/Maildir"

This works (it prints the correct size of /home/support/Maildir dir):

size = subprocess.Popen(["du -s -b " + maildir], shell=True,
                        stdout=subprocess.PIPE).communicate()[0].split()[0]
print size

But, this doesn't work (try it):

size = subprocess.Popen(["du", "-s -b", maildir], shell=True,
                        stdout=subprocess.PIPE).communicate()[0].split()[0]
print size

What's wrong?

Answer 1

it should be ["du", "-s", "-b", maildir]

Answer 2

From document,

On Unix, with shell=True: If args is a string, it specifies the command string to execute through the shell. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional shell arguments.

So, Try

subprocess.Popen("du -s -b " + maildir, ...

or

subprocess.Popen(["du","-s","-b",maildir], ...

Answer 3

From the documentation

On Unix, with shell=True: […] If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])

Which translates in your case to:

Popen(['/bin/sh', '-c', 'du', '-s', '-b', maildir])

This means that -s, -b and maildir are interpreted as options by the shell, not by du (try it on the shell commandline!).

Since shell=True is not needed in your case anyway, you could just remove it:

size = subprocess.Popen(['du', '-s', '-b', maildir],
                    stdout=subprocess.PIPE).communicate()[0].split()[0]

Alternatively you could just use your orignal approach, but you don't need a list in that case. You would also have to take care of spaces in the directory name:

size = subprocess.Popen('du -s -b "%s"' % maildir, shell=True,
                    stdout=subprocess.PIPE).communicate()[0].split()[0]