Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #17014 > unrolled thread

Re: subprocess question

Back to article view | Back to comp.lang.python

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: subprocess question Chris Rebert <clp2@rebertia.com> - 2011-12-11 22:02 -0800
    Re: subprocess question Nobody <nobody@nowhere.com> - 2011-12-12 15:16 +0000

#17014 — Re: subprocess question

On Sun, Dec 11, 2011 at 8:39 PM,  <jyoung79@kc.rr.com> wrote:
> Wondering if anyone could shed some light on the subprocess module?  I'll admit I'm not that great at the shell.
>
> If I was wanting to get the size of the trash (on OS X), I could use:
>
>>>> os.system('du -sh ~/.Trash/')
>  11M    /Users/jay/.Trash/
> 0
>
> Which gives me what I want.  However, I've been reading that it's better to use subprocess.  I did a test like so, but is this a good way to do this?
>
>>>> import subprocess
>>>> p = subprocess.Popen(['du', '-sh'], cwd='/Users/jay/.Trash/', stdout=subprocess.PIPE)
>>>> out, err = p.communicate()
>>>> out
> ' 11M\t.\n'

You might prefer to use subprocess.check_output(); it slightly
simplifies your code:
http://docs.python.org/library/subprocess.html#subprocess.check_output

> And another question - why can't I use the tilde as a shortcut to the home directory?

Because ~ is interpolated by the shell and `subprocess` does not use
the shell by default for reasons that include efficiency and security.
You can expand ~ yourself using os.path.expanduser():
http://docs.python.org/library/os.path.html#os.path.expanduser
Alternatively, you can opt to use the shell by passing shell=True as
an argument.

Cheers,
Chris
--
http://rebertia.com

[toc] | [next] | [standalone]

#17045

On Sun, 11 Dec 2011 22:02:23 -0800, Chris Rebert wrote:

>>>>> p = subprocess.Popen(['du', '-sh'], cwd='/Users/jay/.Trash/', stdout=subprocess.PIPE)

> Alternatively, you can opt to use the shell by passing shell=True as
> an argument.

Except that the OP is talking about a directory passed to the cwd=
parameter, rather than as part of the command, and shell= doesn't affect
that.

But even if the directory was part of the command, just setting shell=True
won't work. On Unix (including MacOSX) the call:

	subprocess.Popen(['du', '-sh'], shell=True)

is equivalent to:

	subprocess.Popen(['/bin/sh', '-c', 'du', '-sh'], shell=False)

This will result in the shell executing "du" with no arguments. The
variable expansion "$1" will evaluate to "-sh", but that's meaningless as
"$1" doesn't occur in the argument to "-c".

The combination of using a list for the "args" parameter along
with shell=True is rarely useful on Unix. And using a string for the
"args" parameter introduces all of the associated reliability and security
issues.

The situation is different on Windows, where list/string and shell= are
orthogonal. A list is always converted to a string according to the
rules by which MSVCRT parses the command line into argv[]. Then, if
shell=True, "cmd.exe /c " is prepended to the string (actually, the value
of the COMSPEC environment variable is used in place of cmd.exe if it is
defined). Setting shell=True allows the "program" to be any file with a
registered extension, rather than just an executable.

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.python

csiph-web