Python subprocess Popen parameter difference

What's the difference between

subpr开发者_开发百科ocess.Popen(['cat','/path/to/file'], stdout=subprocess.PIPE, shell=True)

and

subprocess.Popen(['cat '+'/path/to/file'], stdout=subprocess.PIPE, shell=True)

? I'm doing this in ipython. For the first one, ipython just hang. Maybe not hang, but it's significantly slower. The second is just OK.

Just don't know why.

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The first one is actually only running cat (because of the way that the command is parsed if you use shell = True), and thus hanging because it's waiting for input. The second one is running cat /path/to/file. You can see this by doing:

>>> subprocess.Popen(['ls', '-l'])
<subprocess.Popen object at 0x10048bdd0>
>>> total 8
-rw-------  1 root     wheel  652 Nov 29 09:07 000d94cfc78b4
srwxr-xr-x  1 nbastin  wheel    0 Nov 29 09:06 ics179
srwxr-xr-x  1 nbastin  wheel    0 Nov 29 09:06 icssuis501
drwx------  3 nbastin  wheel  102 Nov 29 09:06 launch-3ZniHd
drwx------  3 nbastin  wheel  102 Nov 29 09:06 launch-8QRgz2
drwx------  3 nbastin  wheel  102 Nov 29 09:06 launch-M5ppWp
drwx------  3 nbastin  wheel  102 Nov 29 09:06 launchd-137.ztQAmI
drwx------  2 nbastin  wheel   68 Nov 29 09:57 ssh-LreGlOZPAR

Versus doing it with shell = True:

>>> subprocess.Popen(['ls', '-l'], shell = True)
<subprocess.Popen object at 0x10048bd90>
>>> 000d94cfc78b4       ics179          icssuis501      launch-3ZniHd       launch-8QRgz2       launch-M5ppWp       launchd-137.ztQAmI  ssh-LreGlOZPAR

If you set shell = True, passing a list as args won't get you the behaviour you want - you need to pass a string.

---

i tested the samples and found that the first worked returning a Popen object in ipython but the second did the same but printed cat: /path/to/file: No such file or directory

i like to avoid using subprocess.Popen with shell=True when possible and use a list ['x', '--version'] as this avoids the need to quote path names with say ` graves in them or some such muck

in a patch to pymp.py i changed:

p = subprocess.Popen(['mplayer -slave -quiet \'' + target + '\' -include \'' + MPLAYERCONFFILE + '\' 2>/dev/null'], shell=True, stdin=subprocess.PIPE, stdout=subprocess.PIPE)

to:

devnull = open('/dev/null', 'w')
p = subprocess.Popen(['mplayer', '-slave', '-quiet', target, '-include', MPLAYERCONFFILE], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=devnull)