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.

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)