Subprocess module from python fails to run bash commands like "<cmd1> | <cmd2>" -

the following python script:

#!/usr/bin/env python  import os cmd = "echo hello world | cut -d' ' -f1" test=os.system(cmd) print(test) 

it runs ok (the output hello). when use subprocess module one:

#!/usr/bin/env python  import subprocess cmd = "echo hello world | cut -d' ' -f1" process = subprocess.popen(cmd.split(), stdout=subprocess.pipe) test = process.communicate()[0] print (test) 

is not ok. output hello world | cut -d' ' -f1 , expect hello. how can correct it?

i saw in general subprocess module fail when i'm using bash command like:

<cmd1> | <cmd2>  

this:

echo hello world | cut -d' ' -f1 

… not command, it's fragment of shell script. need have shell execute it.

you can adding shell=true popen constructor.

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the documentation explains how works. explains better ways same thing without shell. example:

p1 = popen(['echo', 'hello', 'world'], stdout=pipe) p2 = popen(['cut', "-d' '", '-f1'], stdin=p1.stdout, stdout=pipe) p1.stdout.close() test = p2.communicate()[0] 

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meanwhile, never want use split on command line—and in fact, example shows why don't want to:

>>> cmd = "echo hello world | cut -d' ' -f1" >>> cmd.split() ['echo', 'hello', 'world', '|', 'cut', "-d'", "'", '-f1'] 

notice split -d' ' 2 arguments, -d' , '.

if you're using shell=true, don't try split arguments @ all; pass string cmd:

process = subprocess.popen(cmd, stdout=subprocess.pipe, shell=true) 

if you're not using shell, right way shlex module:

>>> shlex.split(cmd) ['echo', 'hello', 'world', '|', 'cut', '-d ', '-f1'] 

notice "-d' '" turned "-d " time. may seem odd @ first glance, it's in fact shell do, , want; cut program space d option. (in other words, quotes shell, not program shell runs.)

(the shlex module has handle quote function can use exact opposite purpose: building command line list of arguments shell=true.)

however, it's better create list of arguments in first place, instead of trying figure out how create string that, when run through shlex.split(), give list wanted.