Python Subprocess: Command Fails in Python but Works in Shell

Question

I am trying to run a command through Python's subprocess, but it won't run properly. If I type into the shell:

pack < packfile.dat

where pack is my software and packfile is the input file, then the software runs fine.

If I try this in python:

import subprocess as sp
import shlex

cmd = 'pack < packfile.dat'.split()
p = sp.Popen(cmd)

The software complains:

Pack must be run with: pack < inputfile.inp 

Reading input file... (Control-C aborts)

and it hangs there.

This last part is specific to my software, but the fact is that the two methods of running the same command give different results, when this shouldn't be the case.

Can anyone tell me what I'm doing wrong?

Actually, I intend to eventually do:

p = sp.Popen(cmd,stdout=sp.PIPE,stderr=sp.PIPE)
stdout, stderr = p.communicate()

Since I am a little new to this, if this is not best-practice, please let me know.

Thanks in advance.

See also stackoverflow.com/questions/3172470/…

tripleee
– tripleee

2021-02-07 10:06:09 +00:00
Commented Feb 7, 2021 at 10:06 — tripleee
– tripleee, Commented Feb 7, 2021 at 10:06

Community · Accepted Answer · 2017-05-23 10:24:52Z

5

I/O redirection is a product of the shell, and by default Popen does not use one. Try this:

p = sp.Popen(cmd, shell=True)

subprocess.Popen() IO redirect

From there you'll also see that some don't prefer the shell option. In that case, you could probably accomplish it with:

with open('input.txt', 'r') as input:
    p = subprocess.Popen('./server.py', stdin=input)

edited May 23, 2017 at 10:24

CommunityBot

11 silver badge

answered Sep 27, 2012 at 15:17

Carl

9155 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Tetsuo Over a year ago

Thanks for your quick reply, I appreciate it. I tried it with shell=True but this unfortunately didn't work. According to the other answers, "<" can't be used in this way.

Wooble · Accepted Answer · 2012-09-27 15:18:46Z

1

"<" isn't a parameter to the command, and shouldn't be passed as one.

Try:

p = sp.Popen(cmd,stdout=sp.PIPE,stderr=sp.PIPE, stdin=open('packfile.dat'))

answered Sep 27, 2012 at 15:18

Wooble

90.5k12 gold badges111 silver badges132 bronze badges

1 Comment

Tetsuo Over a year ago

Thank you! (both for providing the answer and the explanation)

tMC · Accepted Answer · 2012-09-27 15:18:48Z

1

Try this:

import subprocess as sp
p = sp.Popen(['pack'], stdin = open('packfile.dat', 'r'))

answered Sep 27, 2012 at 15:18

tMC

19.5k15 gold badges68 silver badges102 bronze badges

Collectives™ on Stack Overflow

Python Subprocess: Command Fails in Python but Works in Shell

3 Answers 3

1 Comment

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related