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 https://stackoverflow.com/questions/3172470/actual-meaning-of-shell-true-in-subprocess — tripleee, Feb 07 '21 at 10:06

score 5 · Answer 1 · edited May 23 '17 at 10:24

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 '17 at 10:24

Community

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answered Sep 27 '12 at 15:17

Carl

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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. – Tetsuo Sep 27 '12 at 15:36

score 1 · Accepted Answer · answered Sep 27 '12 at 15:18

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 '12 at 15:18

Wooble

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Thank you! (both for providing the answer and the explanation) – Tetsuo Sep 27 '12 at 15:33

score 1 · Answer 3 · answered Sep 27 '12 at 15:18

1

Try this:

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

answered Sep 27 '12 at 15:18

tMC

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Python Subprocess: Command Fails in Python but Works in Shell

3 Answers3