Properly using subprocess.PIPE in python?

Question

I'm trying to use subprocess.Popen to construct a sequence to grab the duration of a video file. I've been searching for 3 days, and can't find any reason online as to why this code isn't working, but it keeps giving me a blank result:

import sys
import os
import subprocess

def main():
  the_file = "/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
  ffmpeg = subprocess.Popen(['/opt/local/bin/ffmpeg', '-i', the_file], stdout = subprocess.PIPE, )
  grep = subprocess.Popen(['grep', 'Duration'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )
  cut = subprocess.Popen(['cut', '-d', ' ', '-f', '4'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )
  sed = subprocess.Popen(['sed', 's/,//'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, )

  duration = sed.communicate()
  print duration

if __name__ == '__main__':
  main()

Why are you using grep, cut and sed to parse the output instead of using builtin Python functions? — Mark Byers
– Mark Byers, Commented Mar 22, 2010 at 16:35

icedwater · Accepted Answer · 2013-08-06 01:19:17Z

25

As others have pointed out, you need to pass the PIPE from one process to the next. The stdout (PIPE) from one process becomes the stdin for the following task.

Something like this (starting from your example):

import sys
import os
import subprocess

def main():
  the_file = "/Volumes/Footage/Acura/MDX/
              2001/Crash Test/01 Acura MDX Front Crash.mov"
  ffmpeg = subprocess.Popen(['/opt/local/bin/ffmpeg', '-i', the_file],
                            stdout = subprocess.PIPE)
  grep = subprocess.Popen(['grep', 'Duration'], 
                          stdin = ffmpeg.stdout, stdout = subprocess.PIPE)
  cut = subprocess.Popen(['cut', '-d', ' ', '-f', '4'],
                         stdin = grep.stdout, stdout = subprocess.PIPE)
  sed = subprocess.Popen(['sed', 's/,//'],
                         stdin = cut.stdout, stdout = subprocess.PIPE)

  duration = sed.communicate()[0]
  print duration

if __name__ == '__main__':
  main()

edited Aug 6, 2013 at 1:19

icedwater

4,8943 gold badges39 silver badges54 bronze badges

answered Aug 6, 2013 at 0:51

KevB

2513 silver badges2 bronze badges

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

2 Comments

Beli Over a year ago

I'd consider this the correct answer, it answers exactly what OP asked and isn't instructing on how you could work around it.

MKesper Over a year ago

There is a pipes module nowadays: docs.python.org/3/library/…

ghostdog74 · Accepted Answer · 2010-03-22 16:49:56Z

15

stderr needs to be redirected to stdout. Also, there's no need to call other tools like cut/sed etc. do your string manipulation in Python

import subprocess
....
the_file = "/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
ffmpeg = subprocess.Popen(['/usr/bin/ffmpeg', '-i', the_file], stderr=subprocess.STDOUT,stdout = subprocess.PIPE )
out, err = ffmpeg.communicate()
if "Duration" in out:
    print out[out.index("Duration"):].split()[1]

If Python is not a must, you can use the shell directly.

the_file="/Volumes/Footage/Acura/MDX/2001/Crash Test/01 Acura MDX Front Crash.mov"
ffmpeg -i "$file" 2>&1 | awk '/Duration/{print $2}'

answered Mar 22, 2010 at 16:49

ghostdog74

346k62 gold badges264 silver badges349 bronze badges

1 Comment

Gordon Fontenot Over a year ago

Thank you. I didn't realize I had to redirect stderr as well.

Aaron Digulla · Accepted Answer · 2010-03-22 16:38:22Z

14

Using subprocess.PIPE will not magically wire the correct pipes for you.

You must pass the output pipe of the first process as the value for the parameter stdin of the second process. See the docs for an example.

answered Mar 22, 2010 at 16:38

Aaron Digulla

330k111 gold badges626 silver badges840 bronze badges

Comments

Alex Martelli · Accepted Answer · 2010-03-22 16:41:21Z

4

Python can't "build a whole pipeline" in this way -- it could delegate the task to the shell, or glue it up more directly using the stdout attributes of previous subprocess objects in the line, but there's really no reason for that in this specific case, since you can code it directly in Python pretty easily. E.g.:

  ffmpeg = subprocess.Popen(['/opt/local/bin/ffmpeg', '-i', the_file],
                            stdout=subprocess.PIPE)
  for line in ffmpeg.stdout:
    if 'Duration' not in line: continue
    fields = line.split()
    duration = fields[4].replace(',', '')
    break

answered Mar 22, 2010 at 16:41

Alex Martelli

887k175 gold badges1.3k silver badges1.4k bronze badges

Collectives™ on Stack Overflow

Properly using subprocess.PIPE in python?

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related