Python popen shell command wait till subprocess has finished

Question

I know this question has been answered before here Python popen command. Wait until the command is finished but the thing is I don't understand the answer and how I can apply it to my code so please dont flag this question as being asked before without a little help please:)

I have a function that takes in a shell command and executes it and returns variable outputs.

It works fine except I dont want the flow of control to continue untill the process has completely finished. The reason for this is that I am creating images using the imagemagick command line tool and when I try and access them for information shortly afterward they are incomplete. This is my code..

def send_to_imagemagick(self, shell_command):

    try:
        # log.info('Shell command = {0}'.format(shell_command))
        description=os.popen(shell_command)
        # log.info('description = {0}'.format(description))            
    except Exception as e:
        log.info('Error with Img cmd tool {0}'.format(e))

    while True:
        line = description.readline()
        if not line: break
        return line

Thank you so much @Ruben This is what I used to finish it off so it returns the outputs correctly.

 def send_to_imagemagick(self, shell_command):

        args = shell_command.split(' ')    
        try:                
            description=Popen(args, stdout=subprocess.PIPE)
            out, err = description.communicate()
            return out

        except Exception as e:
            log.info('Error with Img cmd tool {0}'.format(e))

Why use `os.popen` and not `subprocess.popen`? Subprocess replaces os.popen. And I think you want to do this: `description.communicate()` to wait till it's finished. See https://docs.python.org/2/library/subprocess.html#subprocess.Popen.communicate — Noxeus, Dec 03 '15 at 11:14
So how do I write it? like this ...description= subprocess.call([shell_command]) — whoopididoo, Dec 03 '15 at 11:22
`return line` will break the `while` loop because the function will exit, I think you should use `yield line` here instead of `return`. — Remi Guan, Dec 03 '15 at 11:34

score 3 · Accepted Answer · edited Dec 03 '15 at 11:32

3

Use subprocess.popen:

This module intends to replace several older modules and functions.

So in your case import subprocess and then use popen.communicate() to wait until your command finished.

For the documentation on this see: here

So:

from subprocess import Popen

def send_to_imagemagick(self, shell_command):

    try:
        # log.info('Shell command = {0}'.format(shell_command))
        description=Popen(shell_command)
        description.communicate()

        # log.info('description = {0}'.format(description))            
    except Exception as e:
        log.info('Error with Img cmd tool {0}'.format(e))

    while True:
        line = description.readline()
        if not line: break
        return line

edited Dec 03 '15 at 11:32

Remi Guan

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answered Dec 03 '15 at 11:22

Noxeus

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Sorry for capital error :) – Noxeus Dec 03 '15 at 11:26
`return line` or `yield line`? Also is that `self` argument correct? – Remi Guan Dec 03 '15 at 11:28
I don't know, ask OP. – Noxeus Dec 03 '15 at 11:29
Hmm...okay, didn't see that you just copy-paste OP's code and changed the `os.popen` part of it :P – Remi Guan Dec 03 '15 at 11:30
I didn't even look at that `while`. Looks weird tho, I agree on that. – Noxeus Dec 03 '15 at 11:31
Thanks Ruben. I have tried that but now I get a thiss error "No such file or directory" Does Popen need more arguments in the shell command? – whoopididoo Dec 03 '15 at 11:37
Your command has to be in a `list` so if you want to `echo whoopididoo` your shell_command would be `['echo', 'whoopididoo']`. – Noxeus Dec 03 '15 at 11:41
Ok Thsnks a mill. I see – whoopididoo Dec 03 '15 at 11:43
@Ruben So how would you put this shell command in a list?... "identify -format "%b:%w:%h:%r:%#" myimage.jpg" – whoopididoo Dec 03 '15 at 12:12
You can use `string.split(" ")` to do this so it becomes: `['identify','-format', '"%b:%w:%h:%r:%#"','myimage.jpg']` – Noxeus Dec 03 '15 at 13:07
@Ruben `split(' ')` doesn't do enough, you also have to loose the " around the next to last argument. – BlackJack Dec 03 '15 at 14:13
Thanks guys that seems to be working . the only thing is Im not returning it properly now. Any suggestions? – whoopididoo Dec 03 '15 at 15:48

Python popen shell command wait till subprocess has finished

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