python subprocess.call arguments issue

Question

I am writing a simple python script under Linux to do a batch of concurrent single pings to hosts on my subnet using subprocess.

Manually the command works: ping -c 1 192.168.68.1

The script::

#!/usr/bin/python
from subprocess import call

path = "ping"
flags = "-c 1 "

for i in range(1,3):
    ip_addr = "192.168.68." + str(i)
    args = flags + ip_addr

    print "doing: {} {}".format(path, args)        
    call([path, args])

Commenting out call outputs the expected:

doing: ping -c 1 192.168.68.1
doing: ping -c 1 192.168.68.2

With call(), the script seems to call ping but with unknown arguments. Output as follows:

doing: ping -c 1 192.168.68.1
Usage: ping [-aAbBdDfhLnOqrRUvV64] [-c count] [-i interval] [-I interface]
        [-m mark] [-M pmtudisc_option] [-l preload] [-p pattern] [-Q tos]
        [-s packetsize] [-S sndbuf] [-t ttl] [-T timestamp_option]
        [-w deadline] [-W timeout] [hop1 ...] destination
Usage: ping -6 [-aAbBdDfhLnOqrRUvV] [-c count] [-i interval] [-I interface]
         [-l preload] [-m mark] [-M pmtudisc_option]
         [-N nodeinfo_option] [-p pattern] [-Q tclass] [-s packetsize]
         [-S sndbuf] [-t ttl] [-T timestamp_option] [-w deadline]
         [-W timeout] destination
doing: ping -c 1 192.168.68.2
Usage: ping [-aAbBdDfhLnOqrRUvV64] [-c count] [-i interval] [-I interface]
        [-m mark] [-M pmtudisc_option] [-l preload] [-p pattern] [-Q tos]
        [-s packetsize] [-S sndbuf] [-t ttl] [-T timestamp_option]
        [-w deadline] [-W timeout] [hop1 ...] destination
Usage: ping -6 [-aAbBdDfhLnOqrRUvV] [-c count] [-i interval] [-I interface]
         [-l preload] [-m mark] [-M pmtudisc_option]
         [-N nodeinfo_option] [-p pattern] [-Q tclass] [-s packetsize]
         [-S sndbuf] [-t ttl] [-T timestamp_option] [-w deadline]
         [-W timeout] destination

It seems ping is being called twice for each iteration with different args to what I expect. I note the -6 argument ping complains about.
I would appreciate any help, particularly with regard to using call() properly.

Update:
Added bash script I'm attempting to emulate:

#!/bin/bash
for ip in $(seq 1 2); do
    ping -c 1 192.168.68.$ip &
done

call([path, flags.strip(), ip_addr])

Ozgur Vatansever
– Ozgur Vatansever

2016-05-22 05:47:46 +00:00
Commented May 22, 2016 at 5:47 — Ozgur Vatansever
– Ozgur Vatansever, Commented May 22, 2016 at 5:47

Pablo · Accepted Answer · 2016-05-22 05:52:38Z

3

The arguments passed to the function call should each be a single element in a list, as such:

call(['ping','-c','1','192.168.68.2'])

I could not reproduce the 'double printing' in my environment.

answered May 22, 2016 at 5:52

Pablo

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5 Comments

user1330734 Over a year ago

Hi Pablo, I have tried flags = ["-c", "1"] and then tried unpacking in the call() call([path, *flags, ip_addr]) I get syntax error. Is it not possible to unpack within the array?

jfs Over a year ago

@user1330734: it is Python 3.5+ syntax.

user1330734 Over a year ago

@J.F. Sebastian Interestingly, the python code seem to be synchronous i.e. each ping call seems to wait for previous to complete. In contrast, the forking bash script I added above takes about 5 seconds to do IP range 1-254.

jfs Over a year ago

@user1330734 read the docs for subprocess.call(cmd). Think: what is the difference between subprocess.Popen(cmd)` and subprocess.Popen(cmd).wait()` and which one is closer to subprocess.call(cmd).

user1330734 Over a year ago

@J.F. Sebastian Thank you kindly for pointing me to the right place. In order to achieve concurrency, I need to call subprocess.Popen(cmd). subprocess.call(cmd) seems closer to subprocess.Popen.wait(). The python script now executes as expected, cheers.

Samuel · Accepted Answer · 2016-05-22 06:39:21Z

0

As @Pablo mentioned flagsand path variables should be a list elements, so before calling the call method just convert your command string to list:

path = "ping"
flags = "-c 1"
cmd_list = [path] # convert string to list
cmd_list = cmd_list + flags.split(' ') # converting your flags string to list elements

call(cmd_list)

edited May 22, 2016 at 6:39

answered May 22, 2016 at 6:33

Samuel

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