run linux grep command from python subprocess

Question

I know there are posts already on how to use subprocess in python to run linux commands but I just cant get the syntax correct for this one. please help. This is the command I need to run...

/sbin/ifconfig eth1 | grep "inet addr" | awk -F: '{print $2}' | awk '{print $1}'

Ok this is what I have at the moment that gives a syntax error...

import subprocess
self.ip = subprocess.Popen([/sbin/ifconfig eth1 | grep "inet addr" | awk -F: '{print $2}' | awk '{print $1}'])

Any help greatly appreciated.

Running the useless grep in Python makes no sense at all. Python is superbly equipped to perform the field extraction natively. — tripleee
– tripleee, Commented Sep 22, 2015 at 9:19
@tripleee you don't need the external process at all here. Call get_ip_address('eth1') from How can i get the IP Address of eth0 in Python? — jfs
– jfs, Commented Sep 22, 2015 at 10:07
related: How do I use subprocess.Popen to connect multiple processes by pipes? — jfs
– jfs, Commented Sep 22, 2015 at 10:08

tripleee · Accepted Answer · 2015-09-22 09:33:34Z

1

This has been gone over many, many times before; but here is a simple pure Python replacement for the inefficient postprocessing.

from subprocess import Popen, PIPE
eth1 = subprocess.Popen(['/sbin/ifconfig', 'eth1'], stdout=PIPE)
out, err = eth1.communicate()
for line in out.split('\n'):
    line = line.lstrip()
    if line.startswith('inet addr:'):
        ip = line.split()[1][5:]

edited Sep 22, 2015 at 9:33

answered Sep 22, 2015 at 9:28

tripleee

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1 Comment

jfs Over a year ago

@whoopididoo: ip = re.search(br'inet addr:(\S+)', subprocess.check_output(['ifconfig', 'eth1'])).group(1)

Peter Sutton · Accepted Answer · 2015-09-23 11:17:32Z

0

Here's how to construct the pipe in Python (rather than reverting to Shell=True, which is more difficult to secure).

from subprocess import PIPE, Popen

# Do `which` to get correct paths
GREP_PATH = '/usr/bin/grep'
IFCONFIG_PATH = '/usr/bin/ifconfig'
AWK_PATH = '/usr/bin/awk'

awk2 = Popen([AWK_PATH, '{print $1}'], stdin=PIPE)
awk1 = Popen([AWK_PATH, '-F:', '{print $2}'], stdin=PIPE, stdout=awk2.stdin)
grep = Popen([GREP_PATH, 'inet addr'], stdin=PIPE, stdout=awk1.stdin)
ifconfig = Popen([IFCONFIG_PATH, 'eth1'], stdout=grep.stdin)

procs = [ifconfig, grep, awk1, awk2]

for proc in procs:
    print(proc)
    proc.wait()

It'd be better to do the string processing in Python using re. Do this to get the stdout of ifconfig.

from subprocess import check_output

stdout = check_output(['/usr/bin/ifconfig', 'eth1'])
print(stdout)

edited Sep 23, 2015 at 11:17

answered Sep 22, 2015 at 8:48

Peter Sutton

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4 Comments

tripleee Over a year ago

You are lacking the significant -F : option on awk1. It causes Awk to split the input on colons, instead of whitespace. Of course, the whole pipeline could be simply awk '/inet addr:/ { print substr($2, 6) }'

Peter Sutton Over a year ago

@tripleee I've added the -F: option.

jfs Over a year ago

you should close the pipes in the parent otherwise you may hang child processes. Or start the last command first and pass its stdin pipe to stdout of the preceding command.

jfs Over a year ago

@PeterSutton: you should get the output from awk2 (set stdout=PIPE, call communicate()). You still should close the pipes (call communicate() instead of wait(), to avoid relying on the garbage collection to free file descriptors in the parent).

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