Taking the results of a bash command and using it in python

Question

I am trying to write a code in python that will take some information from top and put it into a file. I want to just write the name of the application and generate the file. The problem i am having is that i can't get the output of the pidof command so i can use it in python. My code looks like this :

import os

a = input('Name of the application')
val=os.system('pidof ' + str(a)) 
os.system('top -d 30 | grep' + str(val) + '> test.txt')
os.system('awk '{print $10, $11}' test.txt > test2.txt')

The problem is that val always has 0 but the command is returning the pid i want. Any input would be great.

You have the name of the application and you want... what? Write something concerning this application into a file? — eumiro
– eumiro, Commented Apr 21, 2011 at 13:56

John Kugelman · Accepted Answer · 2011-04-21 14:15:10Z

8

First up, the use of input() is discouraged as it expects the user to type in valid Python expressions. Use raw_input() instead:

app = raw_input('Name of the application: ')

Next up, the return value from system('pidof') isn't the PID, it's the exit code from the pidof command, i.e. zero on success, non-zero on failure. You want to capture the output of pidof.

import subprocess

# Python 2.7 only
pid = int(subprocess.check_output(['pidof', app]))

# Python 2.4+
pid = int(subprocess.Popen(['pidof', app], stdout=subprocess.PIPE).communicate()[0])

# Older (deprecated)
pid = int(os.popen('pidof ' + app).read())

The next line is missing a space after the grep and would have resulted in a command like grep1234. Using the string formatting operator % will make this a little easier to spot:

os.system('top -d 30 | grep %d > test.txt' % (pid))

The third line is badly quoted and should have caused a syntax error. Watch out for the single quotes inside of single quotes.

os.system("awk '{print $10, $11}' test.txt > test2.txt")

edited Apr 21, 2011 at 14:15

answered Apr 21, 2011 at 14:00

John Kugelman

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3 Comments

ninjagecko Over a year ago

This is the answer you want. You can abstract it into a function if you desire. However note that you may possibly need to call .strip() to remove newlines on the output: .communicate()[0].strip() -- you don't need to in this case, since int('12\n')==12

Florin Dita Over a year ago

Thank you for the solution it works now. You help me a lot and again thank you for your fast time responce. I am using python3 and the raw_input it's not supported or i don't know if has a new form.

Xavier Combelle Over a year ago

the equivalent in python3 of raw_input is input there is no equivalent of input of 2.X

ChrisR · Accepted Answer · 2011-04-21 13:53:50Z

2

Instead of os.system, I recommend you to use the subprocess module: http://docs.python.org/library/subprocess.html#module-subprocess

With that module, you can communicate (input and output) with a shell. The documentation explains the details of how to use it.

Hope this helps!

answered Apr 21, 2011 at 13:53

ChrisR

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