Question

Consider the following examples –

Python 2.4.3 (#1, Jan 14 2011, 00:20:04)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.system("grep -i beatles blur.txt")
Blur's debut album Leisure (1991) incorporated the sounds of Madchester and shoegazing. Following a stylistic change.influenced by English guitar pop groups such as The Kinks, The Beatles and XTC.
0
>>> os.system("grep -i metallica blur.txt")
256
>>>

So, in this case, I don’t want the line having my searched keyword to be printed on the Python shell, I just want the return value i.e. 0 if keyword is present and non-zero if its not. How to achieve that?

Answer 1

You just need to use -q key of the grep:

$ python
Python 2.7.3rc2 (default, Apr  5 2012, 18:58:12) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.system("grep -iq igor /etc/passwd")
0
>>> os.system("grep -iq oleg /etc/passwd")
256
>>> 

I must note that -q is not portable key of the grep, it will work only with GNU grep (Linux and so on).

When you want to make it work on all systems, you must use popen/subprocess.Popen and redirections of streams.

>>> import subprocess
>>> null = open(os.devnull, "w")
>>> grep = subprocess.Popen(shlex.split("grep -i oleg /etc/passwd"), stderr = null, stdout = null)
>>> grep.communicate()
(None, None)
>>> print grep.returncode
1
>>> grep = subprocess.Popen(shlex.split("grep -i igor /etc/passwd"), stderr = null, stdout = null)
>>> grep.communicate()
(None, None)
>>> print grep.returncode
0