Grep and Python

Question

I need a way of searching a file using grep via a regular expression 'RE' from the unix command line. For example when i type in the command line:

python pythonfile.py 'RE' 'file-to-be-searched'

I need the regualr expression 'RE' to be searched in the file and print out the lines that contain the RE.

Ok so heres the coding i have:

import re
import sys

search_term = sys.argv[1]
f = sys.argv[2]

for line in open(f, 'r'):
    if re.search(search_term, line):
        print line,
        if line == None:
            print 'no matches found'

But when i enter a word which isnt present 'no matches found' doesnt print

Answer 1

1. use sys.argv to get the command-line parameters
2. use open(), read() to manipulate file
3. use the Python re module to match lines

Answer 2

The natural question is why not just use grep?! But assuming you can't ...

 import re
 import sys

 file = open(sys.argv[2], "r")

 for line in file:
        if re.search(sys.argv[1], line):
              print line,

Things to note:

• search instead of match to find anywhere in string
• comma , after print removes carriage return (line will have one)
• argv includes python file name - variables need to start at 1

This doesn't multiple arguments (like grep does) or expand wildcards (like the unix shell would). If you wanted this functionality you could get it using the following:

import re
import sys
import glob

for arg in sys.argv[2:]:
    for file in glob.iglob(arg):
     for line in open(file, 'r'):
      if re.search(sys.argv[1], line):
       print line,

Answer 3

adapted from a grep in python ...

accepts shell globs as filename argument ([2:]); no exception handling:

#!/usr/bin/env python
import re, sys, os

for f in filter(os.path.isfile, sys.argv[2:]):
    for line in open(f).readlines():
        if re.match(sys.argv[1], line):
            print line

sys.argv[1] resp sys.argv[2:] works, if you run it as an standalone executable, meaning

chmod +x

first

Answer 4

Here is a nice tutorial for the re module in Python: http://www.amk.ca/python/howto/regex/ .