How can I use grep in Python

Question

There is a file (query.txt) which has some keywords/phrases which are to be matched with other files using grep. The last three lines of the following code are working perfectly but when the same command is used inside the while loop it goes into an infinite loop or something(ie doesn't respond).

import os

f=open('query.txt','r')
b=f.readline()
while b:
    cmd='grep %s my2.txt'%b    #my2 is the file in which we are looking for b
    os.system(cmd)
    b=f.readline()
f.close()

a='He is'
cmd='grep %s my2.txt'%a
os.system(cmd)

Answer 1

First of all, you are not iterating over the file properly. You can simply use for b in f: without the .readline() stuff.

Then your code will blow in your face as soon as the filename contains any characters which have a special meaning in the shell. Use subprocess.call instead of os.system() and pass an argument list.

Here's a fixed version:

import os
import subprocess
with open('query.txt', 'r') as f:
    for line in f:
        line = line.rstrip() # remove trailing whitespace such as '\n'
        subprocess.call(['/bin/grep', line, 'my2.txt'])

---

However, you can improve your code even more by not calling grep at all. Read my2.txt to a string instead and then use the re module to perform the search. In case you do not need a regex at all, you can even simply use if line in my2_content

Hope it works!!

If you are a beginner and need to know more about Python, It's recommended to go for Python Certification course today.

Thanks!

Answer 2

Hi,

According to your description, you have to understand what is the use of grep command? How does grep command works? 

The grep command is used to search text or search the given file for lines containing a match to the given strings or words. By default, grep displays the matching lines. Use grep to search for lines of text that match one or many regular expressions, and outputs only the matching lines.

Answer 3

Hello, 

Here goes a short demonstration on How do I use grep search? Because I found our concerned root are quite similar so, this is a way usage of the grep command which is demonstrated by the following example.

 Use the -r "recursive" option to search a directory and all files included within it. search the "hello" string in all files and # subdirectories of the current directory grep -r "hello"

Comments

An excellent post, congratulations !!

Answer 4

Searching a file using grep will globally search for a regular expression and print the line.

Answer 5

The first part starts with grep , followed by the pattern that you are searching for. After the string comes the file name that the grep searches through. The command can contain many options, pattern variations, and file names. Combine as many options as necessary to get the results you need.