在python中执行管道外壳命令

Question

I need help with python code in executing multiple piped shell commands in python. I have written the following code but I am getting error. As I am passing the file to the command. Please let me know the correct process on how to execute multiple piped commands in python.

EG: cat file|grep -i hostname|grep -i fcid 

is the shell command I want to execute. Here is my python code. I am getting None when I run the code.I am redirecting the final output to a file.

#!/usr/bin/python3

import subprocess

op = open("text.txt",'w')
file="rtp-gw1"

print("file name is {}".format(file))
#cat file|grep -i "rtp1-VIF"|grep -i "fcid"

#cmd ='cat file|grep -i "rtp1-vif"'
p1  = subprocess.Popen(['cat',file],stdout=subprocess.PIPE, stderr=subprocess.PIPE,shell=True)
p2  = subprocess.Popen(['grep','-i', '"rtp1-vif"',file], stdin=p1.stdout, stdout=subprocess.PIPE,stderr= subprocess.PIPE,shell=Ture)
p1.stdout.close()
p3  = subprocess.Popen(['grep', '-i',"fcid"],stdin=p1.stdout, stdout=op,stderr= subprocess.PIPE,shell=Ture)
p2.stdout.close()

result = p3.communicate()[0]

print(result)

Answer 1

you're not getting any result because of this line:

['grep','-i', '"rtp1-vif"']

this is passing "rtp1-vif" literally with quotes: no match.

note that the whole piping process is overkill. No need to use cat as the first grep can take the file as input.

And to go further, it's really easy to perform that task in pure python. Something like:

with open("text.txt",'w') as op:
    file="rtp-gw1"

    for line in file:
        line = line.lower()
        if "rtp1-vif" in line and "fcid" in line:
           op.write(line)

now your code can run on any platform, without the need to issue external commands. It's probably faster too.

Answer 2

Thank you. That how I started I have a files with more than 100,000+ lines. I have to find lines missing between each file. I am not getting the results I want. That is reason why I wanted to try system commands but will definitely try while loop.