file read/write Popen python

Question

I'm trying to Read/Write process file with command grep and cut to get part of the IP address using pipe operator, here is the process simply written.

import subprocess as sub
import shlex

    def discover_Host():    
     with open("Get_IP.txt", "w+") as q_process:
       cmd1 = "grep Host xmap_ip.gnmap"
       cmd2 = "cut -d ' ' -f 2"
       arg1 = shlex.split(cmd1)
       arg2 = shlex.split(cmd2)
    
       proc1 = sub.Popen(args1, stdout = sub.PIPE)
       proc2 = sub.Popen(args2, stdin = proc1.stdout, stdout = q_process)
    
       proc1.stdout.close()
       out, err = proc2.communicate()
    return

Now the file Get_IP.txt contains data like this Host: 172:22:179:156 (abc.efg.net) Status: Up so I'm trying to get only IP address from it as I run the grep and cut command with pipe directly on the terminal it works but as I try it with SubProcess it doesn't write on the file.

Why are you using `grep` and `cut` when Python has a built-in operations for this? — Barmar, May 26 '22 at 19:57
@ahmed That's not needed, he used `shlex.split()` to split it into a list. — Barmar, May 26 '22 at 19:58
What was the point of `proc1.stdout.close()`? proc1 probably hasn't finished (or perhaps even *started* yet) at that point in time. — jasonharper, May 26 '22 at 20:11

score 3 · Answer 1 · answered May 26 '22 at 20:03

There's no reason to use shell commands for this, it can be done entirely in Python.

def discover_Host():
    with open('xmap_ip.gnmap') as xmap, open("Get_IP.txt", "w") as outfile:
        for line in xmap:
            if 'Host' in line:
                field2 = line.split(' ')[1]
                print(field2, file=outfile)

score 2 · Answer 2 · answered May 26 '22 at 20:00

2

You can combine both commands in one call, but you must pass shell to sub.Popen:

   cmd1 = "grep Host xmap_ip.gnmap | cut -d ' ' -f 2"
   proc1 = sub.Popen(cmd1, shell=True)

answered May 26 '22 at 20:00

ahmed

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strange your command does work for `echo 'hello world'` and writes on the file but for `grep Host xmap_ip.gnmap | cut -d ' ' -f 2` it does not write and I assume it does not read as well. – May 26 '22 at 20:19
found out the root cause it was due to method being called synchronously which created the file `xmap_ip.gnmap` and was trying to attempt and read the line using `grep` so in order to halt the process I used the `proc0.wait()` on first method before calling the `cmd1 = "grep Host xmap_ip.gnmap | cut -d ' ' -f 2"` second method. – May 26 '22 at 20:37

file read/write Popen python

2 Answers2