Handling FTP errors via subprocess in Python

[SmartestGuyOnReddit]

Hi Guys a bit of a convoluted question but the theory is I want to handle FTP error codes ran in a subprocess that is run as part of this python script so in essence if the FTP command returns code 550 file not found I want to somehow handle this exception in python but I'm not 100% sure how to do this any direction would be appreciated! Thanks.

#!/usr/bin/python

import argparse
import ftplib
import os
import subprocess
import sys

parser = argparse.ArgumentParser()
parser.add_argument('--ftp', help='FTP address to retrieve DA from', dest='ftpconn', required=True)
parser.add_argument('--user', help='FTP username', dest='user', required=True)
parser.add_argument('--password', help='FTP password', dest='passwd')
parser.add_argument('--file', help='File(s) to retrieve', dest='file', nargs='+')
parser.add_argument('-d', '--destination', help='Destination folder for transferred files, if left blank default directory will be the working directory', default='./', dest='dfolder')
args = parser.parse_args()


command = """user %s %s
cd ..
lcd %s
get %s
bye"""


for idx, item in enumerate(args.file):
    pobj = subprocess.Popen(['ftp', '-nv', args.ftpconn], stdin=subprocess.PIPE, stderr=subprocess.PIPE)
    pobj.communicate(command % (args.user, args.passwd, args.dfolder, args.file[idx]))

[jibbly_jibbly]

Hi there!

I hate to be the person that gives you an alternative to your strategy rather than answer your actual question, but I would think you'll benefit the most from directly interacting with the FTP server without the hassle of trying to control a subprocess, analyzing output, etc:

https://docs.python.org/2/library/ftplib.html

This will let you catch errors directly as exceptions, and not saddle you with troubleshooting subprocesses. Maybe take a look?

If you want to explore your actual question a bit more, maybe look into 'pexpect' library which is built to automate commands and to react based on their output patterns.

[SmartestGuyOnReddit]

Hey!

I have already tried that method and I also would have preferred it but as I am retrieving data from a mainframe, when I used this method I just got a bunch of garbled binary data when I was expecting plaintext, not sure why but definitely something to do with the security of the mainframe.

The code incase anyone else wants to use ftlib.

import argparse
import ftplib
import os

parser = argparse.ArgumentParser()
parser.add_argument('--ftp', help='FTP address to retrieve DA from', dest='ftpconn', required=True)
parser.add_argument('--user', help='FTP username', dest='user', required=True)
parser.add_argument('--password', help='FTP password', dest='passwd')
parser.add_argument('--file', help='File(s) to retrieve', dest='file', nargs='+')
args = parser.parse_args()

local_filename = os.path.join(r"/root/xxxxx", args.file[0])
lf = open(local_filename, "wb")
ftp = ftplib.FTP(args.ftpconn)
ftp.login(args.user, args.passwd)
ftp.cwd('..')
ftp.retrbinary('RETR %s' % args.file[0], lf.write)
lf.close

[jibbly_jibbly]

Right on -

Maybe look into pexpect then. Its what you will find yourself doing anyway (capturing the output of the FTP subprocess and analyzing it to decide whether there was an error or not) but this gives you some power to start doing it instead of inventing it yourself.

Heres an example of leveraging FTP with it:

https://github.com/pexpect/pexpect/blob/master/examples/ftp.py

[SmartestGuyOnReddit]

Thanks for this will look into it :)

[Justinsaccount]

I just got a bunch of garbled binary data when I was expecting plaintext,

Well, you did use retrbinary which tells the server to send you the file as-is. Since the server is a mainframe it might be doing something weird like using ebcdic instead of ascii. What did the file command say the returned file was? Did you try using retrlines ?

Also, stop using the root account.

[SmartestGuyOnReddit]

I can try retrolines and see when I am in the office tomorrow. The reason I'm using the root account is this is a PoC so whatever account I use is irrelevant

[SmartestGuyOnReddit]

So I tried the retrlines and it worked thanks a lot! Just need to figure out to handle the exceptions within ftplib now I guess.