Unable to SSH via Python subprocess

Question

I need to ssh into a machine via a bastion. Therefore the command is rather very long for this:

ssh -i  <pemfile location> -A -o 'proxycommand ssh -i <pemfile location> ec2-user@<bastion ip address> -W %h:%p' hadoop@<machine ip>

This command is rather very long. So I tried to write a python script which takes ip addresses and pemfile location as inputs and does ssh.

#!/usr/local/bin/python3
import argparse
import subprocess
import os
import sys
import errno

parser = argparse.ArgumentParser(description="Tool to ssh into EMR via a bastion host")
parser.add_argument('master', type=str, help='IP Address of the EMR master-node')
parser.add_argument('bastion', type=str, help='IP Address of bastion EC2 instance')
parser.add_argument('pemfile', type=str, help='Path to the pemfile')

args = parser.parse_args()

cmd_list = ["ssh", "-i", args.pemfile, "-A", "-o", "'proxycommand ssh -i {} ec2-user@{} -W %h:%p'".format(args.pemfile, args.bastion), "hadoop@{}".format(args.master)]

command = ""

for w in cmd_list:
    command = command + " " + w

print("")
print("Executing command : ", command)
print("")

subprocess.call(cmd_list)

I get the following error when I run this script :

command-line: line 0: Bad configuration option: 'proxycommand

But I am able to run the exact command via bash.

Why is the ssh from python script failing then?

Answer 1

You are making the (common) mistake of mixing syntactic quotes with literal quotes. At the command line, the shell removes any quotes before passing the string to the command you are running; you should simply do the same.

cmd_list = ["ssh", "-i", args.pemfile, "-A",
    "-o", "proxycommand ssh -i {} ec2-user@{} -W %h:%p".format(
        args.pemfile, args.bastion), "hadoop@{}".format(args.master)]

See also When to wrap quotes around a shell variable? for a discussion of how quoting works in the shell, and perhaps Actual meaning of 'shell=True' in subprocess as a starting point for the Python side.

However, scripting interactive SSH sessions is going to be brittle; I recommend you look into a proper Python library like Paramiko for this sort of thing.