Bash script to read and replace string(external ip) at end of ssh config

Question

Im in need of a script to get external ioaddress and replace at the end of an ssh config file

I have so far

#!/bin/sh
IP=$(wget http://ipecho.net/plain -qO-)

For a variable I can echo but need a way to replace curent external ip with new one in ssh config which looks like

Host $IP

    User UserName
    Port 22
    IdentityFile ~/.ssh/id_rsa

Host home

    HostName 192.168.0.1

Host away

    HostName 97.113.55.62

Away is external

So what I need is replace the external ip in my ssh config ex. HostName 192.168.0.1 (old ip) HostName 192.168.0.2 (new ip)

Answer 1

We also need to determine the OLDIP because that's what we replace:

OLDIP=`grep -w away -A 1 /etc/ssh/ssh_config | awk '/Hostname/ {print $2}'`

It is necessary that here the Hostname line is exactly below the Host away line otherwise you would have to adjust -A 1 to -A 2.

---

-w away matched the line where you have the word away

-A 1 shows one line after the line matched perviously

awk '/Hostname/ {print $2}' from those few lines previously matched we only keep the Hostname line and from that we only keep the second column.

---

Then we just do a sed to replace the OLDIP with IP.

sed -i "s/$OLDIP/$IP/g" /etc/ssh/ssh_config

The hole thing would look something like:

#!/bin/sh
IP=$(wget http://ipecho.net/plain -qO-)
OLDIP=`grep -w away -A 1 /etc/ssh/ssh_config | awk '/Hostname/ {print $2}'`
sed -i "s/$OLDIP/$IP/g" /etc/ssh/ssh_config

Answer 2

My use case is a bit different, because I wanted to get the new IP from an Amazon EC2 instance. Also I wrote the script in python rather than in bash. But I think you will find easy to understand, adapt and maintain:

#!/usr/bin/env python2
# -*- coding: utf-8 -*-

"""
Script to automatically update the SSH host name when
an AWS EC2 instance changes IP after reboot.

Call it like this:

    $ ~/repos/scripts/update_aws_hostname.py i-06bd23cd0514c2c7d windows

"""

# Built-in modules #
import sys, argparse
from os.path import expanduser

# Third party modules #
import sshconf, boto3

# Constants #
ec2 = boto3.client('ec2')

###############################################################################
class UpdateHostnameFromEC2(object):
    """A singleton. EC2 is provided by Amazon Web Services."""

    def __init__(self, instance_id, ssh_shortcut):
       self.instance_id  = instance_id
       self.ssh_shortcut = ssh_shortcut

    def __call__(self):
        # Read SSH config #
        self.config = sshconf.read_ssh_config(expanduser("~/.ssh/config"))
        # Get new DNS #
        self.response = ec2.describe_instances(InstanceIds=[self.instance_id])
        self.new_dns  = self.response['Reservations'][0]['Instances'][0]['PublicDnsName']
        self.tags     = self.response['Reservations'][0]['Instances'][0]['Tags']
        self.instance_name = [i['Value'] for i in self.tags if i['Key']=='Name'][0]
        # Substitute #
        self.config.set(self.ssh_shortcut, Hostname=self.new_dns)
        # Write SSH config #
        self.config.write(expanduser("~/.ssh/config"))
        # Success #
        message = "Updated the ssh config host '%s' for '%s'."
        message = message % (self.ssh_shortcut, self.instance_name)
        print(message)

###############################################################################
if __name__ == "__main__":
    # Parse the shell arguments #
    parser = argparse.ArgumentParser(description=sys.modules[__name__].__doc__)
    parser.add_argument("instance_id",  help="ID of the instance to update from EC2", type=str)
    parser.add_argument("ssh_shortcut", help="The ssh config host name (shortcut)",   type=str)
    args = parser.parse_args()

    # Create the singleton and run it #
    update_config = UpdateHostnameFromEC2(args.instance_id, args.ssh_shortcut)
    update_config()