Introduction

I am currently finishing my last semester of university. My final computer science elective I am taking is Linux Administration. It is a fairly easy and straightforward class, but the reason I am taking this class is to learn more about the Linux Operating System, specifically when it comes to system administration.

One of the topics I wanted to learn about is bash scripting. I have seen many bash scripts online, but I have never taken the time to learn how to read or write them. We did have two lectures about bash scripting with their own lab, which is great, but I wanted more experience with bash scripts.

Halfway through the semester, we transition into the virtual machine portion of the class, where we get to be sysadmins of our own virtual Linux distributions. Most of the students, including me, use the school computers, which are connected together in the same network. You can also SSH from the school computer into your virtual machine if you set the connection type to bridge. Because of this, I can SSH not only into my virtual machine, but also other students’ machines. A mischevious idea brewed in my head that night.

After one long coding session late at night, and a couple rewrites and modifications, I finally had an automated shell script that will SSH into every students’ virtual machine and place a text file on their desktop – a signature from me proclaiming the time and date I “hacked” into their system.

The Bash Script

Here is the bash script:

#!/bin/bash

# The purpose of this script is to ssh into VMs on the network and place a harmless text file on their desktop

OS=$2

show_help() {
        echo "Usage: sshtxtmsg [file] [operating-system]"
        echo "  File must be formated: Last,First"
        echo "  Must have 'sshpass' installed!"
}

ssh() {
        LNAME=`echo $1 | egrep -o '^[[:alpha:]]*'`
        #echo "$LNAME"
        FNAME=`echo $1 | egrep -o '[[:alpha:]]*$'`
        #echo "$FNAME"
        THREEF=`echo $FNAME | egrep -o '^[[:alpha:]]{1,3}'`
        #echo "$THREEF"
        THREEL=`echo $LNAME | egrep -o '^[[:alpha:]]{1,3}'`
        #echo "$THREEL"
        HNAME="${OS}-${THREEF}${THREEL}"
        #echo "$HNAME"
        USERNAME=`echo $FNAME | tr [:upper:] [:lower:]`
        #echo "$USERNAME"
        COMMAND="${USERNAME}@${HNAME}"
        #CMD='sshpass -p MP2650 ssh -o StrictHostKeyChecking=no -q -l root $HNAME "echo "EVNMRK WAS HERE ON " > ~/Desktop/EVNMRK-WAS-HERE.txt; date >> ~/Desktop/EVNMRK-WAS-HERE.txt"'
        CMD='sshpass -p cmps2650 ssh -o StrictHostKeyChecking=no -q -l $USERNAME $HNAME "echo "EVNMRK WAS HERE ON " > ~/Desktop/EVNMRK-WAS-HERE.txt; date >> ~/Desktop/EVNMRK-WAS-HERE.txt"'
        #CMD1="echo '$COMMAND'"
        eval "$CMD"
        if [ $? -eq 0 ]; then
                echo "SUCCESS for $COMMAND - $1"
        elif [ $? -eq 1 ]; then
                echo "FAILURE for $COMMAND"
        else
                echo "UNKNOWN ERROR"
        fi
}

# MAIN

if [ $# -lt 2 ]; then
        show_help
        exit 1
fi
for name in `cat $1`; do
        ssh "$name"
done

Usage

Usage: sshtxtmsg [file] [operating-system]
	File must be formated: Last,First
	Must have 'sshpass' installed!

How It Works

The script requires a text file with every students’ name in the format of “Last,First” as the first argument. Each name is separated by a newline. The script also requires the name of the operating system you are connecting to as the second argument. The reason for this is because of how we set up the virtual machines as a class. The instructor had us follow instructions on how to set up our virtual machines. Part of this instructions is what to put for the hostname, username, password, and root password. These fields are determined by the students’ first and last name, which is why we need the student roster. The hostname field has the operating system’s name inside, too.

• Example Debian hostname for student “Dale Cooper”
  • debian-dalcoo
• Example username
  • dale
• Example ssh command
  • ssh dale@debian-dalcoo

Using regex, we can splice the first and last name to create a username and hostname. But we run into a problem: ssh cannot be automated, since it queries for a password. To solve this, we use the very unsecure sshpass, which can hold the password in a flag. The password of the user is the same for everyone: the classname. We also add the StrictHostKeyChecking=no flag to ignore asking to remember the ssh fingerprint for 100% automation. Here is that command:

sshpass -p cmps2650 ssh -o StrictHostKeyChecking=no -q -l $USERNAME $HNAME

We can append the commands to run once a connection is made to the end of the sshpass command. We just run an echo command for our text and redirect it into a new textfile. Then we append the datetime the same way.

"EVNMRK WAS HERE ON " > ~/Desktop/EVNMRK-WAS-HERE.txt; date >> ~/Desktop/EVNMRK-WAS-HERE.txt"

The script will do the previous commands for every student entry, outputting a success or failure.

Known Issues

This script will not work for users who choose their own unique hostname, username, or password. It will also not work for users who install their virtual machine on their own laptop. Basically it doesn’t work on students who are lazy, cannot RTFM, or are mavericks.

Conclusion

Out of the many times I ran this script, I only had three successful connections.

I also have a version of this script that will run in an infinite loop, but let’s not get into any trouble with that…

Use this script however you like, feel free to modify for your own use.