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Introduction to Bash Scripting

This module covers the basics needed for working with Bash scripts to automate tasks on Linux systems. A strong grasp of Bash is a fundamental skill for anyone working in a technical information security role. Through the power of automation, we can unlock the Linux operating system's full potential and efficiently perform habitual tasks.

4.38

Created by Cry0l1t3

Easy General

Summary

In this module, we will cover:

  • An introduction to Bash scripting
  • Conditional execution
  • Working with arguments, variables, and arrays
  • Comparison operators, arithmetic, and input/output control
  • Loops, branches, and functions
  • Techniques for debugging Bash scripts

This module is broken down into sections with accompanying hands-on exercises to practice each of the tactics and techniques we cover. The module ends with a practical hands-on skills assessment to gauge your understanding of the various topic areas.

As you work through the module, you will see example commands and command output for the various topics introduced. It is worth reproducing as many of these examples as possible to reinforce further the concepts introduced in each section. You can do this in the Pwnbox provided in the interactive sections or your own virtual machine.

You can start and stop the module at any time and pick up where you left off. There is no time limit or "grading," but you must complete all of the exercises and the skills assessment to receive the maximum number of cubes and have this module marked as complete in any paths you have chosen.

The module is classified as "Easy" but assumes a working knowledge of the Linux command line and an understanding of information security fundamentals.

A firm grasp of the following modules can be considered prerequisites for successful completion of this module:

  • Linux Fundamentals
  • Web Requests

Bourne Again Shell


Bash is the scripting language we use to communicate with Unix-based OS and give commands to the system. Since May 2019, Windows provides a Windows Subsystem for Linux that allows us to use Bash in a Windows environment. It is essential to master the language to work efficiently with it. The main difference between scripting and programming languages is that we don't need to compile the code to execute the scripting language, as opposed to programming languages.

As penetration testers, we must be able to work with any operating system, whether it is Windows or Unix-based. Efficiency depends mainly on the knowledge of the systems, especially in the privilege escalation field. On Unix-based systems, it is essential to learn how to use the terminal, filter data, and automate these processes. Especially in large Unix-based enterprise networks, we will have to deal with large amounts of data. We have to sort and filter out accordingly to determine potential gaps and information as fast as possible.

It is also essential to learn how to combine several commands and work with individual results. This is where scripting comes in, increasing our speed and efficiency. Like a programming language, a scripting language has almost the same structure, which can be divided into:

  • Input & Output
  • Arguments, Variables & Arrays
  • Conditional execution
  • Arithmetic
  • Loops
  • Comparison operators
  • Functions

It is often common to automate some processes not to repeat them all the time or process and filter a large amount of information. In general, a script does not create a process, but it is executed by the interpreter that executes the script, in this case, the Bash. To execute a script, we have to specify the interpreter and tell it which script it should process. Such a call looks like this:

Script Execution - Examples

[!bash!]$ bash script.sh <optional arguments>
[!bash!]$ sh script.sh <optional arguments>
[!bash!]$ ./script.sh <optional arguments>

Let us look at such a script and see how they can be created to get specific results. If we execute this script and specify a domain, we see what information this script provides.

CIDR.sh

[!bash!]$ ./CIDR.sh inlanefreight.com

Discovered IP address(es):
165.22.119.202

Additional options available:
	1) Identify the corresponding network range of target domain.
	2) Ping discovered hosts.
	3) All checks.
	*) Exit.

Select your option: 3

NetRange for 165.22.119.202:
NetRange:       165.22.0.0 - 165.22.255.255
CIDR:           165.22.0.0/16

Pinging host(s):
165.22.119.202 is up.

1 out of 1 hosts are up.

Now let us look at that script in detail and read it line by line in the best possible way. In the next sections, we will look at and analyze all the parts of this script.

CIDR.sh

#!/bin/bash

# Check for given arguments
if [ $# -eq 0 ]
then
	echo -e "You need to specify the target domain.\n"
	echo -e "Usage:"
	echo -e "\t$0 <domain>"
	exit 1
else
	domain=$1
fi

# Identify Network range for the specified IP address(es)
function network_range {
	for ip in $ipaddr
	do
		netrange=$(whois $ip | grep "NetRange\|CIDR" | tee -a CIDR.txt)
		cidr=$(whois $ip | grep "CIDR" | awk '{print $2}')
		cidr_ips=$(prips $cidr)
		echo -e "\nNetRange for $ip:"
		echo -e "$netrange"
	done
}

# Ping discovered IP address(es)
function ping_host {
	hosts_up=0
	hosts_total=0
	
	echo -e "\nPinging host(s):"
	for host in $cidr_ips
	do
		stat=1
		while [ $stat -eq 1 ]
		do
			ping -c 2 $host > /dev/null 2>&1
			if [ $? -eq 0 ]
			then
				echo "$host is up."
				((stat--))
				((hosts_up++))
				((hosts_total++))
			else
				echo "$host is down."
				((stat--))
				((hosts_total++))
			fi
		done
	done
	
	echo -e "\n$hosts_up out of $hosts_total hosts are up."
}

# Identify IP address of the specified domain
hosts=$(host $domain | grep "has address" | cut -d" " -f4 | tee discovered_hosts.txt)

echo -e "Discovered IP address:\n$hosts\n"
ipaddr=$(host $domain | grep "has address" | cut -d" " -f4 | tr "\n" " ")

# Available options
echo -e "Additional options available:"
echo -e "\t1) Identify the corresponding network range of target domain."
echo -e "\t2) Ping discovered hosts."
echo -e "\t3) All checks."
echo -e "\t*) Exit.\n"

read -p "Select your option: " opt

case $opt in
	"1") network_range ;;
	"2") ping_host ;;
	"3") network_range && ping_host ;;
	"*") exit 0 ;;
esac

As we can see, we have commented here several parts of the script into which we can split it.

  1. Check for given arguments
  2. Identify network range for the specified IP address(es)
  3. Ping discovered IP address(es)
  4. Identify IP address(es) of the specified domain
  5. Available options

1. Check for given arguments

In the first part of the script, we have an if-else statement that checks if we have specified a domain representing the target company.


2. Identify network range for the specified IP address(es)

Here we have created a function that makes a "whois" query for each IP address and displays the line for the reserved network range, and stores it in the CIDR.txt.


3. Ping discovered IP address(es)

This additional function is used to check if the found hosts are reachable with the respective IP addresses. With the For-Loop, we ping every IP address in the network range and count the results.


4. Identify IP address(es) of the specified domain

As the first step in this script, we identify the IPv4 address of the domain returned to us.


5. Available Options

Then we decide which functions we want to use to find out more information about the infrastructure.

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Relevant Paths

This module progresses you towards the following Paths

Information Security Foundations

Information Security is a field with many specialized and highly technical disciplines. Job roles like Penetration Tester & Information Security Analyst require a solid technical foundational understanding of core IT & Information Security topics. This skill path is made up of modules that will assist learners in developing &/or strengthening a foundational understanding before proceeding with learning the more complex security topics. Every long-standing building first needs a solid foundation. Welcome to Information Security Foundations.

Easy Path Sections 191 Sections
Required: 200
Reward: +120
Path Modules
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This module covers the fundamentals required to work comfortably with the Windows operating system.
Easy
Path Sections 23 Sections
Reward: +10
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Easy
Path Sections 10 Sections
Reward: +10
This module covers the basics needed for working with Bash scripts to automate tasks on Linux systems. A strong grasp of Bash is a fundamental skill for anyone working in a technical information security role. Through the power of automation, we can unlock the Linux operating system's full potential and efficiently perform habitual tasks.
Fundamental
Path Sections 21 Sections
Reward: +10
As an information security professional, a firm grasp of networking fundamentals and the required components is necessary. Without a strong foundation in networking, it will be tough to progress in any area of information security. Understanding how a network is structured and how the communication between the individual hosts and servers takes place using the various protocols allows us to understand the entire network structure and its network traffic in detail and how different communication standards are handled. This knowledge is essential to create our tools and to interact with the protocols.
Medium
Path Sections 15 Sections
Reward: +10
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Path Sections 16 Sections
Reward: +10
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Fundamental
Path Sections 17 Sections
Reward: +10
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Path Sections 8 Sections
Reward: +10
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SOC Analyst Prerequisites

The SOC Analyst Prerequisites path is designed for those looking to become SOC/Security Analysts. It dives into fundamental IT and Information Security subjects including networking, Linux and Windows operating systems, basic programming and scripting, as well as working with Assembly. In addition, students will be exposed to the fundamental concepts of information security and penetration testing. This skill path is made up of modules that will assist learners in developing and strengthening a foundational understanding before proceeding with learning more complex security topics.

Easy Path Sections 173 Sections
Required: 350
Reward: +110
Path Modules
Fundamental
Path Sections 30 Sections
Reward: +10
This module covers the fundamentals required to work comfortably with the Linux operating system and shell.
Easy
Path Sections 10 Sections
Reward: +10
This module covers the basics needed for working with Bash scripts to automate tasks on Linux systems. A strong grasp of Bash is a fundamental skill for anyone working in a technical information security role. Through the power of automation, we can unlock the Linux operating system's full potential and efficiently perform habitual tasks.
Fundamental
Path Sections 14 Sections
Reward: +10
This module covers the fundamentals required to work comfortably with the Windows operating system.
Easy
Path Sections 23 Sections
Reward: +10
As administrators and Pentesters, we may not always be able to utilize a graphical user interface for the actions we need to perform. Introduction to Windows Command Line aims to introduce students to the wide range of uses for Command Prompt and PowerShell within a Windows environment. We will cover basic usage of both key executables for administration, useful PowerShell cmdlets and modules, and different ways to leverage these tools to our benefit.
Fundamental
Path Sections 21 Sections
Reward: +10
As an information security professional, a firm grasp of networking fundamentals and the required components is necessary. Without a strong foundation in networking, it will be tough to progress in any area of information security. Understanding how a network is structured and how the communication between the individual hosts and servers takes place using the various protocols allows us to understand the entire network structure and its network traffic in detail and how different communication standards are handled. This knowledge is essential to create our tools and to interact with the protocols.
Fundamental
Path Sections 16 Sections
Reward: +10
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Reward: +10
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