Notes: Linux Shells | Cyber Security 101 | THM

Writing commands in CLI - more efficient and resource-friendly

How To Interact With a Shell

When interacting with a shell, you must be in the directory where you want to perform operations.

Default = your home directory

Commands

pwd: (Print Working Directory) current working directory

cd: (Change Directory) change directory

ls: show contents of a directory

cat: show contents of a file

grep: search for word or pattern inside a file

Example: grep THM dictionary.txt (searching for THM in dictionary.txt)

Types of Linux Shells

All the different types of shells have their own features and characteristics

Multiple shells are installed in different Linux distributions

Commands

echo $SHELL: see which shell in use

/etc/shells: list all available shells in that Linux OS

Example: cat /etc/shells

Change shell by typing the shell name

Example: zsh

Change default shell: chsh -s

Example: chsh -s /usr/bin/zsh (sets zsh as default shell)

Different Shells

Bash (Bourne Again Shell)

Default shell for most Linux distributions.

Key features:

- widely used with scripting capabilities with extensive documentation available

- basic tab completion feature

- basic level of customization

- less user-friendly (but widely used = familiarity)

- syntax highlighting NOT available

- keeps a history file and logs all of your commands = use up/down arrow

- command: history = display all your previous commands

Fish (Friendly Interactive Shell)

Not installed by default in most Linux distributions.

Focus on user-friendliness

Key features:

- limited scripting features compared to Bash and Zsh

- advanced tab completion

- most user-friends shell - very simple syntax, good for beginner users

- syntax highlighting built-in - improved readability - different colours - helps spot errors

- auto spell correction for writing commands

- customize command prompt with different themes through interactive tools

- command history

Zsh (Z Shell)

Not installed by default in most Linux distributions.

Considered a modern shell.

Key features:

- excellent level of scripting

- advanced tab completion - extended by using plugins

- advanced customisation through oh-my-zsh framework - make it slower than other shells

- can be highly user-friendly with proper customization

- syntax highlighting can be used with plugins

- auto spell correction for commands

- tab completion, and command history

Selecting the best shell depends on your usage and the shell's features.

Shell Scripting and Components

A shell script = set of commands

Scripting helps to automate tasks.

.sh = default extension for bash scripts

create file: nano first:script.sh

Start every script with shebang

shebang = #! + name of interpreter to use while executing the script

Example: #!/bin/bash

Fundamental building blocks of a script (=an efficient script)

Variables

A variable stores a value inside

Then the variable name can be used wherever you need it

Example:

# Defining the Interpreter

#!/bin/bash

echo "Hey, what's your name?"

read name

echo "Welcome, $name"

echo - display the string "Hey, what's your name?"

read - takes input from the used

name - variable to store input from user

Give script execution permissions:

Example: chmod +x scriptname.sh

Use ./ before script name to execute the script in the current directory

(otherwise it will search for the script in the PATH environment variable)

Loops

loop = repeating something

EXAMPLE

# Defining the Interpreter

#!/bin/bash

for i in {1..10};

do

echo $i

done

this is a 'for loop'

i = variable

do = start loop

echo $i = output loop

done = end loop

Conditional Statements

Help execute a code only when a condition is met, otherwise run another code.

EXAMPLE

# Defining the Interpreter

#!/bin/bash

echo "Please enter your name first:"

read name

if [ "$name" = "Stewart" ]; then

echo "Welcome Stewart! Here is the secret: THM:Script"

else

echo "Sorry! You are not authorized to access the secret."

fi

variable = name

if = starts the conditional statement and compare the value of the variable with the string "Stewart"

- if a match = display secret

- if NOT a match = display "Sorry! You are not authorized to access the secret."

fi = end the 'for loop'

Comments

A comment = sentence explaining the following part of the code

Comments don't affect the running of the script

begin with # + space = '# '

Example: # Asking the user to enter a value

Best practice: place comments in major and complex areas of the script

The Locker Script

# Defining the Interpreter

#!/bin/bash

# Defining the variable

username=""

companymame=""

pin=""

# Defining the loop

for i in {1..3}; do

# Defining the conditional statements

if [ "$i" -eq 1 ]; then

echo "Enter your Username:"

read username

elif [ "$i -eq 2 ]; then

echo "Enter your Company name:"

read companyname

else

echo "Enter your PIN:"

read pin

fi

done

# Checking if the user entered the correct details

if [ "$username" = "John" ] && [ "$companyname" = "Tryhackme" ] && [ "$pin" = "7385" ]; then

echo "Authentication Successful. You can now access your locker, John."

else

echo "Authentication Denied!!"

fi

Practical Exercise

change user to root: sudo su

enter user's pw

__

whoami: see who I am

__

cat /var/log/authentication.log