In this article, you’ll get a basic understanding of Linux and its command prompt basic.

## What’s a Command Prompt?

Also called **Terminal **or Bash and is used to** execute commands**.

It is typically a black screen with white font and has a very simple User

Interface. You type in commands, and it interprets these commands and runs

operations on the computer

## SSH and How to do it?

SSH or “Secure Shell” is essentially a magic way to safely access unsafe

networks. This is commonly used to remotely log in to a computer and execute

commands.

To use SSH to remote login:

a. On Windows: Use an SSH client like PuTTY.

b. On Mac or Linux: In the terminal, type “ssh {user}@{host}”. User refers to the

account you want to access and the host is the domain or IP address of the

computer you are trying to access.

For a more in-depth understanding of SSH, Please comment.

Also Read : **How Hackers hack Phone Calls, Whatsapp to read messages without touching the user phone(SS7)**

## The Swiss Army Knife: Netcat!

Netcat is a tool that can help you read or write data over the internet and is

called “The Swiss Army Knife of Information Security” by its fans.

It earned its nickname because one can use Netcat to perform a lot of different

tasks including file transfer, chatting, port scanning and can even serve as both

a client and a server.

The basic syntax for Netcat commands is “nc [options] [destination] [port]”. Here,

a. Options is an optional argument or “flag” that you can use to change the

behavior of netcat. For example “nc -h” prints helpful information about

nc.

b. Destination, is the IP address of the computer you are trying to contact.

c. Port is the endpoint and helps identify the type of communication

happening

There are many different uses of Netcat and in general, you can get all the

information by typing in “nc -h” into your terminal window! If you want to go deeper inside it just comment down we will add more resources there.

## Fun With Terminal

Terminal has a bunch of useful commands that span a wide range of

functionalities. You can navigate between folders (aka directories) using cd

(“change directory”). You can test your net connection(s). You can create, delete

or even edit files from within Terminal! Here are a great list of everyday

(beginner) commands and even some more nuanced ones to help you do some

really cool things.

Warning! Be careful with the rm (“remove”) command. If you execute “rm -rf “

into your terminal, you can delete all your files! So be careful in the terminal.

You have a lot of power (and a lot of responsibility) so make sure you use the

correct command and don’t be afraid to use your favorite search engine

(Google, Bing, DuckDuckGo, etc.) to search for something if you are unsure!

Command-Line Text Editor? One particular section that might confuse some of

you from the article mentioned above is “Intermediate Command

3: nano, vi…”.

If you know what these are, awesome; you can begin your journey doing

everything in the terminal. However, if you think nano is an old iPod, then you

should reference the below section on text editors. For now, it is a text editor

like Notepad that you can access from your terminal.

Root Privileges! Another important command that usually confuses people is

“sudo” or “SuperUserDO”. Essentially, this command allows you to have root

privileges in your terminal. This is very risky as the root user has absolute

power over the system and you can essentially do anything and possibly cause a

lot of harm. But you can also do a lot of cool things so if you’re using sudo, make

sure you know what you are doing and that you’re careful.

We explain some more important commands at the end of this article.

## Who needs a Text Editor when you got Terminal?

There are many terminal text editors out there: Nano, Vim, Emacs, and many

more. They all have similar functionality but each programmer has their own reasons why one is clearly superior to the other. So, what is your choice of editor?

Executing in nano or vim in your terminal. If a file with the same name exists, you will open it and if it doesn’t, it will create a new file.

Here is a basic guide to using GNU Nano in the terminal!

## Numbers, Numbers, Numbers!

You are most probably familiar with the Base-10 Decimal system! However,

computers understand 1’s and 0’s or Binary (aka Base-2)! There are also other

systems, for example, Base-8 or Octal and Base-16 or Hexadecimal. In general, a

The Base-N system has digits from 0 to N-1. For example, Base-2 consists of 0 and 1

as digits.

Let’s do an in-depth analysis of some of the most important number systems.

## Binary (Base -2)

- Digits: 0, 1
- To convert Binary to Decimal:

a. Multiply each digit with 2^(position of that number)

b. Add the above for all digits

c. For example: (10011)

2 = (1*2 4 + 0*2

3 + 0*2 2 + 1*2

1 + 1*2

0

)

10 = (19)

10 - To convert Decimal to Binary:

a. Divide the number by 2

b. Keep track of the remainder

c. Divide the quotient by 2 and keep repeating till you get a quotient of 0

while keeping track of remainders

d. Read off the remainders in reverse order and that is your number in

Binary

e. For example: (6)

10 = 2(3) + 0, (3)

10 = 2(1) + 1, (1)

10 = 2(0) + 1. Therefore, (6)

10 =

(110)

2 - Addition:

a. In binary, there are 4 possibilities when adding 1 bit to another.

i. 0 + 0 = 0 with a carry of 0

ii. 0 + 1 = 1 with a carry of 0

iii. 1 + 0 = 1 with a carry of 0

iv. 1 + 1 = 0 with a carry of 1

b. Addition is commutative and works exactly the same way as in decimal.

Two’s Complement Representation: This is a form of representing Binary

numbers which makes it very easy to store both positive and negative numbers

and to find the binary representation of the negation of a given number.

a. The leftmost bit (most significant bit) is the “sign bit” which means that it

tells you whether the number is positive or negative. ‘1’ means that the

number is negative and ‘0’ means it is positive.

b. The rest of the digits represent the absolute value of the number in the

normal Base 2 way.

c. This means that given n-bits to represent a number, we can only use n-1

bits to represent the number. This leads to the largest positive number of

2

(n-1) – 1 and smallest negative number as -2

(n-1)

. Can you prove this to

yourself? Try a few examples.

d. Now how do you negate a number?

i. Take the original two’s complement representation of the number

and flip all the bits (1 becomes 0 and vice versa)

ii. Add 1 to this new number and this leads to the negative of the

original number.

e. To convert a two’s complement number to decimal, you first check the

leftmost bit to see if it is positive or negative. If it is positive, you convert

to decimal as normal. If it is negative, negate the number, convert to

decimal as normal, then add the negative sign back in front of it.

f. (4)

10 = (0100)

2

in Two’s Complement form. Now let’s apply our little

algorithm! Flipping bits yields (1011)

2 and when you add 1, you get (1100)

2

.

Convert it to decimal and you will see that 1100 is the two’s complement

form of -4.

g. Arithmetic works pretty much the same as normal binary numbers.

## Little Endian and Big Endian!

These are ways to store numbers or data in memory addresses. Let’s use a 16-bit word

as an example, (0xFEED)

16

in this case. Let’s also assume we are storing this word starting

1

at address 0x4000. We store these words in terms of bytes and not bits, so we need

some conversion. Recall that 16 bits are 2 bytes (since 1 byte is 8 bits). The word is stored

in pairs to make up the required 1 byte per memory location so our two parts will be

‘FE’ and ‘ED’.

- Big Endian: This refers to big end first, which means that we store the most

significant byte at the smallest memory location and the rest follow normally.

Therefore, to store our word in this case, memory location 0x4000 will have the

byte FE and memory location 0x4001 will store the byte ED. - Little Endian: This refers to little end first and opposed to Big Endian, we store

the least significant bit at the smallest memory address and the rest follow

normally with the most significant bit at the last memory address. So, 0x4000

will have the byte ED and 0x4001 will have FE.

Big Endian is commonly used in Networking application, while Little Endian is most

commonly used in processors.