Unix/Linux Handbook

WHO INVENTED UNIX?

KEN Thomson & DENNIS Ritchie

WHAT IS THE HISTORY OF UNIX?

Unix was first developed in 1969 at Bell Labs, which was part of AT&T (American Telephone & Telegraph Company) at the time.

It was written in assembly language and later rewritten in C.

HOW DID COMPUTERS WORK BEFORE UNIX?

Users had to interact directly with the hardware.

This was achieved by manually doing things to the computer.

You had to: set switches, punch cards, or tapes to load a program.

Computers ran one job at a time. 

You’d submit a stack of punch cards, and the machine would process them in order …there was no multitasking.

WHY WAS UNIX SUCH A BIG DEAL WHEN IT CAME OUT?

Unix was the first computer to ever have:

• a command line.

• capability to run multiple programs at a time.

• have a clean, portable and general-purpose file system.

It’s file system introduced a hierarchal structure that treated everything like a file. This made using it easier.

It set the standard and became the model for all modern Operating Systems.

What companies are Ubuntu, Debian, CentOS & FreeBSD apart of?

Ubuntu > Canonical

Debian > Software In The Public Interest Inc

CentOS > Red Hat

FreeBSD > The Free BSD Foundation

WHAT DOES A SYSTEM ADMIN DO?

keep systems running, secure, and availabe.

They manage users, software, hardware, and network services.

TO BE A COMPETENT SYSTEM ADMIN, WHAT MUST YOU KNOW?

-shell commands 

-file systems 

-permissions 

-users/groups 

-processes 

-basic networking

-operating systems 

WHAT ARE SHELL COMMANDS?

Text INSTRUCTIONS you type to control the computer.

WHAT IS THE FILE SYSTEM?

The STRUCTURE for storing and organizing data.

WHAT ARE PERMISSIONS?

RULES that control who can read, write, or execute files.

WHAT ARE USERS/GROUPS?

ACCOUNTS that define access rights.

USER ACCOUNT =

individual account that may or may not have permissions

GROUP ACCOUNT =

collection of user accounts that may or may not share permissions

WHAT IS A PROCESSES?

RUNNING instance of a PROGRAM

WHAT IS NETWORKING?

COMMUNICATION between computers

WHAT IS AN OPERATING SYSTEM?

SOFTWARE that manages hardware

HOW IS A PROGRAM RAN FROM CODE TO EXECUTION?

A program is just code stored on disk.

When you run it, it becomes a process.

A process can have one or many threads.

Each thread is a task that the operating system’s scheduler manages.

GIVE A CONCEPTUAL EXAMPLE OF PROCESSES, THREADS & TASKS.

You have a restaurant that’s a running business

(like how a process is a running program)

In the kitchen, you have multiple chefs

(like how a process has multiple threads)

These chefs are each doing individual tasks of making food. Each is making a completely different dish. Yet they’re all sharing the same resources of ingredients, kitchen tools and kitchen space.

(like how tasks share the same storage, and memory space)

GIVE A REAL WORLD EXAMPLE OF PROCESSES, THREADS & TASKS.

When you open a browser, the process of that program starts to run. That’s considered ONE Process.

Inside, there are little workers (threads) doing different jobs at the same time — one handles clicks, one loads pages, one runs code, one plays videos, and others run your tabs.

They all share the same memory but work on different tasks.

EXACTLY WHAT RESOURCES DO THREADS SHARE IN A PROCESS?

1. Memory Space

2. Environment Variables

3. Current Working Directory

4. User and Group IDs

5. Network Connections

Threads in the same process share everything important about the process, except their own execution state.

WHAT IS BOOTING?

(short for bootstraping)

It’s the process your computer goes through when it's turned on to get everything ready for use.

What is the boot process?

When you start the computer the software in the hardware (firmware- BIOS or UEFI) begins the boot sequence. This loader brings the OS's kernel into memory and kicks things off. From there, the system services—known as daemons—are launched, and the system transitions into a runnable state.

WHAT ARE THE 5 STEPS OF THE BOOTING PROCESS:

• Power On – You press the power button.

• BIOS/UEFI Loads – The basic firmware runs some quick hardware checks. (POST – Power-On Self-Test).

• Bootloader Starts – The system looks for a bootloader (like GRUB on Linux, EFI on mac, or BOOTMGR on Windows).

• Operating System Loads – The bootloader loads the operating system (like Windows, macOS, or Linux) into memory.

• User Login – Once the OS is ready, you can log in and use the system.

What is a daemon?

is a background program that runs continuously and performs specific tasks.

…you rarely interact with it.

Examples of daemons?

• httpd – A web server daemon (e.g., Apache).

• sshd – Handles incoming SSH connections.

• crond – Schedules and runs periodic tasks.

How do daemons work?

They can start at boot time, when needed or on demand.

They run silently in the background

And they wait for events or perform regular tasks

Note: On Windows, similar background programs are called "services" instead of daemons.

If the POST test doesn’t work what will happen in the BIOS and UEFI?

BIOS will set off a beep sound

UEFI will display an error message