Operating Systems

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Components
Boot Process
Windows
Linux

Components

Process Scheduling

Non-preemptive - CPU allocated until process completes execution

Preemptive - CPU allocated for fixed time

Process Control Block - Data Structure

PID
State
Pointer
Priority
Program Counter
CPU Registers
I/O Info
Accounting Info

PCBs of particular process states placed in their respective queues:

Job Queue: All processes
Ready Queue: In memory, waiting to execute
Device Queue: Blocked due to unavailability of an I/O device

Scheduler

Loads job from queue into memory
CPU Changes process state (ready → running)

Swapping

Swaps suspended process (e.g., waiting for I/O) out of memory
Reintroduces into memory later

Context Switching

Kernel saves register, memory state of CPU into PCB
Loads saved state of the next process

Algorithms

FCFS, SJN, Priority Scheduling, SRT, Round Robin, Multiple-level Queue, etc

Memory Management

Memory Division

Pages - Fixed-size blocks

Segments - Variable-size logical units

Virtual Memory

Uses reserved storage space (swap) to extend physical memory (RAM)

Device Management

Kernel Modules

Drivers

Hardware devices → Interrupt Request (IRQ)
Register Interrupt Service Routine (ISR):
- Handle interrupts generated by respective devices
- Perform minimal processing and defer extensive processing to quickly return control to the interrupted process
- Use Interrupt Controller:
  - ICs prioritize IRQs for CPU
  - APIC, MSI Controller
Device Drivers:
- CPU, Network Drivers
File System Drivers:
- Manage I/O and buffering
- Example: NTFS

Pluggable Authentication Modules (PAM)

Provides framework for integrating authentication methods into applications

Network Protocol Modules

TCP/IP, UDP

Security Modules - Linux Security Modules (LSM)

Provide hooks for security decisions (file/network operations, process executions)

e.g. SELinux

Kernel Services

Manage software interrupts:
- Exception Handling
- System calls:
  - Examples: read(), write(), getpid(), time(), pipe(), socket(), exec()
  - Processor Mode switch: user to kernel mode (OS core functions)
  - Execute privileged operation
  - Return to user mode

Namespaces

Used to isolate process IDs, network, mount points, users, etc between groups of processes for containerization / sandboxing

Control groups (cgroups)

Manage CPU, memory, I/O resource usage for groups of processes

File System Management

Organize files, metadata hierarchically in directories for easy access
File data storage allocation

Encryption

During storage and transit

Authorization

Discretionary Access Control (DAC)

Manages user permissions (rwx)
Defined in Access Control Lists

Security

Authentication

Verifies identity
Username-password login, biometrics, SSH keys

Authorization

SELinux

Implements Mandatory Access Control (MAC) at kernel level
Establish roles and permissions for users and processes, overriding DAC based on resource ownership
Enforces security policies that restrict access to system resources (e.g., Port access for applications)

Firewall

Block incoming traffic based on rich rules
- Example: block traffic on specific port from certain services/IPs

Boot Process

1. Power ON

CPU fetches reset vector (addr) from Firmware ROM which points to first instruction of firmware

2. Firmware

2.a. BIOS

POST (Power On Self-Test):
1. Checks CPU, RAM, MoBo, I/O devices
2. If successful:
  Locates bootable media
3. Else:
  Stops and shows error (Beeps/LED codes)
Reads storage boot sector (LBA 0) - contains Master Boot Record

MBR (512 B)	-
0x0000–0x01BD, 446 bytes	Bootstrap code: scans parition table
0x01BE–0x01FD, 64 bytes	Partition table (4 entries) - 1 partition marked active (bootable) - contains bootloader
0x01FE–0x01FF, 2 bytes	Boot signature (0x55AA)

2.b. UEFI

SEC (Security Initialization)
PEI (Pre-EFI Initialization)
DXE (Driver Execution Environment)
BDS (Boot Device Selection)
Reads storage primary GPT header (LBA 1) -> partition table -> identifies ESP by its special GUID

GPT

EFI System Partition (ESP) - FAT32

Contains .efi executable files like bootloader e.g. systemd-boot, GRUB2
Later mounted on /boot/efi
Secure boot ensures only signed .efi are executed

3. Bootloader

3.a. GRUB

/boot/efi/EFI/redhat/grub.cfg:

Boot menu
Command line parameters (can customize kernel behavior)
Loads into RAM:
- vmlinuz (supports virtual memory)
- initramfs img

3.b. systemd-boot

/boot/efi/EFI/systemd/systemd-bootx64.efi

Boot menu
Loads vmlinuz and initramfs into RAM and hands over execution

4. Kernel

vmlinuz contains a small stub (decompression code) that extracts itself
Initializes:
- CPU:
  - Configures CPU registers
  - Sets up exception and interrupt handling mechanisms
- Memory:
  - Physical
  - Virtual
- Device detection:
  - Storage controllers
  - Network Interfaces
  - Peripherals
Load modules (e.g., drivers) compiled into kernel:
- Initialized during boot
Mounts temporary RAM-based filesystem (tmpfs) as /
Extracts initramfs img into /
Executes /init script:
- Loads modules (e.g., Intel/AMD microcode)
- Mounts storage / as Read-only -> fsck (file system check) -> remounts / as RW and then chroot
- Executes PID 1 (/sbin/init, sym-linked to /usr/lib/systemd/systemd)

5. Init System

/etc/fstab invoked:
- Filesystem Table - mounts defined filesystems on boot
  - Example: Device Mount_point Filesystem Options Dump Pass
Manages services based on targets (systemd) or runlevels (older init system):
- Reads unit files in /usr/lib/systemd/system and /etc/systemd/system
- Executes them in order

Runlevel target symlinked to	Systemd Target
0	poweroff.target
1	rescue.target (Getty)
2-4	multi-user.target
5	graphical.target
6	reboot.target

Show currently loaded targets/runlevel

systemctl list-units --type=target or

runlevel

Temporarily switch target

systemctl isolate name.target or

init #

Permanent target

systemctl get-default

systemctl set-default name.target

6. User Login & Session Management

Init starts processes like getty or display managers that handle user login

After login, the user’s shell (CLI/GUI) or session manager initializes user-specific configurations

Windows

CLI (Terminal Emulators)

Windows Terminal

Shell

PowerShell

Corporate License

Employees/competitors can report unlicensed software to organizations like BSA through their channels for investigation and potential enforcement actions
Software vendors often engage intermediary audit firms, such as KPMG, whose compliance and regulatory officers conduct audits to ensure corps comply with software license agreements

Linux

Distributions

Individual maintainers, or orgs
Different package managers
- Main packages hosted on distro repos
- Easy update and install through shell instead of searching for binaries

Kernel

Analogous to UNIX kernel written from scratch
Originally developed by Linus Torvalds, written in C
Rust used for new modules
Originally hosted on Git
- Now on GitLab
- Has a GitHub repo without active commits

File Systems

File System	Crash Protection	Data Integrity Checks	Snapshots	Built-in RAID Support	Use Case
ext4	Journaling	None	No	No	Small-med file RW, Desktops
Btrfs	Copy-on-Write (COW)	Checksumming	Yes	Yes	Desktops
XFS	Journaling	Metadata Checksums	No	No	Large file servers
ZFS	Copy-on-Write (COW)	End-to-End Checksums	Yes	Yes	High-capacity servers, data centers

ext4

Linux File System
- Directories stored as files -> point to inodes -> point to data blocks

RAID

Redundant Array of Independent Disks
Data redundancy, performance
- Hardware
  - RAID controller
- Software
  - Linux software RAID
  - LVM RAID

Directory Structure

`/`	Root directory
`/boot`	Bootloader and kernel files
`/dev`	Device files
`/bin`	User command binaries
`/sbin`	System binaries
`/usr`	User binaries, data; typically read only
`/opt`	Optional software applications
`/lib`	Essential shared libraries and kernel modules
`/home`	User home directories
`/root`	Root user home directory
`/etc`	Configuration/setting files
`/var`	Variable data files e.g. logs, tmp (persistent)
`/tmp`	Session (short-lived) temporary files
`/mnt`	Temporary mount point for external file systems
`/proc`	Virtual FS interface for kernel data structures, contains dir for each PID
`/sys`	Virtual FS for kernel data structures

CLI (Terminal Emulators)

Terminal app

Gnome Terminal, st

Shell

(Userspace) interface to interact with applications

Interactive: User can enter commands
Non-interactive: /sbin/nologin
Login
- Loads user environment
Non-login
- Loads only global config without user environment

GNU Utils

FOSS utilities analogous to UNIX OS in function written from scratch

Systemd Service Types

Type	Description
simple	Starts the service binary as a child process running in the foreground immediately; systemd tracks the child process but considers the service started once fork succeeds.
notify	Like simple, runs in foreground but waits for a "ready" signal from the service before marking it started.
exec	Starts a child process that replaces itself with the service binary (execve); systemd waits for successful exec, ensuring accurate startup detection and tracking the actual service process.
forking	Service forks and parent exits; the orphaned child is reparented by the kernel to PID 1 (systemd), which tracks it as the main process. Used for traditional daemonizing services.
oneshot	Runs one or more short tasks sequentially, waits for completion, then stops; ideal for scripts or one-time jobs.

Commands

command [-f] [--flag] arguments #case sensitive

Help Command	Description
`which`	List path of binary
`find dir -name [file]`	Search for files in a directory hierarchy; can use * (wildcard)
`find dir -wholename [*/dir]`	Search for dir in a directory hierarchy
`locate [file/dir]`	List files & directories with substring and subdirectories from database
`updatedb`	Updates locate's database (`/etc/updatedb.conf`);
`whatis cmd`	Short description of a command
`man cmd`	Long manual description of a command
`cmd --help`	List flags and options for a command
`df -h [filename]`	Show space used in filesystems (for specified filenames)
`du -h -d 1` [dir]	Show space used by directory, with max depth for subdirectory list
`lsblk`	List block devices
`lsblk -f`	Display block devices and their filesystems
`findmnt`	Display currently mounted filesystems
`mount /dev/devicename /mountpoint`	Mount block device to mount point
`umount /mountpoint`	Unmount block device from mount point
`w`	List users, host/IP, login time, tty, CPU usage, shell
`who`	List users, tty, and login time
`last`	Show login history with IP and login duration
`whoami`	Show current shell logged-in username
`who am i`	Show username, tty, and login time
`users`	Show logged-in usernames
`finger`	List users, host, tty, user ID details
`stat filename`	Show access/read time, modification/write time, change time (metadata, permissions, ownership)
`date`	Print or set date and time
`hostnamectl [--set-hostname]`	Systemd binary to query/change hostname; displays OS and kernel details (also in `/etc/os-release`)
`uname -a`	Print system information

File Management Command	Description
`cd [absolute / relative path]`	Change directory
`ls`	List directory contents
`-l` -> `ll`	Long listing format
`-a`	Show all entries (including those starting with `.`)
`-h`	Human-readable sizes
`-u`	Show access time instead of modification time
`lsof`	List open files
`cp source dest`	Copy files
`-r dir`	Copy directories recursively
`mv source dest`	Move or rename files/directories
`mkdir [dir]`	Make directory
`mkdir -p [parent/child]`	Make parent directories as needed
`rm file [f2] [f3*] file00{4,5,6}`	Remove file(s); supports wildcards and brace expansion
`-r dir`	Recursively remove directory and its contents
`-f`	Force remove; never prompt - ignore nonexistent files
`rmdir dir`	Remove empty directory
`touch file`	Create empty file
`echo "text"`	Print text on stdout
`cat [file1] [file2]`	Concatenate files and print on stdout
`[1 / 2 / &]> file`	Redirect stdout/stderr/both, overwrite file
`>> file`	Append to file
`grep -v word`	Exclude lines containing word
`-w word`	Search for whole word
`awk '{print $1, $2}' filename`	Print specified columns
`cut -c1 filename`	Print first character of each line
`sed -n '5p' filename`	Print only the 5th line
`stout \| less`	Print output in pages
`head [file]` / `tail [file]`	Print first/last part of file
`tar -cvf archive.tar [files / dirs]`	Create tar archive
`-Avf a1.tar a2.tar`	Append a2 contents to a1
`-rvf a1.tar [files / dirs]`	Append to tar
`-czvf a1.tar.gz dir`	Create gzip-compressed tar archive
`-tf a1.tar.gz`	List contents of archive
`-xvf a1.tar.gz`	Extract archive
`gzip [file]`	Compress file

Vim Command	Description
`vim -o file1 file2`	Open multiple files vertically
`-O file1 file2`	Open multiple files horizontally
`:r file_name`	Append file into current buffer
`:set textwidth=80`	Wrap lines after 80 characters
`:split [file]` / `:vsplit [file]`	Split window horizontally/vertically in vim
`:qa`	Quit all windows
`:only`	Close all but current window
`:wq` ZZ	Save and quit if changes made
Ctrl + w + arrow keys	Switch window

File Permission Management Command	Description
`chown username[:groupname] file/dir`	Change owner (and group owner)
`chgrp groupname file/dir`	Change group owner
`chmod [u,g,o][+/-/=][r][w][x]`	Symbolically change user,group,other read (2) write (4) execute (1) permissions
`+/-t dir`	Sticky bit to dir for denying file deletion/renaming if write provided. `T` only appears if x missing for o (for distinguishing purpose).
`nnn file/dir`	Numerically change perms
`chattr`	Change file attributes. File cannot be modified by even root unless attribute is removed.
`+a`	append only
`lsattr`	List attributes
`umask`	defaults: dir 777 - 022, file 666 - 002

User/Group Management Command	Description / Notes
`adduser username`	Add a user, interactive on debian - create homedir, add passwd
`useradd username`	Add a user
`-g gid/gname`	Custom primary group, else same name as username; gets ownership for user created files
`-G g1,g2`	Supplementary groups; gives files' group perms to user
`-m`	Create home directory if it doesn't exist
`-d custom_homedir`	Use custom home directory
`passwd username`	Set or change password for user
`usermod [-g][-G][-d]`	Modify user account
`-aG group1,group2 username`	Append user to supplementary groups
`userdel username`	Delete user
`gpasswd -M user1,user2,user3 groupname`	Add multiple users to group
`groups username`	List groups user is part of
`id username`	Show UID, GID, and group memberships
`groupadd gname`	Add group
`-g 1003`	Custom group ID (must be unique)
`groupmod`	Modify group
`-n newname oldname`	Change group name
`-g newid gname`	Change group ID
`groupdel gname`	Delete group

Process Management Command	Description
`ps`	Snapshot of current process - UNIX and BSD options
`-ux`	User oriented format, include processes without a tty
`a`	All with tty, including other users
`-a`	All with tty except session leaders
`kill pid`	Send signal SIGTERM(15) by default to terminate a process
`-s`	Specify signal name/number
`-9`	SIGKILL(9) - immediately terminate
`-18`	SIGCONT(18) - resume paused process, fg/bg chosen by process type
`-l`	List signals
`pkill process_name`	Kill by process name
`Ctrl+C`	Send SIGINT(2) - Interrupt and terminate process, can be ignored
`Ctrl+Z`	Send SIGSTP(20) - Suspend and send to background (paused, can have custom handling, not ignored)
`Ctrl+\`	Send SIGQUIT(3) - Interrupt + core dump for debugging, can be ignored
`crontab [-u] [-l / -e]`	List/edit user crontab files
`* * * * * command`	min hour day_of_month month day_of_week
`@monthly command`	Run monthly
`jobs`	List paused process id and names
`fg (%job_id)`	Resume fg process, most recent without argument
`bg (%job_id)`	Resume bg process
`sar [-u] 1 3`	System Activity Report - CPU utilization (-u) by default, interval_time (s) number_of_times

top command	Description
`top -p`	Monitor specific pid
`o` `COMMAND=chrome`	Filter with custom param, superior to L
`L`	Locate process_name
`k`	Kill
`r`	Renice - change priority
`h`	Help

Partition Management Command	Description
`parted`	List and change GPT/MBR partition tables
`fdisk [-l] [/dev/device]`	List and change legacy MBR partition tables
`partprobe`	Make the kernel re-read the partition table without rebooting
`mkfs.ext4 /dev/sda1` / `mke2fs -t ext4 /dev/sda1`	Create an ext4 filesystem on a partition

System Monitoring Command	Description
`vmstat`	Virtual memory statistics
`iostat`	CPU and I/O stats for block devices

System Resource Management Command	Description
`ulimit [-S/H] -a`	View current shell session soft and hard limits
`ulimit [-S/H] <resource_flag> <value>`	Set soft/hard resource limit for current shell session
`/etc/security/limits.conf`	Set resource limits permanently for specific users or groups
`yum autoremove`	Remove orphaned packages
`rpm --rebuilddb`	Rebuild RPM DB in case it becomes inconsistent

Networking

Connection Management Command	Description
`nmcli`	BSD style command line tool for NetworkManager
`d[evice]`	Show network devices
`d show device_name`	Show network device details
`d connect/disconnect device_name`	Connect or disconnect a device
`d wifi connect "SSID" password "your_password"`	Connect to WiFi network with SSID and password
`netstat` / `ss -tu`	Show network connections and sockets: TCP, UDP sockets
`-l`	Listening/open ports
`-n`	Numerical output, show IP instead of hostname

Protocol	Port
`ssh`{`scp` -> `sftp`}	22

SSH Command	Description
`ssh user@host`	Log into remote shell
`scp [-r] user@host1:/path user@host2:/path`	Copy files/directories between hosts - remote or local (doesn't require explicit hostname)
`ssh-keygen -t rsa -b 4096 -C "abc@example.com"`	Generate SSH key pair on client
`ssh-copy-id user@server_ip`	Copy public key to server for passwordless authentication
`ssh-keygen -R server_ip`	Remove server entry from client's known hosts file (fixes host key mismatch errors)

IP Command	Description
`dhclient -r && dhclient p8p1`	DHCP client: release current IP and request new one for interface
`ip a[ddr] show [dev p8p1]`	Show interface IPs
`add/del 192.168.x.x/24 dev p8p1`	Add/delete specific IP to/from interface
`nmcli d reapply p8p1`	Reapply network configuration for interface without restart
`ip a flush dev p8p1`	Remove all IPs from interface, including v6

Password Policy

Shadow Utils Config / Command	Description
`/etc/login.defs`	System-wide account & password policy defaults (e.g., aging, min/max days, warning period)
`/etc/shadow`	Encrypted user passwords and per-user password/account aging policies
`chage -m 7 -M 90 -W 7 username`	Set min, max, and warning days for password expiry for a specific user

PAM Config	Description
`/etc/pam.d/`	Configs for each service to control which PAM modules are used
`/etc/security/pwquality.conf`	Password quality module settings used by services like passwd

Reset root password (via GRUB)

Reboot
GRUB menu -> select boot entry -> press e
Find the line starting with linux16 or linux, and append boot parameters to the end:

rw init=/sysroot/bin/sh # Mount root file system as rw, start a shell (sh) as the first process (init, PID 1)

chroot /sysroot

passwd root
touch /.autorelabel    # re-label SELinux contents

exit

reboot

Table of Contents

Components

Process Scheduling

Process Control Block - Data Structure

Scheduler

Swapping

Context Switching

Algorithms

Memory Management

Memory Division

Virtual Memory

Device Management

Kernel Modules

Drivers

Pluggable Authentication Modules (PAM)

Network Protocol Modules

Security Modules - Linux Security Modules (LSM)

Kernel Services

File System Management

Encryption

Authorization

Discretionary Access Control (DAC)

Security

Authentication

Authorization

SELinux

Firewall

Boot Process

1. Power ON

2. Firmware

2.a. BIOS

2.b. UEFI

3. Bootloader

3.a. GRUB

3.b. systemd-boot

4. Kernel

5. Init System

6. User Login & Session Management

Windows

CLI (Terminal Emulators)

Shell

Corporate License

Linux

Distributions

Kernel

File Systems

ext4

RAID

Directory Structure

CLI (Terminal Emulators)

Shell

GNU Utils

Systemd Service Types

Commands

Networking

Password Policy

Reset root password (via GRUB)