Linux Operating System

Linux is a free, open-source operating system that manages a computer’s hardware and lets you run applications—similar in purpose to Windows or macOS. Linux is the software layer that: Provides a user interface (graphical or command-line), Controls your CPU, memory, disk, and devices and Lets you run programs.

Distributions (distros)

Linux comes in many versions called distributions, which bundle the Linux kernel with tools and software. Examples include:

Ubuntu
Fedora
Debian
Arch Linux

Each distro targets different users—beginners, developers, servers, or power users.

Linux Booting Process

Systemd is a system that is designed specifically for the Linux kernel. It replaces the sysvinit process to become the first process with PID = 1, which gets executed in user space during the Linux start-up process.

Below is the list of some useful systemd utilities along with a brief description of what they do:

systemctl: It Controls the systemd system and services.
journalctl: Used To manage journal, systemd’s own logging system
hostnamectl: Can Control hostname.
localectl: Helps Configure system local and keyboard layout.
timedatectl: Used to Set time and date.
systemd-cgls : It Shows cgroup contents.
systemadm: It is a Front-end for systemctl command.

Process Management

# List all services
systemctl list-unit-files

# Check status of a service
systemctl status service_name

# Stop a service
systemctl stop service_name

# Prevent a service from starting at bot
systemctl disable service_name

# Start a service
systemctl start service_name

# Enable a service after boot
systemctl enable service_name

# List all services
systemctl list-unit-files

# Check status of a service
systemctl status service_name

# Stop a service
systemctl stop service_name

# Prevent a service from starting at bot
systemctl disable service_name

# Start a service
systemctl start service_name

# Enable a service after boot
systemctl enable service_name

All the configuration files of systemd services will :

/lib/systemd/system/
/usr/lib/systemd/system

Disk Management

Show block devices in a tree format, useful to see:

Disks (sda, nvme01)
Partitions (sda1, sda2)
Mount points

# List block devices
lsblk

# List block devices
lsblk

Check used and available disk space

# Check used and available disk space
df -h 

# Show disk usage of directories/files
du -sh /var/log

# Check used and available disk space
df -h 

# Show disk usage of directories/files
du -sh /var/log

# List all disk and their partitions
fdisk -l 

# Perform actions on particular disk
fdisk /dev/sda

# List all disk and their partitions
fdisk -l 

# Perform actions on particular disk
fdisk /dev/sda

Common actions:

n → new partition
d → delete partition
p → print table
w → write changes

Formatting the partitions

# Once the partition is created, we create the file format
mkdir.ext4 /dev/sda1

# Other file formats are
mkdir.ntfs <partition-name>
mkdir.vfat <partition-name>
mkdir.xfs <partition-name>

# Once the partition is created, we create the file format
mkdir.ext4 /dev/sda1

# Other file formats are
mkdir.ntfs <partition-name>
mkdir.vfat <partition-name>
mkdir.xfs <partition-name>

Mounting and unmounting disks

mount /dev/sda1 /mnt/dir1

umount /mnt/dir1

# if unable to umount as it is busy, you can look for open files using the below command:
lsof /mnt/dir1

mount /dev/sda1 /mnt/dir1

umount /mnt/dir1

# if unable to umount as it is busy, you can look for open files using the below command:
lsof /mnt/dir1

Check for bad blocks

badblocks -sv

badblocks -sv

RAID Management

RAID (Redundant Array of Independent Disks) combines multiple disks to improve performance, fault tolerance, or both.

Common RAID Levels

RAID	Purpose	Minimum Disks
RAID 0	Performance (striping, no redundancy)	2
RAID 1	Mirroring (redundancy)	2
RAID 5	Performance + parity	3
RAID 6	Double parity	4
RAID 10	Performance + redundancy	4

Prerequisites

Linux system with multiple disks (e.g. /dev/sdb, /dev/sdc)
Root or sudo access
mdadm installed

RAID Creation

# List Block devices
lsblk

# Wipe existing signatures
wipefs -a /dev/sdb
wifefs -a /dev/sdc

# Create RAID array
mdadm --create /dev/md0 \
      --level=1 \
      --raid-devices=2 \
      /dev/sdb /dev/sdc
      
# Create a filesystem
mkdir.ext4 /dev/md0

# Mount RAID array
mount /dev/md0 /mnt/raid

# Verify
df -h

# Persist RAID configuration
mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
update-initramfs -u 

# Persist mount
blkid /dev/md0

# Add the output to /etc/fstab

# List Block devices
lsblk

# Wipe existing signatures
wipefs -a /dev/sdb
wifefs -a /dev/sdc

# Create RAID array
mdadm --create /dev/md0 \
      --level=1 \
      --raid-devices=2 \
      /dev/sdb /dev/sdc
      
# Create a filesystem
mkdir.ext4 /dev/md0

# Mount RAID array
mount /dev/md0 /mnt/raid

# Verify
df -h

# Persist RAID configuration
mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
update-initramfs -u 

# Persist mount
blkid /dev/md0

# Add the output to /etc/fstab

RAID updation

# Check status
mdadm --detail /dev/md0

# Stop RAID
mdadm --stop /dev/md0

# Replace failed disk
mdadm /dev/md0 --fail /dev/sdb
mdadm /dev/md0 --remove /dev/sdb
mdadm /dev/md0 --add /dev/sdb

# Check status
mdadm --detail /dev/md0

# Stop RAID
mdadm --stop /dev/md0

# Replace failed disk
mdadm /dev/md0 --fail /dev/sdb
mdadm /dev/md0 --remove /dev/sdb
mdadm /dev/md0 --add /dev/sdb

User Management

Basic commands

# Create users

## Low level, script friendly
useradd
## High-level (Debian/Ubuntu)
adduser
# Modify user
usermod 
usermod -aG user group  # Append user to group
# Delete user
userdel

# Password & Authentication
passwd         # Set/change password
chage          # Password aging & expiry
faillog        # Login failure records
lastlog        # Last login per user

# User identity and session info
id             # UID, GID, groups
whoami         # Current user
who            # Logged-in users
w              # Logged-in users + activity
users          # Logged-in usernames only
logname        # Original login user

# Audit and Login history
last           # Login history
lastb          # Failed logins (root only)
uptime

# Create users

## Low level, script friendly
useradd
## High-level (Debian/Ubuntu)
adduser
# Modify user
usermod 
usermod -aG user group  # Append user to group
# Delete user
userdel

# Password & Authentication
passwd         # Set/change password
chage          # Password aging & expiry
faillog        # Login failure records
lastlog        # Last login per user

# User identity and session info
id             # UID, GID, groups
whoami         # Current user
who            # Logged-in users
w              # Logged-in users + activity
users          # Logged-in usernames only
logname        # Original login user

# Audit and Login history
last           # Login history
lastb          # Failed logins (root only)
uptime

Group management

# Add new group
groupadd
# Modify group name or GID
groupmod
# Delete existing group
groupdel
# Administer /etc/group and /etc/gshadow
gpasswd
gpasswd -a username groupname     # Add user to group
gpasswd -d username groupname     # Delete user from group
gpasswd -A username groupname     # Delegates group management rights to the user

# Add new group
groupadd
# Modify group name or GID
groupmod
# Delete existing group
groupdel
# Administer /etc/group and /etc/gshadow
gpasswd
gpasswd -a username groupname     # Add user to group
gpasswd -d username groupname     # Delete user from group
gpasswd -A username groupname     # Delegates group management rights to the user

Core Identity and Membership Files

/etc/passwd: Stores basic user account details like the username, User ID (UID), primary Group ID (GID), home directory path, and default login shell. It is publicly readable by all users.
/etc/group: Defines all local groups and lists their members. Each entry includes the group name, GID, and a comma-separated list of secondary users.
/etc/shadow: Stores encrypted (hashed) user passwords and password expiration/aging policies. For security, it is only readable by the root user.
/etc/gshadow: A secure version of the group file containing encrypted group passwords and a list of group administrators.

PAM (Pluggable Authentication Module)

PAM is a rule engine that decides whether a user action is allowed. Every time someone:

logs in (ssh, console, GUI)
runs sudo
switches user (su)
unlocks a screen

The application asks PAM -> PAM evaluates rules -> Access is allowed or denied

PAM is built from four stacks, evaluated top to bottom.

Stack	Purpose	Example
`auth`	Who are you?	Password, MFA
`account`	Are you allowed now?	Expiry, lock
`password`	Change credentials	Password rules
`session`	What happens after login	Limits, env

PAM Configuration location

/etc/pam.d/
├── sshd
├── login
├── sudo
├── su
├── common-auth
├── common-account
├── common-session
└── common-password

PAM rule syntax:

<stack>  <control>  <module>   [options]
auth   [required | requisite | sufficient | optional ]  pam_unix.so

Essential PAM modules are:

Module	What it does
`pam_unix.so`	Local Linux users
`pam_sss.so`	LDAP / Active Directory
`pam_faillock.so`	Lock after failures
`pam_nologin.so`	Maintenance lock
`pam_access.so`	Allow/deny users
`pam_time.so`	Time-based login
`pam_limits.so`	Resource limits
`pam_env.so`	Set environment

Network Management

Linux networking has three layers you manage:

Link layer – network interfaces (Ethernet, Wi-Fi)
IP layer – addresses, routes, DNS
Connection layer – NetworkManager / Wi-Fi authentication

# List interfaces
ip link

# Show IP addresses
ip addr

# Filter one interface
ip addr show eth0

# Bring interfaces up/down
ip link set eth0 up
ip link set eth0 down

# Wifi
ip link set wlan0 up

# Assign IP Temporarily
ip addr add 192.168.1.50/24 dev eth0

# Remove IP
ip addr del 192.168.1.50/25 dev eth0

# Routing
ip route

# Add default gateway
ip route add default via 192.168.1.1

# Delete route
ip route del default

# List interfaces
ip link

# Show IP addresses
ip addr

# Filter one interface
ip addr show eth0

# Bring interfaces up/down
ip link set eth0 up
ip link set eth0 down

# Wifi
ip link set wlan0 up

# Assign IP Temporarily
ip addr add 192.168.1.50/24 dev eth0

# Remove IP
ip addr del 192.168.1.50/25 dev eth0

# Routing
ip route

# Add default gateway
ip route add default via 192.168.1.1

# Delete route
ip route del default

DNS Configuration

# View DNS
resolvectl status
# Temporarily set DNS
resolvectl dns eth0 8.8.8.8  1.1.1.1
# Test connectivity
ping google.com
# Trace route
traceroute google.com
# Check open ports
ss -tuln

# View DNS
resolvectl status
# Temporarily set DNS
resolvectl dns eth0 8.8.8.8  1.1.1.1
# Test connectivity
ping google.com
# Trace route
traceroute google.com
# Check open ports
ss -tuln

Firewall Management

In Debian based distributions, firewall management is usually done with UFW (Uncomplicated Firewall). It’s basically a friendly wrapper around iptables, and it covers ~95% of common needs without pain. Let’s walk through it cleanly.

# Check the status
ufw status
ufw status verbose

# Install ufw if not installed
apt update
apt install ufw

# Enabling defaults
ufw default deny incoming
ufw default allow outgoing

# Allow by service name
ufw allow ssh
ufw allow http
ufw allow https

# Allow by port
ufw allow 80
ufw allow 8080/tcp
ufw allow 53/udp

# Allow from specific IPs
ufw allow from 192.168.1.100
ufw allow from 192.168.1.100 to any port 22

# Allow a subnet
ufw allow from 192.168.1.0/24

# List firewall rules
ufw status numbered

# Delete rule
ufw delete 2

# Reload firewall
ufw reload

# Reset everything (wipes all rules)
ufw reset

# Enable logging
ufw logging on
tail -f /var/log/ufw.log

# Check the status
ufw status
ufw status verbose

# Install ufw if not installed
apt update
apt install ufw

# Enabling defaults
ufw default deny incoming
ufw default allow outgoing

# Allow by service name
ufw allow ssh
ufw allow http
ufw allow https

# Allow by port
ufw allow 80
ufw allow 8080/tcp
ufw allow 53/udp

# Allow from specific IPs
ufw allow from 192.168.1.100
ufw allow from 192.168.1.100 to any port 22

# Allow a subnet
ufw allow from 192.168.1.0/24

# List firewall rules
ufw status numbered

# Delete rule
ufw delete 2

# Reload firewall
ufw reload

# Reset everything (wipes all rules)
ufw reset

# Enable logging
ufw logging on
tail -f /var/log/ufw.log

Custom rules are stored at :

/etc/ufw/before.rules
/etc/ufw/after.rules

There is also a GUI option that can be install with package gufw

Device Management

// Coming up soon

Linux Hypervisor

LXC (Linux Containers)

LXC (Linux Containers) and Docker are both containerization technologies but differ in their fundamental approach and intended use cases: LXC is for OS-level (system) virtualization, behaving like a lightweight virtual machine, while Docker is for application-level (process) virtualization, focused on packaging and deploying single applications.

Key Architectural Differences

Feature	LXC (Linux Containers)	Docker
Focus	Whole operating system environments	Single applications and their dependencies
Isolation	OS-level isolation, similar to a VM, can have a separate kernel	Process-level isolation, all containers share the host kernel
Management	Managed like a standard Linux server, uses `lxc` command line tool	Uses an extensive ecosystem of tools (`docker build`, `docker run`, Docker Hub, Docker Compose)
Networking	Containers can have their own dedicated IP address and behave like a separate host on the network	Uses NAT and port forwarding by default, with an internal bridge network for communication, but flexible networking options are available
Persistence	Persistent filesystem by default, managed like a standard OS filesystem	Uses a layered, ephemeral filesystem (Copy-on-Write) with data loss on container deletion unless persistent volumes are configured

CGroups

Cgroups is a Linux kernel feature that limits, accounts for, isolates the resource usage of a collection of processes.