Essential Linux Interview Guide: 100+ Questions on Architecture, Commands, and Security

Linux Overview

Linux is a free, open‑source Unix‑like operating system supporting multiple users, multitasking, multithreading, and both 32‑bit and 64‑bit hardware. It runs standard Unix tools, network protocols, and offers a stable, high‑performance environment.

Unix vs Linux

Open source : Linux source is freely available; Unix is proprietary.

Cross‑platform : Linux runs on many hardware platforms; Unix has limited portability.

GUI : Linux provides both command‑line and graphical interfaces; traditional Unix is command‑line only.

Hardware requirements : Linux works on modest hardware; Unix often requires specialized hardware.

User base : Linux is widely used by individuals and enterprises; Unix is common in large, security‑critical environments.

Linux Kernel and Components

The kernel manages hardware, memory, processes, devices, and file systems. Typical components include the kernel, shell, GUI, system utilities, and applications.

Linux Architecture

Divided into user space (applications, C library) and kernel space (system call interface, kernel, architecture‑dependent code). This separation enhances security and stability.

Bash vs DOS

Case‑sensitive commands vs case‑insensitive.

Path separator: / in Bash, \ in DOS.

File naming conventions differ.

Boot Process and Runlevels

Power‑on self‑test (POST) and BIOS initialization.

Load bootloader (GRUB, LILO).

Load Linux kernel.

Start init (PID 1).

Enter appropriate runlevel.

Login prompt.

Default runlevels: 0‑halt, 1‑single‑user, 2‑multi‑user (no network), 3‑multi‑user (network), 5‑graphical, 6‑reboot.

Inter‑Process Communication (IPC)

Pipes, named pipes (FIFO)

Signals

Message queues

Shared memory

Semaphores

Sockets

System Logs

Important log file: /var/log/messages aggregates system and security events.

Security Practices

Add non‑root users, disable root SSH login, change SSH port.

Use key‑based SSH authentication, disable password login.

Enable firewall, disable SELinux if not needed.

Install fail2ban to block brute‑force attacks.

Restrict access to trusted IPs, use VPN.

Limit outbound connections.

Common Attacks

CC (Connection‑Closing) attacks flood web pages; DDoS (Distributed Denial‑of‑Service) attacks use multiple compromised hosts to overwhelm resources.

Shell Scripting Basics

A shell script is a text file containing commands. The default login shell is /bin/bash. Common constructs include variables, if statements, loops ( for, while), case statements, functions, and I/O redirection.

# Example: change default shell
chsh -s /bin/bash username

Essential Linux Commands

File Management

cat

: concatenate and display files. chmod: change file permissions. chown: change file owner/group. cp: copy files/directories. mv: move or rename files. rm: remove files/directories. ln: create hard or symbolic links.

Directory Operations

ls

: list directory contents with options for sorting, showing hidden files, and detailed view. mkdir: create directories (use -p for parent paths). pwd: print current working directory. cd: change directory. rmdir: remove empty directories.

Disk Usage

df

: report file‑system disk space usage. du: estimate file and directory space usage.

Process Management

ps

: display current processes. top: interactive view of running processes. kill: send signals to processes.

Network Tools

ifconfig

/ ip: view/configure network interfaces. netstat: show network connections and routing tables. ping: test reachability of a host. telnet: remote login utility.

Package Management

rpm

: manage Red Hat packages. yum: install, update, and remove packages.

Compression

tar

with -z (gzip) or -j (bzip2) for archiving. gzip, bzip2, unzip for compression/decompression.

Sample Interview Tasks

Write a script to copy character devices to /dev, create a group with 30 users, delete a range of user accounts, and modify files with sed to insert text around a pattern.