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Aisalkyn Aidarova
Aisalkyn Aidarova

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interview questions with answers

This is a very good foundation interview set. A DevOps engineer with 5–7 years of experience should be able to explain all of these clearly.

Infrastructure Fundamentals

What is Hardware?

Answer:
Hardware is the physical component of a computer system.

Examples:

  • CPU
  • RAM
  • SSD
  • Network Card (NIC)
  • Motherboard
  • Hard Drive

Without hardware, software cannot run.

What is Software?

Answer:
Software is a collection of programs and instructions that run on hardware.

Examples:

  • Linux
  • Windows
  • Docker
  • Kubernetes
  • Jenkins
  • Nginx

Hardware = Physical machine

Software = Instructions running on machine

CPU

What is CPU?

Answer:
CPU (Central Processing Unit) is the brain of the computer.

Responsibilities:

  • Execute instructions
  • Process calculations
  • Run applications
  • Manage processes

Example:

When you run:

kubectl get pods
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CPU processes that command.

How do you check CPU usage? 

top
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or

htop
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or

mpstat
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RAM

What is RAM?

Answer:
RAM is temporary memory used by running applications.

Examples:

  • Kubernetes Pods
  • Docker Containers
  • Nginx
  • Java Applications

When server reboots, RAM is cleared.

How do you check memory usage? 

free -h
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Example:

free -h
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Output:

Mem: 16Gi  10Gi  2Gi  4Gi
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How to check memory consumers? 

top
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or

ps aux --sort=-%mem
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SSD

What is SSD?

Answer:
SSD (Solid State Drive) is permanent storage.

Used for:

  • Operating System
  • Logs
  • Databases
  • Files

Advantages:

  • Faster than HDD
  • No moving parts

IP Address

What is IP Address?

Answer:
IP Address uniquely identifies a device on a network.

Example:

192.168.1.10
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Similar to a home address.

Without IP, devices cannot communicate.

Difference between Public and Private IP

Private:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
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Public:

Accessible from internet.

Example:

54.12.34.56
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Port

What is Port?

Answer:
Port identifies a service running on an IP address.

Example:

192.168.1.10:80
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IP = House

Port = Apartment Number

Common Ports

Port Service
22 SSH
80 HTTP
443 HTTPS
53 DNS
3306 MySQL
5432 PostgreSQL
6379 Redis

Protocol

What is Protocol?

Answer:
Protocol is a set of communication rules.

Examples:

  • HTTP
  • HTTPS
  • TCP
  • UDP
  • SSH

Difference between TCP and UDP

TCP:

  • Reliable
  • Ordered
  • Connection oriented

Examples:

  • HTTP
  • HTTPS
  • SSH

UDP:

  • Fast
  • No delivery guarantee

Examples:

  • DNS
  • Video Streaming

OSI Model

Explain OSI Model

Layer Name
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical

Layer 7

Application Layer

Examples:

  • HTTP
  • HTTPS
  • FTP

Layer 4

Transport

Protocols:

  • TCP
  • UDP

Layer 3

Network

Responsible for:

  • Routing
  • IP Addressing

Devices:

  • Router

Layer 2

Data Link

Responsible for:

  • MAC Addresses

Devices:

  • Switch

Layer 1

Physical

Examples:

  • Cables
  • Fiber
  • Ethernet

EC2

What is EC2?

Answer:
Amazon EC2 is a virtual server in AWS.

Used to run:

  • Linux
  • Applications
  • Databases
  • Docker

Example:

Launch Ubuntu server.

Install Nginx.

Access via browser.

ASG

What is Auto Scaling Group?

Answer:
ASG automatically adds or removes EC2 instances.

Benefits:

  • High Availability
  • Scalability
  • Self Healing

Example:

Traffic increases.

ASG:

2 EC2 → 4 EC2 → 8 EC2
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ALB

What is Application Load Balancer?

Answer:
ALB distributes HTTP/HTTPS traffic across multiple targets.

Works at:

OSI Layer 7
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Supports:

  • Host-based routing
  • Path-based routing

Example:

/api/users → User Service

/api/orders → Order Service
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NLB

What is Network Load Balancer?

Answer:
NLB works at:

Layer 4
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Handles:

  • TCP
  • UDP

Very fast.

Millions of connections.

Target Group

What is Target Group?

Answer:
Target Group contains backend servers.

Example:

ALB
 ↓
Target Group
 ↓
EC2 Instances
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Health checks are performed against targets.

Security Group

What is Security Group?

Answer:
Virtual firewall for AWS resources.

Controls:

  • Inbound Traffic
  • Outbound Traffic

Example:

Allow:

22 SSH
80 HTTP
443 HTTPS
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EBS

What is EBS?

Answer:
Elastic Block Store.

Persistent disk attached to EC2.

Similar to:

SSD attached to server
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Use cases:

  • Databases
  • Application storage

EFS

What is EFS?

Answer:
Elastic File System.

Shared storage.

Multiple EC2 instances can mount same filesystem.

Example:

EC2-1
EC2-2
EC2-3
   ↓
   EFS
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S3

What is S3?

Answer:
Object Storage Service.

Stores:

  • Images
  • Videos
  • Backups
  • Logs

Characteristics:

  • Durable
  • Highly Available
  • Unlimited Scale

Cookies

What are Cookies?

Answer:
Small files stored in browser.

Used for:

  • Login Sessions
  • User Preferences
  • Tracking

Example:

User logs in.

Cookie stores session identifier.

Persistent Volume (PV)

What is Persistent Volume?

Answer:
Persistent storage in Kubernetes.

Pod can restart without losing data.

Example:

Pod
 ↓
PVC
 ↓
PV
 ↓
EBS
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Linux Commands

Check Disk Usage 

df -h
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Shows:

Filesystem
Size
Used
Available
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Check Filesystem Size 

lsblk
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Check Memory 

free -h
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Check CPU 

top
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Check Processes 

ps aux
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Kill Process 

kill -9 PID
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Search File 

find / -name nginx.conf
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Check Port 

ss -tulpn
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or

netstat -tulpn
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Current Directory 

pwd
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List Files 

ls -la
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Copy 

cp file1 file2
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Move 

mv old new
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Delete 

rm -rf folder
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Change Permissions 

chmod 755 file
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Change Ownership 

chown ubuntu:ubuntu file
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Systemd

What is systemd?

Answer:
systemd is Linux service manager.

Manages:

  • Services
  • Boot process
  • Logs

Examples:

  • nginx
  • docker
  • kubelet

systemctl

Start Service 

sudo systemctl start nginx
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Stop Service 

sudo systemctl stop nginx
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Restart Service 

sudo systemctl restart nginx
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Service Status 

systemctl status nginx
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Enable On Boot 

sudo systemctl enable nginx
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journalctl

What is journalctl?

Answer:
Reads systemd logs.

View Logs 

journalctl
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Service Logs 

journalctl -u nginx
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Real-Time Logs 

journalctl -fu nginx
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apt

What is apt?

Answer:
Ubuntu package manager.

Install package

sudo apt install nginx -y
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Update repository

sudo apt update
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Upgrade packages

sudo apt upgrade -y
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sudo

What is sudo?

Answer:
Runs command as root.

Example:

sudo apt update
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Network Troubleshooting

Check IP Address 

ip a
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Check Routing Table 

ip route
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Test Connectivity 

ping 8.8.8.8
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DNS Test 

nslookup google.com
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or

dig google.com
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Trace Route 

traceroute google.com
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Check Open Ports 

ss -tulpn
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Test TCP Port 

telnet hostname 80
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or

nc -zv hostname 80
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Real Interview Scenario

Question: Website is down. How do you troubleshoot?

Answer:

  1. Check process 
ps aux | grep nginx
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  1. Check service 
systemctl status nginx
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  1. Check logs 
journalctl -u nginx
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  1. Check port 
ss -tulpn | grep 80
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  1. Check disk 
df -h
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  1. Check memory 
free -h
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  1. Check CPU 
top
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  1. Check connectivity 
ping
curl
nc
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  1. Check firewall/security group

  2. Check DNS

nslookup
dig
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  1. Check load balancer health checks

  2. Verify backend application is responding

This troubleshooting flow is exactly the type of answer expected from a mid-to-senior DevOps/SRE engineer during interviews.

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