Common Mistakes I See Students Make

By Ravindra Bagale Last updated: 18 October 2026

Friends, students almost always make these mistakes in their first week of Linux. Read them once – later, on the server, both your time and your data will be saved.

Typing cd /Home or Index.html. Linux paths are case-sensitive.
Using chmod 777 to fix every error. It is insecure and usually hides a wrong owner or SELinux label.
usermod -G without -a. This replaces all the user's groups instead of adding one — the user may lose sudo access.
Running everything as root with sudo -i and then creating files that the normal user or web server cannot edit.
Using apt on Amazon Linux or yum on Ubuntu. First check cat /etc/os-release.
Getting stuck in vim. Remember: Esc then :q!.
rm -rf with a wrong path. There is no recycle bin — read the command twice before pressing Enter.

Extended notes for this AWS lesson

Why students get stuck here

AWS pages look simple until three things collide: the service setting, the VPC/security group, and the app configuration. The expansions below separate those layers with real stacks and sample commands.

Most "it does not connect" tickets miss one of these three.

The same pattern in app stacks

Whether the data store is RDS MySQL, PostgreSQL, or another engine, apps read an endpoint + user + password from the environment.

Java / Spring Boot

spring.datasource.url=jdbc:mysql://ENDPOINT:3306/db with password from Secrets Manager / env.

Python / Django / Flask

HOST from env; never commit settings with real hosts.

Node.js

mysql2/pg clients with process.env.DB_HOST.

.NET

ConnectionStrings via env or Parameter Store.

PHP / Laravel

DB_HOST in .env, not in Git.

database/sql with env-based DSN.

Indian company (sample)

Suppose we are a junior cloud engineer at Swiggy or Razorpay-style infrastructure. A new microservice cannot reach RDS. You check: 1) SG on RDS allows the app SG on 3306/5432. 2) App is in a private subnet with a route to the DB subnet. 3) The secret in Secrets Manager matches the rotated password. 4) You connect with the mysql/psql client from the app host before blaming the ORM.

Global practice

AWS Well-Architected Security and Reliability pillars: least-privilege SG, Multi-AZ for production relational DBs, automated backups, no public DB reachability.

Everyday analogy

A warehouse (RDS) should not open onto the street. Delivery bikes (app servers) enter through a staff gate (security group) with a badge (password/IAM auth).

Engines and ports

MySQL / MariaDB on RDS

Port 3306. Users and grants like self-managed MySQL.

PostgreSQL on RDS

Port 5432. Prefer scram auth.

SQL Server on RDS

Port 1433. Common with .NET shops.

Amazon Aurora

MySQL or PostgreSQL compatible; learn the endpoint types (cluster vs reader).

Load balancers in one breath

ALB is for HTTP/HTTPS apps (Spring, Node, Django, .NET). NLB is for extreme performance/TCP. GLB is for virtual appliances. Classic LB is legacy—prefer ALB/NLB for new work. Always put TLS on the balancer with ACM when you can.

Common EC2 and Linux mistakes (expanded)

Losing the SSH key → use SSM Session Manager next time.
Editing on the server without backups → keep AMI + user-data scripts.
Opening 0.0.0.0/0 on SSH → restrict to your IP or VPN.
Forgetting Elastic IP / DNS when testing restarts.
Growing EBS without extending the Linux filesystem (growpart + resize2fs/xfs_growfs).

Common mistakes

Publicly accessible RDS "for a quick test" left open.
Mixing Classic LB advice with ALB listener rules.
Hard-coding IPs instead of DNS names for RDS endpoints.
Taking snapshots but never testing restore.

Worksheet

Draw your VPC boxes: public subnet, private subnet, RDS subnet.
Write the SG rules in a table.
From the app host, run a one-line DB client connect.
Take a snapshot and write the restore steps without looking.

Recap

Service + network + app config.
Private databases.
Prefer ALB + ACM for web apps.
Practise restore, not only backup.