Linux Operating System – Complete DevOps Study Notes

Source: Dev.to

Part 1 – Linux Fundamentals & Architecture

1. What is Linux?

Linux is an open‑source, Unix‑like operating‑system kernel. In a DevOps context “Linux” usually refers to a distribution (e.g., Ubuntu, CentOS, Alpine) that bundles the kernel, system utilities, and a package manager.

2. Linux File‑System Hierarchy

Linux uses a single hierarchical tree that starts at the root /.

/
├─ bin & /usr/bin      – Essential user binaries (ls, cp, …)
├─ etc                – Configuration files (e.g., /etc/nginx/nginx.conf)
├─ home               – User home directories (e.g., /home/john)
├─ var                – Variable data (logs, spool files, temporary e‑mail)
├─ tmp                – Temporary files (cleared on reboot)
└─ proc               – Virtual FS exposing process & kernel info

Part 2 – Essential File Management & Permissions

1. File‑Management Commands

2. File Permissions

Permissions are split into User (u), Group (g), and Others (o).

Common commands

chmod 755 file          # u=rwx (7), g=rx (5), o=rx (5)
chmod +x script.sh      # add execute bit
chown user:group file   # change owner & group
chgrp group file        # change group only

Part 3 – User, Group, & Package Management

1. User & Group Management

Typical commands

# Create a user with a home directory and Bash shell
useradd -m -s /bin/bash username

# Add user to the sudo group
usermod -aG sudo username

# Set (or change) password
passwd username

# Show UID/GID information
id username

2. Package Management

Part 4 – Networking & Firewall

1. Network Configuration & Troubleshooting

2. Linux Firewalls

Examples

# UFW
ufw allow 22/tcp          # allow SSH
ufw enable                # enable firewall

# firewalld
firewall-cmd --permanent --add-port=80/tcp
firewall-cmd --reload

# iptables (basic example)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

Part 5 – Processes, Systemd, & Boot

1. The Boot Process

1. BIOS/UEFI – hardware POST, loads bootloader.
2. Bootloader (GRUB) – loads the kernel image.
3. Kernel – mounts the root filesystem, starts init.
4. init (systemd) – PID 1, launches user‑space services.

2. Systemd (Service Management)

systemctl start nginx          # start now
systemctl enable nginx         # start on boot
systemctl status nginx         # view status
journalctl -u nginx            # view service‑specific logs

3. Monitoring & Troubleshooting

Part 6 – Shell Scripting (Bash)

Automation is the heart of DevOps.

#!/bin/bash
# -------------------------------------------------
# Example Bash script
# -------------------------------------------------

# Variables
NAME="DevOps Engineer"
DIR="/var/www/html"

# Conditionals
if [ -d "$DIR" ]; then
    echo "Directory exists."
else
    mkdir -p "$DIR"
    echo "Directory created."
fi

# Loops
for i in {1..5}; do
    echo "Iteration $i"
done

Key concepts

Part 7 – SSH Configuration & Security

Security Best Practices (Hardening)

1. Disable root login – set PermitRootLogin no.
2. Disable password authentication – set PasswordAuthentication no.
3. Change the default port (optional) – e.g., Port 2022.

Key‑Based Authentication

# On the client
ssh-keygen -t rsa -b 4096          # generate key pair
ssh-copy-id user@remote-server    # copy public key to server

# On the server (verify permissions)
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

Part 8 – Advanced Concepts & Cloud

1. Web‑Server Administration

Logs are usually found under /var/log/nginx/ or /var/log/httpd/.

2. Reading Access Logs

• File: /var/log/nginx/access.log
• Purpose: Debug 404/500 errors.

3. Linux in the Cloud (AWS / Azure / GCP)

• Cloud‑Init – runs once when a cloud instance boots to install packages and write files.
• Ephemeral Storage – some cloud disks disappear on termination; be aware of their transient nature.
• Metadata Services – query http://169.254.169.254 from within the VM to retrieve instance information (IP, region, etc.).

4. Text Processing (The “Swiss Army Knives”)

DevOps engineers frequently parse logs using these tools:

• grep – Search for text

  grep "error" file.log

• awk – Print specific columns

  awk '{print $1}' file.txt

• sed – Find and replace text

  sed 's/old/new/g' file.txt

Checkout this for advanced study on Linux for DevOps.