Add Linux DevOps notes (Day 1-3)

01_Linux_Basics.md

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+# Linux – Basic Commands (Day 1)
+
+## What is Linux?
+Linux is an operating system used to run servers and applications.
+
+## Why Linux is important for DevOps?
+Most servers run on Linux, and DevOps engineers manage everything using Linux commands.
+
+## Linux Folder Structure
+- /      → root
+- /home  → user files
+- /etc   → configuration files
+- /var   → logs and data
+- /bin   → system commands
+
+## Navigation Commands
+```bash
+pwd        # show current directory
+ls         # list files
+ls -l      # detailed list
+ls -a      # show hidden files
+cd folder  # move into folder
+cd ..      # go back
+cd ~       # go to home
+```
+
+## Create & Delete Commands
+```bash
+mkdir app
+touch file.txt
+rm file.txt
+rm -r app
+```
+
+## Working with File Content
+```bash
+cat file.txt
+head file.txt
+echo "Linux for DevOps" > info.txt     # write content
+echo "Day 1 completed" >> info.txt    # append content
+cat info.txt                          # view file content
+``` 
+
+## Daily Useful Linux Commands
+```bash
+clear       # clear terminal screen
+history     # show command history
+whoami      # show current user
+uname -a    # show system information
+```
+
+
+

02_Linux_Permissions.md

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+# Linux – File Permissions (Day 2)
+
+## What are File Permissions?
+File permissions control **who can read, write, or execute** a file or folder in Linux.
+
+They protect the system from:
+- accidental deletion
+- unauthorized access
+- hacking
+
+---
+
+## Why Permissions are Important for DevOps?
+DevOps engineers manage **live servers**.
+Wrong permissions can:
+- break applications
+- expose data
+- allow hackers access
+
+So permissions are **critical**.
+
+---
+
+## Permission Types
+
+There are 3 permissions:
+
+- r → read
+- w → write
+- x → execute
+
+---
+
+## Permission Groups
+
+Permissions are set for 3 types of users:
+
+- u → user (owner)
+- g → group
+- o → others
+
+Order is always:
+
+## user | group | others
+
+---
+
+## Checking Permissions
+
+```bash
+ls -l
+-rwxr-xr--
+Meaning:
+
+user → rwx
+
+group → r-x
+
+others → r--
+
+Add values to set permissions.
+
+Examples:
+
+7 = rwx (4+2+1)
+
+6 = rw- (4+2)
+
+5 = r-x (4+1)
+
+4 = r--
+```
+
+---
+
+## Setting Permissions
+
+```bash
+chmod 755 file.txt
+```
+
+Meaning:
+
+user → 7 (rwx)
+
+group → 5 (r-x)
+
+others → 5 (r-x)
+
+## Symbolic Method
+
+```bash
+chmod u+x file.sh
+chmod g-w file.sh
+chmod o+r file.sh
+```
+
+## Common Permission Examples
+
+```bash
+chmod 755 file.txt
+chmod 644 file.txt
+chmod 777 file.txt
+chmod 666 file.txt
+chmod 700 file.txt
+chmod 600 file.txt
+```
+
+## Execute Permission (Very Important)
+
+To run a script:
+
+```bash
+./script.sh
+```
+
+
+If permission denied:
+
+```bash
+chmod +x script.sh
+```
+
+## Folder Permissions Rule
+
+```bash
+r → list files
+w → create/delete files
+x → enter folder
+```
+
+    Without x, you cannot cd into folder.
+
+## Execute Permission (Very Important)
+
+To run a script:
+
+```bash
+./script.sh
+```
+
+
+If permission denied:
+
+```bash
+chmod +x script.sh
+```
+
+## Folder Permissions Rule
+
+```bash
+r → list files
+
+w → create/delete files
+
+x → enter folder
+```
+Without x, you cannot cd into folder.
+
+
+## Mini Practice Task
+
+```bash
+mkdir test
+touch test/app.sh
+ls -l test
+chmod 755 test/app.sh
+ls -l test
+```
+
+## Common Mistakes
+
+- Giving 777 permissions (very dangerous)
+- Not understanding folder execute permission
+- Changing permissions blindly on production servers
+
+
+## Changing Ownership
+
+To change the owner of a file:
+```bash
+sudo chown username file.txt
+```
+
+To change both owner and group:
+```bash
+sudo chown username:groupname file.txt
+```
+
+To change the group ownership:
+```bash
+sudo chgrp groupname file.txt
+```
+
+## Recursive Changes
+Apply permissions or ownership to a directory and all its contents:
+```bash
+chmod -R 755 folder_name/
+chown -R username:groupname folder_name/
+```
+
+## Checking Current Permissions
+To see detailed permissions for all files in a directory:
+```bash
+ls -la
+```
+

03_Linux_Users_Ownership.md

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+# Linux – Users & Ownership (Day 3)
+
+## What are Users in Linux?
+Linux is a multi-user system.
+Each file and process runs under a specific user.
+
+This prevents:
+- accidental damage
+- security issues
+- one app affecting another
+
+---
+
+## Why Users Matter in DevOps?
+Applications **never run as root** in production.
+Each service has its own user for safety.
+
+DevOps engineers:
+- create users
+- assign ownership
+- control access
+
+---
+
+## User Types
+- root → superuser (full access)
+- normal user → regular work
+- system user → runs services (nginx, docker)
+
+---
+
+## Check Current User
+```bash
+whoami
+```
+
+## View All Users
+```bash
+cat /etc/passwd
+```
+
+## Create a New User
+```bash
+sudo useradd devuser
+```
+
+## Set password:
+```bash
+sudo passwd devuser
+```
+
+## Switch User
+```bash
+su devuser
+```
+
+## Exit user
+```bash
+exit
+```
+
+## Groups in Linux
+
+Users belong to groups.
+Groups help manage permissions easily.
+
+## Check groups:
+
+```bash
+groups
+```
+
+## Add user to group:
+
+```bash
+sudo usermod -aG sudo devuser
+```
+
+## File Ownership
+
+Each file has:
+
+owner
+
+group
+
+## Check ownership:
+
+```bash
+ls -l
+```
+
+## Change Ownership (chown)
+```bash
+sudo chown devuser file.txt
+sudo chown devuser:devuser file.txt
+```
+
+## Change Group Only
+```bash
+sudo chgrp devuser file.txt
+```
+
+## Real DevOps Scenario
+
+Application folder:
+
+owned by app user
+
+not by root
+
+correct permissions
+
+Example:
+
+```bash
+sudo mkdir /var/myapp
+sudo chown devuser:devuser /var/myapp
+sudo chmod 755 /var/myapp
+```
+
+## Common Mistakes
+
+Running apps as root
+
+Wrong ownership causing permission denied
+
+Using sudo unnecessarily
+
+DevOps Rule
+
+❌ root everywhere
+✅ least privilege access
+
+
+## Exit user:
+
+```bash
+exit
+```
+
+## Groups in Linux
+
+Users belong to groups.
+Groups help manage permissions easily.
+
+## Check groups:
+
+groups
+
+## Add user to group:
+
+sudo usermod -aG sudo devuser
+
+## File Ownership
+
+Each file has:
+
+owner
+
+group
+
+## Check ownership:
+
+```bash
+ls -l
+```
+
+## Change Ownership (chown)
+```bash
+sudo chown devuser file.txt
+sudo chown devuser:devuser file.txt
+```
+
+## Change Group Only
+```bash
+sudo chgrp devuser file.txt
+```
+
+## Real DevOps Scenario
+
+Application folder:
+
+owned by app user
+
+not by root
+
+correct permissions
+
+Example:
+
+sudo mkdir /var/myapp
+sudo chown devuser:devuser /var/myapp
+sudo chmod 755 /var/myapp
+
+## Common Mistakes
+
+Running apps as root
+
+Wrong ownership causing permission denied
+
+Using sudo unnecessarily
+
+## DevOps Rule
+
+❌ root everywhere
+✅ least privilege access

04_Linux_Networking_Basics.md

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+# Linux – Networking Basics (Day 4)
+
+## What is Networking in Linux?
+Networking means how computers and servers communicate with each other
+using IP addresses, ports, and protocols.
+
+Every DevOps engineer must understand networking
+because applications run on networks.
+
+---
+
+## Why Networking is Important for DevOps?
+If networking fails:
+- app will not open
+- server looks “down”
+- deployment fails
+
+Most DevOps issues are actually network issues.
+
+---
+
+## IP Address
+An IP address identifies a machine on a network.
+
+Check IP:
+```bash
+ip a
+```
+or
+```bash
+ifconfig
+```
+
+---
+
+## Hostname
+```bash
+hostname
+```
+
+---
+
+## Ping Command
+```bash
+ping google.com
+```
+Stop ping:
+```bash
+Ctrl + C
+```
+
+---
+
+## curl Command (VERY IMPORTANT)
+```bash
+curl google.com
+curl -I google.com
+```
+
+---
+
+## wget Command
+```bash
+wget https://example.com/file.txt
+```
+
+---
+
+## Ports (CRITICAL CONCEPT)
+Common ports:
+- 22 → SSH
+- 80 → HTTP
+- 443 → HTTPS
+- 3306 → MySQL
+
+---
+
+## Check Open Ports
+```bash
+netstat -tuln
+```
+or
+```bash
+ss -tuln
+```
+
+---
+
+## Check Which Process Uses a Port
+```bash
+sudo netstat -tulnp
+```
+
+---
+
+## DNS Resolution
+```bash
+nslookup google.com
+dig google.com
+```
+
+---
+
+## Test Server Connectivity
+```bash
+telnet google.com 80
+```
+
+---
+
+## Real DevOps Scenario
+Commands used:
+```bash
+curl
+ping
+netstat
+ss
+```
+
+---
+
+## Common Mistakes
+- Forgetting port number
+- Confusing localhost and public IP
+
+---
+
+## DevOps Rule
+If something doesn’t work → check network first.
+
+---
+
+## Summary
+Linux networking basics: IP, ports, ping, curl, DNS, and open ports.

05_Linux_SSH_Remote_Access.md

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+# Linux – SSH & Remote Server Access (Day 5)
+
+## What is SSH?
+SSH (Secure Shell) is used to securely connect to a remote server.
+DevOps engineers use SSH daily to manage cloud servers.
+
+---
+
+## Why SSH is Important for DevOps?
+- Access cloud servers (AWS EC2, VPS)
+- Deploy applications
+- Debug production issues
+- Manage configurations
+
+No SSH = no DevOps work.
+
+---
+
+## Basic SSH Syntax
+```bash
+ssh user@server_ip
+```
+
+Example:
+```bash
+ssh ubuntu@13.234.56.78
+```
+
+---
+
+## Default SSH Port
+- Port 22
+
+Custom port example:
+```bash
+ssh -p 2222 user@server_ip
+```
+
+---
+
+## SSH First-Time Connection
+You may see:
+```
+Are you sure you want to continue connecting (yes/no)?
+```
+
+Type:
+```text
+yes
+```
+
+---
+
+## SSH Key-Based Authentication (VERY IMPORTANT)
+
+Instead of passwords, DevOps uses SSH keys.
+
+Benefits:
+- More secure
+- No password typing
+- Required by cloud providers
+
+---
+
+## Generate SSH Key
+```bash
+ssh-keygen
+```
+
+Press Enter for defaults.
+
+This creates:
+- private key → ~/.ssh/id_rsa
+- public key → ~/.ssh/id_rsa.pub
+
+---
+
+## Copy SSH Key to Server
+```bash
+ssh-copy-id user@server_ip
+```
+
+Or manually copy:
+```bash
+cat ~/.ssh/id_rsa.pub
+```
+
+Paste into server:
+```bash
+~/.ssh/authorized_keys
+```
+
+---
+
+## Login Using SSH Key
+```bash
+ssh user@server_ip
+```
+
+(No password needed)
+
+---
+
+## SSH Config File (PRO TIP)
+Create:
+```bash
+~/.ssh/config
+```
+
+Example:
+```text
+Host myserver
+    HostName 13.234.56.78
+    User ubuntu
+    IdentityFile ~/.ssh/id_rsa
+```
+
+Now connect using:
+```bash
+ssh myserver
+```
+
+---
+
+## Common SSH Commands
+```bash
+exit            # logout
+whoami          # check user
+hostname        # server name
+uptime          # server running time
+```
+
+---
+
+## Common SSH Errors
+- Permission denied
+- Wrong user
+- Wrong key
+- Port blocked by firewall
+
+---
+
+## DevOps Best Practices
+- Disable password login
+- Use key-based auth only
+- Never share private key
+- Use least privilege users
+
+---
+
+## Real DevOps Scenario
+1. Create EC2 instance
+2. SSH into server
+3. Install Docker
+4. Deploy application
+
+SSH is the entry point.
+
+---
+
+## Summary
+Today I learned SSH basics, key-based authentication,
+and how DevOps engineers securely connect to remote servers.