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🐧 Linux & Command Line Basics

🎯 Overview

Essential Linux command line skills and terminal operations you need to work effectively in DevOps. This covers basic commands, file management, and shell operations that are fundamental to cloud and DevOps work.

📚 Key Concepts

What is the Command Line?

Command Line Interface (CLI) is a text-based interface for interacting with your computer. It's faster, more powerful, and essential for automation and DevOps work.

Why Learn CLI?

  • Automation: Script repetitive tasks
  • Remote access: Connect to servers via SSH
  • DevOps tools: Most DevOps tools are CLI-based
  • Cloud platforms: AWS CLI, Azure CLI, etc.
  • Server management: Most servers are Linux-based

Shell Basics

What is a Shell?

  • Definition: Program that interprets commands
  • Common shells: Bash (default), Zsh, Fish
  • Purpose: Interface between user and operating system

Terminal vs Shell

  • Terminal: Application that runs the shell
  • Shell: Program that executes commands
  • Examples: Terminal.app (macOS), Windows Terminal, iTerm2

🛠️ Essential Commands

Navigation Commands

File System Navigation

pwd          # Print working directory (show current location)
ls           # List files and directories
cd           # Change directory
mkdir        # Make directory
rmdir        # Remove empty directory

Common Navigation Patterns

cd /         # Go to root directory
cd ~         # Go to home directory
cd ..        # Go up one directory
cd -         # Go to previous directory
ls -la       # List all files (including hidden) with details

File Management

Viewing Files

cat          # Display file content
less         # View file content (scrollable)
head         # Show first 10 lines
tail         # Show last 10 lines
tail -f      # Follow file changes (useful for logs)

File Operations

cp           # Copy files/directories
mv           # Move/rename files/directories
rm           # Remove files
touch        # Create empty file or update timestamp

File Permissions

chmod        # Change file permissions
chown        # Change file owner
ls -l        # List with permissions

Permission Examples:

chmod +x script.sh    # Make executable
chmod 755 file.txt    # Set specific permissions
chmod 644 file.txt    # Read/write for owner, read for others

Text Processing

Search and Filter

grep         # Search for text patterns
find         # Find files by name/attributes
which        # Find command location
whereis      # Find command, source, and manual pages

Text Manipulation

echo         # Print text
sort         # Sort lines
uniq         # Remove duplicate lines
wc           # Count lines, words, characters
cut          # Extract sections from lines

System Information

System Commands

ps           # Show running processes
top          # Show system resources (interactive)
htop         # Enhanced top (if available)
df           # Show disk space
du           # Show directory size
free         # Show memory usage

Network Commands

ping         # Test network connectivity
curl         # Transfer data from/to server
wget         # Download files
netstat      # Show network connections
ss           # Show socket statistics

🔧 Shell Features

Command History

history      # Show command history
!n           # Execute command number n
!!           # Execute last command
Ctrl+R       # Search command history

Tab Completion

  • Press Tab to auto-complete commands, filenames, paths
  • Press Tab twice to show all possible completions
  • Essential for efficiency and avoiding typos

Command Chaining

command1 && command2    # Run command2 only if command1 succeeds
command1 || command2    # Run command2 only if command1 fails
command1 ; command2     # Run both commands regardless
command1 | command2     # Pipe output from command1 to command2

Redirection

command > file          # Redirect output to file (overwrite)
command >> file         # Redirect output to file (append)
command < file          # Redirect input from file
command 2>&1            # Redirect stderr to stdout

📁 File System Structure

Linux Directory Structure

/                   # Root directory
├── bin/            # Essential binaries
├── boot/           # Boot loader files
├── dev/            # Device files
├── etc/            # Configuration files
├── home/           # User home directories
├── lib/            # Shared libraries
├── media/          # Mount points for removable media
├── mnt/            # Mount points
├── opt/            # Optional applications
├── proc/           # Process information
├── root/           # Root user home
├── run/            # Runtime data
├── sbin/           # System binaries
├── srv/            # Service data
├── sys/            # System information
├── tmp/            # Temporary files
├── usr/            # User programs and data
└── var/            # Variable data (logs, etc.)

Important Directories for DevOps

  • /etc/: Configuration files
  • /var/log/: System and application logs
  • /home/: User directories
  • /tmp/: Temporary files
  • /opt/: Third-party applications

🔍 Working with Files

File Types

  • Regular files: Text, binary, executable
  • Directories: Contain other files
  • Symbolic links: Pointers to other files
  • Device files: Represent hardware devices

File Permissions

-rw-r--r--  1 user group 1234 Jan 1 12:00 file.txt
││││││││││   │ │    │     │   │        │
││││││││││   │ │    │     │   │        └── Filename
││││││││││   │ │    │     │   └────────── Date
││││││││││   │ │    │     └────────────── Size
││││││││││   │ │    └──────────────────── Group
││││││││││   │ └───────────────────────── Owner
││││││││││   └──────────────────────────── Links
│││││││││└──────────────────────────────── Others (read)
││││││││└───────────────────────────────── Others (write)
│││││││└────────────────────────────────── Others (execute)
││││││└─────────────────────────────────── Group (execute)
│││││└──────────────────────────────────── Group (write)
││││└───────────────────────────────────── Group (read)
│││└────────────────────────────────────── Owner (execute)
││└─────────────────────────────────────── Owner (write)
│└──────────────────────────────────────── Owner (read)
└───────────────────────────────────────── File type (- = file, d = directory, l = link)

🚀 Practical Examples

Common DevOps Tasks

Log Analysis

# View recent log entries
tail -n 50 /var/log/application.log

# Search for errors
grep "ERROR" /var/log/application.log

# Monitor log in real-time
tail -f /var/log/application.log

File Management

# Create backup of configuration
cp /etc/nginx/nginx.conf /etc/nginx/nginx.conf.backup

# Find large files
find /home -type f -size +100M

# Count files in directory
ls -1 | wc -l

System Monitoring

# Check disk space
df -h

# Check memory usage
free -h

# Check running processes
ps aux | grep nginx

📋 Self-Check Questions

Basic Commands

  1. Q: What command shows your current directory? A: pwd

  2. Q: How do you list all files including hidden ones? A: ls -la

  3. Q: What does cd .. do? A: Goes up one directory level

File Operations

  1. Q: How do you copy a file? A: cp source destination

  2. Q: What command removes a file? A: rm filename

  3. Q: How do you make a file executable? A: chmod +x filename

Text Processing

  1. Q: How do you search for text in a file? A: grep "text" filename

  2. Q: What does tail -f do? A: Follows file changes in real-time

🎯 Practice Exercises

Beginner Level

  1. Navigate directories: Use cd, ls, pwd
  2. Create files and directories: Use mkdir, touch
  3. Copy and move files: Use cp, mv
  4. View file contents: Use cat, less, head, tail

Intermediate Level

  1. Search for files: Use find and grep
  2. Check system resources: Use top, df, free
  3. Work with permissions: Use chmod, chown
  4. Use command chaining: Use &&, ||, |

Advanced Level

  1. Write simple scripts: Create bash scripts
  2. Process text data: Use awk, sed
  3. Network troubleshooting: Use ping, curl, netstat
  4. System administration: Monitor processes and logs

🔗 Additional Resources

Online Terminals

Learning Resources

Practice Platforms

🔗 Related Prerequisites

Ready for the next step? Move on to Networking Fundamentals to understand network concepts!