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Topic 5: Linux Bash basics

You need to have CLI skills to work with cloud. We recommend you read Linux Basics for Hackers. You could also go through the list of commands below and research them individually, though we recommend the book.

Foundational Commands​

Before starting the challenges, familiarize yourself with these essential commands:

  • pwd: Print working directory - shows your current location
  • cd: Change directory - navigate between folders
  • ls: List directory contents
  • cat: Display file contents

Challenge 1: Hidden Files​

Learn these commands to find hidden files:

  • ls -a: List all files, including hidden ones
  • ls -la: Detailed listing including hidden files
  • find . -name ".*": Search for hidden files

Key concepts:

  • In Linux, hidden files start with a dot (.)
  • Hidden files are not shown by default
  • Use -a flag with ls to show hidden files

Challenge 2: File Searching​

Master these file search commands:

  • find /path -name "pattern": Search for files by name
  • find /path -type f: Search for regular files
  • locate filename: Quick file search using database
  • grep -r "text" /path: Search file contents recursively

Challenge 3: File Size Analysis​

Commands for analyzing file sizes:

  • ls -lh: List files with human-readable sizes
  • du -h: Display disk usage
  • sort -h: Sort by human-readable sizes
  • find /path -type f -size +1M: Find files larger than 1MB

Challenge 4: User Investigation​

Commands for user management:

  • id username: Display user ID and groups
  • cat /etc/passwd: View user accounts
  • getent passwd uid: Get user by UID
  • ls -l /home/: List home directories

Understanding user files:

  • /etc/passwd: User account information
  • ~/.profile: User's shell profile
  • ~/.bashrc: Shell configuration

Challenge 5: Permission Analysis​

Commands for working with permissions:

  • ls -l: View file permissions
  • stat filename: Detailed file information
  • find / -perm 777: Find files with specific permissions
  • chmod: Change file permissions

Challenge 6: Network Services​

Network investigation commands:

  • netstat -tuln: List listening ports
  • ss -tuln: Modern alternative to netstat
  • lsof -i: List open network files
  • curl localhost:port: Test HTTP service
  • nc -zv host port: Test TCP connection

Challenge 7: Encoding/Decoding​

Text processing commands:

  • base64: Encode/decode base64
  • echo -n "text": Echo without newline
  • |: Pipe operator for command chaining

Challenge 8: SSH Configuration​

SSH-related commands:

  • ls -la ~/.ssh/: List SSH directory contents
  • find ~/.ssh -type f: Find SSH-related files
  • chmod 600: Set correct SSH key permissions
  • ssh-keygen: Generate SSH keys

SSH security best practices:

  • Keep private keys secure
  • Set correct permissions (600 for keys)
  • Check hidden directories
  • Review authorized_keys files

General Tips​

  1. Use command combinations

  2. Navigate efficiently:

    • Use tab completion
    • Use cd - to go to previous directory
    • Use cd .. to go up one level

  3. Debug your commands:

    • Add -v or --verbose for more information
    • Check command syntax with man
    • Use which command to verify command location

Remember: The goal is to learn the commands and understand how they work. Take time to experiment with each command and understand its options and behavior.