Essential Linux Commands for Cybersecurity

Introduction

• Focus: Essential Linux commands and their advanced flags to enhance skills in cybersecurity.
• Necessity: Essential before delving into advanced tools like Metasploit, Nmap, Hydra.
• Approach: Using the terminal, not a file explorer.

Basic Navigation

ls Command

• Usage: List directory contents.
• Flags:
  • -l: Long listing format.
  • -a: Show all files, including hidden ones.
  • -t: Sort by modification time, newest first.
  • -h: Human-readable sizes.
  • Combination: ls -lath for detailed info, human-readable sizes, and hidden files.

cd Command

• Usage: Change directories.
• Shortcuts:
  • cd -: Go back to the previous directory.
  • cd ..: Move up one directory level.
  • cd ~: Go to the home directory.
  • cd /path/to/dir: Directly change to a specified directory.
• Auto-completion: Use the Tab key for auto-completing directory names.
• Displaying current directory: pwd command shows the present working directory.

File and Directory Manipulation

Creating Files and Directories

• touch: Create blank files.
• mkdir: Create directories.
  • mkdir -p /path/to/dir: Creates nested directories in one command.
  • mkdir -m 777 dir: Set permissions while creating a directory.

Copying Files and Directories

• cp:
  • cp source destination: Copy files.
  • cp -r source destination: Copy directories recursively.

Removing Files and Directories

• rm: Remove files.
  • rm -r directory: Remove directories and their contents recursively.
  • rm -rf /path: Forcefully remove files/directories.

Moving and Renaming Files

• mv: Move or rename files and directories.
  • mv source destination: Move/rename files or directories.

Displaying and Viewing File Contents

cat Command

• Usage: Display file contents.
• Advanced:
  • Combine files: cat file1 file2 > combined_file.
  • Append to files: cat file1 >> file2.

less and more Commands

• Usage: View large files page by page.
• Advanced:
  • less file: View file with search capability.
  • more file: Similar to less but with fewer features.

tail and head Commands

• tail: View the end of the file.
  • tail -n 15 file: View the last 15 lines.
  • tail -f file: Follow file updates in real-time.
• head: View the beginning of the file.
  • head -n 15 file: View the first 15 lines.

Editing Files

Text Editors

• vi/vim:
  • Mode-based editor: Insert mode, Command mode.
  • Basic commands: i to insert, :w to save, :q to quit.
• nano:
  • Simpler editor with on-screen navigation.
  • Basic commands: Ctrl + X to exit, Y to confirm changes.

Managing Processes

Viewing Processes

• ps: Display process information.
  • ps aux: Detailed information of all processes.
• top: Interactive view of running processes.
  • Sorting by usage.
  • top -u username: View processes for a specific user.

Killing Processes

• kill: Terminate a process.
  • kill PID: Terminate a process by PID.
  • kill -9 PID: Forcefully terminate a process.

Background Processes

• &: Run processes in the background.
  • command &: Run command in the background.
  • jobs: List background jobs.
  • fg %job_number: Bring background job to foreground.

Networking Commands

IP Configuration

• ifconfig: Display network interface information (deprecated).
• ip: Modern replacement for ifconfig.
  • ip addr: Show IP addresses.
  • ip route: Display routing table.
  • ip route add: Add a new route.
  • ip addr add: Add a new IP address.

Network Statistics

• netstat: Display network connections, routing tables, interface statistics.
  • netstat -l: List listening connections.
• ss: Display network socket information.
  • ss -t: Show TCP sockets.
  • ss -u: Show UDP sockets.
  • ss -p: Show processes using sockets.

Secure Connections

• ssh: Secure Shell for remote login.
  • ssh user@hostname: Connect to a remote server.
  • -p port: Specify port.
• scp: Secure Copy Protocol for transferring files.
  • scp source user@hostname:/path: Copy files to a remote server.

System Information

System Details

• uname -a: Display system information.

Disk Usage

• df: Display disk space usage.
  • df -h: Human-readable format.
  • df -T: Show file system types.

Finding Files

• find: Search for files in a directory hierarchy.

  • find /path -name filename: Find files by name.
  • -size +1M: Find files larger than 1MB.
  • -mtime -30: Find files modified in the last 30 days.

• grep: Search inside files.

  • grep pattern files: Search for a pattern in files.
  • grep -r pattern /path: Recursively search.

Archiving and Compression

tar Command

• Usage: Archive files.
  • tar -czf archive.tar.gz files: Create a compressed archive.
  • tar -xvf archive.tar.gz: Extract files from an archive.
  • tar -rf archive.tar files: Add files to an existing archive.

gzip Command

• Usage: Compress files.
  • gzip file: Compress a file.
  • gunzip file.gz: Decompress a file.
  • gzip -l file.gz: List contents of a compressed file.

Managing Users

• useradd: Add a new user.

  • useradd username: Basic usage.
  • useradd -m username: Create home directory.
  • useradd -g groupname username: Assign user to a group.

• userdel: Delete a user.

  • userdel username: Basic usage.
  • userdel -r username: Remove user and home directory.

• groupadd: Add a new group.

  • groupadd groupname.

• groupdel: Delete a group.

  • groupdel groupname.

Conclusion

• Recap: Covered essential Linux commands for navigation, file manipulation, process management, networking, system info, finding files, archiving, and user management.
• Encouragement: Master these commands before moving on to advanced tools.