A quick primer on common malware types, the GNU project’s role in free software, and two handy Linux tools for viewing disk layout and usage.

Malware family quick definitions

• Malware (umbrella term): Any software designed to harm, steal, spy, or disrupt.
• Virus: Malicious code that attaches to other files/programs and spreads when they run.
• Worm: Self-propagating malware that spreads over networks without user action.
• Trojan horse: Disguised as legitimate software; once run, it performs hidden malicious actions.
• Spyware: Collects user activity/data without consent; can log keystrokes, browser habits, or grab screenshots.

The GNU project (briefly)

• Launched by Richard Stallman to build a completely free (libre) Unix-like OS.
• Gave us core userland tools (gcc, glibc, bash, coreutils) that, combined with the Linux kernel, form “GNU/Linux.”
• Defined the GPL license to ensure software freedoms (use, study, modify, share).

Useful commands: tree and ncdu

• tree: Shows a directory tree structure.

  sudo apt-get install tree   # Debian/Ubuntu
  brew install tree          # macOS (Homebrew)
  
  tree                       # show current dir
  tree -L 2                  # limit depth to 2
  tree -a                    # include hidden files
  

• ncdu: NCurses Disk Usage—interactive disk usage viewer.

  sudo apt-get install ncdu   # Debian/Ubuntu
  brew install ncdu          # macOS (Homebrew)
  
  ncdu                       # analyze current dir
  ncdu /var/log              # analyze a specific path
  

  Use arrow keys to navigate; press d to delete selected entries (careful!).

Safety tips

• Download software from trusted sources; verify signatures/checksums.
• Keep OS and packages patched; run AV/EDR where appropriate.
• Least privilege: avoid running as root unless necessary.
• Back up regularly; have a recovery and incident response plan.

With these fundamentals you can explain common malware, the GNU project’s importance, and quickly inspect file trees and disk usage with tree and ncdu.