• 3 Best Ways to Securely Wipe Disk in Linux: shred, wipe

  sudo shred -v /dev/sda
  # or (if too slow)
  sudo shred -v -n 1 /dev/sda

• show all logged in users: users
• show user/groups assignments and identifiers: id

• /etc/passwd: defines users, their primary groups and login shell
• /etc/shadow: password (hash) database
• /etc/groups: defines all other groups a user is in

File /etc/sudoers defines users (and options) related to command sudo. It might make sense to specify a varying umask in ~/.bashrc.

if [[ $(id -u) -eq 0 ]]; then
   umask 0022
else
   umask 0027
fi

• show iptables firewall rules: iptables -L -n -v
• show Uncomplicated Firewall (UFW) rules: sudo ufw status verbose
• show all predefined UFW application profiles: sudo ufw app list
• establish rules from predefined UFW application profile: sudo ufw allow from 192.168.178.0/24 to any app samba4
• delete allow rules from UFW application profile: sudo ufw delete allow from 192.168.178.0/24 to any app samba4

• /etc/ufw/applications.d/*: predefined UFW application profiles
• /var/log/ufw.log: UFW log (for example blocked packets)