Debian follows the Filesystem Hierarchy Standard (FHS), a standard layout for Linux filesystems. Understanding the directory structure helps you find files, configure systems, and write portable scripts.

Key directories:

• /bin, /usr/bin — executable programs (built-in commands and utilities)
• /sbin, /usr/sbin — system programs (usually require root)
• /lib, /usr/lib — libraries and shared objects
• /etc — system configuration files (editable)
• /home — user home directories (e.g., /home/alice/)
• /root — root user's home directory
• /tmp — temporary files (cleared on reboot)
• /var — variable data: logs (/var/log), cache (/var/cache), databases
• /opt — optional third-party software
• /usr/share — documentation, data files, locale files
• /usr/local — locally compiled software (doesn't get overwritten by package updates)

$ ls /                          # list root directories
$ df -h                         # show disk usage by filesystem
$ mount                         # show mounted filesystems
$ du -sh /var/log              # disk usage of a directory

Tip: Configuration files in /etc usually have .dist backups. Always back up /etc before making changes—it's your system's personality.