1.1.1 Filesystem Hierarchy Standard (FHS) Explained

1.1.1 Filesystem Hierarchy Standard (FHS) Explained #

The Filesystem Hierarchy Standard (FHS) defines the directory structure in Linux, ensuring consistency across distributions. Below is a breakdown of key directories and their purposes:

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1. /boot #

• Purpose: Contains bootloader files (e.g., GRUB), kernel (vmlinuz), and initramfs.

• Example Files:

  • /boot/vmlinuz-* (Linux kernel)

  • /boot/grub/ (GRUB configuration)

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2. /proc #

• Purpose: A virtual filesystem providing process and kernel information as files.

• Key Features:

  • /proc/cpuinfo (CPU details)

  • /proc/meminfo (Memory usage)

  • /proc/[PID]/ (Process-specific data)

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3. /sys #

• Purpose: A virtual filesystem for interacting with kernel devices and drivers (part of sysfs).

• Used For:

  • Hardware detection (/sys/class/)

  • Kernel parameter tuning (/sys/module/)

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4. /var #

• Purpose: Stores variable data (logs, caches, spool files).

• Subdirectories:

  • /var/log/ (System logs)

  • /var/cache/ (Application cache)

  • /var/spool/ (Print queues, mail)

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5. /usr #

• Purpose: Contains user programs and read-only data (secondary hierarchy).

• Key Subdirectories:

  • /usr/bin/ (User commands)

  • /usr/lib/ (Libraries)

  • /usr/local/ (Locally installed software)

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6. /lib #

• Purpose: Stores essential shared libraries for /bin and /sbin.

• Modern Systems: Often replaced by /usr/lib (due to merged /usr).

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7. /dev #

• Purpose: Contains device files (e.g., hard disks, terminals).

• Examples:

  • /dev/sda (First SATA disk)

  • /dev/tty1 (Virtual terminal)

  • /dev/null (Null device)

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8. /etc #

• Purpose: Holds system-wide configuration files.

• Key Files:

  • /etc/passwd (User accounts)

  • /etc/fstab (Filesystem mounts)

  • /etc/ssh/sshd_config (SSH settings)

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9. /opt #

• Purpose: Optional third-party software (e.g., proprietary apps).

• Example:

  • /opt/google/chrome/ (Manually installed Chrome)

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10. /bin #

• Purpose: Essential user binaries (commands) for all users.

• Examples: ls, cp, bash.

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11. /sbin #

• Purpose: Essential system binaries (root-only tools).

• Examples: fdisk, iptables, reboot.

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12. /home #

• Purpose: Personal directories for regular users.

• Example: /home/username/ (User’s files and configs).

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13. /media #

• Purpose: Automatic mount point for removable media (USB, CDs).

• Example: /media/usb-drive/.

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14. /mnt #

• Purpose: Temporary manual mount point for filesystems.

• Example: sudo mount /dev/sdb1 /mnt/data.

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15. /root #

• Purpose: Home directory for the root user (not /home/root).

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16. /tmp #

• Purpose: Temporary files (cleared on reboot).

• Permissions: Global read/write (1777 sticky bit).

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FHS Cheat Sheet #

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Why FHS Matters #

• Consistency: All Linux distros follow a similar structure.

• Troubleshooting: Knowing where files live helps debug issues.

• Scripting: Scripts can rely on standard paths.