📂 Linux Filesystem Hierarchy — Final Comprehensive Summary¶

📚 Overview¶

The Linux filesystem is structured as a single inverted tree, starting at the root directory /. Everything (files, devices, sockets, processes) is represented as a file under /.

🔑 Root Directory /¶

The top-level directory containing all system files, user files, devices, and mounts.

📁 Key Directories Under /¶

1. /bin¶

• Essential user binaries (commands) needed for booting and single-user mode.
• Examples: ls, cp, mv, cat, grep, bash.

2. /sbin¶

• Essential system binaries for administration, mainly used by root.
• Examples: init, fsck, ifconfig, reboot.

3. /boot¶

• Files needed for booting the system.
• Kernel (vmlinuz), initramfs (initrd.img), GRUB files.

4. /dev¶

• Device files (special files that interface with hardware/peripherals).

• Examples:

  • /dev/sda → first SATA disk
  • /dev/null → null device
  • /dev/tty → terminal devices

5. /etc¶

• System-wide configuration files.

• Examples:

  • /etc/passwd → user accounts
  • /etc/fstab → filesystem mount table
  • /etc/hosts → hostname mappings

6. /home¶

• User home directories.
• Example: /home/alice, /home/bob

7. /lib, /lib64¶

• Essential shared libraries for programs in /bin and /sbin.

• Examples:

  • /lib/libc.so → C library
  • /lib/modules → kernel modules

8. /media¶

• Mount point for removable media.
• Examples: /media/usb, /media/cdrom

9. /mnt¶

• Generic temporary mount point (often used for manual mounts).
• Example: /mnt/data

10. /opt¶

• Optional or third-party software packages.
• Example: /opt/google/chrome/

11. /proc¶

• Virtual filesystem providing process and kernel info.

• Examples:

  • /proc/cpuinfo
  • /proc/meminfo
  • /proc/[PID]/

12. /root¶

• Home directory of the root user (not / itself).

13. /run¶

• Volatile runtime data (replaces /var/run in modern systems).
• Example: process IDs, sockets.

14. /srv¶

• Site-specific service data (e.g., web/ftp server files).
• Example: /srv/www/

15. /sys¶

• Virtual filesystem exposing kernel/device info.
• Example: /sys/class/net/

16. /tmp¶

• Temporary files (writable by anyone, usually cleared on reboot).
• Example: session files, sockets.

17. /usr¶

• Secondary hierarchy for user applications and read-only data.

• Often very large.

  • /usr/bin → non-essential user binaries
  • /usr/sbin → non-essential system binaries
  • /usr/lib → libraries
  • /usr/include → header files
  • /usr/share → architecture-independent data (man pages, docs, icons)
  • /usr/local → locally installed software

18. /var¶

• Variable files (expected to grow).

  • /var/log → log files
  • /var/spool → queued tasks (print, mail)
  • /var/tmp → temp files that persist across reboots
  • /var/cache → cached data

🗂 Special Notes¶

• Everything is a file: devices, processes, sockets.
• Hard vs Soft links: Multiple references to files.
• Mount points: Additional filesystems mounted under /.
• Virtual filesystems: /proc and /sys don’t hold real files, but kernel-generated info.

✅ Mnemonic Tip:

• bin/sbin → binaries
• etc → configuration
• var → variable data
• usr → user apps
• lib → libraries
• dev → devices
• proc/sys → system + kernel info

🌳 Full Directory Tree with Annotations¶

/                               # root of the entire filesystem tree
├─ bin/                         # essential user commands (ls, cp, mv, cat, bash)
├─ sbin/                        # essential system binaries for root (init, reboot, fsck)
├─ boot/                        # boot loader + kernel
│   ├─ vmlinuz                  # Linux kernel
│   ├─ initrd.img               # initial RAM disk
│   └─ grub/                    # GRUB bootloader files
├─ dev/                         # device files (interface to hardware/peripherals)
│   ├─ sda                      # first hard disk
│   ├─ null                     # discard output
│   ├─ tty                      # terminals
│   └─ random                   # random number generator
├─ etc/                         # system-wide configuration files
│   ├─ passwd                   # user accounts
│   ├─ shadow                   # secure user passwords
│   ├─ fstab                    # filesystem mount table
│   └─ hosts                    # static hostname mappings
├─ home/                        # user home directories
│   ├─ alice/                   # home dir for user "alice"
│   └─ bob/                     # home dir for user "bob"
├─ lib/                         # essential shared libraries for /bin and /sbin
├─ lib64/                       # 64-bit libraries (on 64-bit systems)
├─ media/                       # mount points for removable media (USB, CD-ROM)
├─ mnt/                         # generic temporary mount point (manual use)
├─ opt/                         # optional / third-party software
├─ proc/                        # virtual filesystem with process & kernel info
│   ├─ cpuinfo                  # CPU details
│   ├─ meminfo                  # memory details
│   └─ [PID]/                   # info for each running process
├─ root/                        # root user’s home directory
├─ run/                         # volatile runtime data (PID files, sockets) — cleared on reboot
├─ srv/                         # service-specific data (web, ftp, etc.)
├─ sys/                         # virtual filesystem exposing kernel & device info
├─ tmp/                         # temporary files (world-writable, cleared on reboot)
├─ usr/                         # user applications and read-only data (very large)
│   ├─ bin/                     # non-essential user commands (awk, gcc, python)
│   ├─ sbin/                    # non-essential system binaries (httpd, named)
│   ├─ lib/                     # libraries for /usr/bin and /usr/sbin
│   ├─ include/                 # header files (C, C++)
│   ├─ share/                   # arch-independent data (docs, man pages, icons)
│   └─ local/                   # locally installed software (safe from package manager)
└─ var/                         # variable data files (changes frequently)
    ├─ log/                     # log files (syslog, dmesg, auth.log)
    ├─ spool/                   # queued tasks (mail, print jobs)
    ├─ tmp/                     # temporary files that persist across reboot
    └─ cache/                   # application cache data

🔑 Key Principles¶

• Everything is a file: devices, sockets, processes, directories.
• Essential binaries → /bin, /sbin.
• Configuration → /etc.
• User data → /home.
• Variable data → /var.
• Kernel + process info → /proc, /sys.
• Boot files → /boot.
• Temporary storage → /tmp, /var/tmp.
• 3rd party software → /opt, /usr/local.

✅ This summary gives you:

• A tree-structured overview (easy to visualize).
• Detailed annotations (purpose + examples).
• A unified cheat sheet for study or quick reference.