Day 1: Linux File System Hierarchy Explained

The Linux file system follows the Filesystem Hierarchy Standard (FHS), a well-defined structure for organizing directories and files. This hierarchy ensures consistency across distributions and simplifies system management. Let’s break down the key directories, their purposes, and real-world use cases.

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1. Root Directory (/)

The root directory (/) is the top-level directory from which all other directories branch out. Think of it as the "trunk" of the filesystem tree.

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2. Essential System Directories

1. /bin (Binary)

   • Purpose: Contains essential user command binaries (executable programs) needed for basic system functionality.

   • Examples: ls, cp, mv, bash, cat.

   • Key Point: Accessible to all users (even in single-user/rescue mode).

2. /sbin (System Binary)

   • Purpose: Holds system administration binaries used for system maintenance and repair.

   • Examples: fdisk (partition tool), iptables (firewall), reboot.

   • Key Point: Typically requires root/sudo privileges to run.

3. /etc (Editable Text Configuration)

   • Purpose: Stores system-wide configuration files and scripts.

   • Examples:

     • /etc/passwd: User account information.

     • /etc/fstab: Filesystem mount points.

     • /etc/nginx/: Configuration for the Nginx web server.

   • Key Point: Never delete/modify files here without understanding their purpose!

4. /var (Variable Data)

   • Purpose: Contains variable data that changes during system operation.

   • Subdirectories:

     • /var/log: System and application logs (e.g., syslog, nginx/access.log).

     • /var/cache: Temporary data for applications (e.g., package manager cache).

     • /var/www: Default directory for web server content (e.g., Apache/Nginx).

     • /var/lib: Databases and state files (e.g., Docker containers, MySQL data).

   • Key Point: Critical for debugging (check logs!) and managing dynamic data.

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3. User & Home Directories

1. /home

   • Purpose: Personal directories for non-root users (e.g., /home/alice).

   • Key Point: Users store personal files, configurations (dotfiles like .bashrc), and projects here.

2. /root

   • Purpose: Home directory for the root user (system administrator).

   • Key Point: Separate from /home for security reasons.

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4. System Data & Devices

1. /dev (Devices)

   • Purpose: Contains device files representing hardware or virtual devices.

   • Examples:

     • /dev/sda: First SATA disk.

     • /dev/null: A "black hole" that discards data.

     • /dev/tty: Represents a terminal.

2. /proc (Processes)

   • Purpose: Virtual filesystem providing real-time system/process information.

   • Examples:

     • /proc/cpuinfo: CPU details.

     • /proc/meminfo: Memory usage.

     • /proc/<PID>/: Directories for running processes (e.g., PID 1 is systemd).

3. /sys (System)

   • Purpose: Virtual filesystem for kernel and hardware configuration (e.g., power management, device drivers).

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5. Temporary & Runtime Data

1. /tmp (Temporary)

   • Purpose: Stores temporary files deleted on reboot.

   • Key Point: All users can write here, but files are not persistent.

2. /run (Runtime)

   • Purpose: Holds volatile runtime data (e.g., PID files, sockets) created after boot.

   • Example: /run/docker.sock (Docker daemon socket).

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6. Application & Software Directories

1. /usr (Unix System Resources)

   • Purpose: Contains read-only user binaries, libraries, and documentation.

   • Subdirectories:

     • /usr/bin: Non-essential user binaries (e.g., python, git).

     • /usr/sbin: Non-essential system admin binaries.

     • /usr/lib: Shared libraries for /usr/bin and /usr/sbin.

     • /usr/local: Locally compiled software (e.g., manually installed apps).

2. /opt (Optional)

   • Purpose: Stores third-party/standalone software (e.g., proprietary apps like Google Chrome).

3. /lib (Libraries)

   • Purpose: Shared libraries for binaries in /bin and /sbin (e.g., libc.so for the C library).

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7. Boot & Mount Points

1. /boot

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

2. /mnt (Mount) & /media

   • Purpose: Temporary mount points for external devices (e.g., USB drives, network shares).

   • Difference:

     • /mnt: Manually mounted devices.

     • /media: Automatically mounted removable media (e.g., by the desktop environment).

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8. Key Takeaways

• Critical Directories:

  • /etc: Configuration files.

  • /var: Logs, databases, and dynamic data.

  • /bin & /sbin: Essential binaries.

• User Data:

  • /home: User files.

  • /root: Admin’s home.

• Temporary Data:

  • /tmp: Non-persistent files.

  • /run: Runtime data.

• Never Delete:

  • /bin, /sbin, /etc, /lib (breaking these can crash your system).

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9. Real-World Example

Imagine deploying a web app:

• Store code in /var/www/myapp.

• Configure Nginx in /etc/nginx/nginx.conf.

• Logs go to /var/log/nginx/access.log.

• Use /tmp for temporary uploads.

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10. Next Steps

• Explore your system with ls, cd, and tree.

• Run man hier for the official manual on the filesystem hierarchy.

• Check disk usage with du -sh /var/log (replace with any directory).

Understanding the Linux file system is foundational for troubleshooting, scripting, and mastering DevOps tools like Docker, Kubernetes, and configuration management (Ansible, Terraform). 🐧