Linux file system hierarchy

The Linux file system hierarchy is structured in a way that organizes files and directories based on their purpose and function. Each directory in the root (`/`) directory serves a specific role, making it easier for users and the system to locate files. Here’s an overview of the most important directories:

1. `/` (Root Directory)

- The root directory is the top-level directory in the Linux file system. All other directories are branches of this directory. The root directory contains system-critical files and directories necessary for the system to function.

- It is the starting point of the file system and holds the entire directory structure.

2. `/home`

- User's Personal Directory: This directory contains personal files and directories for each user account on the system.

- Inside /home, you will find subdirectories for each user, such as /home/username. Users can store their documents, downloads, and configuration files here.

- Example: Files like Documents, Downloads, and hidden configuration files (e.g., .bashrc, .ssh) are stored in the user's home directory.

3. `/etc`

- Configuration Files: This directory contains all system-wide configuration files and shell scripts used to configure the system.

- Typically, settings for system services (like network configurations, daemons, or authentication) are found here. For example, /etc/hostname stores the system's hostname, and /etc/passwd contains user account information.

- Example:

- /etc/fstab: Defines how disk partitions and storage devices are mounted.

- /etc/network/interfaces: Network configuration settings.

4. `/var`

- Variable Files: Contains files that are expected to grow and change over time, such as logs, caches, and spool files (for mail, print jobs, etc.).

- Subdirectories within /var store:

- Logs (`/var/log`), like system logs, kernel logs, and application logs.

- Web server data (`/var/www`).

- Temporary files related to services and background processes.

- Example:

- /var/log/syslog or /var/log/messages stores system and service logs.

- /var/spool: Directory for spool files (e.g., print jobs waiting to be printed).

5. `/tmp`

- Temporary Files: This directory is used for temporary files that applications and the system generate at runtime.

- Files stored here are usually automatically deleted after the system restarts or after a set period.

- Example: A file opened in a text editor without being saved might create a temporary backup file in /tmp.

6. `/usr`

- User System Resources: This directory holds user-installed software, libraries, documentation, and binaries (executable files) that are not critical for the base system but are used in a user environment.

- Subdirectories in /usr include:

- /usr/bin: Common user binaries (executables) like standard command-line tools (e.g., grep, ls).

- /usr/lib: Libraries for programs stored in /usr/bin.

- /usr/share: Architecture-independent data, like icons and documentation.

- Example:

- /usr/bin/vim: The Vim text editor binary.

- /usr/share/doc: Contains documentation for installed software.

7. `/bin`

- Essential User Binaries: This directory contains essential command-line utilities and binaries needed for all users. These binaries are necessary to boot the system and bring it up in a minimal working state.

- Example: Commands like ls, cp, mv, cat, and echo are located here.

8. `/sbin`

- System Binaries: This directory holds essential system administration binaries and commands, usually used by the root user for system maintenance.

- These are crucial for system maintenance and include commands to manage system processes and hardware.

- Example: Commands like ifconfig, fdisk, and reboot.

9. `/lib`

- Shared Libraries: Contains shared libraries required by essential binaries in /bin and /sbin, as well as kernel modules.

- It includes dynamically linked libraries (.so files), which are similar to Windows' .dll files.

- Example: /lib/libc.so (the C standard library), which is required by almost every program on the system.

10. `/opt`

- Optional Software: This directory is for installing optional or add-on software that is not part of the default system setup. Many third-party packages are installed here.

- Example: If you install a commercial software package or proprietary driver, it may go into /opt.

11. `/boot`

- Boot Files: This directory contains the files needed to boot the system, including the Linux kernel, the initial RAM disk (`initrd`), and the bootloader (like GRUB).

- Example:

- /boot/vmlinuz: The Linux kernel binary.

- /boot/grub: Configuration files for the GRUB bootloader.

12. `/dev`

- Device Files: This directory contains files that represent devices (hardware) on the system. Linux treats devices like files, so /dev contains special files that correspond to different system devices such as hard drives, USB drives, and peripherals.

- Example:

- /dev/sda1: Represents the first partition on the first hard drive.

- /dev/null: A special device file that discards all data written to it.

13. `/mnt` and /media

- Mount Points for Temporary File Systems: These directories are used for mounting external storage devices like USB drives or network shares.

- /mnt is traditionally used for manually mounting file systems, while /media is typically used for removable media such as CD-ROMs or USB drives that the system automatically detects and mounts.

- Example:

- /mnt/external_drive: A manually mounted external hard drive.

- /media/username/USB: A USB drive automatically mounted by the system.

14. `/root`

- Root User's Home Directory: This is the home directory for the root user, which is the system's superuser with administrative privileges.

- Example: While regular users have their home directories under /home/, the root user’s home directory is /root.

15. `/proc`

- Process Information: This virtual directory provides an interface to kernel data structures. It contains information about system and running processes, in real-time. It is dynamically generated and doesn't occupy actual disk space.

- Example:

- /proc/cpuinfo: Information about the CPU.

- /proc/[PID]: Information about processes with a specific process ID (PID).

16. `/sys`

- System Information: This is another virtual file system, similar to /proc, but focuses more on representing and interfacing with the kernel, device drivers, and hardware components.

- Example: /sys/class/net/ contains information about network devices.

17. `/srv`

- Service Data: This directory is used to store data for services hosted on the system, such as web servers or FTP servers.

- Example: A web server might store its web content in /srv/www/.

18. `/run`

- Runtime Data: This directory stores runtime data for processes and services that are started after the system boots. It typically contains PID files, sockets, and other temporary data that is cleared on reboot.

- Example: /run/systemd/ stores information related to the systemd init system during runtime.

Conclusion

The Linux file system hierarchy is designed to provide a well-organized structure where each directory serves a specific purpose. Understanding this layout helps users navigate, manage, and configure the Linux operating system efficiently. Knowing what each directory contains also allows for better system maintenance, troubleshooting, and effective use of Linux commands.