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The Linux Filesystem

In Linux, everything is a file. This includes text documents, directories, hardware devices, and even network sockets. Understanding the filesystem hierarchy is crucial for navigating and administrating a Linux system.

The Filesystem Hierarchy Standard (FHS)​

Linux distributions follow the Filesystem Hierarchy Standard (FHS). It defines the directory structure and directory contents. The entire filesystem originates from a single root directory represented by a forward slash /.

Here is a breakdown of the most important directories:

  • / (Root): The top-level directory. Everything resides under this directory.
  • /bin (User Binaries): Contains essential command-line utilities available to all users (e.g., ls, cp, bash).
  • /sbin (System Binaries): Contains essential administrative commands typically used by the root user (e.g., fdisk, reboot, iptables).
  • /boot: Contains the files needed to boot the system, including the bootloader configuration (GRUB), the kernel (vmlinuz), and the initramfs.
  • /dev (Device Files): Contains special files that represent hardware devices. For example, /dev/sda represents the first hard drive, and /dev/null is a special device that discards data written to it.
  • /etc (Configuration Files): Contains system-wide configuration files and scripts. You will spend a lot of time here configuring services (e.g., /etc/nginx/nginx.conf, /etc/fstab).
  • /home (Home Directories): Contains the personal directories for standard users. If your username is "karasu", your home directory is /home/karasu.
  • /root (Root Home): The home directory for the root user. Do not confuse this with the top-level / directory.
  • /lib, /lib64 (Shared Libraries): Contains shared library files needed by the binaries in /bin and /sbin. Similar to DLLs in Windows.
  • /mnt, /media (Mount Points): Empty directories used as temporary mount points for external filesystems (like USB drives or network shares).
  • /opt (Optional Software): Used for installing third-party or unbundled software packages. Many enterprise applications install here.
  • /proc (Process Information): A virtual filesystem created in RAM. It contains information about running processes and system resources. For example, cat /proc/cpuinfo displays CPU details.
  • /sys (System Information): Another virtual filesystem similar to /proc, providing a unified view of hardware devices and drivers attached to the system.
  • /tmp (Temporary Files): Used for temporary files created by the system and users. Data here is often wiped upon reboot.
  • /usr (User Programs): Contains multi-user applications, utilities, and libraries. It has its own hierarchy (/usr/bin, /usr/lib, /usr/local). /usr/local is specifically for software compiled from source locally.
  • /var (Variable Data): Contains files that change frequently during system operation, such as logs (/var/log), databases (/var/lib/mysql), and web server files (/var/www).

Mount Points and Partitions​

Unlike Windows, which uses drive letters (C:\, D:\), Linux mounts physical partitions into the virtual directory tree. For example, you might have a dedicated physical hard drive for databases. You would format the drive and mount it to /var/lib/mysql. The system seamlessly treats it as part of the unified directory tree.

The /etc/fstab (filesystem table) file defines how partitions are mounted automatically at boot time.

Inodes​

Every file and directory in Linux has an associated data structure called an inode (index node). The inode stores metadata about the file, such as:

  • File type
  • Permissions (read, write, execute)
  • Owner and Group IDs
  • File size
  • Pointers to the physical blocks on the disk where the actual data is stored.

Importantly, the inode does not store the filename. The directory file acts as a table linking filenames to their respective inodes.

  • Hard Link: Creates a new filename that points to the exact same inode as the original file. Both filenames are completely equal. If you delete one, the data remains accessible via the other. Hard links cannot span across different partitions.
  • Soft Link (Symlink): Creates a new, special file whose content is simply a path pointing to another file. If the original file is deleted, the symlink becomes "broken" and points to nothing. Symlinks can cross partitions and link to directories.

Best Practices​

  • Separate Partitions: For servers, it is best practice to place directories like /var and /home on separate partitions. If an application spams logs and fills /var, it won't fill up the root / partition and crash the entire system.
  • Avoid running as root: Limit your time in /root. Operate in your /home directory and use sudo for administrative tasks.