Table of Contents Link to heading

• The Filesystem Hierarchy Standard
• Directory Classification
• Filesystem Hierarchy Structure
  • Top-Level (Root) Hierarchy: /
  • Second-Level Hierarchy: /usr
  • Third-Level Hierarchy: /usr/local
  • Fourth-Level Hierarchy: /var
• Operational Directory Reference
  • User Home Directories: /home
  • Binary and Command Directories
  • Library Directories
  • Software Application File Locations
  • Variable Data and Log Directories

The Filesystem Hierarchy Standard Link to heading

The Filesystem Hierarchy Standard (FHS) defines where different types of files should be placed on a Linux system. Distributions are not required to implement it, but virtually all major distributions comply, which means a sysadmin familiar with the FHS can navigate any compliant system without needing to memorise distribution-specific layouts.

Understanding the FHS has direct operational value: when troubleshooting, you know where to find log files (/var/log), configuration files (/etc), binaries (/usr/bin, /usr/sbin), and temporary storage (/tmp, /var/tmp) without guessing.

Directory Classification Link to heading

The FHS classifies directories along two axes:

By shareability: Can the directory be NFS-mounted and shared across multiple machines?

• Shareable: /usr (read-only software), /opt, /var/mail — content that can be served from one host to many
• Not shareable: /etc (host-specific config), /var/lock — content that must be local to each machine

By variability: Does the content change during normal operation?

• Static: binaries, libraries, documentation — content that only changes when explicitly updated
• Variable: logs, spools, caches — content that changes as the system runs

Filesystem Hierarchy Structure Link to heading

The hierarchy has four main levels of organisation. The root filesystem and everything under it is required to boot the system; the /usr and /var hierarchies may be on separate partitions and may not be available until after the root filesystem is mounted.

Top-Level (Root) Hierarchy: / Link to heading

Second-Level Hierarchy: /usr Link to heading

/usr holds the majority of user-facing programs and data. It is the largest directory on most systems and is frequently mounted read-only and shared across multiple machines via NFS.

Key subdirectories:

Third-Level Hierarchy: /usr/local Link to heading

/usr/local mirrors the /usr structure and is intended for software compiled and installed locally — software not managed by the distribution’s package manager. This prevents custom software from conflicting with distribution-managed packages.

/usr/local/bin      # locally installed executables
/usr/local/lib      # locally installed libraries
/usr/local/etc      # configuration for locally installed software
/usr/local/share    # architecture-independent data for local software

When you compile software from source and run make install, it typically installs into /usr/local by default.

Fourth-Level Hierarchy: /var Link to heading

/var contains files whose content changes continuously during system operation — log files grow, mail spools accumulate, cache data is written and expired.

Operational Directory Reference Link to heading

User Home Directories: /home Link to heading

Each user account has a dedicated directory under /home with the same name as the username. A user has full ownership of their home directory and can create, modify, and delete any files within it.

Without special permission grants, a regular user can only write to:

• Their home directory (/home/username/)
• /tmp and /var/tmp (world-writable temporary directories)

Binary and Command Directories Link to heading

Linux separates binaries across several directories based on their audience and whether they are essential for system recovery:

Third-party applications may also install binaries to /opt/applicationname/bin or /usr/local/application/bin.

If a command is not found, check whether its directory is in the current PATH variable:

echo $PATH
which command_name      # shows which directory the shell finds it in

Library Directories Link to heading

Libraries (.so — shared object files) follow the same essential/non-essential split as binaries:

Multiple versions of the same library may coexist — different applications may require different versions, and both 32-bit and 64-bit versions may be present on 64-bit systems.

Software Application File Locations Link to heading

Unlike Windows (where an application typically installs all files to a single directory under Program Files), Linux applications distribute files across the FHS:

To find all files belonging to an installed package:

dpkg -L packagename         # Debian/Ubuntu
rpm -ql packagename         # Red Hat/RHEL/CentOS

Variable Data and Log Directories Link to heading

/var/log is the primary location for log files. Key logs:

Logs are managed by logrotate, which compresses and rotates log files on a schedule. Compressed logs use .gz extension and can be read with zcat or zless.