Understanding Linux File System and Basic Description

The Filesystem Hierarchy Standard (FHS)

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux and Unix-like operating systems. It is maintained by the Linux Foundation and provides a predictable layout that software and users can depend on.

Understanding the FHS is fundamental for any Linux engineer. Whether you're troubleshooting a production server at 3 AM or architecting a new deployment, knowing exactly where files live—and why—separates competent administrators from exceptional ones.

Root Directory Structure

Every Linux system starts at / — the root directory. From here, the filesystem branches into a well-defined hierarchy:

/
├── bin/      # Essential user command binaries
├── boot/     # Boot loader files, kernel images
├── dev/      # Device files (block, character devices)
├── etc/      # System-wide configuration files
├── home/     # User home directories
├── lib/      # Essential shared libraries
├── media/    # Mount points for removable media
├── mnt/      # Temporary mount points
├── opt/      # Optional/third-party software
├── proc/     # Virtual filesystem for process info
├── root/     # Root user's home directory
├── run/      # Runtime variable data
├── sbin/     # Essential system binaries
├── srv/      # Data for services (HTTP, FTP)
├── sys/      # Virtual filesystem for kernel objects
├── tmp/      # Temporary files (cleared on reboot)
├── usr/      # Secondary hierarchy (user programs)
└── var/      # Variable data (logs, mail, spool)

Critical Directories in Detail

/etc — The Configuration Nexus

This is where all system-wide configuration lives. Key files include:

/etc/passwd        # User account information
/etc/shadow        # Encrypted password data
/etc/fstab         # Filesystem mount table
/etc/hosts         # Static hostname resolution
/etc/resolv.conf   # DNS resolver configuration
/etc/ssh/          # SSH server/client configuration
/etc/nginx/        # Nginx web server configuration
/etc/systemd/      # SystemD unit file overrides

/var — Variable Runtime Data

Logs, caches, mail spools, and other data that changes during operation:

/var/log/       # System and application logs
/var/cache/     # Application cache data
/var/lib/       # State information for applications
/var/spool/     # Queued data (mail, print jobs)
/var/tmp/       # Temporary files preserved across reboot

/proc — The Virtual Window into the Kernel

This pseudo-filesystem exposes kernel and process information as files. Every running process gets a numbered directory:

cat /proc/cpuinfo       # CPU information
cat /proc/meminfo       # Memory statistics
cat /proc/loadavg       # System load averages
cat /proc/1/status      # Status of PID 1 (init/systemd)
cat /proc/version       # Kernel version string

The /usr Hierarchy

The /usr directory contains the majority of user-space programs and data. It mirrors the root filesystem structure:

/usr/bin/       # Non-essential user commands
/usr/sbin/      # Non-essential system commands
/usr/lib/       # Libraries for /usr/bin and /usr/sbin
/usr/local/     # Locally compiled software
/usr/share/     # Architecture-independent data
/usr/include/   # C/C++ header files

Practical Tips for Engineers

• Always check /var/log first when troubleshooting — the answer is usually in the logs
• Never store application data in /tmp — it's wiped on reboot on most distributions
• Use /opt for third-party software that doesn't follow the standard package layout
• Mount /var on a separate partition in production to prevent log floods from filling the root filesystem
• The /proc/sys interface lets you tune kernel parameters at runtime with sysctl

Mastering the filesystem hierarchy isn't just academic knowledge—it's the foundation that every other Linux skill builds upon.