Mastering Linux Filesystem Hierarchy: A Complete Guide to FHS Directories

Introduction

For every Linux learner, understanding the directory structure is essential. The Linux filesystem appears as a tree rooted at /, the root directory.

Filesystem Hierarchy Standard (FHS)

The FHS was created by a community of Linux enthusiasts to standardize where files should be placed, making systems portable and easier to maintain.

Four Types of Directories

Shareable : directories that can be mounted and shared across systems (e.g., /usr, /var/mail).

Unshareable : machine‑specific files such as device nodes and sockets (e.g., /etc, /dev).

Static : rarely changed data that follows the distribution (e.g., libraries, documentation).

Variable : frequently changing data like logs, mail spools, lock files.

Top‑Level Directories

/ (root) : entry point of the filesystem; contains essential boot files and should reside on a small partition.

/usr : “Unix Software Resource”, holds shareable and static software; analogous to Windows C:\Windows and Program Files.

/var : “variable”, stores data that grows during operation such as logs, caches, mail, and run‑time files.

Important Subdirectories

/bin : essential user commands usable in single‑user mode.

/boot : kernel and bootloader files.

/dev : device files presented as files.

/etc : system configuration files (readable by all, writable only by root).

/home : user home directories.

/lib : libraries needed by binaries in /bin and /sbin.

/media and /mnt : mount points for removable media.

/opt : third‑party optional software.

/root : home directory of the root account.

/sbin : system binaries for administration.

/srv : data for network services.

/tmp : temporary files, cleared at boot.

/lost+found : recovery directory for ext2/3 filesystems.

/proc and /sys : virtual filesystems exposing kernel and hardware information.

Path Types

Absolute paths start with / (e.g., /home/user/.bashrc). Relative paths are expressed from the current directory (e.g., ./script.sh or ../var/log).

Examples

To move from /var/spool/mail to /var/spool/cron you can use:

cd /var/spool/mail
cd ../cron

Running a script in the current directory requires ./ before the script name, e.g., ./run.sh.

Understanding the FHS helps developers place files correctly, simplifies system administration, and ensures compatibility across Linux distributions.