What is the Filesystem Hierarchy Standard?

The FHS is a specification, maintained by the Linux Foundation, that defines the purpose of the top-level directories on a Linux system. It is why /etc holds configuration and /var holds variable data on nearly every distribution, which makes scripts and tools portable across systems.

What is the difference between /bin and /usr/bin?

Historically /bin held essential binaries needed for early boot and single-user mode, while /usr/bin held the rest. On most modern distributions /bin is a symlink to /usr/bin (the usr-merge), so the split is now mostly historical, but the FHS still documents the original intent.

Are /proc and /sys real directories on disk?

No. They are virtual filesystems generated by the kernel in memory. /proc exposes process and kernel information, and /sys exposes devices and kernel objects. Reading a file there queries live kernel state, and writing to certain files changes kernel settings at runtime.

Where should I install my own software?

Software you install manually, outside the package manager, belongs under /usr/local (binaries in /usr/local/bin, etc.) or /opt for self-contained third-party packages. This keeps your additions separate from files the distribution's package manager owns under /usr.

What is the difference between /tmp and /var/tmp?

Both hold temporary files, but /tmp is typically cleared on every reboot (and is often a memory-backed tmpfs), while /var/tmp is meant to persist across reboots. Use /var/tmp for temporary data that must survive a restart.

Linux Filesystem Hierarchy Reference

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Know where everything lives on Linux

Every Linux system follows the Filesystem Hierarchy Standard (FHS), which assigns a clear purpose to each top-level directory. Configuration goes in /etc, logs and spools in /var, user programs in /usr, and live kernel state in the virtual /proc and /sys. This reference lets you search any path and instantly see what it is for, what files you will find there, and whether it is safe to write to.

How it works

The FHS separates files along two axes: shareable vs unshareable (can it be served to other hosts) and static vs variable (does it change at runtime). That is why read-only program files live under /usr while changing data lives under /var, and why machine-specific configuration sits in /etc. The virtual filesystems /proc and /sys are special: they hold no real files on disk but expose kernel and device state that you can read, and in some cases write, to tune the running system. Filtering matches the path, its purpose and example contents, so searching logs, /var, or config finds the right entry.

Tips and notes

Put your own manually-installed software in /usr/local or /opt, never directly in /usr, so the package manager does not overwrite it. Treat /etc as the place to look first when configuring a service. Remember /proc and /sys are live windows into the kernel — cat /proc/cpuinfo and cat /sys/class/net/*/address read real state, and writing to files like /proc/sys/... (better done via sysctl) changes kernel behaviour immediately. Use /tmp for throwaway files and /var/tmp only when the data must survive a reboot.