Unix Filesystem Hierarchy
The Unix filesystem is structured as a single tree starting at the root (/). This tree organizes system files, user data, configuration, binaries, libraries, and temporary data in well-defined directories.
Root Directory /
All files and directories stem from /, the root of the filesystem. There are no drive letters like on Windows — all volumes and devices are mounted somewhere within this tree.
Core Directories
| Directory | Description |
|---|---|
/bin |
Essential system binaries (e.g., ls, cp, sh) needed for basic boot and repair. |
/sbin |
System binaries used for administration (e.g., fsck, reboot). Typically root-only. |
/lib |
Essential shared libraries required by binaries in /bin and /sbin. |
/lib64 |
64-bit libraries on 64-bit systems. |
/etc |
System-wide configuration files (e.g., /etc/passwd, /etc/ssh/sshd_config). |
/var |
Variable data like logs, mail, cache files (e.g., /var/log, /var/spool). |
/tmp |
Temporary files; cleared on reboot. |
/home |
Home directories for users (e.g., /home/lynn). |
/root |
Home directory for the root user. |
/dev |
Device files representing hardware (e.g., /dev/sda, /dev/null). |
/proc |
Virtual filesystem exposing kernel and process info. |
/sys |
Interface to kernel and hardware data structures. |
/mnt |
Temporary mount point for manually mounted filesystems. |
/media |
Mount point for removable media (USB drives, CDs, etc.). |
/opt |
Optional third-party software and packages. |
/usr |
“User system resources” — contains most userland programs and data. |
/usr/bin |
Most non-essential user binaries (e.g., python, vim). |
/usr/sbin |
Non-essential system binaries (e.g., apachectl). |
/usr/lib |
Libraries for binaries in /usr/bin and /usr/sbin. |
/usr/local |
Locally compiled software (not managed by package manager). |
Conceptual Layers
-
Boot-critical layer:
/bin,/sbin,/lib,/etc -
User-space layer:
/usr/bin,/usr/lib,/usr/share -
Local override layer:
/usr/local/*(for sysadmins/devs) -
User-specific layer:
/home/username(config, data, personal files)
User-Level Directory Conventions (XDG)
While not part of the classic Unix hierarchy, modern systems often include:
| Directory | Purpose | Default Path |
|---|---|---|
$XDG_CONFIG_HOME |
User config files | ~/.config |
$XDG_DATA_HOME |
User data files | ~/.local/share |
$XDG_CACHE_HOME |
Non-essential cache data | ~/.cache |
$XDG_STATE_HOME |
Persistent user state (logs, etc.) | ~/.local/state |
~/.local/bin |
User-installed executables | Not standard, but common |
Finding Executables and Libraries
-
Executables are searched via the
$PATHvariable. -
Shared libraries are located using
ld.so,LD_LIBRARY_PATH, and/etc/ld.so.conf.
Summary
The Unix directory hierarchy is designed for separation of concerns:
-
System binaries/config:
/bin,/etc -
Userland apps:
/usr/bin,/usr/lib -
User files:
/home -
Ephemeral data:
/tmp,/var -
Custom/optional:
/opt,/usr/local -
User-controlled:
~/.config,~/.local