Sovereign Filesystem Hierarchy

Complete

Nexus OS replaces the POSIX Filesystem Hierarchy Standard (FHS) with the Sovereign FSH — a clean hierarchy designed around capability-based access, not Unix conventions.

The Five Root Directories

Path	Purpose	POSIX Equivalent
/Cas	System binaries and libraries (current version)	/usr, /bin, /lib
/Nexus	Kernel, boot, and system identity	/boot, /etc
/Cell	Per-process private state	/proc, /tmp (per-process)
/Bus	Inter-process communication endpoints	/dev, /sys, /run
/Data	Persistent user and application data	/home, /var

/Cas — The Current System

/Cas (Content-Addressable System) holds the currently active system binaries:

/Cas/
├── current/
│   ├── bin/       # System binaries (toybox, nip, dinit, etc.)
│   └── lib/       # Shared libraries
└── versions/
    └── <hash>/    # Previous system versions (CAS-addressed)

The current symlink points to the active version. System updates atomically switch this symlink to point to a new CAS-addressed version. Rollback is instant — just switch the symlink back.

/Nexus — System Identity

/Nexus holds the kernel image, boot configuration, and system identity:

/Nexus/
├── kernel         # Rumpk kernel image
├── manifest.bkdl  # Boot manifest
├── identity/      # System keys (Ed25519)
└── provchain/     # ProvChain ledger

This directory is read-only after boot. Only the init system can write here, and only during verified update operations.

/Cell — Process State

Each process (fiber) gets its own /Cell namespace:

/Cell/
├── <fiber-id>/
│   ├── heap/      # Process heap
│   ├── stack/     # Process stack
│   └── tmp/       # Temporary files (cleared on exit)

A process cannot see another process's /Cell directory. This provides isolation without containers.

/Bus — Communication

/Bus is the inter-process communication plane:

/Bus/
├── ion/           # ION Ring descriptors
├── hud/           # HUD visual interface endpoints
├── net/           # Network socket endpoints
└── dev/           # Device capability endpoints

Everything in /Bus is a communication endpoint, not a "device file" in the Unix sense. You don't open() a device — you request a capability grant and communicate through ION Rings.

/Data — Persistent Storage

/Data holds all persistent user and application data:

/Data/
├── users/         # Per-user data
├── apps/          # Per-application persistent state
└── shared/        # Explicitly shared data

FHS Compatibility

For grafted POSIX applications, Nexus creates compatibility symlinks:

/bin     → /Cas/current/bin
/lib     → /Cas/current/lib
/tmp     → /Cell/<current>/tmp
/home    → /Data/users
/etc     → /Nexus/config  (limited)

These symlinks exist only within the Membrane compatibility layer. Native Nexus applications use the Sovereign FSH directly.