---
name: "unionfs"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/unionfs/4"
---

# unionfs(4)

union mount file system

## NAME

**unionfs** – union mount file system

## SYNOPSIS

**option** **UNIONFS**

In [loader.conf(5)](/loader.conf/5.md): **unionfs\_load="YES"**

## DESCRIPTION

The UNIONFS driver is an implementation of a stackable unification filesystem.

## SEE ALSO

[mount\_unionfs(8)](/mount_unionfs/8.md)

## STANDARDS

J. S. Pendry and M. K. McKusick, *Union mounts in 4.4BSD-Lite*, Proceedings of the USENIX Technical Conference on UNIX and Advanced Computing Systems, December 1995.

P. H. Kamp and R. N. M. Watson, *Jails: Confining the omnipotent root*, Proceedings of the Second International System Administration and Networking Conference (SANE2000), May 2000.

## HISTORY

The **unionfs** device driver first appeared in FreeBSD 5.0.

## AUTHORS

The **unionfs** device driver was written by Jan-Simon Pendry for 4.4BSD and Masanori OZAWA ⟨*ozawa@ongs.co.jp*⟩ reimplemented the handling of the locking for FreeBSD 7.0. The manual page was written by Gordon Bergling ⟨*gbe@FreeBSD.org*⟩.

## BUGS

Please see the [mount\_unionfs(8)](/mount_unionfs/8.md) manual page for a list of bugs regarding the **unionfs** filesystem.