Filesystem in Userspace

Filesystem in Userspace (FUSE) is a software interface for Unix and Unix-like computer operating systems that lets non-privileged users create their own file systems without editing kernel code. This is achieved by running file system code in user space while the FUSE module provides only a "bridge" to the actual kernel interfaces.
FUSE is available for Linux, FreeBSD, OpenBSD, NetBSD (as puffs), OpenSolaris, Minix 3, Android, macOS, and Windows.
FUSE is free software originally released under the terms of the GNU General Public License and the GNU Lesser General Public License.
History.
The FUSE system was originally part of "AVFS" ("A Virtual Filesystem"), a filesystem implementation heavily influenced by the translator concept of the GNU Hurd. It superseded Linux Userland Filesystem, and provided a translational interface using in libfuse1.
FUSE was originally released under the terms of the GNU General Public License and the GNU Lesser General Public License, later also reimplemented as part of the FreeBSD base system and released under the terms of Simplified BSD license. An ISC-licensed re-implementation by Sylvestre Gallon was released in March 2013, and incorporated into OpenBSD in June 2013.
FUSE was merged into the mainstream Linux kernel tree in kernel version 2.6.14.
The userspace side of FUSE, the library, generally followed the pace of Linux kernel development while maintaining "best effort" compatibility with BSD descendants. This is possible because the kernel FUSE reports its own "feature levels", or versions. The exception is the FUSE fork for macOS, OSXFUSE, which has too many differences for sharing a library. A break in libfuse history is libfuse3, which includes some incompatible improvements in the interface and performance, compared to the older libfuse2 now under maintenance mode.
As the kernel-userspace protocol of FUSE is versioned and public, a programmer can choose to use a different piece of code in place of and still communicate with the kernel's FUSE facilities. On the other hand, and its many ports provide a portable high-level interface that may be implemented on a system without a "FUSE" facility.
Operation and usage.
To implement a new file system, a handler program linked to the supplied codice_1 library needs to be written. The main purpose of this program is to specify how the file system is to respond to read/write/stat requests. The program is also used to mount the new file system. At the time the file system is mounted, the handler is registered with the kernel. If a user now issues read/write/stat requests for this newly mounted file system, the kernel forwards these IO-requests to the handler and then sends the handler's response back to the user.
FUSE is particularly useful for writing virtual file systems. Unlike traditional file systems that essentially work with data on mass storage, virtual filesystems don't actually store data themselves. They act as a view or translation of an existing file system or storage device.
In principle, any resource available to a FUSE implementation can be exported as a file system.
Applications.
On-disk file systems.
Conventional on-disk file systems can be implemented in user space with FUSE, e.g. for compatibility or licensing reasons.
Layering file systems.
FUSE filesystems can create a view of an underlying file system, transforming the files in some way.
Archive and backup file systems.
FUSE filesystems can expose the contents of archives or backup sets without having to first extract them.