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A filesystem is a subsystem of a UNIX system. It is built on top of secondary storage devices but not limited to that and can also be built on top of primary storage devices (RAM) or be an entirely virtual filesystem. A UNIX system cannot operate without bootstrapping itself and programs from the filesystem.

All filesystems available in a UNIX system are mounted in a tree hierarchy that starts with the root filesystem at the top and branches out to other filesystems toward the branches. The structure of the entire filesystems tree is made up of directories with files, character devices, block devices, sockets and symlinks making up the branches along the tree. From the root to the farthest branch from the root only 1023 bytes may be spanned in writing the path delimited by '/' characters, between the '/' characters a directory or file may only be up to 255 bytes long.


To view files in a filesystem the command ls is used, to view mounted filesystems the mount command is used, to see properties about filesystems one would use df.

List of filesystems available in UNIX and UNIX-like Operating Systems

Unix File System (ufs)

Most UNIX'es have the ufs filesystem it is derived from the Berkeley ffs filesystem.

Berkeley Fast File System (FFS)

The ffs filesystem was written at UCB by Marshall Kirk McKusick for 4.2BSD. It is still widely used in the BSD family of Operating Systems.

Memory File System (mfs)

The mfs filesystem resides in RAM but is available as a file system. When the system powers off any files left in the mfs filesystem will vanish.

Proc File System (procfs)

The procfs filesystem is a filesystem that represents the process namespace of a system. You can see all processes listed here and when you change into the directories of the processes and list further attributes about the specific process.

Device File System (devfs)

Union File System (unionfs)

Kern File System (kernfs)

Bind File System (bindfs)

Null File System (nullfs)

Ext2 File System (ext2fs)

Ext3 File System (ext3fs)

Reiser File System (ReiserFS)

This is a filesystem written and architected by a man named Hans Reiser. Development will is stalled since Hans Reiser was found guilty of murdering his wife even though her body was never found.

X File System (xfs)