Editing UnionFS (section)

{{Short description|A filesystem service for Linux}}
{{Use dmy dates|date=November 2021}}
{{Infobox filesystem
| name                    = Unionfs
| full_name               = Unification File System
| developer               = [[Open source community]]
| introduction_os         = 
| introduction_date       =
| partition_id            = 
| directory_struct        = 
| file_struct             = 
| bad_blocks_struct       = 
| max_filename_size       = 
| max_files_no            = 
| max_volume_size         = 
| max_file_size           = 
| filename_character_set  = 
| dates_recorded          = 
| date_range              = 
| date_resolution         = 
| forks_streams           = 
| attributes              = 
| file_system_permissions = [[POSIX]]
| compression             = No
| encryption              = No (but can be provided at the block device level)
| OS                      = [[Linux]], [[FreeBSD]] and [[NetBSD]]
| website                 = [https://unionfs.filesystems.org/ unionfs.filesystems.org]
}}
'''Unionfs''' is a filesystem service for [[Linux]], [[FreeBSD]] and [[NetBSD]] which implements a [[union mount]] for other [[file systems]]. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.

When mounting branches, the priority of one branch over the other is specified. So when both branches contain a file with the same name, one gets priority over the other.

The different branches may be either ''read-only'' or ''read/write'' file systems, so that writes to the virtual, merged copy are directed to a specific real file system. This allows a file system to appear as writable, but without actually allowing writes to change the file system, also known as [[copy-on-write]]. This may be desirable when the media is physically read-only, such as in the case of [[Live CD]]s.

Unionfs was originally developed by Erez Zadok and his team at [[Stony Brook University]].<ref>{{Cite journal
| title = Kernel Korner: Unionfs: Bringing Filesystems Together
| url = https://dl.acm.org/citation.cfm?id=1044970.1044978
| journal = Linux Journal
| date = December 2004
| issn = 1075-3583
| page = 8
| volume = 2004
| issue = 128
| first1 = Charles P.
| last1 = Wright
| first2 = Erez
| last2 = Zadok}}</ref><ref>{{Cite web
 |url         = http://git.fsl.cs.sunysb.edu/
 |title       = Git server of File systems and Storage Lab
 |publisher   = [[Stony Brook University]]
 |quote       = Project: unionfs-3.9.y.git; Owner: Erez Zadok
 |access-date  = 2017-07-17
 
 |archive-url  = https://archive.today/20160617073405/http://git.fsl.cs.sunysb.edu/
 |archive-date = 17 June 2016
}}</ref><ref>{{Cite web
| url = http://www.fsl.cs.sunysb.edu/
| title = File systems and Storage Lab: CS Department
| publisher = [[Stony Brook University]]
| quote = Lab Coordinator and Principal Investigator: Prof. Erez Zadok
}}</ref>