whenever an application opens a file for writing, the file system automatically creates a new instance of the file, with a version number appended to the name. Version numbers start at 1 and count upward as new instances of a file are created. When an application opens a file for reading, it can either specify the exact file name including version number, or just the file name without the version number, in which case the most recent instance of the file is opened. The "purge" DCL/CCL command can be used at any time to manage the number of versions in a specific directory. By default, all but the highest numbered versions of all files in the current directory will be deleted; this behavior can be overridden with the /keep=n switch and/or by specifying directory path(s) and/or filename patterns. VMS systems are often scripted to purge user directories on a regular schedule; this is sometimes misconstrued by end-users as a property of the versioning system. Linux On February 8, 2004, Kiran-Kumar Muniswamy-Reddy, Charles P. Wright, Andrew Himmer, and Erez Zadok (all from Stony Brook University) proposed an application that was user friendly to many of the users who tested the app. The system was developed with Linux software, so it was first operated on Linux. NILFS - A log-structured file system supporting versioning of the entire file system and continuous snapshotting. In this list, this is the only one that is stable and included in the mainline kernel. Tux3 - Most recent change was in 2014. Next3 - Most recent update was in 2012. ext3cow - Most recent release was in 2005. LMFS The Lisp Machine File System supports versioning. This was provided by implementations from MIT, LMI, Symbolics and Texas Instruments. Such an operating system was Symbolics Genera. macOS Starting with Lion (10.7), macOS has a feature called Versions which allows Time Machine-like saving and browsing of past versions of documents for applications written to use Versions. This functionality, however, takes place at the application layer, not the filesystem layer; Lion and later releases do not incorporate a true versioning file system. SCO OpenServer HTFS, adopted as the primary filesystem for SCO OpenServer in 1995, supports file versioning. Versioning is enabled on a per-directory basis by setting the directory's setuid bit, which is inherited when subdirectories are created. If versioning is enabled, a new file version is created when a