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In multiplayer online games, a MUSH (a backronymed pun on MUD most often expanded as Multi-User Shared Hallucination, though Multi-User Shared Hack, Habitat, and Holodeck are also observed) is a text-based online social medium to which multiple users are connected at the same time. MUSHes are often used for online social intercourse and role-playing games, although the first forms of MUSH do not appear to be coded specifically to implement gaming activity. MUSH software was originally derived from MUDs; today's two major MUSH variants are descended from TinyMUD, which was fundamentally a social game. MUSH has forked over the years and there are now different varieties with different features, although most have strong similarities and one who is fluent in coding one variety can switch to coding for the other with only a little effort. The source code for most widely used MUSH servers is open source and available from its current maintainers.
A primary feature of MUSH codebases that tends to distinguish it from other multi-user environments is the ability, by default, of any player to extend the world by creating new rooms or objects and specifying their behavior in the MUSH's internal scripting language. Another is the default lack of much player or administrative hierarchy imposed by the server itself. Over the years, both of these traits have become less pronounced, as many server administrators choose to eliminate or heavily restrict player-controlled building, and several games have custom coded systems to restore more of a hierarchal system.
The programming language for MUSH, usually referred to as "MUSHcode" or "softcode" (to distinguish it from "hardcode" – the language in which the MUSH server itself is written) was developed by Larry Foard. TinyMUSH started life as a set of enhancements to the original TinyMUD code. "MUSHcode" is similar in syntax to Lisp. Most customization is done in "softcode" rather than by directly modifying the hardcode.
Roleplay on MUSHes 
Traditionally, roleplay consists of a series of "poses". Each character makes a "pose" – that is, writes a description of speech actions, etc. which the character performs. Special commands allow players to print OOC (out of character) messages, distinguished by a prefixed tag from IC (in character) action. This medium borrows traits from both improvisational stage acting and writing. Roleplaying is one of the primary activities of MUSHes, along with socializing.
There is nothing in the code base that restricts a new MUSH from being a traditional hack-and-slash MUD-style game. However, the earliest uses of MUSH servers were for roleplaying and socializing, and these early trends have largely governed their descendants. In addition, due to this pressure, code updates have tended to emphasize improvements of value to the roleplayer. MUSH servers have one combat command out of the box, aptly called "kill". Despite the name, it simply gives a player a chance of killing another player dependent on how much OOC currency that player put into the effort. If successful, the "killed" player is sent to his home and given some money for his trouble. If unsuccessful, nothing interesting occurs. This command is frequently disabled by server administrators as it is in practice not conducive to roleplay or theme.
A large number of roleplaying MUSHes have custom combat systems coded by their administrators. However, these are usually intended to provide a vehicle for roleplayed combat, and not as the main objective of the game. Many MUSHes with combat systems in fact discourage their players from using them.
Administration of MUSHes 
All MUSH servers provide a flag that, when set on a player, bestows the ability to view and modify nearly everything in the game's database. Such players are usually called Wizards, and typically form the basis for the MUSH administration.
Although MUSH servers do not impose strong administrative hierarchies, most MUSH games establish additional levels of management besides Wizards. Some do so on a purely organizational basis, naming some Wizards "Head Wizards" or "Junior Wizards" or assigning sphere of responsibility to Wizards, despite the technical equality of their abilities in the game world. Others provide finer-grained control over capabilities that can be assigned to players so that some players can be granted the ability to view, but not modify, the entire game world, or to perform limited modifications. Other levels of power can include added control over one's own character, or fewer limits on resources. PennMUSH, TinyMUSH, and TinyMUX include the "Royalty" flag, which gives a player the powers to do most anything that doesn't involve modifying the database. RhostMUSH has a wide array of staff flags that differ in many ways from its sister servers.
MUSH Software 
Maintainers and developers of MUSH servers have traditionally shared ideas with one another, so most MUSH servers include concepts or code developed originally in other servers. There is particular interest in ensuring that common MUSHcode features work similarly across servers.
PennMUSH, TinyMUSH, TinyMUX and RhostMUSH are all open-source MUSH servers. Some enthusiasts[who?] may exclude one or more of the above on the basis of distribution method, name, or parentage, but all are free-form MUSH servers. Differences in the software tend to focus more on the administrative or softcode side (slightly different function syntax; or different functions altogether; more, or less, administrative controls). The set of commands that players use to interface to the game are essentially standard amongst servers bearing the appellation MUSH.
See also 
- TinyMUD – the ancestor to MUSH servers.
- Online text-based roleplaying game
- Online creation
- Burka, Lauren P. (1995). "The MUDline". "MUSH didn't stand for anything, but its authors later decided it meant "Multi-User Shared Hallucination.""
- "Beleriand MUSH FAQ". "MUSH stands for "Multi-User Shared Hallucination.""
- "Internet Games About". "MUSH stands for "Multi-User Shared Hallucination.""
- Shah, Rawn; Romine, James (1995). Playing MUDs on the Internet. John Wiley & Sons, Inc. p. 278. ISBN 0-471-11633-5. "Its name, MUSH, stands for Multiuser Shared Hallucination."
- Ito, Joi (2006). "Goodbye Privacy Presentation". "They [MUDS] spun off MOOs (MUD object oriented) and MUSHes (Multi-User Shared Hack) where it was more about creating things and creating objects. The kinds of people who participated in MOOs and MUSHes were very different from the kinds of people who participated in MUDs. MUDs were more about constraints and limitations and game-play."
- Bartle, Richard (2003). Designing Virtual Worlds. New Riders. p. 47. ISBN 0-13-101816-7. "MUSHes. Socially oriented, mostly focused on role-playing, but occasionally non-gaming in nature. MUSHes tend to have a Science Fiction setting based on books, comics, or movies, with Fantasy some way behind."
- Schwartz, Alan (2006). "PennMush FAQ".
- Leong, Lydia (1999). "TinyMush Development FAQ".
- Burka, Lauren P. (1995). "The MUDline". "Unlike most previous MUDs, TinyMUD emphasizes world creation over competition."
- Leong, Lydia (1995). "MUSH Manual Version 2.008". "MUSH is a derivative of the original TinyMUD. [...] The programming language of MUSH is most similar to LISP, in its emphasis on lists and the way functions are evaluated."
- One Ring Mush Staff. "MUSH and Roleplaying Etiquette Guidelines".
- StarWars Mush staff. "Interaction – The Power of the POSE".
- One Ring Mush staff. "RolePlay for Newbies".
- Ervin Hearn III (2000). "An Introduction to Mush".
- Platt, Wes (2004). "Taking the Stage".
- Smith, Jennifer (1990). "rec.games.mud FAQ". "Each type has its own unique style, and players are rarely forced to stick to one type of playing – there's no rule that says an LPMUD _must_ be a combat-oriented MUD, or that a TinyMUSH must not be a combat-oriented MUD. [...] The Tiny- and Teeny- family of MUDs are usually more social in orientation; the players on those MUDs tend to gather, chat, meet friends, make jokes, and discuss all kinds of things"
- PennMUSH developed from TinyMUD and PernMUSH at University of Pennsylvania, and later at University of California, Berkeley and University of Illinois at Chicago.
- TinyMUSH 3.0 through 3.3 (currently in alpha stage of development) derive from a merger of TinyMUSH 2.2.5 and TinyMUX 1.6, both of which ultimately derive from TinyMUD.
- TinyMUX 2.0 through 2.9 derive from TinyMUX 1.6. TinyMUX 1.0 through 1.6 were developed from a fork of TinyMUSH 2.0 10p6 and ultimately from TinyMUD.
- BattletechMUX A TinyMUX 1.6 derived codebase with real-time Battletech extensions.
- RhostMUSH developed from TinyMUD, then TinyMUSE. It is currently opensource.
- MUSH Resources at the Open Directory Project