The ''es'' (for extensible shell)<ref> is an [[Open source]], [[command line interpreter]] developed by Byron Rakitzis and Paul Haahr, that uses a [[scripting language]] syntactically similar to the [[rc shell]] of the [[Plan 9 from Bell Labs|Plan 9]] [[operating system]].{{cite web | url=http://manpages.ubuntu.com/manpages/hardy/man1/es.1.html |title=Ubuntu Manpage: es - extensible shell |publisher=Manpages.ubuntu.com |date=1992-03-05 |accessdate=2012-08-24}}</ref><ref>{{cite web | url=http://foldoc.org/Extensible+Shell |title=Extensible Shell |publisher=FOLDOC |date= |accessdate=2012-08-24}}</ref> It was originally based on code from Byron Rakitzis's clone of [[rc shell|rc]] for Unix, and released in 1997.<ref>{{cite web | url=http://luv.asn.au/overheads/shells-talk.html |title=Shells Available for Linux |publisher=LUV |date= |accessdate=2012-08-24}}</ref>
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Extensible shell is intended to provide as fully a [[functional programming|functional]] [[programming language]] as a [[Unix shell]].<ref>{{cite web | url=http://nnc3.com/lj/LJ/LJ12/0062.html |title=Linux Journal 12: What's GNU |date= |accessdate=2012-08-24}}</ref> The bulk of es development occurred in the early 1990s, after the shell was introduced at the Winter 1993 [[USENIX]] conference in [[San Diego]],<ref>[http://stuff.mit.edu/afs/sipb/user/yandros/doc/es-usenix-winter93.html Es: A shell with higher-order functions] by Byron Rakitzis, [[NetApp|NetApp, Inc]], and Paul Haahr, [[Adobe Systems Incorporated]]; <u>Archived</u> at [http://web.archive.org/web/20090415213858/http://192.220.96.201/es/es-usenix-winter93.html Archive.Org].</ref> Official releases appear to have ceased after 0.9-beta-1 in 1997,<ref>[ftp://ftp.sys.utoronto.ca/pub/es/ ]</ref> and standard es lacks features as compared to more popular shells, such as [[zsh]] and [[bash]],<ref>{{cite web | url=http://www.faqs.org/faqs/unix-faq/shell/shell-differences/ |title=UNIX shell differences| publisher=Faqs.org |date= | accessdate=2012-08-24}}</ref> but unofficial development has been continued with job control and history patches,<ref>[http://web.archive.org/web/20070626160532/http://theorie1.physik.uni-erlangen.de/rogalsky/es/es.html ahh]</ref> and a more ambitious renamed fork, Xs<ref>[http://github.com/frytvm/XS Xs]</ref> (including syntax changes and [[C++]] code).
