''es'' (for "extensible shell") is an [[open source]], [[command line interpreter]] developed by Rakitzis and Paul Haahr,<ref>{{cite book | url=https://books.google.com/books?id=iEK6eQQwOF0C&pg=PA43&redir_esc=y#v=onepage&q&f=false | title=Spatial Analytical Perspectives on GIS}}</ref> that uses a [[scripting language]] syntactically similar to the rc shell.<ref>{{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 |deadurl=yes |archiveurl=https://web.archive.org/web/20140224211945/http://manpages.ubuntu.com/manpages/hardy/man1/es.1.html |archivedate=2014-02-24 |df= }}</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<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><ref>{{cite web|last=Jones|first=Tim|title=Evolution of shells in Linux|url=http://www.ibm.com/developerworks/library/l-linux-shells/|publisher=IBM|accessdate=14 March 2014}}</ref>
Extensible shell is intended to provide a fully [[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|deadurl=yes|archiveurl=https://web.archive.org/web/20130117064248/http://nnc3.com/lj/LJ/LJ12/0062.html|archivedate=2013-01-17|df=}}</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 [https://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 (Unix shell)|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>
