View source for Rexx

{{Short description|A programming language used for scripting, application macros and application development}}
{{about|classic Rexx|oRexx and ooRexx|Object Rexx}}
{{use mdy dates|date=May 2021}}
{{use American English|date=May 2021}}
{{how-to|date=September 2022}}
{{trivia|date=September 2022}}
{{Infobox programming language
| logo = Rexx-img-lg.png
| paradigm = [[multi-paradigm programming language|multiparadigm]]: [[procedural programming|procedural]], [[structured programming|structured]]
| year = {{Start date and age|1979}}
| designer = [[Mike Cowlishaw]]
| developer = Mike Cowlishaw, [[IBM]]
| latest release version = [[ANSI]] X3.274
| latest release date = {{Start date and age|1996}}
| typing = Dynamic
| implementations = VM/SP R3,<ref>{{cite book
 | title       = Virtual Machine/System Product - System Product - Interpreter Reference - Release 3 
 | id          = SC24-5239-0
 | date        = September 1983
 | edition     = First
 | url         = http://bitsavers.org/pdf/ibm/370/VM/SP/Release_3.0_Jul83/SC24-5239-0_VM_SP_Interpreter_Reference_Rel_3_Sep83.pdf
 | publisher   = [[IBM]]
 | access-date = November 6, 2024
 }}
</ref> TSO/E V2,<ref>{{cite book
 | title       = TSO Extensions Version 2 - Procedures Language MVS/REXX Reference
 | id          = SC28-1883-4
 | date        = August 1991
 | edition     = Fifth
 | url         = http://bitsavers.org/pdf/ibm/370/TSO_Extensions/SC28-1883-4_TSO_Extensions_Version_2_Procedures_Langage_MVS_REXX_Reference_Aug1991.pdf
 | publisher   = [[IBM]]
 | access-date = November 6, 2024
 }}
</ref> SAAREXX,<ref>{{cite book
 | title       = Systems Application Architecture - An Overview 
 | id          = GC26-4341-0
 | date        = May 1987
 | edition     = First
 | section     = Procedures Language
 | section-url = http://bitsavers.org/pdf/ibm/saa/GC26-4341-0_System_Application_Architecture_An_Overview_198705.pdf@page=46
 | page        = 40
 | url         = http://bitsavers.org/pdf/ibm/saa/GC26-4341-0_System_Application_Architecture_An_Overview_198705.pdf
 | publisher   = [[IBM]]
 | access-date = November 6, 2024
 }}
</ref><ref>{{cite book
 | title       = Procedures Language/2 - REXX Reference - Version 2.00 
 | id          = S10G-6268-00
 | date        = December 1991
 | edition     = First
 | url         = http://bitsavers.org/pdf/ibm/pc/os2/OS2_2.x/IBM_OS2_2.0_Technical_Library_1992/S10G-6268-00_OS2_2.0_Procedures_Language_2_REXX_Reference_199203.pdf
 | publisher   = [[IBM]]
 | series      = OS/2 2.00 - Technical Library
 | access-date = November 6, 2024
 }}
</ref> [[ARexx]], BREXX, Regina,<ref name="regina">{{cite web
      |url=http://regina-rexx.sourceforge.net
      |title=Regina Rexx Interpreter
      |author=Mark Hessling |date=October 25, 2012
      |publisher=[[SourceForge]] project regina-rexx |access-date=2014-02-10}}</ref> Personal REXX, REXX/imc
| dialects = [[NetRexx]], [[Object REXX]], now <!-- same page -->[[ooREXX]], [[XEDIT#KEXX|KEXX]] <!-- also mentions THE and Regina -->
| influenced_by = [[PL/I]], [[ALGOL]], [[CMS EXEC|EXEC]], [[EXEC 2]]
| influenced = [[NetRexx]], [[Object REXX]]
| file_ext = .cmd, .bat, .exec, .rexx, .rex, EXEC
| wikibooks = Rexx Programming
}}

'''Rexx''' ('''restructured extended executor''') is a [[high-level programming language]] developed at [[IBM]] by [[Mike Cowlishaw]].<ref name=RexHist/><ref name=RexMay95/> Both proprietary and [[open-source software|open source]] Rexx [[interpreter (computing)|interpreters]] exist for a wide range of [[computing platform]]s, and [[compiler]]s exist for IBM [[mainframe computer]]s.<ref>{{cite web
     |url=http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/FINDBOOK?filter=rexx
     |title=Catalog of All Documents (filter=rexx)
     |publisher=[[IBM]] library server
     |year=2005
     |access-date=2014-02-10
     |archive-date=February 15, 2013
     |archive-url=https://web.archive.org/web/20130215214058/http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/FINDBOOK?filter=rexx
     |url-status=dead
     }}</ref> Rexx is used for [[scripting language|scripting]], [[application software|application]] [[macro (computer science)|macro]]s and [[Software development|application development]]. As a general purpose scripting language, Rexx is considered a precursor to [[Tcl]] and [[Python (programming language)|Python]].

Rexx is supported in a variety of environments. It is the primary scripting language in some [[operating system]]s including [[OS/2]], [[MVS]], [[VM (operating system)|VM]], [[AmigaOS]] and is used for macros in some software including [[SPF/PC]], [[XEDIT#PC and Unix adaptations|KEDIT]], [[The Hessling Editor|THE]] and [[ZOC (software)|ZOC]]. With an engine installed, Rexx can be used for scripting and macros in programs that use a [[Windows Scripting Host]] [[ActiveX]] scripting engine (such as [[VBScript]] or [[JScript]]). Rexx is supplied with VM/SP Release 3 on up, TSO/E Version 2 on up, OS/2 (1.3 and later, where it is officially named ''Procedures Language/2''), [[AmigaOS#AmigaOS 2.0, 2.1|AmigaOS Version 2]] on up, [[PC DOS]] ([[PC DOS 7.0|7.0]] or [[PC DOS 2000|2000]]), [[ArcaOS]],<ref>{{cite web
|url=https://www.arcanoae.com/faqwd/does-arcaos-include-rexx-support
|title=Does ArcaOS include REXX support?
|access-date=2020-09-03}}</ref> and Windows NT 4.0 (Resource Kit: Regina). In the late 1980s, Rexx became the common scripting language for [[IBM Systems Application Architecture]], where it was renamed "SAA Procedure Language REXX".

A script is associated with a Rexx interpreter at runtime in various ways based on context. In mainframe computing, a Rexx script or command is sometimes referred to as an ''EXEC'' since that is the name of the file type used for similar [[Conversational Monitor System|CMS]] [[CMS EXEC|EXEC]],<ref name=x>{{cite book|title=IBM Virtual Machine Facility /370: EXEC User's Guide|id=GC20-1812-1|edition=Second|date=April 1975|publisher=International Business Machines Corporation |url=http://bitsavers.org/pdf/ibm/370/VM_370/Release_2/GC20-1812-1_VM370_EXEC_Users_Guide_Rel_2_Apr75.pdf}}</ref> and [[EXEC 2]]<ref name=X2>{{cite book | title=EXEC 2 Reference | id=SC24-5219-1 | pages=92 | edition=Second | date=April 1982 | publisher=International Business Machines Corporation | url=http://bitsavers.org/pdf/ibm/370/VM_SP/Release_2_Jun82/SC24-5219-1_VM_SP_EXEC_2_Rel_2_Reference_Apr82.pdf | access-date=March 28, 2019 | archive-date=April 2, 2020 | archive-url=https://web.archive.org/web/20200402040956/http://bitsavers.org/pdf/ibm/370/VM_SP/Release_2_Jun82/SC24-5219-1_VM_SP_EXEC_2_Rel_2_Reference_Apr82.pdf | url-status=dead }}</ref> scripts and for Rexx scripts on [[VM/SP]] R3 through [[z/VM]]. The first line of a script specifies the use of a Rexx interpreter in a comment either by identifying the code as Rexx language or by file path via <code>EXTPROC</code>. On MVS, Rexx scripts may{{efn|The TSO EXEC command with an unqualified dataset name, and neither the CLIST nor EXEC option, examines the low level qualifier for "EXEC".}} be recognized by the low level qualifier "EXEC" or if the first line fetched from SYSPROC is a comment containing "REXX" then it is treated as Rexx (rather than CLIST), and a script fetched from SYSEXEC must be Rexx. On OS/2, Rexx scripts share the filename extension ".cmd" with other scripting languages, and the first line of the script specifies the interpreter to use. On [[Linux]], Rexx scripts generally begin with a [[shebang (Unix)|shebang]]. Rexx macros for Rexx-aware applications use extensions determined by the application.

==Name==
Originally, the language was called ''REX'', short for ''Reformed Executor'', but an extra "X" was added to avoid confusion with other products. The name was originally all uppercase because that was the only way to represent it in mainframe code at the time. Both editions of Mike Cowlishaw's first book on the language use all-caps, ''REXX'', although the cover graphic uses mixed case. His book on NetRexx uses mixed case but all caps in the cover graphic with large and small caps, {{big|N}}{{small|ET}}{{big|R}}{{small|EXX}}. An expansion that matches the abbreviation, ''REstructured eXtended eXecutor'', was used for the system product in 1984.<ref name="POLA"/> The name ''Rexx'' (mixed case) is used in this article, and is commonly used elsewhere.

== Attributes ==
Objective and subjective attributes of Rexx include:
* Simple syntax
* Ability to route commands to multiple environments
* Ability to support functions, procedures and commands associated with a specific invoking environment.
* Built-in stack with the ability to interoperate with the host stack if there is one
* Small instruction set
* [[Free-form language|Free-form syntax]]; indentation is optional but can help readability
* Case-insensitive tokens, including variable names
* [[Character string]] basis <!--what does basis imply?-->
* [[Type system#Dynamic typing|Dynamic data typing]]; no declarations
* No [[reserved word|reserved keyword]]s, except in local context{{efn|It is best practice to avoid using keywords as labels or names.{{cn|date=May 2025}}}}
* No include file facility
* [[Arbitrary-precision arithmetic]]
* Decimal arithmetic, [[floating-point]]
* Rich selection of built-in functions, especially string and word processing
* Automatic storage management
* Crash protection
* Content addressable data structures
* [[Associative array]]
* Straightforward access to system commands and facilities
* Simple error-handling, and built-in tracing and debugger
* Few artificial limitations
* Simplified I/O facilities
* Unconventional operators <!--like what?-->
* Only partly supports Unix style command line parameters, except specific implementations
* Provides no basic terminal control as part of the language, except specific implementations
* Provides no generic way to include functions and subroutines from external libraries, except specific implementations

Some claim that Rexx is a relatively simple language. With only 23 instructions (such as <code>call</code>, <code>parse</code>, and <code>select</code>), it has a relatively small instruction set. Rexx has limited punctuation and formatting requirements. Rexx has only one [[data type]], the character string. Some claim that such simplicities make Rexx relatively easy to debug.

Some claim that Rexx code looks similar to [[PL/I]] code, but has fewer notations. With fewer notations, it tends to be is harder to parse via a translator, but is easier to write. Simplifying coding was intentional as noted by the Rexx design goal of the [[principle of least astonishment]].<ref name="POLA">{{cite journal
|url=https://www.cs.tufts.edu/~nr/cs257/archive/mike-cowlishaw/rexx.pdf|title=The design of the REXX language|author=M. F. Cowlishaw|journal=IBM Systems Journal |page=333|type=PDF|publisher=[[IBM Research]]|year=1984
|volume=23|issue=4|quote=Could there be a high astonishment factor associated with the new feature? If a feature is accidentally misapplied by the user and causes what appears to him to be an unpredictable result, that feature has a high astonishment factor and is therefore undesirable. If a necessary feature has a high astonishment factor, it may be necessary to redesign the feature.
|doi=10.1147/sj.234.0326|author-link=Mike Cowlishaw|access-date=2014-01-23}}</ref>

==History==
===pre{{En dash}}1990===
On his own time, [[Mike Cowlishaw]] developed the language and an interpreter for it in [[assembly language]] between 20 March 1979 and mid-1982 with the intent to replace the languages [[EXEC]] and [[EXEC 2]].<ref name=RexHist>{{cite web|url=http://www.rexxla.org/links/IBM_historical_pages/rexxhist.html|title=IBM REXX Brief History|author=M. F. Cowlishaw|publisher=[[IBM]]|author-link=Mike Cowlishaw|access-date=2006-08-15}}</ref> Mike also intended Rexx to be a simplified and easier to learn version of [[PL/I]], but some claim that Rexx has problematic differences from PL/I.

Rexx was first described in public at the SHARE 56 conference in Houston, Texas, in 1981,<ref>{{cite web|url=http://speleotrove.com/rexxhist/share56.txt <!-- http://www-306.ibm.com/software/awdtools/rexx/library/share56.html -->|title=REX -- A Command Programming Language |date=1981-02-18 |author=M. F. Cowlishaw |publisher=[[SHARE (computing)|SHARE]] |author-link=Mike Cowlishaw |access-date=2006-08-15}}</ref> where customer reaction, championed by Ted Johnston of [[SLAC National Accelerator Laboratory|SLAC]], led to it being shipped as an IBM product in 1982.

Over the years IBM included Rexx in almost all of its operating systems ([[VM (operating system)|VM/CMS]], [[MVS|MVS TSO/E]], [[IBM OS/400]], [[VSE (operating system)|VSE/ESA]], [[MUSIC/SP]], [[IBM AIX|AIX]], [[PC DOS]], and [[OS/2]]), and has made versions available for [[Novell NetWare]], [[Microsoft Windows|Windows]], [[Java (programming language)|Java]], and [[Linux]].

The first non-IBM version was written for [[PC DOS]] by Charles Daney in 1984/5<ref name=RexMay95>{{cite web |author=Melinda Varian
|url=http://www.rexxla.org/events/1995/report.html |title=REXX Symposium, May 1995}}</ref> and marketed by the Mansfield Software Group (founded by Kevin J. Kearney in 1986).<ref name=RexHist/> The first Rexx compiler appeared in 1987, written for CMS by Lundin and Woodruff.<ref name="TR">{{cite journal|url=http://www.copyrightsearch.org/trexx-a-rexx-compiler-for-cms/TXu000295377|title=T/REXX, a REXX compiler for CMS|last=Lundin|first=Leigh|author2=Woodruff, Mark|date=1987-04-23|journal=U.S. Copyright Office|publisher=Independent Intelligence Incorporated|location=Washington, DC|issue=TXu000295377|access-date=February 20, 2010|archive-date=March 3, 2016|archive-url=https://web.archive.org/web/20160303204104/http://www.copyrightsearch.org/trexx-a-rexx-compiler-for-cms/TXu000295377|url-status=dead}}</ref> Other versions have also been developed for [[Atari]], [[AmigaOS]], [[Unix]] (many variants), [[Solaris (operating system)|Solaris]], [[Digital Equipment Corporation|DEC]], [[Microsoft Windows|Windows]], [[Windows CE]], [[Pocket PC]], [[DOS]], [[Palm OS]], [[QNX]], [[OS/2]], [[Linux]], [[BeOS]], [[EPOC32]]/[[Symbian]], [[AtheOS]], [[OpenVMS]],<ref name=Fosdick2025>{{cite book|last1=Fosdick|first1=Howard|year=2025|location=[[Apex, North Carolina]]|title=Rexx Programmer's Reference|url=https://rexxinfo.org/Rexx_Programmers_Reference.pdf|language=en-us|edition=2|publisher=Rexx Language Association|isbn=9789403745527|oclc=1475017186|url-status=live|archive-url=https://web.archive.org/web/20250403173024/https://rexxinfo.org/Rexx_Programmers_Reference.pdf|archive-date=2025-04-03|access-date=2025-05-26}}</ref>{{rp|p=309}} Apple [[Macintosh]], and [[Mac OS X]].<ref>{{cite web |url=http://www.rexxla.org/About_Rexx/mfc/rexxplat.html
|title=Rexx Implementations |publisher=RexxLA |access-date=2006-08-15
|archive-url=https://web.archive.org/web/20060924072512/http://www.rexxla.org/About_Rexx/mfc/rexxplat.html
|archive-date=2006-09-24}}</ref>  <!-- Bot retrieved archive -->

[[ARexx]], a Rexx interpreter for [[Amiga]], was included with AmigaOS 2 onwards and was popular for scripting and application control. Many Amiga applications have an "ARexx port" which allows control of the application via a Rexx script. Notably, a Rexx script can switch between Rexx ports to control multiple applications.

===1990 to present===
In 1990, Cathie Dager of SLAC organized the first independent Rexx symposium, which led to the forming of the Rexx Language Association. Symposia are held annually.

In 1992, the two most widely used [[open-source software|open-source]] ports appeared: Ian Collier's REXX/imc for Unix and Anders Christensen's Regina<ref name="regina"/> (later adopted by Mark Hessling) for Windows and Unix. BRexx was developed by Vasilis N Vlachoudis, a nuclear scientist at [[CERN]]. It runs on a range of operating systems, including Unix, Linux, BSD, macOS and Windows. Its small size means it can run on an [[Android (operating system)|Android]] mobile phone. BRexx/370 is a version that runs on IBM mainframes.<ref name=Fosdick2025/>{{rp|pp=359&ndash;383}}<ref>{{cite web|last1=Vlachoudis|first1=Vassilis|date=June 2011|location=[[CERN]]|title=BRexx|url=https://github.com/vlachoudis/brexx|language=en|publication-place=[[San Francisco|San Francisco, California]]|publisher=[[GitHub]]|url-status=live|archive-url=https://web.archive.org/web/20250612085939/https://github.com/vlachoudis/brexx|archive-date=2025-06-12|access-date=2025-06-24}}</ref>

OS/2 has a visual development system from Watcom [[VX-REXX]]. Another dialect was VisPro REXX from Hockware.

''Portable Rexx'' by Kilowatt and ''Personal Rexx'' by Quercus are two Rexx interpreters designed for DOS and can be run under Windows as well using a command prompt. Since the mid-1990s, two newer variants of Rexx have appeared:
* [[NetRexx]]: compiles to [[Java (programming language)|Java]] [[byte-code]] via Java source code; this has no reserved keywords at all, and uses the Java object model, and is therefore not generally upwards-compatible with 'classic' Rexx.
* [[Object REXX]]: an [[object-oriented]] generally upwards-compatible version of Rexx.

In 1996 the [[American National Standards Institute]] (ANSI) published a standard for Rexx: ANSI X3.274–1996 "Information Technology&nbsp;– Programming Language REXX".{{efn|text=While ANSI INCITS 274-1996/AMD1-2000 (R2001) and ANSI INCITS 274-1996 (R2007) are chargeable, a free draft can be downloaded.<ref>{{cite web|location=[[Washington D.C.]]|title=American National Standard for Information Systems – Programming Language R{{small|EXX}}|url=https://www.rexxla.org/rexxlang/standards/j18pub.pdf|website=The Rexx Language Association|language=en-us|publisher=[[American National Standards Institute|American National Standards Institute (ANSI)]]|id=X3J18-199X|url-status=live|archive-url=https://web.archive.org/web/20250403153402/https://www.rexxla.org/rexxlang/standards/j18pub.pdf|archive-date=2025-04-03|access-date=2025-06-07}}</ref>}} More than two dozen books on Rexx have been published since 1985.

Rexx marked its 25th anniversary on 20 March 2004, which was celebrated at the Rexx Language Association's 15th International REXX Symposium in Böblingen, Germany, in May 2004.

On October 12, 2004, IBM announced their plan to release their [[Object REXX]] implementation's sources under the [[Common Public License]]. Recent releases of Object REXX contain an ActiveX [[Windows Scripting Host]] (WSH) scripting engine implementing this version of the Rexx language.

On February 22, 2005, the first public release of Open Object Rexx (ooRexx) was announced. This product contains a WSH scripting engine which allows for programming of the Windows operating system and applications with Rexx in the same fashion in which [[Visual Basic]] and [[JScript]] are implemented by the default WSH installation and [[Perl]], [[Tcl]], [[Python (programming language)|Python]] third-party scripting engines.

In January 2018 the [[TIOBE index]] listed Rexx at position 30.<ref>{{cite web|date=January 2018|location=[[Eindhoven|Eindhoven, Netherlands]]|title=TIOBE Index for January 2018|url=https://web.archive.org/web/20180203212305/https://www.tiobe.com/tiobe-index#:~:text=0.569%-,30,0.562%,-31|website=TIOBE|language=en|access-date=2025-06-07}}</ref> Since 2018 it has been either outside the top 50, or, more frequently, outside the top 100.

In 2019, the 30th Rexx Language Association Symposium marked the 40th anniversary of Rexx. The symposium was held in Hursley, England, where Rexx was first designed and implemented.<ref>{{Cite web|url=https://www.rexxla.org/events/schedule.rsp?year=2019|title = RexxLA - Symposium Schedule}}</ref>

==Toolkits==
*RexxUtil {{endash}} a package of file and directory functions, windowed I/O, and functions to access system services such as WAIT and POST {{endash}} is available for most Rexx environments.<ref>{{cite book|last1=Ashley|first1=W&nbsp;David|last2=Flatscher|first2=Rony&nbsp;G|last3=Hessling|first3=Mark|last4=McGuire|first4=Rick|last5=Peedin|first5=Lee|last6=Sims|first6=Oliver|last7=Steinbock|first7=Erich|last8=Wolfers|first8=Jon|date=2024-01-19|chapter=8. Rexx Utilities (RexxUtil)|chapter-url=https://rexxinfo.org/reference/articles/oorexxref.pdf#page=530|title=Open Object Rexx: Reference|url=https://rexxinfo.org/reference/articles/oorexxref.pdf|language=en-us|edition=5.1.0|publisher=Rexx Language Association|pages=512&ndash;566|url-status=live|archive-url=https://web.archive.org/web/20240525080605/https://rexxinfo.org/reference/articles/oorexxref.pdf#page=530|archive-date=2024-05-25|access-date=2025-05-26}}</ref><ref>{{cite web |title=REXX Tips & Tricks:REXXUTIL functions |url=http://www.edm2.com/index.php/REXX_Tips_%26_Tricks:REXXUTIL_functions |website=EDM2: The Electronic Developer Magazine for OS/2 |access-date=October 14, 2023}}</ref><ref>{{cite web |title=Regina Rexx Interpreter |url=https://regina-rexx.sourceforge.io/ |website=Sourceforge |access-date=October 14, 2023}}</ref>
*Rexx/Tk {{endash}} a toolkit for graphics to be used in Rexx programmes in the same fashion as [[Tcl/Tk]] {{endash}} is widely available.
*RexxEd {{endash}} an [[integrated development environment]] (IDE) for Rexx {{endash}} was developed for [[Windows]].<ref name=Fosdick2025/>{{rp|p=390}}
*RxSock for network communication as well as other add-ons to and implementations of Regina Rexx have been developed, and a Rexx interpreter for the Windows command line is supplied in most resource kits for various versions of Windows and works in [[DOS]] as well.

==Host environment==
A Rexx host environment is a named interface for sending [[#Commands|commands]] to an, e.g., application, operating system, subsystem. The Rexx interpreter initially has a list of known environments; the first in the list is known as the ''default environment''. A Rexx script use the <code>ADDRESS</code> statement to change the default environment and may also use it to send a single command to a specific environment without changing the default environment.

==Syntax==
{{how-to|section|date=December 2012}}

===Instruction types===
Rexx has three instruction types<ref name=Cowlishaw1990>{{cite book|last1=Cowlishaw|first1=Michael|author-link1=Mike Cowlishaw|year=1990|location=[[Englewood Cliffs, New Jersey]]|title=The REXX Language: A Practical Approach to Programming|url=https://rexxinfo.org/info/articles/the_rexx_language_2nd_edition.pdf|language=en-us|edition=2|publisher=[[Prentice Hall]]|isbn=9780137806515|lccn=89071130|oclc=20826616|ol=2226199M|url-status=live|archive-url=https://web.archive.org/web/20240421142252/https://rexxinfo.org/info/articles/the_rexx_language_2nd_edition.pdf|archive-date=2024-04-21|access-date=2025-06-13}}</ref>{{rp|p=31}}
*Assignments &ndash; Single clause of the form <code>''symbol''=''expression''</code>, assigns a value to a variable; e.g. <code>count=count+1</code>, would add 1 to value in variable "count". 
*Keyword instructions &ndash; The keyword is the first word of the instruction; e.g. <code>'''say''' "Message"</code>, would print the word 'Message'.
*{{anchor|Commands}}Commands &ndash; an expression that the interpreter evaluates and passes as a command to the default [[#Host environment|environment]]; e.g. <code>"sleep 10"</code> would cause a Unix-like host environment to produce a ten seconds delay.

===Address instruction===
The <code>address</code> instruction has three roles:
*An <code>address</code> instruction with no operand interchanges the first two host environments; normally this reinstate the previous default environment.
*An <code>address</code> instruction with only an environment moves it to the beginning of the list, making it the default environment.
*An <code>address</code> instruction with an environment and an expression passes the value of the expression as a command to the specified environment.
The ANSI standard added options for redirecting the input and output of commands.

===Do groups===
The language provides <code>do</code> groups for two purposes:
*To treat a group of instructions within an <code>if</code> or <code>select</code> statement as a unit for purposes of flow control.
*For [[For loop|loop control]], similar to many other languages. A single <code>do</code> group may optionally contain repetitor phrases, conditional phrases, or both, with termination whenever any of them is satisfied.

A <code>do</code> group begins with <code>do</code> and ends with <code>end</code>. A single group may serve both purposes. In the related ooRexx and NetRexx, there is both a <code>do</code> and a <code>loop</code> keyword, with almost identical semantics; they differ in that a simple <code>do</code> is equivalent to <code>do 1</code> while a simple  <code>loop</code> is equivalent to <code>loop forever</code>.

An iteration of a <code>do</code> group may be terminated with an <code>iterate</code> statement and the entire group may be terminated with a <code>leave</code> statement.

====Simple do====
Although it is valid anywhere, a simple do is specifically useful inside conditional statements:
<syntaxhighlight lang=rexx>
if foo=bar
then
  do
    i=1;j=3
  end
else
  do
    i=2;j=4
  end
</syntaxhighlight>

====Conditional loop====
The language supports testing a condition either before (<code>do while</code>) or after (<code>do until</code>) executing a group of code via syntax:
 '''do while''' [''condition'']
   [''instructions'']
 '''end'''

 '''do until''' [''condition'']
   [''instructions'']
 '''end'''

====Simple repetitive loop====
The language permits counted loops, where an expression is computed at the start of the loop and the instructions within the loop are executed that many times:
 '''do''' ''expression''
   [''instructions'']
 '''end'''

====Controlled Repetitive Loops====
A loop can increment a variable and stop when a limit is reached.
 '''do''' ''index''=''start'' ['''to''' ''limit''] ['''by''' ''increment''] ['''for''' ''count'']
   [''instructions'']
 '''end'''

The increment value is 1 if the <code>by</code> clause is omitted. The loop continues forever if the limit <code>to</code> clause is omitted, unless terminated earlier by another clause or by a <code>leave</code> statement.

====Unconditional loop====
The language supports an unconditional loop via <code>forever</code> that continues until the loop is terminated or the program is terminated.
 '''do forever'''
   [''instructions'']
 '''end'''

====Combined loop====
Repetitive elements and conditional phrases can be combined in the same loop;<ref name=Cowlishaw1990/>{{rp|p=50}} e.g.:<syntaxhighlight lang=rexx>do i=1 while i<=3; say i; end</syntaxhighlight>
 '''do''' ''index''=''start'' ['''to''' ''limit''] ['''by''' increment] ['''for''' ''count''] ['''while''' ''condition'']
   [''instructions'']
 '''end'''

 '''do''' ''expression'' ['''until''' ''condition'']
   [''instructions'']
 '''end'''

=== Conditional ===
The language provides for [[Conditional (computer programming)|conditional execution]] via <code>if</code>, <code>then</code> and <code>else</code> for a group delimited by <code>do</code> and <code>end</code>.

 '''if''' [''condition''] '''then'''
   '''do'''
   [''instructions'']
   '''end'''
 '''else'''
   '''do'''
   [''instructions'']
   '''end'''

For a single instruction, <code>do</code> and <code>end</code> can be omitted.

 '''if''' [''condition''] '''then'''
   [''instruction'']
 '''else'''
   [''instruction'']

=== Multiple condition branching ===
The language provides [[Switch statement|multiple condition branching]] via <code>select</code> which derives from the <code>SELECT;</code> form of the PL/I <code>SELECT</code> statement
.{{efn|Rexx has no equivalent to the <code>SELECT (''expression'');</code> form.}} Like similar constructs in other dynamic languages, Rexx's <code>when</code> clauses specify full conditions {{endash}} not equality tests of a single value for the statement as some languages do. In that, they are more like cascading <code>if-then-else</code> code than like the C or Java <code>switch</code> statement.

 '''select'''
   '''when''' [''condition''] '''then'''
     [''instruction'' or '''nop''']
   '''when''' [''condition''] '''then'''
     '''do'''
     [''instructions'' or '''nop''']
     '''end'''
   '''otherwise'''
     [''instructions'' or '''nop''']
 '''end'''

The <code>nop</code> instruction is required if no action is associated with a <code>when</code> condition.

The <code>otherwise</code> clause is optional. If omitted and no <code>when</code> conditions are met, then the <code>syntax</code> condition is raised.

=== Variable ===
==== Typing system ====
Variables are typeless and initially are evaluated as their names in upper case. Thus a variable's type can vary with its use in the program:
<syntaxhighlight lang="rexx">
say hello /* => HELLO */
hello = 25
say hello /* => 25 */
hello = "say 5 + 3"
say hello /* => say 5 + 3 */
interpret hello /* => 8 */
drop hello
say hello /* => HELLO */
</syntaxhighlight>

==== Evaluation ====
If no <code>novalue</code> condition handler is configured, then an undefined variable evaluates to its name, in upper case. The built-in function <code>SYMBOL</code> returns "VAR" for a defined variable and does ''not'' trigger <code>novalue</code> even if not defined. The <code>VALUE</code> function gets the value of a variable without triggering a <code>novalue</code> condition, but its main purpose is to read and set [[env (shell)|environment]] variables, similar to [[POSIX]] <code>getenv</code> and <code>putenv</code>.

==== Compound variable ====
The language provides the '''compound variable''' construct which supports adding fields (called tails) to a variable (called a stem in this context) to support data structures such as lists, arrays, n-dimensional arrays, sparse or dense arrays, balanced or unbalanced trees and records.

The language does not provide special support for numeric array indexing like many other languages do. Instead, a compound variable with numeric tails produce a similar effect.<ref>{{Cite web|url=https://rexxinfo.org/info/articles/how_to_code_arrays.pdf|title=How to Code Arrays and Other Data Structures In Rexx}}</ref>

The following code defines variables <code>stem.1 = 9, stem.2 = 8, stem.3 = 7</code>...
<syntaxhighlight lang="rexx">
do i = 1 to 10
  stem.i = 10 - i
end
</syntaxhighlight>
Unlike a typical array, a tail (index) need not identify (be named) an integer value. For example, the following code is valid:
<syntaxhighlight lang="rexx">
i = 'Monday'
stem.i = 2
</syntaxhighlight>
A default value can be assigned to a stem via <code>.</code> but no tail.
<syntaxhighlight lang="rexx">
stem. = 'Unknown'
stem.1 = 'USA'
stem.44 = 'UK'
stem.33 = 'France'
</syntaxhighlight>
In this case <code>stem.3</code>, for example, evaluates to the default value, <code>'Unknown'</code>.

The whole stem (including any default value) can be erased with the <code>drop</code> statement.
<syntaxhighlight lang="rexx">
drop stem.
</syntaxhighlight>
By convention (not part of the language) the compound <code>stem.0</code> is often used to keep track of how many items are in a stem, for example a procedure to add a word to a list might be coded like this:
<syntaxhighlight lang="rexx">
add_word: procedure expose dictionary.
  parse arg w
  n = dictionary.0 + 1
  dictionary.n = w
  dictionary.0 = n
return
</syntaxhighlight>
A stem can have multiple tails. For example:
<syntaxhighlight lang="rexx">
m = 'July'
d = 15
y = 2005
day.y.m.d = 'Friday'
</syntaxhighlight>
Multiple numerical tail elements can be used to provide the effect of a multi-dimensional array.

Features similar to the compound variable are found in other languages including [[associative array]]s in [[AWK programming language|AWK]], [[hash table|hashes]] in [[Perl]] and [[hash table]]s in [[Java (programming language)|Java]], dynamic objects in [[JavaScript]]. Most of these languages provide a mechanism to iterate over the keys (tails) of such a construct, but this is lacking in classic Rexx. Instead, it is necessary to store additional information. For example, the following procedure might be used to count each occurrence of a word.
<syntaxhighlight lang="rexx">
add_word: procedure expose count. word_list
  parse arg w .
  count.w = count.w + 1 /* assume count. has been set to 0 */
  if count.w = 1 then word_list = word_list w
return
</syntaxhighlight>
and then later:
<syntaxhighlight lang="rexx">
do i = 1 to words(word_list)
  w = word(word_list,i)
  say w count.w
end
</syntaxhighlight>
More recent Rexx variants, including Object REXX and ooRexx, provide a construct to iterate over the tails of a stem.
<syntaxhighlight lang="rexx">
do i over stem.
  say i '-->' stem.i
end
</syntaxhighlight>

=== Parse ===
The <code>parse</code> instruction provides string-handling via syntax:

<code>
'''parse''' ['''upper'''] ''origin'' [''template'']
</code>

If <code>upper</code> is included then the input is converted to upper case before parsing.

''origin'' describes the input as one of the following:

* <code>arg</code> {{endash}} arguments, at top level tail of command line
* <code>linein</code> {{endash}} standard input, e.g. keyboard
* <code>pull</code> {{endash}} Rexx data queue or standard input
* <code>source</code> {{endash}} info on how program was executed
* <code>value</code> ''expression'' <code>with</code> {{endash}} <code>with</code> indicates the end of the expression
* <code>var</code> {{endash}} a variable
* <code>version</code> {{endash}} version/release number

''template'' can be a combination of variables, literal delimiters, and column number delimiters.

==== Examples ====
Using a list of variables as template:
<syntaxhighlight lang="rexx">
myVar = "John Smith"
parse var myVar firstName lastName
say "First name is:" firstName
say "Last name is:" lastName
</syntaxhighlight>
displays:
<syntaxhighlight lang="output">
First name is: John
Last name is: Smith
</syntaxhighlight>
Using column number delimiters:
<syntaxhighlight lang="rexx">
myVar = "(202) 123-1234"
parse var MyVar 2 AreaCode 5 7 SubNumber
say "Area code is:" AreaCode
say "Subscriber number is:" SubNumber
</syntaxhighlight>
displays:
<syntaxhighlight lang="output">
Area code is: 202
Subscriber number is: 123-1234
</syntaxhighlight>

=== Interpret ===
The <code>interpret</code> instruction evaluates its argument as a Rexx statement allowing for evaluation of code formatted at runtime. Uses include passing a function as a parameter, arbitrary precision arithmetic, use of the <code>parse</code> statement with programmatic templates, stemmed arrays, and sparse arrays.{{how|date=January 2021}} The following example displays 16 and exits.
<syntaxhighlight lang="rexx">
X = 'square'
interpret 'say' X || '(4) ; exit'
SQUARE: return arg(1)**2
</syntaxhighlight>
The [[Valour (software)|Valour software]] package relied upon Rexx's interpretive ability to implement an [[object-oriented programming|OOP]] environment.{{citation needed|date=January 2025}} Another use was found in an unreleased [[Westinghouse Electric (1886)|Westinghouse]] product called ''Time Machine'' that was able to fully recover following an otherwise fatal error.{{citation needed|date=January 2025}}

=== Numeric ===
<syntaxhighlight lang="rexx">
say digits() fuzz() form() /* => 9 0 SCIENTIFIC */
say 999999999+1 /* => 1.000000000E+9 */
numeric digits 10 /* only limited by available memory */
say 999999999+1 /* => 1000000000 */

say 0.9999999999=1 /* => 0 (false) */
numeric fuzz 3
say 0.99999999=1 /* => 1 (true) */
say 0.99999999==1 /* => 0 (false) */

say 100*123456789 /* => 1.23456789E+10 */
numeric form engineering
say 100*123456789 /* => 12.34567890E+9 */

say 53 // 7   /* => 4   (rest of division)*/
</syntaxhighlight>
Calculation of the value [[Square root of 2|&radic;2]]:
<syntaxhighlight lang="rexx">
numeric digits 50
n=2; r=1
do forever /* Newton's method */
  rr=(n/r+r)/2
  if r=rr then leave
  r=rr
end
say "SqRt" n "=" r 
</syntaxhighlight>
Output: {{samp|1=SqRt 2 = 1.414213562373095048801688724209698078569671875377}}

Calculation of the value [[e (mathematical constant)|{{mvar|e}}]]: 
<syntaxhighlight lang="rexx">
numeric digits 50
e=2.5; f=0.5
do n=3
  f=f/n
  ee=e+f
  if e=ee then leave
  e=ee
end
say "e =" e
</syntaxhighlight>
Output: {{samp|1=e = 2.7182818284590452353602874713526624977572470936998}}

=== Error handling ===
The <code>signal</code> instruction configures the runtime to run custom code to handle a system condition if triggered. Conditions include:

* <code>error</code> {{endash}} Positive return code from a system command <!--indicating runtime error?-->
* <code>failure</code> {{endash}} Negative return code from a system command (e.g. command doesn't exist) <!--indicating coding error?-->
* <code>halt</code> {{endash}} Abnormal termination
* <code>novalue</code> {{endash}} A variable name was used but the variable is not defined
* <code>notready</code> {{endash}} Input or output error (e.g. read attempts beyond end of file)
* <code>syntax</code> {{endash}} Invalid program syntax, or some other error condition
* <code>lostdigits</code> {{endash}} Significant digits were lost (ANSI Rexx, not in TRL second edition)

The following fragment prints a message when the user terminates (halts) it:
<syntaxhighlight lang="rexx">
signal on halt;
do a = 1
  say a
  do 100000 /* a delay */
  end
end
halt:
say "The program was stopped by the user"
exit
</syntaxhighlight>
Since Rexx version 4, a handler can be named. In the following example, the handler <code>ChangeCodePage.Trap</code> is configured to handle a <code>syntax</code> condition.
<syntaxhighlight lang="rexx">
ChangeCodePage: procedure
  signal on syntax name ChangeCodePage.Trap
return SysQueryProcessCodePage()

ChangeCodePage.Trap: return 1004
</syntaxhighlight>
When a condition is handled (as configured via <code>signal on</code>), the condition can be analyzed via <code>RC</code>  which indicates the last error code and <code>SIGL</code> which indicates the line number of the code that triggered the condition.

== See also ==
{{Portal|Free and open-source software}}
* [[ISPF]]
* [[XEDIT]]
* [[Comparison of computer shells]]
* [[Comparison of programming languages]]

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* Callaway, Merrill. ''The ARexx Cookbook: A Tutorial Guide to the ARexx Language on the Commodore Amiga Personal Computer''. Whitestone, 1992. {{ISBN|978-0963277305}}.
* Callaway, Merrill. ''The Rexx Cookbook: A Tutorial Guide to the Rexx Language in OS/2 & Warp on the IBM Personal Computer''. Whitestone, 1995. {{ISBN|0-9632773-4-0}}.
* Cowlishaw, Michael. ''The Rexx Language: A Practical Approach to Programming''. Prentice Hall, 1990. {{ISBN|0-13-780651-5}}.
* Cowlishaw, Michael. ''The NetRexx Language''. Prentice Hall, 1997. {{ISBN|0-13-806332-X}}.
* Daney, Charles. ''Programming in REXX''. McGraw-Hill, TX, 1990. {{ISBN|0-07-015305-1}}.
* Ender, Tom. ''Object-Oriented Programming With Rexx''. John Wiley & Sons, 1997. {{ISBN|0-471-11844-3}}.
*{{cite book|last1=Fosdick|first1=Howard|year=2025|location=[[Apex, North Carolina]]|title=Rexx Programmer's Reference|url=https://rexxinfo.org/Rexx_Programmers_Reference.pdf|language=en-us|edition=2|publisher=Rexx Language Association|isbn=9789403745527|oclc=1475017186|url-status=live|archive-url=https://web.archive.org/web/20250403173024/https://rexxinfo.org/Rexx_Programmers_Reference.pdf|archive-date=2025-04-03|access-date=2025-05-03}}
* Gargiulo, Gabriel. ''REXX with OS/2, TSO, & CMS Features''. MVS Training, 1999 (third edition 2004). {{ISBN|1-892559-03-X}}.
* Goldberg, Gabriel and Smith, Philip H. ''The Rexx Handbook ''. McGraw-Hill, TX, 1992. {{ISBN|0-07-023682-8}}.
* Goran, Richard K. ''REXX Reference Summary Handbook''. CFS Nevada, Inc.,1997. {{ISBN|0-9639854-3-4}}.
* IBM Redbooks. ''Implementing Rexx Support in Sdsf''. Vervante, 2007. {{ISBN|0-7384-8914-X}}.
* Kiesel, Peter C. ''Rexx: Advanced Techniques for Programmers''. McGraw-Hill, TX, 1992. {{ISBN|0-07-034600-3}}.
* Marco, Lou ''ISPF/REXX Development for Experienced Programmers''. CBM Books, 1995. {{ISBN|1-878956-50-7}}
* O'Hara, Robert P. and Gomberg, David Roos. ''Modern Programming Using Rexx''. Prentice Hall, 1988. {{ISBN|0-13-597329-5}}.
* Rudd, Anthony S. 'Practical Usage of TSO REXX'. CreateSpace, 2012. {{ISBN|978-1475097559}}.
* Schindler, William. ''Down to Earth Rexx''. Perfect Niche Software, 2000. {{ISBN|0-9677590-0-5}}.
{{refend}}

== External links ==
{{commons category|REXX (programming language)}}
* [https://speleotrove.com/mfc/mfc.html Mike Cowlishaw's home page]
* [https://www.rexxla.org/ Rexx Language Association]
* [https://rexxinfo.org/ Rexxinfo.org]
* [https://speleotrove.com/rexxhist/TRL-background.pdf The Rexx Language (Background)]
* [https://www.ibm.com/docs/en/zos/2.5.0?topic=programming-rexx-language REXX programming language] language reference at IBM
* [https://rexxinfo.org/Rexx_Programmers_Reference.pdf Rexx Programmer's Reference 2nd Edition]

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