The Rexx programming language was first created in 1979, as a very high level scripting language that had a particularly strong facility for text processing tasks. Since Rexx's inception, IBM has included versions of Rexx with most of its operating systems -- all the way from its mainframes, to its mid-level systems, to end user OS's like OS/2 and PC-DOS. Other OS makers, such as Amiga, have also integrated Rexx as an always-available system scripting language. A number of ISVs, moreover, have created Rexx environments for many platforms. Somewhat late in the game, ANSI officially adopted a standard for Rexx in 1996.
Nowadays (especially on Linux or BSD-derived OS's), most of those
older implementations of Rexx are primarily interesting as historical
footnotes. However, two currently maintained implementations of Rexx
remain available across a wide range of platforms, including Linux,
MacOSX and Windows: Regina and NetRexx. Regina is a native executable,
available as Free Software source code, or pre-compiled to a large
number of platforms -- install it pretty much as you would any other
programming language interpreter. NetRexx is an interesting hybrid. The language is a derivative of plain Rexx. Much like Jython or Jacl, NetRexx compiles Rexx-like source code into Java bytecodes, and (optionally) runs the resulting
.class file within a JVM.
NetRexx is an IBM project for compiling Rexx-like code for a Java Virtual Machine. In capabilities and programming level, Rexx can be compared most
bash plus the GNU text utilities
sed for good measure); or maybe to
awk or Perl. Certainly Rexx has
more of a quick-and-dirty feel to it than do, e.g., Python, Ruby, or
Java. The verbosity -- or rather, conciseness -- of Rexx is similar to
that of Perl, Python, Ruby or TCL. And Rexx is certainly
Turing-complete, enables modules and structured programming, and has
libraries for tasks such as GUI interfaces, network programming, and
database access. But its most natural target is in automation of
system scripting and text processing tasks. As with shell scripting,
Rexx allows very natural and transparent control of
application; but compared to
ksh, etc.), Rexx
contains a much richer collection of built-in control structures and
(text processing) functions.
Stylistically, the IBM/mainframe roots of Rexx show in its case-insensitive commands; and to a lesser degree in the relative sparsity of punctuation it uses (preferring keywords to symbols). I tend to find that these qualities aid readability; but this is mostly a matter of individual taste.
A start at streams and stacks
As a simple conceit, let me present a number of versions of a very
simple utility that lists files and numbers them. One feature that
Rexx has in common with shell scripting is that it has a relatively
impoverished collection of functions for working with the underlying
operating system -- mostly limited to the the ability to open, read, and
modify files. For most anything else, you rely on external utilities
to perform the job at hand. The utility
Listing 1. numbered-1.rexx
#!/usr/bin/rexx DO i=1 UNTIL lines()==0 PARSE LINEIN line IF line\="" THEN SAY i || ") " || line END
The ubiquitous instruction
PARSE can read
from various sources. In
this case, it puts the next line of STDIN into the variable
We also check if a line is blank, and skip showing and numbering it
if so. For example, combined with
ls we can get:
Listing 2. Piping command to numbered-1
$ ls | ./numbered-1.rexx 1) ls-1.rexx 2) ls-2.rexx 3) ls-3.rexx 4) ls-4.rexx 5) ls-5.rexx 6) ls-6.rexx 7) numbered-1.rexx 8) numbered-2.rexx
You can just as easily pipe any other command in.
A concept at the core of Rexx is juggling multiple stacks or streams. In
bash-like fashion, anything in Rexx that is
not recognized as an
internal instruction or function is assumed to be an external utility.
There is no special function or syntax for calling an external
command. Taking advantage of the Regina utility
rxqueue, which puts
output onto the Rexx stack, we can write a "numbered ls" utility as:
Listing 3. ls-1.rexx
#!/usr/bin/rexx "ls | rxqueue" DO i=1 WHILE queued() \= 0 PARSE PULL line SAY i || ") " || line END
Some instructions in Rexx may explicitly specify a stack to operate
on; but other instructions operate within an environment which you
configure with the
instruction. STDIN, STDOUT, STDERR,
files, and in-memory data stacks are all handled in a uniform and
elegant fashion. Above we used the external
rxqueue utility, but we
can similarly redirect output of utilities right within Rexx. For
Listing 4. ls-2.rexx
#!/usr/bin/rexx ADDRESS SYSTEM ls WITH OUTPUT FIFO '' ERROR NORMAL DO i=1 WHILE queued() \= 0 PARSE PULL line; SAY i || ") " || line; END
It might appear that the
ADDRESS command is
grabbing the output of
ls utility; but it is actually
changing the general
execution environment for later external calls. These examples were run on a
case-retentive/case-insensitive filesystem; under many systems, you will
have to quote "ls" to maintain its case. This behaves
Listing 5. ls-5.rexx
#!/usr/bin/rexx ADDRESS SYSTEM WITH OUTPUT FIFO '' ERROR NORMAL ls DO i=1 WHILE queued()\=0; PARSE PULL ln; SAY i||") "||ln; END
Any subsequent external commands, if they are run in the default
SYSTEM environment, will direct their output
to the default
FIFO.(first-in-first-out). You could also output to a LIFO instead
(either named or default) -- the difference being that a FIFO adds to
the "bottom" of the stack, and a LIFO to the "top." The instructions
correspond to LIFO and FIFO operations on the
stack. The instruction
PARSE PULL take a string off the
top of the stack.
Another useful stack to look at is that of the command-line arguments
to a Rexx script. For example, we might want to execute an arbitrary
command in our numbering utility, not always
Listing 6. numbered-1.rexx
#!/usr/bin/rexx PARSE ARG cmd ADDRESS SYSTEM cmd WITH OUTPUT FIFO '' DO i=1 WHILE queued()\=0; PARSE PULL ln; SAY i||") "||ln; END
The script in action:
Listing 7. Passing command to numbered-1
$ ./numbered-2.rexx ps -a -x 1) PID TT STAT TIME COMMAND 2) 1 ?? Ss 0:00.00 /sbin/init 3) 2 ?? Ss 0:00.19 /sbin/mach_init 4) 51 ?? Ss 0:01.95 kextd [...]
PARSE PULL can be used to pull lines from
user input. Following the
example of the execution of the argument
cmd, you could write a
shell or interactive environment in Rexx (perhaps running either
external utilities or built-in commands, much like
Stem variables and associative arrays
In Rexx -- somewhat like in TCL -- to a large extent everything is a string. Having stacks and streams composed of lines gives you a simple list or array of strings. But mostly, strings simply act like other datatypes as needed. For example, a string that contains a suitable representation of a number (digits, decimal, an exponent "e", etc.) can be used in arithmetic operations. For processing reports, log files, and the like, this is exactly the behavior you want.
Rexx, however, does have one additional standard datatype: associative arrays. In Rexx they go under the name "stem variables," but the concept is very similar that of dictionaries in many other languages. The syntax for stem variables will be oddly familiar to users of OOP languages like Java, Python, or Ruby: a dot separates "objects" and their "attributes." This is not really object-orientation, but the syntax does (accidentally) highlight the degree to which an object resembles a particularly robust dictionary; there are OOP extensions to Rexx out there, but this article will not address them.
Not every string is valid Rexx symbol -- which restricts the keys in the dictionary -- but Rexx is pretty liberal about its symbol names, compared to most languages. E.g.
Listing 8. Using stem variables in Rexx
$ cat stems.rexx #!/usr/bin/rexx foo.X_!1.bar = 1 foo.X_!1.23 = 2 foo.fop.fip = 3 foo.fop = 4 SAY foo.X_!1.bar # foo.X_!1.23 # foo.fop.fip # foo.fop # foo.fop.NOPE $ ./stems.rexx 1 # 2 # 3 # 4 # FOO.FOP.NOPE
A couple features stand out in this example. We set a value for both a
stem and its compound (e.g.
foo.fop.fip). Also notice
that the undefined symbol
simply stands for its own
spelling, absent an assignment to the contrary. This lets us skip
quotes in most situations. Case of names is normalized to upper case
in most Rexx contexts.
One useful trick is to set a value for the dotted stem, which then
acts as a default value for compound names based on the stem. For the
next example, we also make use of the capability to
sequential numbered symbols of a compound name as an output
Listing 9. ls-3.rexx
#!/usr/bin/rexx ls. = UNDEF ADDRESS SYSTEM ls WITH OUTPUT STEM ls. DO i=1 IF ls.i == UNDEF THEN LEAVE SAY i || ") " || ls.i END
As soon as the loop gets to some compound variable name that was not
populated by the output of the external
utility, we detect the
default value of "UNDEF" and leave the loop (if the output might
contain that string, a false collision could occur, however).
Rexx also has an error-handling system that lets you
conditions and handle them appropriately. Instead of checking for a
default compound value, you can also catch the access to an undefined
Listing 10. ls-6.rexx
#!/usr/bin/rexx ADDRESS SYSTEM ls WITH OUTPUT STEM ls. SIGNAL ON NOVALUE NAME quit DO i=1 SAY i || ") " || ls.i END quit:
Just to round our
ls variants out, here is
one more that uses a file
for its I/O:
Listing 11. ls-4.rexx
#!/usr/bin/rexx ADDRESS SYSTEM ls WITH OUTPUT STREAM files DO i=1 line = linein(files) IF line = "" THEN LEAVE SAY i || ") " || line END rm files
Since the output stream is a regular file, it is probably good to remove it at the end.
Text processing functions
The brief examples above will give readers a bit of the feel of Rexx
as a programming language. You can also, of course, define your own
procedures and functions -- in separate module files, if you wish -- and
call them either with the
or using parenthesized
arguments, as with some of the examples in this article that use
Perhaps the greatest strength in Rexx as a text processing language is its useful collection of built-in string manipulation functions. Somewhere over half of all the standard Rexx functions are for working with strings, with a chunk of others thrown in for quite readable manipulation of bit vectors. Moreover, even bit vectors are often manipulated (or read in) as strings of ones and zeros:
Listing 12. bits.rexx
#!/usr/bin/rexx SAY b2c('01100001') b2c('01100010') /* --> a b */ SAY bitor(b2c('01100001'), b2c('01100010')) /* --> c */ SAY bitor('a','b') /* --> c */ EXIT /* Function in ARexx, but not ANSI Rexx */ b2c: PROCEDURE ARG bits return x2c(b2x(bits))
A nice feature of Rexx's text handling functions is the naturalness of
treating lines as being composed of whitespace-separated
textual reports and log files, easily ignoring extraneous whitespace
is quite useful -- 'awk' does something similar, but Python's
string.split() quickly gets more "busy" in
describing the same
operations. In fact, "arrays" in Rexx just amount to
whitespace-separated strings. The
instruction will pull out variables from
a general template pattern for a line, which at a minimal case allows
Listing 13. pushpull.rexx
#!/usr/bin/rexx PUSH "a b c d e f" PULL x y " C " z /* pull x and y before the C, remainder into z */ SAY x # y # z /* --> A # B # D E F */
Further dividing strings that may or may not have been pulled with a
template is elegant. Functions like
subword() let you refer to the "words"
in a string as if they made up a list, e.g.:
Listing 14. Working with words in a string
seuss = "The cat in the hat came back" thehat = wordpos('the hat', seuss) SAY "'came'" is wordlength(seuss, thehat+2) letters long /* --> 'came' IS 4 LETTERS LONG */
Of course, you also get a rich collection of character-oriented
functions as well. It is equally easy to work with character
positions using functions like
substr() (and others).
Another batch of the built-in functions let you work with dates and numbers in a flexible, report-oriented, way. That is, numbers can be read and written in a variety of formats, with arbitrary (configurable) precision. Dates, similarly, can be read, written and converted among many formats with standard function calls (e.g. day-of-week, days-in-century, European versus US dates, etc.). The flexibility with dates and numbers is probably less often necessary in writing system scripts and processing log files than it is in working with semi-structured output reports from database applications. But when you need it, it is much more robust to have well-tested built-in functions than to write your own ad-hoc converters and formatters.
Coming more out of an IBM "big-iron" environment than from Unix
systems, Rexx is little-known to many Linux programmers and systems
administrators. But there remains an important Linux niche where
Rexx is a better scripting solution than either the "too-light"
ksh shells, or the
"too-heavy" interpreted programming languages
like Python, Perl, Ruby, TCL, or maybe Scheme. For quick and easily
readable scripts that perform text manipulation on the inputs and
outputs of external processes, Rexx is hard to beat, and not hard to
learn or install.
- Regina is the LGPL Rexx implementation used in writing and testing the examples in this article, and for most people will be the best choice for an environment to install. Regina is available on an extremely wide range of platforms, and is fully ANSI compliant (with a few extensions added). The site also contains links to a number of useful Rexx libraries for working with application areas like Tk, Curses, Sockets, SQL, and others.
- NetRexx is an IBM project for compiling Rexx code for a Java Virtual Machine. While it is certainly possible to use NetRexx for client/workstation scripting needs, the main focus of NetRexx is to allow more rapid development of server-side Java applications (JSP and related technologies).
- The Rexx Language Association is a a general advocacy group for the Rexx programming language. Their site contains miscellaneous links to useful libraries and other resources, including details on the Rexx ANSI Standard.
- IBM Hursley also maintains a nice list of Rexx resources. You can find tutorials, references, and links to a variety of modern libraries. The Hursley site also has a page on the history of Rexx.
- The IBM REXX Family runs on host systems, such as VM/ESA, VSE/ESA, and MVS/ESA, and workstation environments, such as AIX, OS/2, Linux, and Windows.
- There is even a Mod_Rexx package for the Apache Web server.
- Find more resources for Linux developers in the developerWorks Linux zone.
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