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History of pipes concept

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in C
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Pipes rank alongside the hierarchical file system and regular expressions as one of the three most powerful contributions of Unix to operating systems design.  They appeared in Unix around 1972-1973, almost simultaneously with rewriting the kernel in C. This is an elegant implementation of coroutines in operating system as well as shell notation that allow to use them in shell programming.


Coroutines  are classic programming-in-the-large methodology that most know via pipes concept which was brought to mainstream by Unix.

The term "coroutine" was coined by Melvin Conway in 1958, when he invented the concept and applied it to the construction of a compiler. He is also famous for formulation of so called Conway Law (first published in Datamation in 1968 and popularized by F. Books in The Mythical Man-Month) and Think Pascal debugger

Any organization that designs a system (defined broadly) will produce a design whose structure is a copy of the organization's communication structure.

The fate of coroutine concept was much more difficult then the fate of Conway Law, which instantly found almost universal recognition.  Like often happens with innovative concepts, Conway has difficulties in publishing his paper and that happened much later in Conway's article 'Design of a Separable Transition-Diagram Compiler', CACM 6 (1963), 396-408.  In this paper Melvin Convey considered coroutines as a natural compiler writing paradigm, which was tremendous breakthrough in compiler design for the time: you can write multipass compiler and debug it using intermediate files one stage at the time (as output of previous stage can just be saved). Then you can integrate all passes using coroutines achieving much greater efficiency and eliminating and writing to the disk except by the final stage of the compiler.

But the concept itself was discovered much earlier and probably by multiple people. A primitive form of coroutine linkage had already been noted briefly as a 'programming tip' in an early UNIVAC publication Coroutines were independently studied by J. Erdwinn and J. Merner, at about the same time as Conway. The first academic discussion was in the first volume of the Art of Computer programming by Donald Knuth (1968).

Simula 67 was the first language that implement coroutines as language constructs. Here is explanatiuon of this concept from OOSC 2 28.9 EXAMPLES

Coroutines emulate concurrency on a sequential computer. They provide a form of functional program unit ("functional" as opposed to "object-oriented") that, although similar to the traditional notion of routine, provides a more symmetric form of communication. With a routine call, there is a master and a slave: the caller starts a routine, waits for its termination, and picks up where it left; the routine, however, always starts from the beginning. The caller calls; the routine returns. With coroutines, the relationship is between peers: coroutine a gets stuck in its work and calls coroutine b for help; b restarts where it last left, and continues until it is its turn to get stuck or it has proceeded as far as needed for the moment; then a picks up its computation. Instead of separate call and return mechanisms, there is a single operation, resume c, meaning: restart coroutine c where it was last interrupted; I will wait until someone e  

This is all strictly sequential and meant to be executed on a single process (task) of a single computer. But the ideas are clearly drawn from concurrent computation; in fact, an operating system that provide such schemes as time-sharing, multitasking (as in Unix) and multithreading, mentioned at the beginning of this chapter as providing the appearance of concurrency on a single computer, will internally implement them through a coroutine-like mechanism.

Coroutines may be viewed as a boundary case of concurrency: the poor man's substitute to concurrent computation when only one thread of control is available. It is always a good idea to check that a general-purpose mechanism degrades gracefully to boundary cases; so let us see how we can represent coroutines. The following two classes will achieve this goal.

Later it was desribed with several interesting examples in the influential 1972 book Structured Programming  by Edsger W Dijkstra, C. A. R. Hoare, and Ole-Johan Dahl 

Coroutines is a natural programming technique for assembler programmers. Actually in some ways more natural then subroutines. 

Unix Pipes

Pipes were first suggested by M. Doug McIlroy, when he was a department head in the Computing Science Research Center at Bell Labs, the research arm of AT&T (American Telephone and Telegraph Company), the former U.S. telecommunications monopoly. McIlroy had been working on macros since the latter part of the 1950s, and he was a ceaseless advocate of linking macros together as a more efficient alternative to series of discrete commands. A macro is a series of commands (or keyboard and mouse actions) that is performed automatically when a certain command is entered or key(s) pressed.

McIlroy's persistence led Ken Thompson, who developed the original UNIX at Bell Labs in 1969, to rewrite portions of his operating system in 1973 to include pipes. This implementation of pipes was not only extremely useful in itself, but it also made possible a central part of the Unix philosophy, the most basic concept of which is modularity (i.e., a whole that is created from independent, replaceable parts that work together efficiently).

Let me state it again: the pipe is the the most elegant and powerful features of UNIX.  In The Creation of the UNIX Operating System /Connecting streams like a garden hose the authors wrote: 

Another innovation of UNIX was the development of pipes, which gave programmers the ability to string together a number of processes for a specific output.

Doug McIlroy, then a department head in the Computing Science Research Center, is credited for the concept of pipes at Bell Labs, and Thompson gets the credit for actually doing it.

McIlroy had been working on macros in the later 1950s, and was always theorizing to anyone who would listen about linking macros together to eliminate the need to make a series of discrete commands to obtain an end result.

"If you think about macros," McIlroy explained, "they mainly involve switching data streams. I mean, you're taking input and you suddenly come to a macro call, and that says, 'Stop taking input from here and go take it from there.'

"Somewhere at that time I talked of a macro as a 'switchyard for data streams,' and there's a paper hanging in Brian Kernighan's office, which he dredged up from somewhere, where I talked about screwing together streams like a garden hose. So this idea had been banging around in my head for a long time."

 ... ... ... ...

While Thompson and Ritchie were at the chalkboard sketching out a file system, McIlroy was at his own chalkboard trying to sketch out how to connect processes together and to work out a prefix notation language to do it.

It wasn't easy. "It's very easy to say 'cat into grep into...,' or 'who into cat into grep,'" McIlroy explained. "But there are all these side parameters that these commands have; they just don't have input and output arguments, but they have all these options."

"Syntactically, it was not clear how to stick the options into this chain of things written in prefix notation, cat of grep of who [i.e. cat(grep(who))]," he said. "Syntactic blinders: I didn't see how to do it." 

Although stymied, McIlroy didn't drop the idea. "And over a period from 1970 to 1972, I'd from time to time say, 'How about making something like this?', and I'd put up another proposal, another proposal, another proposal. And one day I came up with a syntax for the shell that went along with the piping, and Ken said, 'I'm going to do it!'"

"He was tired of hearing this stuff," McIlroy explained. "He didn't do exactly what I had proposed for the pipe system call. He invented a slightly better one that finally got changed once more to what we have today. He did use my clumsy syntax."

"Thompson saw that file arguments weren't going to fit with this scheme of things and he went in and changed all those programs in the same night. I don't know how...and the next morning we had this orgy of one-liners."

"He put pipes into UNIX, he put this notation into shell, all in one night," McElroy said in wonder.

Next: Creating a programming philosophy from pipes and a tool box

Here is how Dennis M. Ritchie in his paper Early Unix history and evolution describes how pipes were introduced in Unix:

One of the most widely admired contributions of Unix to the culture of operating systems and command languages is the pipe, as used in a pipeline of commands. Of course, the fundamental idea was by no means new; the pipeline is merely a specific form of coroutine. Even the implementation was not unprecedented, although we didn't know it at the time; the `communication files' of the Dartmouth Time-Sharing System [10] did very nearly what Unix pipes do, though they seem not to have been exploited so fully.

Pipes appeared in Unix in 1972, well after the PDP-11 version of the system was in operation, at the suggestion (or perhaps insistence) of M. D. McIlroy, a long-time advocate of the non-hierarchical control flow that characterizes coroutines. Some years before pipes were implemented, he suggested that commands should be thought of as binary operators, whose left and right operand specified the input and output files. Thus a `copy' utility would be commanded by

 inputfile copy outputfile

To make a pipeline, command operators could be stacked up. Thus, to sort input, paginate it neatly, and print the result off-line, one would write

 input sort paginate offprint

In today's system, this would correspond to

 sort input | pr | opr

The idea, explained one afternoon on a blackboard, intrigued us but failed to ignite any immediate action. There were several objections to the idea as put: the infix notation seemed too radical (we were too accustomed to typing `cp x y' to copy x to y); and we were unable to see how to distinguish command parameters from the input or output files. Also, the one-input one-output model of command execution seemed too confining. What a failure of imagination!

Some time later, thanks to McIlroy's persistence, pipes were finally installed in the operating system (a relatively simple job), and a new notation was introduced. It used the same characters as for I/O redirection. For example, the pipeline above might have been written

 sort input >pr>opr>

The idea is that following a `>' may be either a file, to specify redirection of output to that file, or a command into which the output of the preceding command is directed as input. The trailing `>' was needed in the example to specify that the (nonexistent) output of opr should be directed to the console; otherwise the command opr would not have been executed at all; instead a file opr would have been created.

The new facility was enthusiastically received, and the term `filter' was soon coined. Many commands were changed to make them usable in pipelines. For example, no one had imagined that anyone would want the sort or pr utility to sort or print its standard input if given no explicit arguments.

Soon some problems with the notation became evident. Most annoying was a silly lexical problem: the string after `>' was delimited by blanks, so, to give a parameter to pr in the example, one had to quote:

 sort input >"pr -2">opr>

Second, in attempt to give generality, the pipe notation accepted `<' as an input redirection in a way corresponding to `>'; this meant that the notation was not unique. One could also write, for example,

 opr <pr<"sort input"<

or even

 pr <"sort input"< >opr>

The pipe notation using `<' and `>' survived only a couple of months; it was replaced by the present one that uses a unique operator to separate components of a pipeline. Although the old notation had a certain charm and inner consistency, the new one is certainly superior. Of course, it too has limitations. It is unabashedly linear, though there are situations in which multiple redirected inputs and outputs are called for. For example, what is the best way to compare the outputs of two programs? What is the appropriate notation for invoking a program with two parallel output streams?

I mentioned above in the section on IO redirection that Multics provided a mechanism by which IO streams could be directed through processing modules on the way to (or from) the device or file serving as source or sink. Thus it might seem that stream-splicing in Multics was the direct precursor of Unix pipes, as Multics IO redirection certainly was for its Unix version. In fact I do not think this is true, or is true only in a weak sense. Not only were coroutines well-known already, but their embodiment as Multics spliceable IO modules required that the modules be specially coded in such a way that they could be used for no other purpose. The genius of the Unix pipeline is precisely that it is constructed from the very same commands used constantly in simplex fashion. The mental leap needed to see this possibility and to invent the notation is large indeed.

 Dartmouth Time-Sharing System "communication files"  

Dartmouth Time Sharing System  or DTSS for short, was the first large-scale time-sharing system to be implemented successfully

DTSS was inspired by a PDP-1-based time-sharing system at Bolt, Beranek and Newman. In 1962, John Kemeny and Thomas Kurtz at Dartmouth College submitted a grant for the development of a new time-sharing system to NSF (funded in 1964).[1] Its implementation began in 1963 by a student team [2] under the direction of Kemeny and Kurtz with the aim of providing easy access to computing facilities for all members of the college.[3] On May 1, 1964, at 4:00 a.m., the system began operations. It remained in operation until the end of 1999.[4][5] DTSS was originally implemented to run on a GE-200 series computer with a GE Datanet 30 as a terminal processor that also managed the 235. Later, DTSS was reimplemented on the GE 635,[1] still using the Datanet 30 for terminal control. The 635 version provided interactive time-sharing to up to nearly 300 simultaneous users in the 1970s, a very large number at the time.

Because of the educational aims, ease of use was a priority in DTSS design.

DTSS implemented the world's first Integrated Design Environment: a command-based system implementing the following commands.

These commands were often believed to be part of the Dartmouth BASIC language by users but in fact they were part of the time sharing system and were also used when preparing ALGOL[6] or FORTRAN programs via the DTSS terminals.

Any line typed in by the user, and beginning with a line number, was added to the program, replacing any previously stored line with the same number; anything else was immediately compiled and executed. Lines which consisted solely of a line number weren't stored but did remove any previously stored line with the same number. This method of editing provided a simple and easy to use service that allowed large numbers of teleprinters as the terminal units for the Dartmouth Timesharing system.

By 1968 and into the mid-1970s, the nascent network included users at other schools and institutions around the East Coast (including Goddard College, Phillips Andover and the U.S. Naval Academy), connected with Teletype Model 33 machines and modems. The system allowed email-type messages to be passed between users and real-time chat via a precursor to the Unix talk program.

In 2000 a project to recreate the DTSS system on a simulator was undertaken and as a result DTSS is now available for Microsoft Windows systems and for the Apple Macintosh computer.[7]

According to Dennis M. Ritchie (Early Unix history and evolution), "communication files" of the Dartmouth Time-Sharing System  did very nearly what Unix pipes do, though they seem not to have been exploited so fully. They were developed by Ken Lochner  in the 1960s (DTSS.doc)

Ken Lochner continually pushed the "right" abstractions Probably a major reason for the success of the second system Inventer of "communication files" that later became UNIX pipes

CMS Pipes

In early 1990 pipes were re-implemented in CMS by John P. Hartmann (IBM Denmark). In his implementation it was possible to merge different pipe streams making pipelines more complex. The same was possible with Unix named pipes but in less consistent way.  A very interesting feature was that they were available from REXX which is a more powerful and elegant scripting language then Borne shell. That make the whole concept more powerful as in REXX allow writing more complex filters.  In his Share paper CMS Pipelines Enhancements for Shared File System Byte File System published in 1995 he described the concept in the following way:

CMS Pipelines implements the dataflow model of programming. It passes records through programs in a multistream topology. This allows for selective processing of records based on their contents, or based on the previous history of input data.

VM/ESA Version 2 Release 1 contains many enhancements to CMS Pipelines. This session describes enhancements to:

A good introduction to CMS pipes can be found in Melinda Varian article PLUNGING INTO PIPES written in 1992(V4617004.TXT):

The pipeline concept  has not been integrated into  command parsing in
CMS,  as it has in UNIX.   Instead,   CMS Pipelines adds the new CMS
command, PIPE:
                      pipe  pipeline-specification
The argument  to PIPE  is a  "pipeline specification".    A pipeline
specification is a string listing the stages to be run.   The stages are
separated by  the "stage separator  character",  which is  usually a
vertical bar ("|"):
           pipe  stage-1  |  stage-2  |  stage-3  |  stage-4
When CMS sees  this PIPE command (whether  in an EXEC or  typed on the
command line),  it passes control to the PIPE module,  which interprets
the argument string as a pipeline containing four stages.   The pipeline
parser locates the  four programs and checks for correct  syntax in the
invocations of any that are built-in  programs.   If all the stages are
specified correctly, the pipeline is executed;  otherwise, the pipeline
parser issues useful error messages and exits.
Device Drivers
In UNIX,  a program can do I/O to  a device in exactly the same way it
does I/O to a file.   Under the covers, the system has "device drivers"
to make  this work.    Because CMS  does not  provide such  device
transparency, CMS Pipelines has its own device drivers,  pipeline stages
that connect  the pipeline to  host interfaces,  thus  allowing other
pipeline stages to be completely independent of host interfaces.
CMS Pipelines provides a large number of device drivers.   A very simple
pipeline might  contain only  device drivers.   We  may as  well be
traditional and start with this one:
                pipe  literal  Hello, World!  |  console
Here,  the device driver literal inserts a record containing the phrase
"Hello,  World! " into the pipeline.    The device driver console then
receives that record and displays it on the console.
This pipeline reads lines from the console and writes them to the punch:
                        pipe  console  |  punch
(It continues reading from the console and writing to the punch until it
reaches end-of-file, i.e., until it receives a null line as input.)
Plunging into Pipes                                               page 4
As the use of console in these two examples shows,  some device drivers
can be used for either reading or writing.   If they are the first stage
in the pipeline, they read from the host interface.   If they come later
in the pipeline,   they write to the host  interface.   This pipeline
performs a simple echo operation:
                       pipe  console  |  console
It just  reads lines from  the console and  writes them back  to the
console.   A similar pipeline performs a more useful task;  it copies a
file from one tape to another:
                      pipe  tape  |  tape tap2 wtm
The first tape stage knows to read,  because it can sense that it is the
first stage in  the pipeline;  the second tape stage  knows to write,
because it can sense  that it is not the first  stage in the pipeline.
tap2 and wtm are arguments to the second tape stage.   When the pipeline
dispatcher invokes  the second  tape stage,   it passes  along those
arguments,  which tape recognizes as instructions to use the CMS device
TAP2 and to write a tapemark at the end of the data.
There are several device drivers to read and write CMS files.   Some of
them will look familiar to you if  you know UNIX,  but may look rather
strange if you do not:
   * The  <  ("disk read") device driver reads a CMS file and inserts
     the records from  the file into the  pipeline.   Thus,  this
     pipeline copies a file from disk to tape:
                        pipe  <  fn ft fm  |  tape
   * >  ("disk replace")  writes records from the pipeline to the CMS
     file specified by its arguments,  replacing any existing file of
     the same name, so this pipeline copies a file from tape to disk:
                        pipe  tape  |  >  fn ft fm
   * >>  ("disk append") is the same as >,  except that it appends an
     existing file of  the specified name,  if  any,  rather than
     replacing it.   Thus, this pipeline also copies a file from tape
     to disk, but if the named file already exists,  it is appended,
     not replaced:
                       pipe  tape  |  >>  fn ft fm
(Note that  although <,  >,  and  >> look like the  UNIX redirection
operators,  they are actually the names  of programs;  like other CMS
program names, they must be delimited by a blank.)
An output device driver is not necessarily the last stage of a pipeline.
Output device drivers write the records  they receive from the pipeline
Plunging into Pipes                                               page 5
to their host interface,  but they also  pass those records back to the
pipeline, which then presents them as input to the following stage,  if
there is one.   For example,  this pipeline reads a CMS file and writes
the records to a CMS file, to the console, to the punch, and to a tape:
                  pipe  <  fn ft fm  |  >  outfn outft
                outfm  |  console  |  punch  |  tape wtm
If you wanted to include that PIPE  command in a REXX EXEC,  you would
need to keep in mind that the entire command is a string,  only portions
of which should have variables substituted.   Thus, in an EXEC you would
write that PIPE command something like this:
             'PIPE  <'  infn inft infm  '|  >'  outfn outft
               outfm  '|  console  |  punch  |  tape wtm'
That is, you would quote the parts that are not variable, while allowing
REXX to substitute  the correct values for the  variable fields,  the
As PIPE commands grow longer,  using  the linear form in EXECs becomes
somewhat awkward.   Most  experienced "plumbers" prefer to  put longer
pipelines into "portrait format", with one stage per line, thus:
           |                                                 |
           |      'PIPE (name DRIVERS)',                     |
           |         '<' infn inft infm '|',                 |
           |         '>' outfn outft outfm '|',              |
           |         'console |',                            |
           |         'punch |',                              |
           |         'tape wtm'                              |
           |                                                 |
You can use the FMTP XEDIT macro,  which comes with CMS Pipelines,  to
reformat a PIPE command into portrait  format.   Note the commas at the
ends of the  lines;  those are REXX  continuation characters.   This
pipeline specification  will still be a  single string once  REXX has
interpreted it.
Note also the "global option"  name in parentheses immediately following
the PIPE command.   This gives the pipeline  a name by which it can be
referenced in a  traceback,  should an error occur while  the pipe is
running.   (There are a number of other global options, but this is the
only one we will meet in this session.)
Once you have the pipeline in portrait format,  you can key in comments
on each line and then invoke the  SC XEDIT macro,  which comes with CMS
Pipelines, to line them up nicely for you:
Plunging into Pipes                                               page 6
|                                                                      |
|    'PIPE (name DRIVERS)',              /* Name for tracing */        |
|       '<' infn inft infm '|',          /* Read CMS file    */        |
|       '>' outfn outft outfm '|',       /* Copy to CMS file */        |
|       'console |',                     /* And to console   */        |
|       'punch |',                       /* And to punch     */        |
|       'tape wtm'                       /* And to tape      */        |
|                                                                      |
You will notice  that all the device  drivers observe the rule  that a
program that runs in a pipeline should  be able to connect to any other
program.   Although the device drivers are specialized on the side that
connects to the host, they are standard on the side that connects to the
There are four very useful device drivers  to connect a pipeline to the
REXX environment:
   * var,  which reads a REXX variable  into the pipeline or sets a
     variable to the contents of the first record in the pipeline;
   * stem,  which retrieves  or sets the values in  a REXX stemmed
   * rexxvars,  which  retrieves the  names and  values of  REXX
     variables; and
   * varload, which sets the values of the REXX variables whose names
     and values are defined by the records in the pipeline.
All four of these stages allow you  to specify which REXX environment is
to be accessed.    If you do not specify the  environment,  then the
variables you set or retrieve are from  the EXEC that contains your PIPE
command.   But you may instead specify that the variables are to be set
in or retrieved from  the EXEC that called the EXEC  that contains your
PIPE command or another EXEC further up the chain,  to any depth.   For
example, this pipeline:
                 'PIPE  stem parms.  1  |  stem parms.'
retrieves the stemmed array "parms" from  the environment one level back
(that is,  from the EXEC that called  this EXEC)  and stores it in the
stemmed array "parms" in this EXEC.   (If these two stages are reversed,
then the array is copied in the opposite direction.)
rexxvars retrieves the  names and values of all  exposed REXX variables
from the specified REXX environment and  writes them into the pipeline,
starting with the source string:
Plunging into Pipes                                               page 7
|                                                                      |
|    'PIPE rexxvars 1 | var source1'    /* Get caller's source. */     |
|    'PIPE rexxvars 2 | var source2'    /* And his caller's.    */     |
|                                                                      |
|    Parse Var source1 . . . fn1 .                                     |
|    Parse Var source2 . . . fn2 .                                     |
|                                                                      |
|    Say 'I was called from' fn1', which was called from' fn2'.'       |
|                                                                      |
In this example, rexxvars is used twice,  once to retrieve the variables
from the EXEC that  called this one and once to  retrieve the variables
from the EXEC that called that one.   In each case, a var stage is then
used to store the first record  produced by rexxvars (the source string)
in a variable in this EXEC,  where it  can be used like any other REXX
Another very useful  group of stages issue host commands  and route the
responses into the pipeline.  Among these "host command processors" are:
   * cp, which issues CP commands;
   * cms,  which issues  CMS commands with full  command resolution
     through the CMS subcommand environment,   just as REXX does for
     the Address CMS instruction; and
   * command,  which issues CMS commands using a program call with an
     extended parameter list,   just as REXX does  for the Address
     Command instruction.
Each of these stages  issues its argument string as a  command and then
reads any records  from its input stream and issues  those as commands,
too.  The command responses are captured, and each response line becomes
a record in the pipeline.  For example, in this pipeline:
                'PIPE  cp  query  dasd  |  stem  dasd.'
the cp stage issues a CP QUERY DASD command and writes the response into
the pipeline,  where the stem stage receives  it and writes it into the
stemmed array "DASD",  setting "DASD.0" to the count of the lines in the
There are a great variety of other device drivers, for example:
   * xedit,  which  writes records from  an XEDIT session  to the
     pipeline or vice versa;
   * stack, which reads or writes the CMS program stack;
   * sql and ispf, which interface to SQL and ISPF;
   * qsam, which reads MVS files (and writes them under MVS);
   * storage, which reads or writes virtual machine storage; and
   * subcom, which sends commands to a subcommand environment.
The list of device drivers goes on and on, and it continues to grow.
Plunging into Pipes                                               page 8
Other Built-in Programs
Pipelines built only of device drivers do  not really show the power of
CMS Pipelines (although they may be  quite useful,  especially as they
often out-perform the equivalent native CMS commands).  There are dozens
of other CMS Pipelines built-in programs.   Most of these are "filters",
programs that can be put into  a pipeline to perform some transformation
on the records flowing through the pipeline.
Using Pipeline Filters:   A simple pipeline  consisting of a couple of
device drivers wrapped  around a few filter stages  provides an instant
enhancement to the CMS command set.    Once you have had some practice,
you will find yourself typing lots  of little "throwaway" pipes right on
the command line.
Many CMS Pipelines filters are  self-explanatory (especially as many of
them behave just  like the XEDIT subcommand of the  same name).   For
example,  this  pipeline displays the  DIRECTORY statement from  a CP
          pipe  <  user direct  |  find  DIRECTORY  |  console
The find filter selects records using the  same logic as the XEDIT FIND
This pipeline displays  all the occurrences of the string  "GCS" in the
CMS Pipelines help library:
    pipe  <  pipeline helpin  |  unpack  |  locate /GCS/  |  console
The unpack filter checks whether its input is a packed file and,  if it
is,  does the  same unpack operation that the CMS  COPYFILE and XEDIT
commands do.   The locate filter selects records using the same logic as
the XEDIT LOCATE subcommand.
This pipeline tells  you how many words  there are in one  of your CMS
          pipe  <  plunge script a  |  count words  |  console
A slightly more  elaborate pipeline tells you how  many different words
there are in that same file:
pipe  <  plunge  script  a  |  split  |  sort  unique  |  count  lines  |  console
split writes one  output record for every blank-delimited  word in its
input;  sort unique then sorts those  one-word records and discards the
duplicates,  passing the  unique records on to count  lines to count.
count writes a single record containing  the count to its output stream.
console reads that record and displays it on the console.
Plunging into Pipes                                               page 9
This pipeline writes a CMS file containing fixed-format, 80-byte records
to a tape, blocking it in a format suitable to be read by other systems:
           pipe  <  gqopt fortran a  |  block 16000  |  tape
This pipeline writes a list of the commands used with "SMART" (RTM) to a
CMS file:
pipe  literal next|  vmc smart help|  strip trailing  |  >  smart commands a
literal writes a  record containing the word "next".    The vmc device
driver sends a help  command to the SMART service machine  via VMCF and
writes the response to the pipeline.    It then reads the single record
from its input and  sends a next command to the  SMART service machine,
again writing the  response to the pipeline.    strip trailing removes
trailing blanks from the records that pass through it,  thus turning the
blank lines in  the response from SMART into null  records.   > reads
records from its input,  discards those that are null,  and writes the
others to the file SMART COMMANDS A.
And here is a pipeline I especially like; it would be typed on the XEDIT
command line:
     pipe  cms query search  |  change  //INPUT  /  |  subcom xedit
In this pipeline,   the cms device driver issues the  CMS QUERY SEARCH
command and routes the response into  the pipeline;  the change filter
(which works like the XEDIT CHANGE subcommand)  changes each line of the
response into an XEDIT INPUT subcommand; and then subcom sends each line
to XEDIT,  which executes it as a command.   This is a very easy way to
incorporate the response from a command into  the text of a file you are
The Specs Filter:   Now, let's look at one of the less obvious filters,
specs.   specs selects pieces of an input  record and puts them into an
output record.   It is very useful and not really as complex as it looks
at first.   Its syntax was derived from the syntax for the SPECS option
of the CMS COPYFILE command,  but it has long since expanded far beyond
the capabilities of that option:
   * The basic syntax of specs is:
                  specs  input-location  output-location
     with as many input/output pairs as you need.
   * The input location  may be a column range,   such as "10-14".
     "10.5" means the same thing as "10-14".   "1-*" means the whole
     record.  "words 1-4" means the first four blank-delimited words.
     The input may also be a literal field,  expressed as a delimited
     string, such as "/MSG/", or it may be "number",  to get a record
Plunging into Pipes                                              page 10
   * The output location may be a starting column number,  or "next",
     which means the next column,  or "nextword",  which leaves one
     blank before the output field.
   * A conversion routine,  such as "c2d",  may be specified between
     the input  location and  the output  location.   The  specs
     conversion routines are similar to the REXX conversion functions
     and are applied to the value from  the input field before it is
     moved into the output field.
   * A placement option,  "left",  "center",  or "right",  may be
     specified following the output location;  for example,  "number
     76.4 right" puts a 4-digit  record number right-aligned starting
     in column 76.
|                                                                      |
|    /* PIPEDS EXEC:                Find lrecl of an OS dataset */     |
|                                                                      |
|    Parse Upper Arg dsname fm                                         |
|                                                                      |
|    'PIPE (name PIPEDS)',                                             |
|       'command LISTDS' fm '( FORMAT |',    /* Issue LISTDS.   */     |
|       'locate /' dsname '/ |',       /* Locate file we want.  */     |
|       'specs  word 2  1 |',          /* Lrecl is second word. */     |
|       'console'                      /* Display lrecl.        */     |
|                                                                      |
PIPEDS EXEC is a simple example  of using specs.   PIPEDS displays the
logical record length of an OS dataset.   The command stage issues a CMS
LISTDS command with the FORMAT option  and routes the response into the
pipeline,  where locate selects the  line that describes the specified
dataset, e.g.:
         U           6447  PO    02/25/80 RES342 B  SYS5.SNOBOL
specs selects only  the second word of that line,   the logical record
length ("6447"),  and moves it to column 1 of its output record,  which
console then reads and displays.
pipe  <  cms exec a  |  specs  1-27  1  8-27  nextword  |  >  cms exec a
This is another simple example of using specs.   The arguments to specs
here  are two  pairs of  input-output  specifications.   The  first
input-output pair ("1-27 1")  copies the  data from columns 1-27 of the
input record  to columns  1-27 of the  output record.    The second
input-output pair ("8-27 nextword") copies the data from columns 8-27 of
the input record to columns 29-48 of the output record; that is, a blank
is left between the first output field and the second output field.  So,
this pipeline would be  used to duplicate the filenames in  a CMS EXEC
created by the EXEC option of the CMS LISTFILE command.   (This pipeline
Plunging into Pipes                                              page 11
is almost 500 times as fast as the XEDIT  macro I used to use to do this
same thing.)
Augmenting REXX:  People often start in gradually using CMS Pipelines in
EXECs,  first just taking advantage of the built-in programs that supply
function that is missing or awkward in  REXX.   Here is a function that
has been implemented a zillion times in REXX or Assembler:
             'PIPE  stem bananas.  |  sort  |  stem bunch.'
That sorts the values in the stemmed  array "bananas" and puts them into
the array "bunch".
Here is an example  of using specs to augment REXX  (which has no "c2f"
      'PIPE  var cpu2busy  |  specs  1-*  c2f  1  |  var cpu2busy'
The device driver  var picks up the REXX  variable "cpu2busy",  which
contains a  floating-point number stored  in the  System/370 internal
representation (e.g.,  '4419B600'x),  and writes  it to the pipeline.
specs reads the record passed from var  and converts it to the external
representation of the floating-point number  (6.582E+03),  and then the
second var stage stores the new  representation back into the same REXX
variable, allowing it to be used in arithmetic operations.
Another function CMS Pipelines brings to REXX programmers is an easy way
to process all the variables that have  a given stem.   In the example
below,  rexxvars writes two records into  the pipeline for each exposed
variable.  One record starts with "n " and contains the variable's name;
the other starts  with "v " and contains its value.    The find stage
selects only  the name records for  variables with the  stem "THINGS".
specs removes  the "n ",  and  stem puts  the names of  the "THINGS"
variables into the stemmed array "vars", where they can be accessed with
a numeric  index.   (The buffer stage  prevents the stem  stage from
creating new  variables while rexxvars  is still loading  the existing
|                                                                      |
|    'PIPE',                    /* Discover stemmed variables:  */     |
|       'rexxvars |',           /*   Get all variables.         */     |
|       'find n THINGS.|',      /*   Select names of THINGS.    */     |
|       'specs 3-* 1 |',        /*   Remove record type prefix. */     |
|       'buffer |',             /*   Hold all records.          */     |
|       'stem vars.'            /*   Names of THINGS into stem. */     |
|                                                                      |
|    Do i = 1 to vars.0                                                |
|       Say vars.i '=' Value(vars.i)                                   |
|    End                                                               |
|                                                                      |
Plunging into Pipes                                              page 12
Replacing EXECIO:   EXECIO  is usually the first thing to  go when one
learns CMS Pipelines.  Anything that can be done with EXECIO can be done
with CMS Pipelines,  generally faster and always more straightforwardly.
(And replacing EXECIO with  a pipeline makes it easier to  port an EXEC
between CMS and MVS.)   Let's look at a few EXECIO examples from various
IBM manuals, along with the equivalent pipelines:
   * These both read the first three records  of a CMS file into the
     stemmed array "X" and set the value of "X.0" to 3:
          'EXECIO  3  DISKR  MYFILE DATA *  1  ( STEM X.'
          'PIPE  <  myfile data *  |  take 3  |  stem x.'
   * These both issue  a CP QUERY USER  command in order to  set a
     return code (without saving the response):
          +++ RC(1045) +++
          'PIPE  cp query user glorp'
          +++ RC(45) +++
   * These both put a blank-delimited list of the user's virtual disk
     addresses into the REXX variable "used":
          Signal Off Error
          Signal On Error
          theirs = Queued()
          'EXECIO * CP ( STRING Q DASD'
          used = ''
          Do While Queued() > theirs
             Pull . cuu .
             used = used cuu
          'PIPE  cp q dasd  |  specs  word  2  1  |  join  *  /  /  |  var used'
     The EXECIO case comes from the REXX User's Guide.   Admittedly,
     it is rather old-fashioned code;  nevertheless, its eleven lines
     make up  an all too familiar  example of manipulating  the CMS
     stack.   In the pipeline,  the cp  device driver issues the CP
     QUERY DASD command  and routes the response  into the pipeline.
     specs selects the second word from  each input record and makes
     it the first (and only) word in an output record.   join * joins
     all these  records together into  one record,   inserting the
     delimited string in its argument  (a blank)  between the values
     from the individual input records.   And var stores this single
     record into the variable "used".
Plunging into Pipes                                              page 13
Pipeline Programs:   After a while,  you  will find yourself not just
augmenting your EXECs with small pipes,  but also writing EXECs that are
predominantly pipes, such as REACCMSG EXEC:
|                                                                      |
| /* REACCMSG EXEC:       Notify users to re-ACCESS a changed disk */  |
|                                                                      |
| Parse Arg vaddr .                                                    |
|                                                                      |
| 'PIPE (name REACCMSG)',                                              |
|    'cp query links' vaddr '|',        /* Issue CP QUERY LINKS.   */  |
|    'split at , |',                    /* Get one user per line.  */  |
|    'strip |',                         /* Remove leading blanks.  */  |
|    'sort unique 1-8 |',               /* Discard duplicates.     */  |
|    'specs /MSG/ 1',                   /* Make into MSG commands. */  |
|       'word 1 nextword',              /* Fill in userid.         */  |
|       '/Please re-ACCESS your/ nextword',                            |
|       'word 2 nextword',              /* Fill in virtual address.*/  |
|       '/disk./ nextword |',                                          |
|    'cp'                               /* Issue MSG commands.     */  |
|                                                                      |
REACCMSG is  used to  send a message  to all the  users linked  to a
particular CMS disk to let them know that they should re-ACCESS the disk
because it has been changed.   It  uses built-in programs we have seen
before, but in a slightly more sophisticated manner:  split receives the
response from the CP QUERY LINKS command:
 PIPMAINT 320 R/O, MAINT    420 R/O, TDTRUE   113 R/O, Q0606    320 R/O
 Q0606    113 R/O, SERGE    420 R/O
and splits those  records into multiple records by breaking  them up at
the commas between items;  strip removes the leading blanks;  and sort
unique sorts the records on the userid  field in the first eight columns
and discards any duplicates,  so that each  user will be sent only one
message.   This example shows a more elaborate use of specs than before,
but it is not difficult to understand  if you keep in mind that specs's
arguments are always pairs of definitions  for input and output.   This
specs stage has been written in  portrait format with each input-output
pair on a separate line.   You will  note that the input definitions in
three of the five pairs here are for literals.   The first input-output
pair puts the literal "MSG" into columns 1-3 of the output record;  the
second pair puts  the userid from the  first word of the  input record
("word 1") into columns 5-12 of the output record; and so on.   Then as
each record flows from the specs stage to the cp stage,  cp issues it as
a CP MSG command.
The next  example is a simple  service machine that uses  the starmsg
device driver to connect to the CP *ACCOUNT system service,  so that it
can monitor attempts  to LOGON to the system with  an invalid password.
Plunging into Pipes                                              page 14
Each time CP produces an accounting record,  this starmsg stage receives
that record via IUCV and writes it to the pipeline (prefacing it with an
8-byte header).  The locate stage discards all but the "Type 4" records,
which are the  ones that CP produces  when the limit of  invalid LOGON
passwords is reached.   specs formats a message containing a literal and
three fields from the accounting  record,  which console then displays.
(Note the stage separators on the left side here.  This is a widely used
alternative portrait format.)
This pipeline runs  until you stop it by using  the haccount immediate
command,  which CMS Pipelines sets up  for you when it establishes the
connection to the *ACCOUNT system service.   starmsg can also be used to
connect to several other CP system services, including *MSG and *MSGALL.
|                                                                      |
|   /* HACKER EXEC:           Display Type 4 Accounting Records. */    |
|                                                                      |
|   'PIPE (name STARMSG)',                                             |
|      '| starmsg *account CP RECORDING ACCOUNT ON LIMIT 20',          |
|      '| locate 88 /4/',            /* Only Type 4 records.     */    |
|      '| specs',                    /* Format warning message:  */    |
|      '     /Hacker afoot? / 1',    /*    literal,              */    |
|      '     9.8 next',              /*    ACOUSER,              */    |
|      '     37.4 nextword',         /*    ACOTERM@,             */    |
|      '     79.8 nextword',         /*    ACOLUNAM.             */    |
|      '| console'                   /* Display on console.      */    |
|                                                                      |
|   If Userid() <> 'OPERACCT'                                          |
|   Then 'CP RECORDING ACCOUNT OFF PURGE QID' Userid()                 |
|                                                                      |
The next example may be a bit arcane,   but it can be very useful;  it
reads a file containing textual material of arbitrary content and record
length and  produces a  file containing the  same text  formatted as
Assembler DC instructions for use, say, as messages:
Plunging into Pipes                                              page 15
|                                                                      |
|   /* MAKEDC EXEC:   Reformat text into Assembler DC statements */    |
|                                                                      |
|   Parse Arg fn ft fm .          /* File to be processed.       */    |
|                                                                      |
|   "PIPE (name MAKEDC)",                                              |
|      "<" fn ft fm "|",          /* Read the file.              */    |
|      "change /&/&&/ |",         /* Double the ampersands.      */    |
|      "change /'/''/ |",         /* Double the single quotes.   */    |
|      "specs",                   /* Reformat to DC statement:   */    |
|      "   /DC/ 10",              /*    literal "DC" in col 10;  */    |
|      "   /C'/ 16",              /*    literal "C'" in col 16;  */    |
|      "   1-* next",             /*    entire record next; and  */    |
|      "   /'/ next |",           /*    terminate with quote.    */    |
|      "asmxpnd |",               /* Split to continuations.     */    |
|      ">" fn "assemble a"        /* Write the new file.         */    |
|                                                                      |
The two change filters double any ampersands or quotes in the text.  For
each input record, specs builds an output record that has "DC" in column
10 and "C" in column 16, followed by the input record enclosed in single
quotes.   asmxpnd then  examines each record to  determine whether it
extends beyond column 71;  if so,  it breaks the record up into two or
more records  formatted in accordance  with the Assembler's  rules for
continuations.   And finally, > writes the reformatted records to a CMS
file.  Thus, if the input file were to contain the line:
     The PACKAGE file records have ' &1 &2 ' in columns 1-7 and a filename,
then these two records would appear in the output file:
         DC    C'The PACKAGE file records have '' &&1 &&2 '' in columns*
                1-7 and a filename,'
Selection Filters:   There are many more CMS Pipelines filters to learn,
but I want to mention one class in particular, the selection filters:
 between        frlabel        nfind          outside        unique
 drop           inside         nlocate        take           whilelab
 find           locate         notinside      tolabel
The selection  filters are used to  select certain records  from among
those passing through the pipeline,   while discarding all others.   A
cascade of selection  filters can quickly select the  desired subset of
even a very  large file.   I routinely use pipelines  to filter files
containing tens (or even hundreds) of thousands of records to select the
records I need for some purpose.
One simple example is a filter I  use with the NETSTAT CLIENTS command.
NETSTAT CLIENTS produces hundreds of lines of output,  several lines for
Plunging into Pipes                                              page 16
each user who has used TCP/IP since the last IPL.  The first line of the
response for each user begins with the string "Client:"  followed by the
userid;  and  one of the  other lines  begins with the  string "Last
Touched:".   Usually, when I issue a NETSTAT CLIENTS command, I need to
see only these two lines for each  of four servers.   The eight lines I
want are easily isolated using two selection filters:
|                                                                      |
|    /* STATPIPE EXEC:      Display "Last Touched" for BITFTPn. */     |
|                                                                      |
|    'PIPE',                                                           |
|       'command NETSTAT CLIENTS |',                                   |
|       'between   /Client: BITFTP/ /Last Touched:/ |',                |
|       'notinside /Client: BITFTP/ /Last Touched:/ |',                |
|       'console'                                                      |
|                                                                      |
The command  stage issues  a NETSTAT CLIENTS  command and  routes the
response into  the pipeline.   The  between filter selects  groups of
records;  its arguments are two delimited strings, describing the first
and last records to be selected for each group.   So, the between stage
here selects  groups of records that  begin with a record  that begins
"Client: BITFTP" and that end with a record that begins "Last Touched:".
notinside then further refines the data  by selecting only those records
that are not between a record that  begins with "Client:  BITFTP" and a
record that begins with "Last Touched:".  That leaves us with only those
two lines for each client I am  interested in,  the ones whose userids
start "BITFTP".
You will likely find that many of  your pipelines process the output of
CP or CMS  commands or CMS or  MVS programs.   The output  from UNIX
commands and programs is generally designed  to be processed by a pipe,
so it  tends to be  essentially "pure  data",  with few  headers and
trailers.   With CP, CMS, and MVS output, however, you generally need to
winnow out the chaff to get down to the data.  Although I cannot go over
the selection filters in detail today,  they  are easy to use and quite
powerful,  so you should not hesitate to process listing files that were
designed to  be read by humans  and that have complicated  headers and
trailers and carriage control.   It is very  easy to write a pipe that
reads such a file and pares it down to the bare data.
LIST2SRC EXEC is an example of  really using the selection filters;  I
will leave the  detailed interpretation of LIST2SRC as  an exercise for
you.   Basically, LIST2SRC reads a LISTING file produced by Assembler H
and passes it through a series of selection filters,  winnowing out the
chaff in order to reconstruct the original source file.   Although this
is a "quick &  dirty" program (and not quite complete),   it is a good
example of "pipethink",  of solving a complex problem by breaking it up
into simple steps:
Plunging into Pipes                                              page 17
|                                                                      |
| /* LIST2SRC EXEC:       Re-create the source from a LISTING file */  |
| Signal On Novalue                                                    |
|                                                                      |
| Parse Arg fn .                                                       |
|                                                                      |
| 'PIPE (name LIST2SRC)',                                              |
|    '| <' fn 'listing *',          /* Read the LISTING file       */  |
|    '| mctoasa',                   /* Machine carriage ctl => ASA */  |
|    '| frlabel -  LOC',            /* Discard to start of program */  |
|    '| drop 1',                    /* Drop that '-  LOC' line too */  |
|    '| tolabel - POS.ID',          /* Keep only up to relocation  */  |
|    '| tolabel -SYMBOL',           /*    dictionary or cross-ref  */  |
|    '| tolabel 0THE FOLLOWING STATEMENTS',  /*    or diagnostics  */  |
|    '| outside /1/ 2',             /* Drop 1st 2 lines on each pg */  |
|    '| nlocate 5-7 /IEV/',         /* Discard error messages      */  |
|    '| nlocate 41 /+/',            /* Discard macro expansions    */  |
|    '| nlocate 40 /                  /',  /* Discard blank lines  */  |
|    '| specs 42.80 1',             /* Pick out source "card"      */  |
|    '| >' fn 'assemble a fixed'    /* Write new source (RECFM F)  */  |
|                                                                      |
Subroutine Pipelines
Once you have been using CMS Pipelines  for a while,  you may find that
there are some sequences of stages that you use often:
     pipe  stage-a  |  stage-b  |  stage-c  |  stage-d  |  stage-e
     pipe  stage-x  |  stage-b  |  stage-c  |  stage-d  |  stage-y
In that  case,  it is  time to move  those stages into  a subroutine
pipeline, polish them a bit, generalize them a bit,  and create your own
little gem:
    |                                                                  |
    |  /* MYSUB REXX */                                                |
    |                                                                  |
    |  'CALLPIPE  *:  |  stage-b  |  stage-c  |  stage-d  |  *:'       |
    |                                                                  |
Then whenever you need the function  performed by your subroutine,  you
simply use its name as a stage name ("mysub" in this case):
                  pipe  stage-a  |  mysub  |  stage-e
                  pipe  stage-x  |  mysub  |  stage-y
Plunging into Pipes                                              page 18
The subroutine may look a bit mysterious,   but it is simply a pipeline
stage written in REXX.   If we look at it again in portrait format,  it
can be demystified quickly:
|                                                                      |
|    /* MYSUB REXX:                Generic subroutine pipeline  */     |
|                                                                      |
|    'callpipe',                       /* Invoke pipeline       */     |
|       '*: |',                        /* Connect input stream  */     |
|       'stage-b |',                                                   |
|       'stage-c |',                                                   |
|       'stage-d |',                                                   |
|       '*:'                           /* Connect output stream */     |
|                                                                      |
|    Exit RC                                                           |
|                                                                      |
There are just  a few things one needs to  understand about subroutine
1.   The CMS  Pipelines command  callpipe says  to run  a subroutine
  pipeline;  callpipe has the same syntax  and the same options as the
  PIPE command itself.
2.   Those "*:"  sequences are called  "connectors".   The connector at
  the beginning tells the pipeline dispatcher to connect the output from
  the previous stage of the calling pipeline  to the input of the first
  stage of this subroutine pipeline, stage-b.   The connector at the end
  says to  connect the output from  the last stage of  this subroutine
  pipeline,  stage-d,  to the input of  the next stage in the calling
3.   When you  use REXX to write  an XEDIT subroutine,   the default
  subcommand environment is XEDIT.   Similarly,   when you use REXX to
  write a CMS Pipelines subroutine,  the default subcommand environment
  executes CMS Pipelines commands.  Thus, if you wish to issue CP or CMS
  commands in your subroutine,  you will  need to use the REXX Address
4.   When you use REXX to write an XEDIT subroutine,  the subroutine has
  a filetype of XEDIT,  but when you  use REXX to write a CMS Pipelines
  subroutine, the filetype is not PIPE.  It is REXX.
5.   Arguments passed to  a subroutine are available to  the REXX Parse
  Arg instruction.
Let's look at an example of a real subroutine pipeline, HEXSORT,  which
sorts hexadecimal  numbers.   An  ordinary sort  does not  work for
hexadecimal numbers  (i.e.,  base 16  numbers,  expressed  with the
"numerals" 0-9,  A-F),  because the EBCDIC collating sequence sorts A-F
Plunging into Pipes                                              page 19
before 0-9.   This handy little  subroutine pipeline sorts hexadecimal
data correctly  by using the trick  of temporarily translating  A-F to
characters higher in the collating sequence than 0-9 (which are F0-F9 in
|                                                                      |
| /* HEXSORT REXX:           Hexadecimal sort,   0123456789ABCDEF  */  |
|                                                                      |
| Parse Arg sortparms                     /* Get parms, if any     */  |
|                                                                      |
| 'callpipe (name HEXSORT)',              /* Invoke pipeline       */  |
|    '*: |',                              /* Connect input stream  */  |
|    'xlate 1-* A-F fa-ff fa-ff A-F |',   /* Transform for sort    */  |
|    'sort' sortparms '|',                /* Sort w/caller's parms */  |
|    'xlate 1-* A-F fa-ff fa-ff A-F |',   /* Restore               */  |
|    '*:'                                 /* Connect output stream */  |
|                                                                      |
| Exit RC                                                              |
|                                                                      |
The arguments to the xlate stages here are a column range, "1-*",  which
means the  entire record,   followed by  pairs of  character ranges
specifying "to" and  "from" translations.   Records flow  in from the
calling pipeline through the beginning  connector;  they are processed
through the xlate,  sort,  and xlate  stages;  and then they flow out
through the end connector back into the calling pipeline.  If the caller
specifies an argument,   that argument is passed to the  sort stage to
define a non-default sort operation.  Here is a typical invocation:
          'PIPE  stem mdisk.  |  hexsort 7.3  |  stem mdisk.'
That sorts an array of minidisk records  from a CP directory into device
address order.  (The device addresses are hexadecimal numbers in columns
7-9 of the minidisk records.)
Of course,   it is  not necessary  to put  these operations  into a
subroutine.   You could simply use the xlate-sort-xlate sequence in all
your pipelines,  whenever you need to do a hexadecimal sort,  but it is
much better to  hide such complexity.   Once you  have this subroutine
built,  you can invoke it by name from any number of pipelines and need
never think about the problem again.
Furthermore,  by building the subroutine  with a simple,  well-defined
interface and  at the same  time making  its function as  generic as
possible,  you create a  piece of code that can be  used over and over
again.  Here is another example of invoking HEXSORT:
  'PIPE  cp q nss map  |  drop 1  |  hexsort  33-44  |  >  nss map a'
Plunging into Pipes                                              page 20
That issues a CP  QUERY NSS command,  drops the header  line from the
response,  and sorts  the remaining lines to produce a  list of saved
systems in memory  address order.   (The virtual  memory addresses are
hexadecimal numbers starting in column 33 of the response lines.)
A subroutine pipeline  is often the cleanest way to  package a function
that you  have implemented with  CMS Pipelines.    If you make  it a
subroutine pipeline, then the people you give it to can easily invoke it
from their own pipes.
Writing REXX Filters
The time will come when you have a  problem that cannot be solved by any
reasonable combination of CMS Pipelines  built-in programs.   You will
need to write a filter of your own, preferably in REXX.   A REXX filter
is similar to the simple subroutine  pipelines we have just been looking
at.   It has a filetype of REXX; its subcommand environment executes CMS
Pipelines commands;  it is  invoked by using its name as  a stage in a
pipeline; and it can receive passed arguments.
You will find writing your own pipeline  filters in REXX to be very easy
once you understand the basics.   When I am writing a filter,  I always
start with  this dummy filter  that does  nothing at all  except pass
records through unchanged:
|                                                                      |
|     /* NULL REXX:                    Dummy pipeline filter  */       |
|     Signal On Error                                                  |
|                                                                      |
|     Do Forever                          /* Do until EOF     */       |
|        'readto record'                  /* Read from pipe   */       |
|        'output' record                  /* Write to pipe    */       |
|     End                                                              |
|                                                                      |
|     Error: Exit RC*(RC<>12)             /* RC = 0 if EOF    */       |
There are only a  few new things one needs to  learn to understand this
REXX filter:
1.   The CMS Pipelines  command readto reads the next  record from the
  pipeline into the specified REXX variable ("record" in this case).
2.   The CMS Pipelines command output writes  a record to the pipeline.
  The contents  of the  record are  the results  of evaluating  the
  expression following the output command  (again,  in this case,  the
  value of the REXX variable "record").
Plunging into Pipes                                              page 21
3.   The  pipeline dispatcher  sets  return  code 12  to  indicate
  end-of-file.  A readto command completes with a return code of 12 when
  the stage before it in the pipeline has  no more records to pass on to
  it.   An output command completes with a  return code of 12 when the
  stage following it in the pipeline has decided to accept no more input
So, this filter,  NULL,  reads a record from the pipeline and writes it
back to the pipeline unchanged.   It keeps on doing that until an error
is signalled, i.e., until a non-zero return code is set.   That causes a
transfer to the label "Error" in the  last line of the EXEC.   The most
likely non-zero return code  would be a return code 12  from the readto
command, which would indicate end-of-file on the input stream,  but the
output command could get return code 12  instead,  or there could be a
real error.   If the return code is 12,  then before exiting the filter
sets its own return code to 0 to indicate normal completion.   Any other
return code is passed back to the caller.
The effect of including the NULL filter in a pipeline:
             pipe  stage-a  |  null  |  stage-b  |  stage-c
is simply  to make the  pipeline run a  bit slower.   But  once you
understand NULL, you can quickly go on to writing useful filters,  such
as REVERSE, which reverses the contents of the records that pass through
|                                                                      |
|     /* REVERSE REXX:          Filter that reverses records  */       |
|     Signal On Error                                                  |
|                                                                      |
|     Do Forever                          /* Do until EOF     */       |
|        'readto record'                  /* Read from pipe   */       |
|        'output' Reverse(record)         /* Write to pipe    */       |
|     End                                                              |
|                                                                      |
|     Error: Exit RC*(RC<>12)             /* RC = 0 if EOF    */       |
|                                                                      |
We can make that example slightly more complex,  to illustrate one more
concept that you will need when writing filters.   This filter reverses
only the even-numbered lines passing through it:
Plunging into Pipes                                              page 22
|                                                                      |
|   /* BOUSTRO REXX:   Filter that writes records boustrophedon */     |
|   Signal On Error                                                    |
|                                                                      |
|   Do recno = 1 by 1                      /* Do until EOF      */     |
|      'readto record'                     /* Read from pipe    */     |
|      If recno // 2 = 0                   /* If even-numbered  */     |
|         Then record = Reverse(record)    /*    line, reverse  */     |
|      'output' record                     /* Write to pipe     */     |
|   End                                                                |
|                                                                      |
|   Error: Exit RC*(RC<>12)                /* RC = 0 if EOF     */     |
|                                                                      |
Each stage in a pipeline runs as  a "co-routine",  which means that it
runs concurrently with the other stages in the pipeline.   It is invoked
once, when the pipeline is initiated, and remains resident.   So,  when
BOUSTRO is  ready for  another record,  it  calls upon  the pipeline
dispatcher by  doing a readto.   The  dispatcher may then  decide to
dispatch some other co-routine, but it will eventually return control to
this one, which will continue reading and writing records until an error
is signalled.   Thus, when you are writing a CMS Pipelines filter,  you
need not worry (as I did at first)  about where to save local variables,
such as "recno" here,  between "calls" to your filter.   Your filter is
called only once and then runs concurrently with the other stages in the
pipeline.   There is  nothing special that your filter needs  to do in
order to run concurrently with the other stages; the pipeline dispatcher
takes care of all that for you.
I would like to show one more example of a simple REXX filter,  AVERAGE,
which illustrates the point that your filter  can decide not to write a
record back to the pipeline for every record it reads from the pipeline.
AVERAGE first  reads all  the input records;   then,  when  it gets
end-of-file on its input,  it calculates the contents of a single output
record and writes that to the pipeline:
Plunging into Pipes                                              page 23
|                                                                      |
|  /* AVERAGE REXX:                  Filter that averages input  */    |
|  Signal On Error                                                     |
|                                                                      |
|  acum = 0                                 /* Initialize        */    |
|                                                                      |
|  Do nobs = 0 by 1                         /* Do until EOF      */    |
|     'readto record'                       /* Read from pipe    */    |
|     Parse Var record number .             /* Get number        */    |
|     acum = acum + number                  /* Accumulate        */    |
|  End                                                                 |
|                                                                      |
|  Error:  If RC = 12                       /* If EOF, then      */    |
|     Then 'output' Format(acum/nobs,,2)    /*    write average  */    |
|  Exit RC*(RC<>12)                         /* RC = 0 if EOF     */    |
|                                                                      |
Differences from UNIX Pipes
That is as many examples of using pipelines  in CMS as we have time for
right now.   I have pointed out some of the differences between the UNIX
and CMS implementations of pipelines.   You may have noticed some of the
* As you would expect,  CMS Pipelines is record-oriented,  rather than
* CMS Pipelines implements asynchronous input, immediate commands,  and
  dynamic reconfiguration of pipeline topology.
* CMS Pipelines implements multi-stream  pipelines.   These networks of
  interconnected pipelines allow selection filters  to split a file into
  streams that are processed in different ways.  The streams can then be
  recombined for further processing.
* Most CMS Pipelines stages run unbuffered;  that is, they process each
  input record as soon as it is received and pass it on to the following
  stage immediately.   (Of course, some pipeline stages,  such as sort,
  must, by their nature, be buffered.)  Running the stages unbuffered is
  necessary to allow records flowing  through a multi-stream pipeline to
  arrive at the end in a predictable order.   It can have the advantage
  of greatly reducing  the virtual memory requirements.    Thus,  CMS
  Pipelines can often be used to  perform operations that cannot be done
  with XEDIT because of virtual memory constraints.
* CMS Pipelines  runs a pipeline only  after all its stages  have been
  specified correctly.
Plunging into Pipes                                              page 24
* CMS Pipelines  programs can co-ordinate  their progress  via "commit
  levels" and can stop the pipeline when a program encounters an error.
* When the CMS Pipelines PIPE command completes, it sets its return code
  to the worst of the return codes set by the stages in the pipeline.
To sum up the differences between UNIX pipes and CMS Pipelines,  let me
quote a colleague of mine who said  recently,  "You know what I really
miss in UNIX?  CMS Pipelines!"
Advanced Topics
I have had time  to give you only a flavor of  CMS Pipelines.   I have
barely alluded to multi-stream pipelines,  a very powerful extension to
the basic pipeline concept with which  you will want to become familiar.
I also have not mentioned that CMS Pipelines can be run under GCS,  TSO,
and MUSIC.  CMS Pipelines can now be ordered with MUSIC, and although it
is not officially supported for TSO and GCS,  it contains device drivers
developed specifically for those  environments.   The Program Directory
for the PRPQ  has an appendix that discusses installing  and using CMS
Pipelines with all three of these systems.

See also

CMS Pipelines Explained, PDF format (117K), revised 2007-09-10 

CMS Pipelines Enhancements for Shared File System and Byte File System, LIST3820 format, packed (24K); PDF format (44K)

Writing Assembler Filters for Use with CMS Pipelines, LIST3820 format, packed (104K); PDF format (92K) Note that the information in this paper is to a large extent obsolete.

PIPE Command Programming Interface, LIST3820 format, packed (196K), revised 05/20/97

CMS Pipelines Procedures Macro Language, LIST3820 format, packed (360K) PDF format (449K) Note that while the PDF file shows a formatting date of July 18 2006, the contents are unchanged from the 1997 edition. In particular, the paper makes no mention of the current location of things, such as FPLGPI and FPLOM MACLIBs


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