Crucial to "[How Tcl is special]" is the pipeline.  Through the
syntax
  set channel [open |$external_command r+]
Tcl [open]s an external process, and connects to the [stdin] and [stdout]
of $external_command.  A special syntax is needed in external_command to get [stderr] to be concantenated to stdout.  Reading and writing to stdin and
stdout are with the usual Tcl I/O commands:  [gets], [puts], and [read]. (It may be necessary to [flush] resp. ''fflush'' to prevent deadlocks caused by buffering).

This makes for exceptionally handy [glue] for many situations.

The concept of pipeline, however, requires the external command to be designed to interact with data either on its stdin or stdout.  Many [GUI] applications (and occasionally non-graphical applications) do not do so.  In those cases, the pipeline provides little benefit.

The concept of the pipeline is opimally utilitized when writing tools using a [filter] paradigm.

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Many Wiki pages and other references obliquely discuss or illustrate
Tcl process pipelines.  While the construction is crucial to a proper
understanding of Tcl's capabilities, many newcomers take quite a while
to come across an explanation that "gets through" to them.  Among
the pertinent available writings are
   * "[Concepts of Architectural Design for Tcl Applications]"
   * "Scripted wrappers for legacy applications", a series of articles from ITworld, which appears to be gone.  Temporarily at least, check out google's cached version at [http://www.google.com/search?q=cache:qasxiAR1z-4C:www.itworld.com/AppDev/4061/swol-1218-regex/+%22scripted+wrappers+for+legacy+applications%22&hl=en&ie=UTF-8] and part 2 of the series at [http://www.google.com/search?q=cache:nG2xX4cRxZoC:www.itworld.com/AppDev/4061/swol-0105-regex/+%22scripted+wrappers+for+legacy+applications%22&hl=en&ie=UTF-8].
   * "[client/server with fileevent]"
   * [Pipe servers in C from Tcl]
   * [VFS, exec and command pipelines]
   * ...

----
This transcript of an interaction session illustrates a simple pipeline
with Unix's bc executable:
    % set channel [open |bc r+]
    file3
    % puts $channel "1234567890 * 987654321"
    % flush $channel
    % puts [gets $channel]
    1219326311126352690


----
with winnt cmd:
% set channel [open |cmd r+]
    file3
% gets $channel
% gets $channel
    % puts $channel "hostname"
    % flush $channel
   % gets $channel
% gets $channel
    % puts [gets $channel]
    % close $channel


















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While (classic) MacOS supports no Tcl pipelines, see "[Inventory of IPC methods]"
for generalizations that apply there and elsewhere.







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[[Explain "named pipe" as specialization of concept.]]


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[idiom]