Crucial to "[How Tcl is special]" is the pipeline.  Through the
syntax
  set channel [open |$external_command]
Tcl [open]s an external process, and connects to the stdin, stdout,
and stderr of $external_command.  Reading and writing to stdin and
stdout are with the usual Tcl I/O commands:  [gets], [puts], and [read].

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

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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"
   * ...


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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


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Do pipelines work on Windows and MacOS?  On Windows NT and XP, the
OS provides pipeline support.