Crucial to "How Tcl is special" is the pipeline. Through the syntax
set channel [open |$external_command r+]
Tcl opens 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.
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
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
[Give example of Win* pipeline.]
While (classic) MacOS supports no Tcl pipelines, see "Inventory of IPC methods" for generalizations that apply there and elsewhere.
[Explain "named pipe" as specialization of concept.]
category idiom (?)