Crucial to "How Tcl is special" is the pipeline. Through the syntax
set channel [open |$external_command]
Tcl opens 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.
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 (?)