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The Tool Command Language (Tcl) is a scripting language. Togther with the graphical user interface toolkit Tk it is a programming tool which is ported to many operating systems. While not as efficient as compiled programming languages or compiled user interfaces, it allows fast development of code in a high level programming language.
What you can do with Tcl is best seen with a few examples. After having downloaded a file say "command.tcl" and after having made it executable with chmod "755 command.tcl" it can be executed like a shellscript.

You might find Tkl/Tk is especially useful to test programs which output numerical values. The program duffing.c for example produces orbits of a ordinary differential equation. With input.tcl and output.tcl , you can interact with that program graphically instead of staring at numbers only.
You can download this whole directory as a gziped tarball tcl.tgz .

Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".