The Tcl-only interpreter is called tclsh, but we will only use Tcl in
combination with Tk, and therefore we don't use tclsh, but wish .
wish is simply started with
When wish is started, the wish prompt (%) and an empty window
appear. Commands are typed at the prompt and results of Tcl/Tk commands will
mostly become visible in the window.
The interpreter is exited by typing exit or by pressing Control-C.