When tclsh is invoked interactively, it normally prompts for each command with % . You can change the prompt by setting the Tcl global variables tcl_prompt1 and tcl_prompt2. If variable tcl_prompt1 exists, then it must consist of a Tcl script to output a prompt; instead of outputting the % prompt, tclsh will evaluate the script in tcl_prompt1.
The standard prompt is equivalent to this:
puts -nonewline "% "
If you have a useful example of a tcl_prompt1 script, perhaps you might consider sharing it with us?
See also magic names.
This link has a few "tcl_prompt1" examples in it also.