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.