Comp.lang.tcl questioners '''frequently''' demand some sort of
gadget to monitor-an-ongoing-process-and-display-the-result-in-a-[text].
[Tailing widget] is the simplest answer CL knows that meets the
common intent of these requests.

There are lots of variations on this theme, though.
[tailf] carefully implements
the functionality of the Unix tail(1), and adds a bit of filtering
capability.  I've written a few examples
[http://phaseit.net/claird/comp.lang.tcl/tcl-examples.html#tail]
that show how different combinations of IPC, fileevent, after, ...
can co-operate to monitor continuing processes.

A utility to filter the output of tail is [greptail].  It lazily uses the unix tail(1) program.

Also see [directory notification package in Tcl] (Linux 2.4 and higher).