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).

And a more generic approach is outlined on the [file and directory change notifications] page.

Here is another version. It continuously opens and closes the file and seems to work where a normal "tail -f" fails. This is the case at my internet provider's ssh-account (Linux infong159 2.4.24-grsec-20040109a). Issuing a "tail -f" on the shell there will produce no output when I access the homepage, but using this little proc, I get instant response (well, within a second at least). I don't think this is a logfile-rotation issue, though:
 proc tail-f {filename} {
    # get an initial last position:
    set fh [open $filename r]
    seek $fh 0 end
    set pos [tell $fh]
    close $fh
    # ask again all second:
    while 1 {
         after 1000
         set fh [open $filename r]
         fconfigure $fh -blocking no -buffering line
         seek $fh $pos start
         while {[eof $fh] == 0} {
             gets $fh line
             if {[string length $line] > 0} {puts $line}
        }
        set pos [tell $fh]
        close $fh
    }
 }