Starting with version 8.3.4 Tk can be loaded dynamically from a tclsh and automatically enters the event loop. The plain Tk8.3.4 is still missing a pkgIndex.tcl file, so for now one has to load tk by hand:
load libtk8.3.so
This works for Linux. Please add the correct commands for other platforms.
Once Tk does ship with a propper pkgIndex.tcl, it will be loadable platfrom independently with:
[package require Tk]
The possibility to load Tk on the fly opens up some interesting possibilities. For example one can start a tclsh outide of an X session and later load Tk into it to connect to an X server:
# start a tclsh that doesn't know anything about X
$ DISPLAY="" tclsh
# prove that the display really isn't availible
% package require Tk
couldn't connect to display ""
# now tell it about the display and try loading Tk again.
% set argv {-display :0}
-display :0
% package require Tk
8.3
# A window pops up - we have succeeded.By using subinterpreters, it is even possible to have Tk sessions to different displays at the same time while having no Tk at all in the master interpreter: