When you start ''tclsh'' or ''wish'' (on Unix and perhaps a few other platforms) 
'''without''' a filename, you will get a prompt so you can interact with Tcl. (For ''wish'' on Windows, use ''console show'' instead). If you start them '''with''' a filename, that script is executed, but the interactive prompt does not appear.

What if you want both? [Donald Porter] presented this solution on [the comp.lang.tcl newsgroup]:

 set cmd ""
 while 1 {
    if { "$cmd" != "" } {
            # Why "format catch"?  It "is a legacy workaround
            #     for an old bytcode compiler bug."  We should
            #     find out when it's fixed, and "package require
            #     Tcl ..." appropriately.
        catch {[format catch] $::tcl_prompt2}
    } else {
        if {[catch {[format catch] $::tcl_prompt1}]} {
          puts -nonewline "% "
        }
    }
    flush stdout
    append cmd \n[gets stdin]
    if [ info complete $cmd ] {
        catch $cmd res
        puts $res
        set cmd ""
    }
 }

Other improvements that could still be made:
   * avoid using global variables cmd and res
   * properly handle scripts that return codes other than 0 (TCL_OK) or 1 (TCL_ERROR) (for example, [[break]])
----
Note that the code above is best understood as an improvement on
the implementation developed in this Usenet thread:
http://groups.google.com/groups?hl=en&th=79c056a40deb476d&rnum=4

Given the original problem description, the right solution in
wish is [[console show]].  With a little hacking you can
have a [console for Unix], or better yet, make use of
TkCon [http://www.purl.org/net/hobbs/tcl/script/tkcon/].

For tclsh only, no console widget is available, so then I 
would go ahead and use [[vwait forever]] to get an event loop
going and use [[fileevent stdin readable]] to collect and
evaluate interactive commands from stdin rather than the
[[while]] loop above.

'''DGP'''

There is similar code at [Integrating Tcl and Emacs on Windows] for a
more specific purpose.

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[Arts and crafts of Tcl-Tk programming]