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.

----
'''Simple example: from stdio to variable'''

Instead, this is simple example may be useful for newbies (like myself, GV).
This reads multiple lines from the standard input (stdin) and keeps 
all the lines in a variable. Last input line must be a dot (".").
======
  set n 0
  while {$n<3000} {
    gets stdin row
    puts $row
    if { [string equal $row "."] } { break };
    append message $row "\n"
  }
  puts "---------"
  puts $message
======

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