[Tclx] documents commandloop as:


     commandloop ?-async? ?-interactive on | off

          Create an interactive command loop reading commands from
          stdin and writing results to stdout. Command loops are maybe
          either be blocking or event oriented. This command is useful
          for Tcl scripts that do not normally converse interactively
          with a user through a Tcl command interpreter, but which
          sometimes want to enter this mode, perhaps for debugging or
          user configuration. The command loop terminates on EOF.

          The following options are available:

          -async
               A command handler will be associated with stdin. When
               input is available on stdin, it will be read and
               accumulated until a full command is available. That
               command will then be evaluated. An event loop must be
               entered for input to be read and processed.

          -interactive on | off | tty
               Enable or disable interactive command mode. In
               interactive mode, commands are prompted for and the
               results of comments are printed. The value maybe any
               boolean value or tty. If tty is used, interactive mode
               is enabled if stdin is associated with a terminal or
               terminal emulator. The default is tty.

          -prompt1 cmd
               If specified, cmd is used is evaluate and its result
               used for the main command prompt. If not specified, the
               command in tcl_prompt1 is evaluated to output the prompt.
               Note the difference in behavior, cmd results is used,
               while tcl_prompt1 outputs. This is to allow for future
               expansion to command loops that write to other than
               stdout.

          -prompt2 cmd
               If specified, cmd is used is evaluate and its result used
               for the secondary (continuation) command prompt. If not
               specified, the command in tcl_prompt2 is evaluated to
               output the prompt.

          -endcommand cmd
               If specified, cmd is evaluated when the command loop
               terminates.

               In interactive mode, the results of set commands with two
               arguments are not printed.

               If SIGINT is configured to generate a Tcl error, it can
               be used to delete the current command being type without
               aborting the program in progress.
----
[KBK] 2004-05-18 - Something very similar can be done in pure Tcl; the signal handling and 'isatty' checking is Unix and hence unportable, but the following code works for many purposes:


 namespace eval ::mainloop {
     variable partialCommand {}
 }
 
 proc ::mainloop::prompt {} {
     variable partialCommand
 
     if { [info complete $partialCommand] } {
         set status [catch {
             uplevel \#0 $partialCommand
         } result]
         if { $result ne {} } {
             if { $status != 0 } {
                 puts stderr $result
             } else {
                 puts stdout $result
             }
         }
         set partialCommand {}
         if { [info exists ::tcl_prompt1] } {
             catch { uplevel \#0 $::tcl_prompt1 } prompt
         } else {
             set prompt "% "
         }
     } else {
         append partialCommand \n
         if { [info exists ::tcl_prompt2] } {
             catch { uplevel \#0 $::tcl_prompt2 } prompt
         } else {
             set prompt "> "
         }
     }
     puts -nonewline stderr $prompt
 
     return
 }
 
 proc ::mainloop::readable {} {
     variable partialCommand
     variable eof
 
     if { [gets stdin text] < 0 } {
         fileevent stdin readable {}
         set eof 1
     } else {
         append partialCommand $text
         prompt
     }
     return
 }
 
 proc ::mainloop::mainloop {} {
 
     variable eof
 
     set ::tcl_interactive 1
     info script ""
 
     fconfigure stdin -buffering line -blocking 0
     fileevent stdin readable ::mainloop::readable
     
     ::mainloop::prompt
 
     vwait [namespace which -variable eof]
 
     return
 }
 
 ::mainloop::mainloop
----

[JH] wrote the following code that you tack on to the end of a script to get the near equivalence of the tclsh prompt.  For those who really want an interactive loop and are doing UI apps, look into the [console] megawidget or [tkcon], both of which can be embedded into apps and provide many more features and polish.
 
 if {!$tcl_interactive} {
     set long_command ""
     set verbose_history 0
     if {![catch {rename unknown tcl_unknown}]} {
 	proc unknown {cmdname args} {
 	    if {[regexp "^!..*" $cmdname]} {
 		banghist [string range $cmdname 1 end]
 	    } else {
 		tcl_unknown $cmdname $args
 	    }
 	}
     }
 
     proc banghist {val} {
 	global verbose_history
 	if {![string compare $val "!"]} {set val ""}
 	if {$verbose_history} {puts "[history event $val]"}
 	history redo $val
     }
 
     if {![info exists tcl_prompt1]} {
 	set tcl_prompt1 {puts -nonewline "tclsh ([history nextid]) % "}
     }
     proc read_stdin {} {
 	global eventLoop tcl_prompt1 long_command
 	set l [gets stdin]
 	if {[eof stdin]} {
 	    set eventLoop "done"     ;# terminate the vwait eventloop
 	} else {
 	    if {[string compare $l {}]} {
 		append long_command "\n$l"
 		set l $long_command
 		if {[info complete $l]} {
 		    if {[catch {uplevel \#0 history add [list $l] exec} err]} {
 			puts stderr $err
 		    } elseif {[string compare $err {}]} {
 			puts $err
 		    }
 		    set long_command ""
 		    catch $tcl_prompt1
 		} else {
 		    puts -nonewline "> "
 		}
 	    } elseif {![string compare $long_command {}]} {
 		catch $tcl_prompt1
 	    } else {
 		puts -nonewline "> "
 	    }
 	    flush stdout
 	}
     }
 
     # set up our keyboard read event handler:
     # Vector stdin data to the socket
     fileevent stdin readable read_stdin
 
     catch $tcl_prompt1
     flush stdout
     # wait for and handle or stdin events...
     vwait eventLoop
 }

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----
[Category Command], a part of [Tclx]