Organize the code so it doesn't depend on globals but puts data in namespaces and code in another namespace.  Separate creation of widgets from all the other logic.  And add support for Unix lauching by embedded command.   And you get.

#!/bin/sh

# \

exec wish "$0" ${1+"$@"}

# The /bin/sh stuff is for Unix systems to so you can launch it by "> editor.tcl"

package require Tk ; # Make sure the Tk library is loaded

namespace eval ::ej {                         ; # Create a namespace for our editor

   namespace eval text000 {                   ; # Create a namespace for instance data

      set top ""                              ; # If we have widgets above .t put it here

      set rightScrollbar ${top}.y             ; # Name of right scrollbar

      set textWin ${top}.t                    ; # Name of text widget

   }

   proc getV {this name} { return [set ${this}::${name}] } ; # Get value from ::ej::text000 namespace

   proc setup {this} {                        ; # Setup widgets

      catch { console show } str                 ; # Show a console for command input

      set sb [getV $this rightScrollbar]         ; # Get name of scrollbar

      set tx [getV $this textWin]                ; # Get name of text widget

      scrollbar $sb -command "$tx yview"         ; # create a scrollbar

      text $tx -yscrollcommand "$sb set"         ; # create a text window

      pack $sb -side right -fill y               ; # put the scrollbar on right

      pack $tx -side right -fill both -expand 1  ; # create a text window

   }

   proc loadText {this fn} {                  ; # routine to load text

      if {$fn==""} return                        ; # check for bad input

      set w [getV $this textWin]                 ; # get name of text widget

      global fileName                            ; # make a global named fileName

      set fileName $fn                           ; # save file name for later

      wm title . [file tail $fn]                 ; # set title of window to filename

      set fp [open $fn]                          ; # open the file

      $w delete 1.0 end                          ; # delete any current text

      $w insert end [read $fp]                   ; # insert new text

      close $fp                                  ; # close file

   }

   proc saveText {this fn} {                 ; # routine to save text

      if {$fn==""} return                        ; # check for bad input

      set w [getV $this textWin]                 ; # get name of text widget

      set fp [open $fn w]                        ; # open file

      puts -nonewline $fp [$w get 1.0 end]       ; # write to file

      close $fp                                  ; # close file

   }

   proc main {this argv} {                   ; # startup routine

      set tx [getV $this textWin]                ; # get name of text widget

      if {[llength $argv] > 0} {                 ; # if started with filename

          loadText $tx [lindex $argv 0]          ; # load file

      }

      focus -force $tx                           ; # give text window the focus

   }

}


set name ::ej::text000


::ej::setup $name

::ej::main $name $argv