NAME

       unknown - Handle attempts to use non-existent commands

SYNOPSIS

       unknown cmdName ?arg arg ...?
_________________________________________________________________

DESCRIPTION

       This command is invoked by the Tcl interpreter whenever a script tries to invoke a command
       that doesn't exist.  The default implementation of unknown is a library procedure  defined
       when  Tcl  initializes an interpreter.  You can override the default unknown to change its
       functionality.  Note that there  is  no  default  implementation  of  unknown  in  a  safe
       interpreter.

       If the Tcl interpreter encounters a command name for which there is not a defined command,
       then Tcl checks for the existence of a  command  named  unknown.   If  there  is  no  such
       command, then the interpreter returns an error.  If the unknown command exists, then it is
       invoked with arguments consisting of the fully-substituted  name  and  arguments  for  the
       original  non-existent  command.  The unknown command typically does things like searching
       through library directories for a command procedure with the name  cmdName,  or  expanding
       abbreviated  command  names to full-length, or automatically executing unknown commands as
       sub-processes.  In some cases (such as expanding abbreviations) unknown  will  change  the
       original  command slightly and then (re-)execute it.  The result of the unknown command is
       used as the result for the original non-existent command.

       The default implementation of unknown behaves as follows.  It first  calls  the  auto_load
       library  procedure  to  load the command.  If this succeeds, then it executes the original
       command  with  its  original  arguments.   If  the  auto-load  fails  then  unknown  calls
       auto_execok  to see if there is an executable file by the name cmd.  If so, it invokes the
       Tcl exec command with cmd and all the args as arguments.  If cmd can't  be  auto-executed,
       unknown  checks  to see if the command was invoked at top-level and outside of any script.
       If so, then unknown takes two additional steps.  First, it sees if  cmd  has  one  of  the
       following  three  forms:  !!,  !event,  or  ^old^new?^?.   If so, then unknown carries out
       history substitution in the same way  that  csh  would  for  these  constructs.   Finally,
       unknown checks to see if cmd is a unique abbreviation for an existing Tcl command.  If so,
       it expands the command name and executes the command with the original arguments.  If none
       of  the  above  efforts  has  been able to execute the command, unknown generates an error
       return.  If the global variable  auto_noload  is  defined,  then  the  auto-load  step  is
       skipped.   If  the  global  variable  auto_noexec  is  defined  then the auto-exec step is
       skipped.  Under normal circumstances the return value from unknown  is  the  return  value
       from the command that was eventually executed.

EXAMPLE

       Arrange for the unknown command to have its standard behavior except for first logging the
       fact that a command was not found:

              # Save the original one so we can chain to it
              rename unknown _original_unknown

              # Provide our own implementation
              proc unknown args {
                  puts stderr "WARNING: unknown command: $args"
                  uplevel 1 [list _original_unknown {expand}$args]
              }

SEE ALSO

       info(3tcl), proc(3tcl), interp(3tcl), library(3tcl)

KEYWORDS

       error, non-existent command