NAME

       unknown - Handle attempts to use non-existent commands

SYNOPSIS

       unknown cmdName ?arg arg ...?
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DESCRIPTION

       This command is invoked by the Tcl interpreter whenever a script tries to invoke a command
       that does not 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, or you can register a new  handler  for  individual  namespaces  using  the
       namespace  unknown  command.  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
       (in  either  the  current  namespace,  or  the  global namespace), then Tcl checks for the
       existence of an unknown handler for the current namespace. By default, this handler  is  a
       command  named  ::unknown.   If  there is no such command, then the interpreter returns an
       error.  If the unknown command exists (or a  new  handler  has  been  registered  for  the
       current  namespace), 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 cannot 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: $args"
                  uplevel 1 [list _original_unknown {*}$args]
              }

SEE ALSO

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

KEYWORDS

       error, non-existent command, unknown