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unknown(n)	      Tcl Built-In Commands	       unknown(n)

_________________________________________________________________

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
       implementation  of  unknown  isn't  part	 of the Tcl core;
       instead, it is a library procedure defined by default when
       Tcl  starts  up.	  You can override the default unknown to
       change its functionality.

       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

Tcl								1

unknown(n)	      Tcl Built-In Commands	       unknown(n)

       skipped.	  If  the  global variable auto_noexec is defined
       then the auto-exec step is skipped.  Under normal  circum-
       stances	the return value from unknown is the return value
       from the command that was eventually executed.

KEYWORDS
       error, non-existent command

Tcl								2

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