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

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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 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  inter-
       preter.

       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  com-
       mand,  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_exe-
       cok 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 substi-
       tution 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 circum-
       stances the return value from unknown is the return value from the command that was  even-
       tually executed.

SEE ALSO
       info(n), proc(n), interp(n), library(n)

KEYWORDS
       error, non-existent command

Tcl										       unknown(n)
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