unknown(3tcl)						       Tcl Built-In Commands						     unknown(3tcl)

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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 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  func-
       tionality,  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  proce-
       dure  with the name cmdName, or expanding abbreviated command names to full-length, or automatically executing unknown commands as sub-pro-
       cesses.	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 nor-
       mal 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

Tcl																     unknown(3tcl)