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 and Tcl is run interactively 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

Tcl                                                                                                unknown(3tcl)