unknown(3tcl)

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

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