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 interpreter.

       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
       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: $args"
                  uplevel 1 [list _original_unknown {expand}$args]
              }

SEE ALSO

       info(3tcl), proc(3tcl), interp(3tcl), library(3tcl)

KEYWORDS

       error, non-existent command

Tcl                                                                                                unknown(3tcl)