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Tcl Built-In Commands                                 unknown(1T)



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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 over-
     ride 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 typ-
     ically  does  things  like  searching through library direc-
     tories 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 execut-
     able  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 sub-
     stitution in the same way that  csh  would  for  these  con-
     structs.   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



Tcl                       Last change:                          1






Tcl Built-In Commands                                 unknown(1T)



     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(1T), proc(1T), interp(1T), library(1T)


KEYWORDS
     error, non-existent command

ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:

     _______________________________________
    |   ATTRIBUTE TYPE   |  ATTRIBUTE VALUE|
    |____________________|__________________|_
    | Availability       |  runtime/tcl-8  |
    |____________________|__________________|_
    | Interface Stability|  Uncommitted    |
    |____________________|_________________|

NOTES
     Source for Tcl is available on http://opensolaris.org.












Tcl                       Last change:                          2