unknown

unknown(n)                   Tcl Built-In Commands                  unknown(n)



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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
       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(n), proc(n), interp(n), library(n), namespace(n)

KEYWORDS
       error, non-existent command, unknown



Tcl                                                                 unknown(n)