redhat man page for 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 doesn't exist.  The implementation of
       unknown isn't part of the Tcl core;  instead, it is a library procedure defined by default when	Tcl  starts  up.   You	can  override  the
       default unknown to change its functionality.

       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  abbrevia-
       tion 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 circum-
       stances the return value from unknown is the return value from the command that was eventually executed.

SEE ALSO
       info(n), proc(n)

KEYWORDS
       error, non-existent command

Tcl																	unknown(n)