unknown - Handle attempts to use non-existent commands
unknown cmdName ?arg arg ...?
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
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
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
error, non-existent command
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.
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