source: downloads/tcl8.5.2/doc/unknown.n @ 35

'\"
'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\" RCS: @(#) $Id: unknown.n,v 1.9 2007/12/13 15:22:33 dgp Exp $
'\" 
.so man.macros
.TH unknown n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
unknown \- Handle attempts to use non-existent commands
.SH SYNOPSIS
\fBunknown \fIcmdName \fR?\fIarg arg ...\fR?
.BE
.SH DESCRIPTION
.PP
This command is invoked by the Tcl interpreter whenever a script
tries to invoke a command that does not exist.  The default implementation
of \fBunknown\fR is a library procedure defined when Tcl initializes an
interpreter.  You can override the default \fBunknown\fR to change its
functionality, or you can register a new handler for individual namespaces
using the \fBnamespace unknown\fR command.  Note that there is no default 
implementation of \fBunknown\fR in a safe interpreter.
.PP
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 \fB::unknown\fR.  If there is no such 
command, then the interpreter returns an error.
If the \fBunknown\fR 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 \fBunknown\fR command typically does things like searching
through library directories for a command procedure with the name
\fIcmdName\fR, or expanding abbreviated command names to full-length,
or automatically executing unknown commands as sub-processes.
In some cases (such as expanding abbreviations) \fBunknown\fR will
change the original command slightly and then (re-)execute it.
The result of the \fBunknown\fR command is used as the result for
the original non-existent command.
.PP
The default implementation of \fBunknown\fR behaves as follows.
It first calls the \fBauto_load\fR 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 \fBunknown\fR calls \fBauto_execok\fR
to see if there is an executable file by the name \fIcmd\fR.
If so, it invokes the Tcl \fBexec\fR command
with \fIcmd\fR and all the \fIargs\fR as arguments.
If \fIcmd\fR cannot be auto-executed, \fBunknown\fR checks to
see if the command was invoked at top-level and outside of any
script.  If so, then \fBunknown\fR takes two additional steps.
First, it sees if \fIcmd\fR has one of the following three forms:
\fB!!\fR, \fB!\fIevent\fR, or \fB^\fIold\fB^\fInew\fR?\fB^\fR?.
If so, then \fBunknown\fR carries out history substitution
in the same way that \fBcsh\fR would for these constructs.
Finally, \fBunknown\fR checks to see if \fIcmd\fR 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, \fBunknown\fR generates an error return.
If the global variable \fBauto_noload\fR is defined, then the auto-load
step is skipped.
If the global variable \fBauto_noexec\fR is defined then the
auto-exec step is skipped.
Under normal circumstances the return value from \fBunknown\fR
is the return value from the command that was eventually
executed.
.SH EXAMPLE
Arrange for the \fBunknown\fR command to have its standard behavior
except for first logging the fact that a command was not found:
.PP
.CS
# Save the original one so we can chain to it
rename \fBunknown\fR _original_unknown

# Provide our own implementation
proc \fBunknown\fR args {
    puts stderr "WARNING: unknown command: $args"
    uplevel 1 [list _original_unknown {*}$args]
}
.CE
.SH "SEE ALSO"
info(n), proc(n), interp(n), library(n), namespace(n)
.SH KEYWORDS
error, non-existent command