'\" '\" 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. '\" '\" RCS: @(#) $Id: unknown.n,v 1.2 2001/09/14 01:42:29 zlaski 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 doesn't exist. The implementation of \fBunknown\fR 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 \fBunknown\fR to change its functionality. .PP 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 \fBunknown\fR. If there is no such command, then the interpreter returns an error. If the \fBunknown\fR command exists, 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 can't 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 KEYWORDS error, non-existent command