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markom |
'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" RCS: @(#) $Id: unknown.n,v 1.1.1.1 2002-01-16 10:25:25 markom Exp $
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'\"
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.so man.macros
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.TH unknown n "" Tcl "Tcl Built-In Commands"
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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unknown \- Handle attempts to use non-existent commands
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.SH SYNOPSIS
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\fBunknown \fIcmdName \fR?\fIarg arg ...\fR?
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.BE
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.SH DESCRIPTION
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.PP
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This command is invoked by the Tcl interpreter whenever a script
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tries to invoke a command that doesn't exist. The implementation
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of \fBunknown\fR isn't part of the Tcl core; instead, it is a
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library procedure defined by default when Tcl starts up. You
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can override the default \fBunknown\fR to change its functionality.
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.PP
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If the Tcl interpreter encounters a command name for which there
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is not a defined command, then Tcl checks for the existence of
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a command named \fBunknown\fR.
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If there is no such command, then the interpreter returns an
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error.
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If the \fBunknown\fR command exists, then it is invoked with
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arguments consisting of the fully-substituted name and arguments
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for the original non-existent command.
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The \fBunknown\fR command typically does things like searching
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through library directories for a command procedure with the name
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\fIcmdName\fR, or expanding abbreviated command names to full-length,
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or automatically executing unknown commands as sub-processes.
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In some cases (such as expanding abbreviations) \fBunknown\fR will
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change the original command slightly and then (re-)execute it.
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The result of the \fBunknown\fR command is used as the result for
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the original non-existent command.
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.PP
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The default implementation of \fBunknown\fR behaves as follows.
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It first calls the \fBauto_load\fR library procedure to load the command.
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If this succeeds, then it executes the original command with its
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original arguments.
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If the auto-load fails then \fBunknown\fR calls \fBauto_execok\fR
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to see if there is an executable file by the name \fIcmd\fR.
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If so, it invokes the Tcl \fBexec\fR command
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with \fIcmd\fR and all the \fIargs\fR as arguments.
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If \fIcmd\fR can't be auto-executed, \fBunknown\fR checks to
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see if the command was invoked at top-level and outside of any
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script. If so, then \fBunknown\fR takes two additional steps.
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First, it sees if \fIcmd\fR has one of the following three forms:
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\fB!!\fR, \fB!\fIevent\fR, or \fB^\fIold\fB^\fInew\fR?\fB^\fR?.
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If so, then \fBunknown\fR carries out history substitution
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in the same way that \fBcsh\fR would for these constructs.
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Finally, \fBunknown\fR checks to see if \fIcmd\fR is
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a unique abbreviation for an existing Tcl command.
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If so, it expands the command name and executes the command with
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the original arguments.
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If none of the above efforts has been able to execute
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the command, \fBunknown\fR generates an error return.
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If the global variable \fBauto_noload\fR is defined, then the auto-load
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step is skipped.
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If the global variable \fBauto_noexec\fR is defined then the
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auto-exec step is skipped.
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Under normal circumstances the return value from \fBunknown\fR
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is the return value from the command that was eventually
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executed.
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.SH KEYWORDS
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error, non-existent command
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