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 default implementation of unknown is a library procedure defined when Tcl ini-tializes initializes
tializes an interpreter. You can override the default unknown to change its functionality. Note
that there is no default implementation of unknown in a safe interpreter.
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 inter-preter interpreter
preter 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 com-mand command
mand 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 pro-cedure procedure
cedure to load the command. If this succeeds, then it executes the original command with its origi-nal original
nal arguments. If the auto-load fails then unknown calls auto_execok to see if there is an exe-cutable executable
cutable 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 toplevel
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 executed.
SEE ALSO
info(n), proc(n), interp(n), library(n)
KEYWORDS
error, non-existent command
Tcl unknown(n)
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