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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 does not exist. The default implementation of unknown is a library procedure defined
when Tcl initializes an interpreter. You can override the default unknown to change its
functionality, or you can register a new handler for individual namespaces using the
namespace unknown command. 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
(in either the current namespace, or the global namespace), then Tcl checks for the exis-
tence of an unknown handler for the current namespace. By default, this handler is a com-
mand named ::unknown. If there is no such command, then the interpreter returns an error.
If the unknown command exists (or a new handler has been registered for the current names-
pace), 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 original arguments. If the auto-load fails then unknown calls auto_exe-
cok 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 cannot 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 substi-
tution 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 circum-
stances the return value from unknown is the return value from the command that was even-
tually executed.
EXAMPLE
Arrange for the unknown command to have its standard behavior except for first logging the
fact that a command was not found:
# Save the original one so we can chain to it
rename unknown _original_unknown
# Provide our own implementation
proc unknown args {
puts stderr "WARNING: unknown command: $args"
uplevel 1 [list _original_unknown {*}$args]
}
SEE ALSO
info(n), proc(n), interp(n), library(n), namespace(n)
KEYWORDS
error, non-existent command
Tcl unknown(n)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/2.4.6 (CentOS)
Under GNU General Public License
2025-06-23 22:12 @127.0.0.1 CrawledBy Wget/1.21.2