unknown(n) Tcl Built-In Commands unknown(n) ______________________________________________________________________________
NAME
unknown - Handle attempts to use non-existent commands
SYNOPSIS
unknown cmdName ?arg arg ...?
______________________________________________________________________________
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 existence of an unknown handler for
the current namespace. By default, this handler is a command 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 namespace), 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 and Tcl is run interactively then
unknown calls auto_execok 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 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.
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, unknown
Tcl unknown(n)
tcl 8.6.15 - Generated Wed Dec 4 11:01:32 CST 2024