Provided by: tcl8.5-doc_8.5.19-4_all 
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 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(3tcl), proc(3tcl), interp(3tcl), library(3tcl), namespace(3tcl)
KEYWORDS
error, non-existent command