unknown(3tcl) Tcl Built-In Commands unknown(3tcl)
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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 inter-
preter. You can override the default unknown to change its functional-
ity, or you can register a new handler for individual namespaces using
the namespace unknown command. Note that there is no default implemen-
tation 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 name-
space), then Tcl checks for the existence of an unknown handler for the
current namespace. By default, this handler is a command named ::un-
known. If there is no such command, then the interpreter returns an
error. If the unknown command exists (or a new handler has been regis-
tered 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 origi-
nal command slightly and then (re-)execute it. The result of the un-
known command is used as the result for the original non-existent com-
mand.
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 argu-
ments. 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 addi-
tional 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 con-
structs. Finally, unknown checks to see if cmd is a unique abbrevia-
tion 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, unknown
Tcl unknown(3tcl)
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