unknown(n) Tcl Built-In Commands unknown(n)_________________________________________________________________NAMEunknown - Handle attempts to use non-existent commands
SYNOPSISunknown cmdName ?arg arg ...?
_________________________________________________________________DESCRIPTION
This command is invoked by the Tcl interpreter whenever a
script tries to invoke a command that doesn't exist. The
implementation of unknown isn't part of the Tcl core;
instead, it is a library procedure defined by default when
Tcl starts up. You can override the default unknown to
change its functionality.
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 interpreter 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 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 can't 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
Tcl 1
unknown(n) Tcl Built-In Commands unknown(n)
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 eventually executed.
KEYWORDS
error, non-existent command
Tcl 2