tcl7.6 User Commands - unknown
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 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 typ-
ically does things like searching through library direc-
tories 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 execut-
able 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 sub-
stitution in the same way that csh would for these con-
structs. 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.
Many extensions redefine the unknown command to include
their own special command processing. If more than one
extension tries to do this, however, the results conflict.
The standard unknown command is now set up to handle this
correctly. Each extension can register its own command
handler using the following command:
unknown_handler name command ?arg arg ...?
Here, name is a symbolic name identifying the handler. The
command string and any extra arg arguments are integrated
into the unknown command.
Each time an unknown command is encountered, the list of
handlers is consulted to determine how to handle the com-
mand. If a handler recognizes the command, then it should
load, define, or otherwise handle the command and return.
Otherwise, it should return "-code continue", which tells
the unknown facility to continue with another handler.
Handlers registered first are consulted last. This way,
extensions that are loaded later have higher priority for
handling special cases.
Usually command is the name of a handling procedure. But
extra arg arguments can be specified when the handler is
registered, and these will be kept as part of the handler.
The actual unknown command and its arguments are appended
after these when the handler is invoked.
KEYWORDS
error, non-existent command