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 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 autoload 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 command. 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