unknown(n) Tcl Built-In Commands unknown(n) 







unknown - Handle attempts to use non-existent commands








unknown cmdName ?arg arg ...?










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.








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]
}










info(n), proc(n), interp(n), library(n), namespace(n)








error, non-existent command







  

    
    Tcl