unknown(n) Tcl unknown(n)
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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 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
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unknown(n) Tcl unknown(n)
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.
KEYWORDS
error, non-existent command
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