unknown(n) Tcl Built-In Commands 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 default implementation
of unknown is a library procedure defined when Tcl initializes an interpreter. You can override the default unknown to change its func-
tionality. 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, 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 abbrevia-
tion 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 circum-
stances the return value from unknown is the return value from the command that was eventually executed.
SEE ALSO
info(n), proc(n), interp(n), library(n)
KEYWORDS
error, non-existent command
Tcl unknown(n)