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 default implementation of
unknown is
a library procedure defined when Tcl initializes an interpreter. You can
override the default
unknown to change its functionality. 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 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.
EXAMPLE¶
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: $args"
uplevel 1 [list _original_unknown {expand}$args]
}
SEE ALSO¶
info(3tcl), proc(3tcl), interp(3tcl), library(3tcl)
KEYWORDS¶
error, non-existent command