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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

unknown (1t)


unknown - existent commands


unknown cmdName ?arg arg ...?


unknown(1t)                  Tcl Built-In Commands                 unknown(1t)


       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 inter-
       preter.  You can override the default unknown to change its functional-
       ity,  or you can register a new handler for individual namespaces using
       the namespace unknown command.  Note that there is no default implemen-
       tation 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 argu-
       ments 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  proce-
       dure  with  the name cmdName, or expanding abbreviated command names to
       full-length, or automatically executing unknown  commands  as  sub-pro-
       cesses.   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 argu-
       ments.  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 addi-
       tional 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  con-
       structs.   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 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]

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

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

       error, non-existent command, unknown

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                                                unknown(1t)