       dup fileId ?targetFileId?
              Duplicate  an  open  file.  A new file id is opened
              that addresses the same file as fileId.

              If targetFileId is specified, the the file  is  dup
              to this specified file id.  Normally this is stdin,
              stdout, or stderr.  The  dup  command  will  handle
              flushing  output and closing this file.  The target
              file should be open if its one of stdin, stdout, or
              stderr and the process is not going to do an execl.
              Otherwise internal C code that uses  one  of  these
              files  via  direct  access to stdio FILE struct may
              behave strangely or fail.

              This command is provided by Extended Tcl.
