copyfile ?-bytes num|-maxbytes num? fromFileId toFileId
              Copies  the  rest  of  the  file specified by from-
              FileId, starting from its current position, to  the
              file  specified by toFileId, starting from its cur-
              rent position.

              If -bytes is specified, then num bytes are  copied.
              If  less  than num bytes are available, an error is
              returned.  If  -maxbytes  is  specified,  then  num
              bytes  are  copied but no error is returned if less
              are available.

              The command returns the number of bytes  that  were
              copied.

              The -bytes option is particularly useful for mixing
              binary data in with ASCII commands  or  data  in  a
              data stream.

              This command is provided by Extended Tcl.