sync ?fileId?
              If  fileId  is  not specified, or if it is and this
              system does not  support  the  fsync  system  call,
              issues a sync system call to flush all pending disk
              output.  If fileId is specified and the system does
              support  the  fsync system call, issues an fsync on
              the file corresponding to the specified Tcl  fileId
              to force all pending output to that file out to the
              disk.

              If fileId is specified, the file must be  writable.
              A  flush  will  be issued against the fileId before
              the sync.

              The infox have_fsync command can be used to  deter-
              mine if "sync fileId" will do a sync or a fsync.

              This command is provided by Extended Tcl.