sync —
force completion of pending disk writes (flush cache)
The sync program can be called to ensure that all disk
  writes have been completed before the processor is halted in a way not
  suitably done by reboot(8) or
  halt(8). Generally, it is
  preferable to use reboot(8) or
  halt(8) to shut down the system,
  as they may perform additional actions such as resynchronizing the hardware
  clock and flushing internal caches before performing a final
  sync.
The sync program simply invokes the
    sync(2) system call.
A sync command appeared in
  Version 4 AT&T UNIX.