DESCRIPTION
 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.