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.