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SYNC(8) System Manager's Manual SYNC(8)

sync
force completion of pending disk writes (flush cache)

sync

The sync utility 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.
sync utilizes the sync(2) function.

fsync(2), sync(2), halt(8), reboot(8)

A sync command appeared in Version 4 AT&T UNIX.
August 16, 2016 OpenBSD-6.1