NAME
sync – force completion of pending disk writes (flush cache)
SYNOPSIS
sync
DESCRIPTION
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.
The sync utility utilizes the sync(2) function call.
SEE ALSO
HISTORY
A sync utility appeared in Version 4 AT&T UNIX.
On systems older than 4.0BSD, commands like reboot(8) and halt(8) were unavailable. The shutdown procedure involved running sync, waiting for the lights to stop, and turning off the machine.
Issuing three separate sync commands (one line each) was a placebo that would generally suffice in Version 7 AT&T UNIX machines that were otherwise quiesced systems. It replaced the one-per-line sync as a substitute for waiting.
4.0BSD introduced reboot(2) and sync(2) which rendered this trick obsolete.