xref: /freebsd/bin/sync/sync.8 (revision 177ba183076bd6fe6b29b1d657b954cca256506b)

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.Dd April 25, 2024
.Dt SYNC 8
.Os
.Sh NAME
.Nm sync
.Nd force completion of pending disk writes (flush cache)
.Sh SYNOPSIS
.Nm
.Sh DESCRIPTION
The
.Nm
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
.Xr reboot 8
or
.Xr halt 8 .
Generally, it is preferable to use
.Xr reboot 8
or
.Xr 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
.Nm .
.Pp
The
.Nm
utility utilizes the
.Xr sync 2
function call.
.Sh SEE ALSO
.Xr fsync 2 ,
.Xr sync 2 ,
.Xr syncer 4 ,
.Xr halt 8 ,
.Xr reboot 8
.Sh HISTORY
A
.Nm
utility appeared in
.At v4 .
.Pp
On systems older than
.Bx 4.0 ,
commands like
.Xr reboot 8
and
.Xr halt 8
were unavailable.
The shutdown procedure involved running
.Nm ,
waiting for the lights to stop,
and turning off the machine.
.Pp
Issuing three separate
.Nm
commands (one line each) was a placebo that would generally suffice in
.At v7
machines that were otherwise quiesced systems.
It replaced the one-per-line
.Nm
as a substitute for waiting.
.Pp
.Bx 4.0
introduced
.Xr reboot 2
and
.Xr sync 2
which rendered this trick obsolete.