1.\"- 2.\" Copyright (c) 1980, 1991, 1993 3.\" The Regents of the University of California. All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 3. Neither the name of the University nor the names of its contributors 14.\" may be used to endorse or promote products derived from this software 15.\" without specific prior written permission. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 21.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.\" @(#)sync.8 8.1 (Berkeley) 5/31/93 30.\" 31.Dd April 25, 2024 32.Dt SYNC 8 33.Os 34.Sh NAME 35.Nm sync 36.Nd force completion of pending disk writes (flush cache) 37.Sh SYNOPSIS 38.Nm 39.Sh DESCRIPTION 40The 41.Nm 42utility 43can be called to ensure that all disk writes have been completed before the 44processor is halted in a way not suitably done by 45.Xr reboot 8 46or 47.Xr halt 8 . 48Generally, it is preferable to use 49.Xr reboot 8 50or 51.Xr halt 8 52to shut down the system, 53as they may perform additional actions 54such as resynchronizing the hardware clock 55and flushing internal caches before performing a final 56.Nm . 57.Pp 58The 59.Nm 60utility utilizes the 61.Xr sync 2 62function call. 63.Sh SEE ALSO 64.Xr fsync 2 , 65.Xr sync 2 , 66.Xr syncer 4 , 67.Xr halt 8 , 68.Xr reboot 8 69.Sh HISTORY 70A 71.Nm 72utility appeared in 73.At v4 . 74.Pp 75On systems older than 76.Bx 4.0 , 77commands like 78.Xr reboot 8 79and 80.Xr halt 8 81were unavailable. 82The shutdown procedure involved running 83.Nm , 84waiting for the lights to stop, 85and turning off the machine. 86.Pp 87Issuing three separate 88.Nm 89commands (one line each) was a placebo that would generally suffice in 90.At v7 91machines that were otherwise quiesced systems. 92It replaced the one-per-line 93.Nm 94as a substitute for waiting. 95.Pp 96.Bx 4.0 97introduced 98.Xr reboot 2 99and 100.Xr sync 2 101which rendered this trick obsolete. 102