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.Dd April 25, 2024 30.Dt SYNC 8 31.Os 32.Sh NAME 33.Nm sync 34.Nd force completion of pending disk writes (flush cache) 35.Sh SYNOPSIS 36.Nm 37.Sh DESCRIPTION 38The 39.Nm 40utility 41can be called to ensure that all disk writes have been completed before the 42processor is halted in a way not suitably done by 43.Xr reboot 8 44or 45.Xr halt 8 . 46Generally, it is preferable to use 47.Xr reboot 8 48or 49.Xr halt 8 50to shut down the system, 51as they may perform additional actions 52such as resynchronizing the hardware clock 53and flushing internal caches before performing a final 54.Nm . 55.Pp 56The 57.Nm 58utility utilizes the 59.Xr sync 2 60function call. 61.Sh SEE ALSO 62.Xr fsync 2 , 63.Xr sync 2 , 64.Xr syncer 4 , 65.Xr halt 8 , 66.Xr reboot 8 67.Sh HISTORY 68A 69.Nm 70utility appeared in 71.At v4 . 72.Pp 73On systems older than 74.Bx 4.0 , 75commands like 76.Xr reboot 8 77and 78.Xr halt 8 79were unavailable. 80The shutdown procedure involved running 81.Nm , 82waiting for the lights to stop, 83and turning off the machine. 84.Pp 85Issuing three separate 86.Nm 87commands (one line each) was a placebo that would generally suffice in 88.At v7 89machines that were otherwise quiesced systems. 90It replaced the one-per-line 91.Nm 92as a substitute for waiting. 93.Pp 94.Bx 4.0 95introduced 96.Xr reboot 2 97and 98.Xr sync 2 99which rendered this trick obsolete. 100