xref: /freebsd/bin/sync/sync.8 (revision 9bc300465e48e19d794d88d0c158a2adb92c7197)
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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