sync
writes any data buffered in memory out to disk.
This can include (but is not limited to) modified superblocks,
modified inodes, and delayed reads and writes.
This must be implemented by the kernel;
The
sync
program does nothing but exercise the
sync(2)
system call.
The kernel keeps data in memory to avoid doing (relatively slow) disk
reads and writes.
This improves performance, but if the computer
crashes, data may be lost or the file system corrupted as a result.
sync
ensures that everything in memory is written to disk.
sync
should be called before the processor is halted in an unusual manner
(e.g., before causing a kernel panic when debugging new kernel code).
In general, the processor should be halted using the
shutdown(8)
or
reboot(8)
or
halt(8)
commands, which will attempt to put the system in a quiescent state
before calling
sync(2).
(Various implementations of these commands exist; consult your
documentation; on some systems one should not call
reboot(8)
and
halt(8)
directly.)
OPTIONS
--help
Print a usage message on standard output and exit successfully.
--version
Print version information on standard output, then exit successfully.
--
Terminate option list.
ENVIRONMENT
The variables LANG, LC_ALL, LC_CTYPE,
and LC_MESSAGES have the usual meaning.
CONFORMING TO
POSIX.2.
NOTES
On Linux,
sync
is only guaranteed to schedule the dirty blocks for writing; it can
actually take a short time before all the blocks are finally written.
The
reboot(8)
and
halt(8)
commands take this into account by sleeping for a few seconds after
calling
sync(2).
This page describes
sync
as found in the fileutils-4.0 package;
other versions may differ slightly.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
