sync

Overview

sync flushes filesystem buffers: dirty page cache and pending metadata are written to storage. Use it before removing USB media, after bulk dd/cp to removable devices, or when you need a durable checkpoint before power loss. Modern kernels and umount already sync as part of a clean unmount; calling sync explicitly is still a useful operator habit for removable media and scripts.

Syntax

sync [options] [file...]

With no arguments, flushes all filesystems (global sync). With file arguments (GNU), flushes those files and the filesystems that hold them.

Common Options

Option	Description
(none)	Sync all (classic usage)
FILE...	Sync data for listed files (GNU coreutils)
-d, --data	Sync data only, not metadata (when supported)
-f, --file-system	Sync the filesystems that contain the files

BusyBox/sync on minimal systems may only support the no-argument form.

Safety

• sync is not a substitute for unmounting. Always umount (or eject) removable volumes when possible so the kernel finishes metadata updates cleanly.
• sync does not make a crash-consistent snapshot of multi-file databases by itself — use application flush/quiesce or LVM/ZFS snapshots for that class of problem.
• Calling sync under heavy write load can stall the system briefly while I/O drains — expected, not a hang in most cases.

Examples with Explanations

Flush everything (classic)

sync

Blocks until outstanding dirty data is queued to devices (and typically completed for the global case). Common after imaging USBs or large copies.

After writing a USB with dd

sudo dd if=image.iso of=/dev/sdX bs=4M status=progress conv=fsync
sync
# safely remove when activity LED is idle

conv=fsync already fsyncs the output; an extra sync is cheap insurance before unplug.

Sync specific file (GNU)

cp bigfile /media/usb/bigfile
sync /media/usb/bigfile
# or
sync -f /media/usb/bigfile

Narrows flush to the file/filesystem of interest instead of the whole machine.

Safe removable-media workflow

cp -a project/ /media/$USER/STICK/
sync
sudo umount /media/$USER/STICK
# or: udisksctl unmount -b /dev/sdX1

Copy → sync → unmount is the reliable order for thumb drives.

In scripts before reboot/power events

install -m 644 app.conf /etc/myapp/app.conf
sync
sudo systemctl restart myapp

Ensures config hit disk before a service that might race with a crash/reboot path.

Drop caches (admin debug — not for production routine)

sync
echo 3 | sudo tee /proc/sys/vm/drop_caches

sync first so drop_caches does not discard dirty data. Used for I/O benchmarks; not a general cleanup step.

Notes

• Journaling filesystems reduce risk of total corruption, but applications can still lose unflushed writes on power loss without fsync/fdatasync.
• dd … conv=fsync and cp with certain flags interact with durability differently than a bare write + delayed writeback.
• Desktop “eject” / file-manager safe-remove typically unmounts (which syncs); CLI operators should mirror that with umount.
• Network filesystems: sync semantics depend on the protocol and mount options (nfs, cifs); local flush may not equal server durability.

Related Commands

• dd — block copy; pair with sync/fsync
• umount / findmnt — clean detach
• mount -o sync — mount with synchronous I/O (slow)
• fsync (syscall) / language APIs — per-fd durability
• hdparm / device caches — hardware write-cache caveats

Additional Resources

• man sync
• man 2 fsync