sync - Linux

Overview

The sync command in Linux is used to flush the filesystem buffers by forcing changed blocks to disk and updating the superblock. This ensures that all buffered modifications to files and metadata are written to the disk, which is crucial for preventing data loss especially before system shutdown or reboot. Using sync can be particularly important in environments dealing with critical data or when using systems vulnerable to power failures.

Syntax

The basic syntax for the sync command is:

sync [OPTION]

The sync command can be used without options, which will sync all file systems.

Options/Flags

-f, --file-system: Limit the sync to file systems accessible from within the specified directory. This is particularly useful when dealing with mounted external storage or particular partitions.
--help: Display a help message and exit.
--version: Output version information and exit.

Most commonly, sync is used without any options to ensure all buffered changes to the disk are safeguarded.

Examples

Basic Syncing:
```
sync
```
This command will sync all data from buffer to disk across all mounted file systems.
Sync Specific Filesystem:
```
sync -f /mnt/external
```
Sync only the file systems accessible from within /mnt/external.

Common Issues

Performance Slowdown: Using sync may temporarily slow down system performance if there is a large amount of data to write to disk. This is normal, but frequent syncing may indicate that you need a different workflow or more powerful hardware.
Using with USB Drives: Devices like USB drives might still not be safe to remove immediately after running sync, as the device itself might cache writes. It’s generally recommended to unmount the device first.

Integration

Combine sync with other commands to ensure data integrity in scripts. For instance:

cp largefile /mnt/backup && sync && echo "Backup completed."

This line copies a large file to a backup directory, ensures the copy is flushed to disk, and then outputs that the backup is completed.

rsync: Useful for copying files. It has more features than cp, such as syncing directories across networks.
dd: Often used for making disk images; using sync after dd can ensure that the disk image is written completely.