Added KB about the 'sync' command

knowledge base/sync.md

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+# Sync
+
+```sh
+# Flush all cached file data of the current user only.
+sync
+
+# Flush all pending write operations on all disks and mounted filesystems.
+sudo sync
+
+# Flush all pending write operations on given files only to disk.
+sync path/to/first/file path/to/second/file
+
+# Flush all pending write operations on all files in a directory, recursively.
+sync path/to/directory
+
+# Flush only the file data and its minimal metadata to disk.
+sync -d path/to/file
+
+# Flush all pending write operations on only the filesystem on mounted partition
+# '/dev/sdc1'.
+sudo sync /dev/sdc1
+
+# Flush all pending write operations on all mounted filesystem from '/dev/sdb'.
+sudo sync /dev/sdb
+
+# Flush all pensing write operations on the entire file system which contains
+# '/var/log/syslog'.
+sudo sync -f /var/log/syslog
+```
+
+## Sources
+
+- [cheat.sh]
+- [Linux sync command]
+
+[cheat.sh]: https://cheat.sh/sync
+[linux sync command]: https://www.computerhope.com/unix/sync.htm