To preserve hard links when using rsync to copy a directory tree, you can use the --link-dest option and specify the path to a directory that contains a previous version of the tree. rsync will then create hard links between files that are identical in both the source and destination directories, rather than copying the files themselves.
For example, let's say you want to use rsync to create a backup of the /etc directory on your system. You can create a directory to store the backup, such as /backup/etc, and use the --link-dest option to create hard links between the files in the /backup/etc directory and the previous version of the directory:
rsync -a --link-dest=/backup/etc /etc /backup/etc
This command will create a new version of the /backup/etc directory, with hard links to any files that are identical to the previous version. This can be useful for creating space-efficient backups using tools like rsnapshot, which use rsync to create incremental backups of a directory tree.
Keep in mind that the --link-dest option only works if the source and destination directories are on the same filesystem. If the directories are on different filesystems, rsync will not be able to create hard links and will instead copy the files themselves.
Additionally, the --link-dest option is only supported by rsync version 3.0.0 or newer. If you are using an older version of rsync, you may need to use a different method to preserve hard links when creating backups.