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The xtrabackup binary does not have any functionality for restoring a backup. That is up to the user to do. You might use rsync or cp to restore the files. You should check that the restored files have the correct ownership and permissions.
Note that xtrabackup backs up only the InnoDB data. You must separately restore the MySQL system database, MyISAM data, table definition files (.frm files), and everything else necessary to make your database functional – or innobackupex can do it for you.