Fail to restore data with mariabackup without specified data dir

Fail to restore data with mariabackup without specified data dir

expected : depend on the doc --datadir option and path to datadir are not required
https://mariadb.com/kb/en/library/full-backup-and-restore-with-mariadb-backup/
$ mariabackup --copy-back --target-dir /var/mariadb/backup/ \
--user backup_user --password backup_passwd

note: the datadir is the default datadir : /var/lib/mysql/

how to repeat;
1 Create and load database
2. Perform mariabackup with option --backup
3. Perform mariabackup with option --prepare
4. Drop the created database
5, Restore database with mariabackup and option --move-back or --copy-back
it's returned Error that datadir must be specified

1.2. Perform mariabackup with option --backup
[root@t4w6 ~]# mariabackup -u root -p1 --backup --databases=boxoffice_mdb --target-dir /var/mariadb/backup/
180116 19:06:23 Connecting to MySQL server host: localhost, user: root, password: set, port: not set, socket: not set
Using server version 10.2.12-MariaDB
mariabackup based on MariaDB server 10.2.12-MariaDB Linux (x86_64)
mariabackup: uses posix_fadvise().
mariabackup: cd to /var/lib/mysql/
mariabackup: open files limit requested 0, set to 1024
mariabackup: using the following InnoDB configuration:
mariabackup: innodb_data_home_dir = .
mariabackup: innodb_data_file_path = ibdata1:12M:autoextend
mariabackup: innodb_log_group_home_dir = ./
2018-01-16 19:06:23 140574305568896 [Note] InnoDB: Number of pools: 1
180116 19:06:23 >> log scanned up to (347323590)
mariabackup: Generating a list of tablespaces
Skipping db: ./mysql
Skipping db: ./test
Skipping db: ./performance_schema
2018-01-16 19:06:23 140574305568896 [Warning] InnoDB: Allocated tablespace ID 4 for boxoffice_mdb/dim_date, old maximum was 0
180116 19:06:23 [01] Copying ./ibdata1 to /var/mariadb/backup/ibdata1
180116 19:06:23 [01] ...done
180116 19:06:23 [01] Copying ./boxoffice_mdb/dim_date.ibd to /var/mariadb/backup/boxoffice_mdb/dim_date.ibd
180116 19:06:23 [01] ...done
180116 19:06:23 [01] Copying ./boxoffice_mdb/dim_person.ibd to /var/mariadb/backup/boxoffice_mdb/dim_person.ibd
180116 19:06:23 [01] ...done
180116 19:06:24 [01] Copying ./boxoffice_mdb/dim_showtime.ibd to /var/mariadb/backup/boxoffice_mdb/dim_showtime.ibd
180116 19:06:24 [01] ...done
180116 19:06:24 >> log scanned up to (347323590)
180116 19:06:24 [01] Copying ./boxoffice_mdb/dim_theater_mdc.ibd to /var/mariadb/backup/boxoffice_mdb/dim_theater_mdc.ibd
180116 19:06:24 [01] ...done
180116 19:06:24 [01] Copying ./boxoffice_mdb/dim_theater.ibd to /var/mariadb/backup/boxoffice_mdb/dim_theater.ibd
180116 19:06:24 [01] ...done
180116 19:06:24 [01] Copying ./boxoffice_mdb/dim_ticket_type.ibd to /var/mariadb/backup/boxoffice_mdb/dim_ticket_type.ibd
180116 19:06:24 [01] ...done
180116 19:06:25 [01] Copying ./boxoffice_mdb/dim_title.ibd to /var/mariadb/backup/boxoffice_mdb/dim_title.ibd
180116 19:06:25 [01] ...done
180116 19:06:25 [01] Copying ./boxoffice_mdb/fact_crew.ibd to /var/mariadb/backup/boxoffice_mdb/fact_crew.ibd
180116 19:06:25 [01] ...done
180116 19:06:25 [01] Copying ./boxoffice_mdb/fact_tickets.ibd to /var/mariadb/backup/boxoffice_mdb/fact_tickets.ibd
180116 19:06:26 [01] ...done
180116 19:06:27 Executing FLUSH NO_WRITE_TO_BINLOG TABLES...
180116 19:06:27 Executing FLUSH TABLES WITH READ LOCK...
180116 19:06:27 Starting to backup non-InnoDB tables and files
Skipping db: ./mysql
Skipping db: ./test
Skipping db: ./performance_schema
180116 19:06:27 [01] Copying ./boxoffice_mdb/db.opt to /var/mariadb/backup/boxoffice_mdb/db.opt
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_date.frm to /var/mariadb/backup/boxoffice_mdb/dim_date.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_person.frm to /var/mariadb/backup/boxoffice_mdb/dim_person.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_showtime.frm to /var/mariadb/backup/boxoffice_mdb/dim_showtime.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_theater_mdc.frm to /var/mariadb/backup/boxoffice_mdb/dim_theater_mdc.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_theater.frm to /var/mariadb/backup/boxoffice_mdb/dim_theater.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_ticket_type.frm to /var/mariadb/backup/boxoffice_mdb/dim_ticket_type.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/dim_title.frm to /var/mariadb/backup/boxoffice_mdb/dim_title.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/fact_crew.frm to /var/mariadb/backup/boxoffice_mdb/fact_crew.frm
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying ./boxoffice_mdb/fact_tickets.frm to /var/mariadb/backup/boxoffice_mdb/fact_tickets.frm
180116 19:06:27 [01] ...done
180116 19:06:27 Finished backing up non-InnoDB tables and files
180116 19:06:27 [01] Copying aria_log_control to /var/mariadb/backup/aria_log_control
180116 19:06:27 [01] ...done
180116 19:06:27 [01] Copying aria_log.00000001 to /var/mariadb/backup/aria_log.00000001
180116 19:06:27 [01] ...done
180116 19:06:27 Executing FLUSH NO_WRITE_TO_BINLOG ENGINE LOGS...
mariabackup: The latest check point (for incremental): '347323581'
mariabackup: Stopping log copying thread.

180116 19:06:27 >> log scanned up to (347323590)
180116 19:06:28 Executing UNLOCK TABLES
180116 19:06:28 All tables unlocked
180116 19:06:28 [00] Copying ib_buffer_pool to /var/mariadb/backup/ib_buffer_pool
180116 19:06:28 [00] ...done
180116 19:06:28 Backup created in directory '/var/mariadb/backup/'
180116 19:06:28 [00] Writing backup-my.cnf
180116 19:06:28 [00] ...done
180116 19:06:28 [00] Writing xtrabackup_info
180116 19:06:28 [00] ...done
mariabackup: Redo log (from LSN 347323581 to 347323590) was copied.
180116 19:06:28 completed OK!

3. Perform mariabackup with option --prepare
[root@t4w6 ~]# mariabackup -u root -p1 --prepare --databases=boxoffice_mdb --target-dir /var/mariadb/backup/
mariabackup based on MariaDB server 10.2.12-MariaDB Linux (x86_64)
mariabackup: cd to /var/mariadb/backup/
mariabackup: This target seems to be not prepared yet.
mariabackup: using the following InnoDB configuration for recovery:
mariabackup: innodb_data_home_dir = .
mariabackup: innodb_data_file_path = ibdata1:12M:autoextend
mariabackup: innodb_log_group_home_dir = .
mariabackup: Starting InnoDB instance for recovery.
mariabackup: Using 104857600 bytes for buffer pool (set by --use-memory parameter)
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Uses event mutexes
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Compressed tables use zlib 1.2.7
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Number of pools: 1
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Using generic crc32 instructions
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Initializing buffer pool, total size = 100M, instances = 1, chunk size = 100M
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Completed initialization of buffer pool
2018-01-16 19:08:34 139835872372480 [Note] InnoDB: page_cleaner coordinator priority: -20
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Highest supported file format is Barracuda.
2018-01-16 19:08:34 139836222560384 [Note] InnoDB: Starting crash recovery from checkpoint LSN=347323581
mariabackup: Recovered WSREP position: 714e1bbe-face-11e7-a64d-120419d03514:103
180116 19:08:34 completed OK!

4. Drop the created database
MariaDB [(none)]> drop database boxoffice_mdb ;
Query OK, 9 rows affected (0.82 sec)

5.Restore database with mariabackup and option --move-back or --copy-back
it's returned Error that datadir must be specified

[root@t4w6 ~]# mariabackup -u root -p1 --copy-back --databases=boxoffice_mdb --target-dir /var/mariadb/backup/
mariabackup based on MariaDB server 10.2.12-MariaDB Linux (x86_64)
Error: datadir must be specified.

[root@t4w6 ~]# mariabackup -u root -p1 --move-back --databases=boxoffice_mdb --target-dir /var/mariadb/backup/
mariabackup based on MariaDB server 10.2.12-MariaDB Linux (x86_64)
Error: datadir must be specified.

[root@t4w6 ~]# mysql -u root -p1 information_schema -B -N -e 'SELECT Variable_Value FROM GLOBAL_VARIABLES WHERE Variable_Name = "datadir" '
/var/lib/mysql/