RMAN> CATALOG START WITH '/tmp/ForStandby';

using target database control file instead of recovery catalog
searching for all files that match the pattern /tmp/ForStandby

List of Files Unknown to the Database
=====================================
File Name: /tmp/ForStandby_d7ue11id_5_1
File Name: /tmp/ForStandby_d7ue11id_4_1
File Name: /tmp/ForStandby_d7ue11id_3_1
File Name: /tmp/ForStandby_d7ue11id_1_1
File Name: /tmp/ForStandby_d8ue12rt_1_1
File Name: /tmp/ForStandby_d7ue11id_2_1

Do you really want to catalog the above files (enter YES or NO)? YES
cataloging files...
cataloging done

List of Cataloged Files
=======================
File Name: /tmp/ForStandby_d7ue11id_5_1
File Name: /tmp/ForStandby_d7ue11id_4_1
File Name: /tmp/ForStandby_d7ue11id_3_1
File Name: /tmp/ForStandby_d7ue11id_1_1
File Name: /tmp/ForStandby_d8ue12rt_1_1
File Name: /tmp/ForStandby_d7ue11id_2_1

RMAN> RECOVER DATABASE NOREDO;

Starting recover at 12-OCT-19
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=254 device type=DISK
channel ORA_DISK_1: starting incremental datafile backup set restore
channel ORA_DISK_1: specifying datafile(s) to restore from backup set
destination for restore of datafile 00001: /opt/oracle/oradata/fbdb/system01.dbf
destination for restore of datafile 00002: /opt/oracle/oradata/fbdb/sysaux01.dbf
destination for restore of datafile 00003: /opt/oracle/oradata/fbdb/undotbs01.dbf
destination for restore of datafile 00004: /opt/oracle/oradata/fbdb/users01.dbf
destination for restore of datafile 00005: /opt/oracle/oradata/fbdb/example01.dbf
channel ORA_DISK_1: reading from backup piece /tmp/ForStandby_d7ue11id_1_1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of recover command at 10/12/2019 02:43:30
ORA-19870: error while restoring backup piece /tmp/ForStandby_d7ue11id_1_1
ORA-19573: cannot obtain exclusive enqueue for datafile 1

SQL> RECOVER DATABASE NOREDO;
ORA-00905: missing keyword