RMAN commands for different purposes.

Recover whole oracle database .

SQL> startup force mount;
Now from the RMAN prompt.
run{
restore database;
recover database;
alter database open;
}

Backup database.

run {
configure device type disk backup type to compressed backupset parallelism 2; 
backup database plus archivelog;
}

Whenever we issue the above mentioned command to have a backup by default first of all RMAN will  check each and every block for physical block corruption then it will allow to backup the block on disk, the moment it have encountered, backup will be terminated. To avoid check for physical corruption go with option NOCHECKSUM as well as to avoid creating checksum for each block in backup. Logical block corruption is not done at the time of backup, for this to happen we have to provide command as mentioned below.

run {
configure device type disk backup type to compressed backupset parallelism 2;
backup check logical database plus archivelog;   # validate each and every block for physical
}                                                                       # as well as logical corruption

Validate database;
It would validate the whole database for physical corruption.

validate check logical database;
 It would validate the whole database for physical as well as for logical corruption.