SQL> alter database register logfile
'/var/arch/arch_1_101.arc';
So this is a straightforward way of registering a logfile giving the full path to the logfile. However, what if you have a very large number of logfiles to register, the above does not really scale all that well. Thankfully there is an rman command that enables you to register all logfiles within a directory:
rman> catalog start with '/var/arch';
This can save you a lot of time if you need to register a large number of files.