Two ways to do it

A) WHILE STARTING RMAN
$ rman log=’tmp/rman_log.log’ OR $ rman log /tmp/rman_log.log append
(use with append clause if you don’t wanna OVERWRITE existing FILE)

Also If you are running RMAN interactively and you want to see output on your terminal screen as well as have it written to a log file, use TEE command:
$ rman | tee /tmp/rman.log

B) WHILE INSIDE RMAN
$rman
RMAN> spool log to ‘/tmp/rman_log.log’;
(use with append clause if you don’t wanna overwrite existing file “spool log to ‘/tmp/rman_log.log’ append.)
..
RMAN> spool log off;
** you can use any writeable directory and log file NAME.

Two ways to do it

A) WHILE STARTING RMAN
$ rman log=’tmp/rman_log.log’ OR $ rman log /tmp/rman_log.log append
(use with append clause if you don’t wanna overwrite existing file)

Also If you are running RMAN interactively and you want to see output on your terminal screen as well as have it written to a log file, use tee command:
$ rman | tee /tmp/rman.log

B) WHILE INSIDE RMAN
$rman
RMAN> spool log to ‘/tmp/rman_log.log’;
(use with append clause if you don’t wanna overwrite existing file “spool log to ‘/tmp/rman_log.log’ append.)
..
RMAN> spool log off;
** you can use any writeable directory and log file name.