这两者这间有如下区别：
rman target / 是连接了 target DB并进入了rman环境，这时还可以连接 catalog DB

rman nocatalog target /  是以nocatalog模式连接进入了target DB，这时是不可以再连接catalog DB了


[oracle@prod ~]$ rman nocatalog target /

Recovery Manager: Release 9.2.0.4.0 – Production

Copyright (c) 1995, 2002, Oracle Corporation.  All rights reserved.

connected to target database: TESTDB (DBID=2567316959)
using target database controlfile instead of recovery catalog
RMAN>
[oracle@prod ~]$ rman target /

Recovery Manager: Release 9.2.0.4.0 – Production

Copyright (c) 1995, 2002, Oracle Corporation.  All rights reserved.

connected to target database: TESTDB (DBID=2567316959)