If a database is in a different path than the default, or if you want to use an alias that is different than the database name, you can catalog the global database. You can use the command line processor CATALOG GLOBAL DATABASE command as follows:
db2 CATALOG GLOBAL DATABASE database_global_name
AS alias
USING DIRECTORY DCE
The alias will be used by any application program that accesses the database.
For example:
db2 CATALOG GLOBAL DATABASE /.../cdscell2/subsys/database/dbmvs12 AS NYC3
USING DIRECTORY DCE