problem calling psql multiple times from a script ?

I am executing a script which contains multiple executions of psql.
The last execution of psql renames the database. It looks something
like the following

psql -f create_tables.sql db_name1

psql -f modify_tables.sql db_name1

psql -f add_indexes.sql db_name1

psql template1 <<XX_rename
ALTER DATABASE db_name1 RENAME TO db_name2;
XX_rename

I notice sometimes that the "ALTER DATABASE ... RENAME ..." statement
fails with an error that a user has the database (db_name1) open.

I am wondering if one of the previous executions of psql is doing some
"back room" work in the database while allowing the script to continue.
I am wondering if this "back room" work prevents the database from being
renamed. Which system table could I check to see if the database is open?

I have seen this type of "back room" behavior with our Informix
databases. Multiple calls to dbaccess followed by a database rename
caused the rename to fail in some cases. I had to add a loop with a
"sleep" followed by a check if the database was open.

We are using postgres Version 7.4.8.

TIA.

Paul Tilles