If you are using
psql 9.6+, there is a very convenient command called
Sends the current query input buffer to the server, then treats each
column of each row of the query's output (if any) as a SQL statement
to be executed. [...] The generated queries are executed in the order
in which the rows are returned, and left-to-right within each row if
there is more than one column. NULL fields are ignored. The generated
queries are sent literally to the server for processing, so they
cannot be psql meta-commands nor contain psql variable references.
So, your script could look like:
SELECT format('create database slave%s template master_db', l_num)
FROM generate_series(1, 30) t(l_num);
Note that you can use the newer
psql version, even if the server version is lower. There might be some rare cases of incompatibility (when using
\d* commands, because some system views may change between server versions), but I haven't yet seen such a case.
If you are still stuck on an earlier client version, you can just use the
SELECT and copy the output manually to a script file.
If your database names are in a file, you have multiple options. Either treat that file with an editor (something like
sed -i -e 's/^/CREATE\ DATABASE\ /' -e 's/$/\ TEMPLATE\ master_db' in any OS where
sed exists), and then run the resulting script.
Alternatively, you can also do something like this:
CREATE TEMPORARY TABLE db_names (db text);
\copy db_names FROM 'path/to/your/file'
SELECT format('create database slave%s template master_db', db)
Important note: only use \gexec when you absolutely trust the source you are working with. The above approach is prone to SQL injection (which, interestingly, not so easy when one of the commands on the same line is
CREATE DATABASE, but it's always better to be careful than sorry later).