You can put in your file something like:

\c first_db_name
select * from t; --- your sql
\c second_db_name
select * from t; --- your sql
...

When you get a connection to PostgreSQL it is always to a particular database. To access a different database, you must get a new connection.

Using \c in psql closes the old connection and acquires a new one, using the specified database and/or credentials. You get a whole new back-end process and everything.

Example:

yourUser=# \c newDatabaseName
You are now connected to database "newDatabaseName" as user "yourUser".

PL/pgSQL is server language. \c meta command is psql feature. PostgreSQL server know nothing about \c command.

This is a task for some client application. It is impossible to read or change something in another database. Connection is always open only for one database.

However, you can use dblink to open connection to another database and initiate execution of arbitrary commands in that database. dblink will act as a simple libpq client.

In 'psql', you can change the database you are connected to by using the \c metacommand. So it would look like this:

CREATE DATABASE my_database;
\c my_database
CREATE TABLE my_table (
  id UUID primary key,
  data json
);

This works from '\i' included files. When the \i file is done, you will be left in the new database unless you do something to change back to the old one.

The client program createdb does not support all those options.
Create a file db_create.sql:

CREATE DATABASE MydatAbseName
   WITH OWNER myadmin 
   TEMPLATE template0
   ENCODING 'SQL_ASCII'
   TABLESPACE  pg_default
   LC_COLLATE  'C'
   LC_CTYPE  'C'
   CONNECTION LIMIT  -1;

Call it:

psql -U postgres postgres -f C:/path/to/db_create.sql

The trick here is to connect to the default database named "postgres" and create the new database from there. I do it with the default superuser named "postgres" in my example.
psql -f executes the SQL commands in the given file.

You could also just execute a single command with psql -c (no file involved):

psql -U postgres postgres -c "CREATE DATABASE MydatAbseName WITH OWNER Myadmin
EMPLATE template ENCODING 'SQL_ASCII' TABLESPACE  pg_default LC_COLLATE  'C'
LC_CTYPE  C' CONNECTION LIMIT  -1"

Read the fine manual on creating a database here and here.
More on psql.

On Windows, it looks something like this:

"C:\Program Files\PostgreSQL\verson_number\bin\psql.exe" -U user -f C:/path/to/db_create.sql postgres

The last "postgres" is the name of the default db. If you want to use it in a batch file you have to answer a password prompt or connect with a user that is allowed access without providing a password. Basics in the manual chapters "The Password File" and "The pg_hba.conf File". More here:

• Run batch file with psql command without password

Try this one:

#!/bin/bash
psql -U postgres -d database_name -c "SELECT c_defaults  FROM user_info WHERE c_uid = 'testuser'"

Or using su:

#!/bin/bash
su -c "psql -d database_name -c \"SELECT c_defaults  FROM user_info WHERE c_uid = 'testuser'\"" postgres

And also sudo:

#!/bin/bash
sudo -u postgres -H -- psql -d database_name -c "SELECT c_defaults  FROM user_info WHERE c_uid = 'testuser'"