Shell script to execute pgsql commands in files

you can echo your commands to the psql input:

for dbname in foo foofoo foobar barbar
do
    echo """
CREATE DATABASE $dbname TEMPLATE mytemplate1
""" | psql
done

First off, do not mix psql meta-commands and SQL commands. These are separate sets of commands. There are tricks to combine those (using the psql meta-commands \o and \\ and piping strings to psql in the shell), but that gets confusing quickly.

• Make your files contain only SQL commands.
• Do not include the CREATE DATABASE statement in the SQL files. Create the db separately, you have multiple files you want to execute in the same template db.

Assuming you are operating as OS user postgres and use the DB role postgres as (default) Postgres superuser, all databases are in the same DB cluster on the default port 5432 and the role postgres has password-less access due to an IDENT setting in pg_hba.conf - a default setup.

psql postgres -c "CREATE DATABASE mytemplate1 WITH ENCODING 'UTF8'
                  TEMPLATE template0"

I based the new template database on the default system template database template0. Basics in the manual here.

Your questions

How to (...) run a set of pgsql cmds from file

Try:

psql mytemplate1 -f file

Example script file for batch of files in a directory:

#! /bin/sh

for file in /path/to/files/*; do
    psql mytemplate1 -f "$file"
done

The command option -f makes psql execute SQL commands in a file.

How to create a database based on an existing template at the command line

psql -c 'CREATE DATABASE my_db TEMPLATE mytemplate1'

The command option -c makes psql execute a single SQL command string. Can be multiple commands, terminated by ; - will be executed in one transaction and only the result of the last command returned.
Read about psql command options in the manual.

If you don't provide a database to connect to, psql will connect to the default maintenance database named "postgres". In the second answer it is irrelevant which database we connect to.

If you're willing to go the extra mile, you'll probably have more success with sqlalchemy. It'll allow you to build scripts with python instead of bash, which is easier and has better control.

As requested in the comments: https://github.com/srathbun/sqlCmd