How to create a config file bash script with variables inside

I'm learning BASH and trying to create a script that generates a configuration file in which I want to use variables. But of course, like this is a text file, the variable ${MONGO_PORT} is not being replaced by the value i need.

(
cat << MONGOD_CONF
# /etc/mongod.conf

systemLog:
    destination: file
    path: /datos/log/mongod.log
    logAppend: true

storage:
    dbPath: /datos/db
    engine: wiredTiger

net:
    port: ${MONGO_PORT}

security:
    authorization: enabled    
MONGOD_CONF
) > /etc/mongod.conf

The first problem is about permission to create the file that I'm avoiding by creating the file in /tmp and moving it to /etc. Because this doesn't work.
```
    sudo cat << MONGOD_CONF
```
The second and more important is about how to replace de variable by the value

Any way to make this work?

Thank you!

Solution

If MONGO_PART is set, the here-document is subject to parameter expansion and you'll get the parameter value in your file.

Use tee instead of cat, which can open the output file itself:

sudo tee /etc/mongod.conf > /dev/null << MONGOD_CONF
    ...
MONGOD_CONF