How to specify environment variable in docker-compose.yml ?

Here is my docker-compose.yml file which is using environment

variable from .env file.I am also providing my .env file below this.

docker-compose.yml

version: "3.5"

services:

database_master:

image: mysql:8.0.30

stdin_open: true

tty: true

container_name: "database_master"

restart: unless-stopped

ports:

- 3307:3306

volumes:

- mysqldata_master:/var/lib/mysql

- ./master.cnf:/etc/my.cnf

environment:

- MYSQL_ROOT_PASSWORD=${MYSQL_ROOT_PASSWORD}

- MYSQL_USER=${MYSQL_USER}

- MYSQL_DATABASE=${MYSQL_DATABASE}

- MYSQL_PASSWORD=${MYSQL_PASSWORD}

networks:

- mynetwork

database_slave:

image: mysql:8.0.30

stdin_open: true

tty: true

container_name: "database_slave"

restart: unless-stopped

depends_on:

- database_master

ports:

- 3308:3306

volumes:

- mysqldata_slave:/var/lib/mysql

- ./slave.cnf:/etc/my.cnf

environment:

- MYSQL_ROOT_PASSWORD=${MYSQL_ROOT_PASSWORD}

- MYSQL_USER=${MYSQL_USER}

- MYSQL_DATABASE=${MYSQL_DATABASE}

- MYSQL_PASSWORD=${MYSQL_PASSWORD}

networks:

- mynetwork

networks:

mynetwork:

volumes:

mysqldata_master:

mysqldata_slave:

create .env file as

MYSQL_ROOT_PASSWORD='S3cret'

MYSQL_USER='my_db_user'

MYSQL_DATABASE='my_db'

MYSQL_PASSWORD='S3cret'

docker-compose.yml we are using bash syntax to get value of variable in .env file in environment section as

environment:

- MYSQL_ROOT_PASSWORD=${MYSQL_ROOT_PASSWORD}

- MYSQL_USER=${MYSQL_USER}

- MYSQL_DATABASE=${MYSQL_DATABASE}

- MYSQL_PASSWORD=${MYSQL_PASSWORD}