docker-compose local variable declare and use

Question

In the programming language we can define and declare a variable and use it later in the program, can we do same in docker-compose file?

For example in Java:

int x = 100; System.out.println("Number is: "+ x);
Similarly, in docker-compose file, can we declare a local variables and use it later.

docker-compose file

In the docker-compose file we want to declare tomcat_home as a variable and use it later as shown in example.

values: - tomcat_home: "/usr/local/tomcat"

# Creating Tomcat Server 1

Server1: image: tomcat hostname: TomcatServer1 container_name: TomcatServer1 volumes: - ${PWD}/tomcat-users.xml:${tomcat_home}/conf/tomcat-users.xml - ${PWD}/server.xml:${tomcat_home}/conf/server.xml

Answer 1

https://docs.docker.com/compose/environment-variables/#the-env-file

One could set a variable in an .env file:

TAG=1.2.3

and call it in a docker-compose file

docker-compose.yml

version: "3"
services:
  web:
    image: nginx:${TAG}
    ports:
    - 80:80

docker-compose up

returns:

Pulling web (nginx:1.2.3)...
ERROR: manifest for nginx:1.2.3 not found

No need to run docker-compose to check the config

According to the documentation one could run:

docker-compose config

to verify the docker-compose config

networks: {}
services:
  web:
    image: nginx:1.2.3
    ports:
    - 80:80/tcp
version: '3.0'
volumes: {}

Answer 2

For docker-compose, you can use either the environment or a .env file. For swarm mode, you can only use the environment. So for portability, I like to export variables into the shell or environment of the command being run.

From a bash shell, this would look like:

$ export tomcat_home=/usr/local/tomcat
$ docker-compose up

Depending on what you use to deploy your containers (shell script, custom code, some other CI/CD tool), exporting your environment variable will vary.