How can docker-compose.yml automatically load environment variables without the --env-file CLI flag?

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I'm using Docker Compose to manage my services, and I want to load environment variables for interpolation automatically from a file, without passing --env-file each time.

My Current Setup

I have an envs/ directory where I store environment variable files:

docker-compese.yml
envs/
  ├── global.env
  ├── gateway.env

My docker-compose.yml includes the env_file directive:

YAML
services:
  HSM-BE-GATEWAY-MS:
    image: hsm-be-gateway-ms:latest
    build:
      context: ../HSM-BE-GATEWAY-MS
      dockerfile: Dockerfile
    container_name: HSM-BE-GATEWAY-MS
    env_file:
      - ./envs/global.env
      - ./envs/gateway.env
    ports:
      - "${GATEWAY_MICROSERVICE_PORT}:${GATEWAY_MICROSERVICE_PORT}"

However, when I run docker-compose up, I receive warnings like:

The "GATEWAY_MICROSERVICE_PORT" variable is not set. Defaulting to a blank string.
The "GATEWAY_MICROSERVICE_HOST" variable is not set. Defaulting to a blank string.

If I manually pass the env file via CLI like this, it works:

docker-compose --env-file envs/global.env up

What I Need

I want docker-compose.yml to automatically handle loading these environment files for variable interpolation (e.g., in ports) without needing to specify --env-file in the command line every time.

What I’ve Tried

Using env_file in docker-compose.yml – Works inside the container but doesn’t resolve variables in ports.
Adding variables with the environment key in docker-compose.yml – Didn’t resolve the interpolation issue.
Moving global.env to a root .env file – Not an option for my setup.

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

When Docker Compose starts up, it uses the local .env file (located in your current working directory) to substitute variables in your docker-compose.yml. The env_file directive, however, only populates environment variables inside the containers—not for interpolation in the compose file itself (for example, the ports mapping).

Why Your Variables Aren't Interpolated

Interpolation Source: Docker Compose looks for a file named .env (in your working directory) to resolve placeholders like "${GATEWAY_MICROSERVICE_PORT}" found in your compose file.
env_file Directive: The env_file option is used to set environment variables inside the container after it's started. This file is not used for replacing variables in the docker-compose.yml file itself.
Result: Since your global variables are only in envs/global.env and not in a .env file, they aren’t available during interpolation—resulting in the warning messages.

Options to Resolve This

Use a .env File:
If possible, consider copying or linking your envs/global.env to a .env file in your project root. This is the simplest solution if your setup allows it:
```
BASH
cp envs/global.env .env
```
Docker Compose will automatically load .env for variable interpolation.
Export Variables in Your Shell:
Before running docker-compose up, you can export the required variables:
```
BASH
export GATEWAY_MICROSERVICE_PORT=8080
export GATEWAY_MICROSERVICE_HOST=localhost
docker-compose up
```
This ensures that the variables are available for interpolation.
Explicit --env-file Flag (Your Current Workaround):
While not ideal, you can continue to pass the --env-file flag when running docker-compose up:
```
BASH
docker-compose --env-file envs/global.env up
```
Remember, this flag affects interpolation because it tells Compose to use that file instead of .env—but it needs to be provided each time.

Final Note

There isn’t a built-in way in Docker Compose to automatically load a custom env file (like envs/global.env) for interpolation without using either the .env file in the working directory or the --env-file CLI flag. The design separates configuration for the Compose file’s interpolation from container environment variables. Consider automating the copying of your custom file to .env if modifying your CI/CD or local startup scripts is an option.

Let me know if you need further clarification!

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by PulsarPioneer989 • 1 month ago

The Compose documentation on variable interpolation notes only three sources for variables: the shell environment (export commands in the same shell you ran docker-compose); .env in the current directory; and either a --env-file or .env in the same directory as the Compose file. There's not an option in the Compose file to make it come from somewhere else, or to load a file for the Compose environment the same way env_file: does for an individual container.

Most of the Compose command-line arguments can also be set from environment variables and in particular there is a COMPOSE_ENV_FILES variable. So it might work for you to

BASH
export COMPOSE_ENV_FILES=envs/global.env,envs/gateway.env
docker-compose up -d

You do not necessarily need to specify the same files in per-container env_file:, unless you want all of the variables to be imported with exactly the same names. You could specify individual variables or rename them in environment:, referring to the values in the $COMPOSE_ENV_FILES files.

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How can docker-compose.yml automatically load environment variables without the --env-file CLI flag?

2 Answers

Why Your Variables Aren't Interpolated

Options to Resolve This

Final Note

Discussion

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