How to Merge Docker Compose file with Bash

The Docker Compose config command does exactly what you need, it takes multiple compose file and merges them.

Just pass them using multiple -f flags:

docker-compose -f docker-compose.yml -f docker-compose2.yml config

or using an environment variable:

COMPOSE_FILE=docker-compose.yml:docker-compose2.yml docker-compose config

The same approach is valid for every Docker Compose command, so if your final target is, for example, to set up your project, you can directly run:

docker-compose -f docker-compose.yml -f docker-compose2.yml up

Check the documentation for further details on how to specify multiple compose files.

I don't think you can do this (easily as a one-liner) in native bash without writing a script. I was curious so I did a quick search and found a yaml manipulation tool which supports merging yaml (docker-compose) files and looks like it fits your use-case.

I used brew to install on MacOS but there are instructions for Linux as well - https://mikefarah.github.io/yq/.

brew install yq

Showing existing files:

$ cat file1.yaml
version: "3"

services:
  nexus:
    image: sonatype/nexus3
    volumes:
      - "/opt/nexus3/nexus-data:/nexus-data"
    ports:
      - "8081:8081"

volumes:
  nexus-data: {}

$ cat file2.yaml
version: "3"

services:
  nexus2:
    image: sonatype/nexus3
    volumes:
      - "/opt/nexus3/nexus-data:/nexus-data"
    ports:
      - "8082:8082"

volumes:
  nexus-data: {}

Merge both files outputting to stdout:

$ yq m file1.yaml file2.yaml
services:
  nexus:
    image: sonatype/nexus3
    ports:
    - 8081:8081
    volumes:
    - /opt/nexus3/nexus-data:/nexus-data
  nexus2:
    image: sonatype/nexus3
    ports:
    - 8082:8082
    volumes:
    - /opt/nexus3/nexus-data:/nexus-data
version: "3"
volumes:
  nexus-data: {}

There may be a native way but I just redirected the stdout to a file:

$ yq m file1.yaml file2.yaml > file3.yaml
$ cat file3.yaml
services:
  nexus:
    image: sonatype/nexus3
    ports:
    - 8081:8081
    volumes:
    - /opt/nexus3/nexus-data:/nexus-data
  nexus2:
    image: sonatype/nexus3
    ports:
    - 8082:8082
    volumes:
    - /opt/nexus3/nexus-data:/nexus-data
version: "3"
volumes:
  nexus-data: {}

There are a lot of examples in their documentation for you to explore - https://mikefarah.github.io/yq/merge/.

There is a new way to do it, you can use docker compose convert here is example fit to what you're trying to do:

docker compose -f docker-compose.yml -f docker-compose2.yml convert > output.yml

Official documentation