docker-minecraft-server/docs/configuration/interpolating.md

title
Modifying config files

Replacing variables inside configs

Sometimes you have mods or plugins that require configuration information that is only available at runtime. For example if you need to configure a plugin to connect to a database, you don't want to include this information in your Git repository or Docker image. Or maybe you have some runtime information like the server name that needs to be set in your config files after the container starts.

For those cases there is the option to replace defined variables inside your configs with environment variables defined at container runtime.

When the environment variable REPLACE_ENV_IN_PLACE is set to true (the default), the startup script will go through all files inside the container's /data path and replace variables that match the container's environment variables. Variables can instead (or in addition to) be replaced in files sync'ed from /plugins, /mods, and /config by setting REPLACE_ENV_DURING_SYNC to true (defaults to false).

Variables that you want to replace need to be declared inside curly brackets and prefixed with a dollar sign, such as ${CFG_YOUR_VARIABLE}, which is same as many scripting languages.

You can also change REPLACE_ENV_VARIABLE_PREFIX, which defaults to "CFG_", to limit which environment variables are allowed to be used. For example, with "CFG_" as the prefix, the variable ${CFG_DB_HOST} would be substituted, but not ${DB_HOST}. The prefix can be set to an empty string to allow for matching any variable name.

If you want to use a file's content for value, such as when using secrets mounted as files, declare the placeholder named like normal in the file and declare an environment variable named the same but with the suffix _FILE.

For example, a my.cnf file could contain:

[client]
password = ${CFG_DB_PASSWORD}

...a secret declared in the compose file with:

secrets:
  db_password:
    external: true

...and finally the environment variable would be named with a _FILE suffix and point to the mounted secret:

    environment:
      CFG_DB_PASSWORD_FILE: /run/secrets/db_password

Variables will be replaced in files with the following extensions: .yml, .yaml, .txt, .cfg, .conf, .properties.

Specific files can be excluded by listing their name (without path) in the variable REPLACE_ENV_VARIABLES_EXCLUDES.

Paths can be excluded by listing them in the variable REPLACE_ENV_VARIABLES_EXCLUDE_PATHS. Path excludes are recursive. Here is an example:

REPLACE_ENV_VARIABLES_EXCLUDE_PATHS="/data/plugins/Essentials/userdata /data/plugins/MyPlugin"

Here is a full example where we want to replace values inside a database.yml.

---
database:
  host: ${CFG_DB_HOST}
  name: ${CFG_DB_NAME}
  password: ${CFG_DB_PASSWORD}

This is how your docker-compose.yml file could look like:

version: "3.8"
# Other docker-compose examples in /examples

services:
  minecraft:
    image: itzg/minecraft-server
    ports:
      - "25565:25565"
    volumes:
      - "mc:/data"
    environment:
      EULA: "TRUE"
      ENABLE_RCON: "true"
      RCON_PASSWORD: "testing"
      RCON_PORT: 28016
      # enable env variable replacement
      REPLACE_ENV_VARIABLES: "TRUE"
      # define an optional prefix for your env variables you want to replace
      ENV_VARIABLE_PREFIX: "CFG_"
      # and here are the actual variables
      CFG_DB_HOST: "http://localhost:3306"
      CFG_DB_NAME: "minecraft"
      CFG_DB_PASSWORD_FILE: "/run/secrets/db_password"

volumes:
  mc:
  rcon:

secrets:
  db_password:
    file: ./db_password

Patching existing files

JSON path based patches can be applied to one or more existing files by setting the variable PATCH_DEFINITIONS to the path of a directory that contains one or more patch definition json files or a patch set json file.

Variable placeholders in the patch values can be restricted by setting REPLACE_ENV_VARIABLE_PREFIX, which defaults to "CFG_".

The following example shows a patch-set file were various fields in the paper.yaml configuration file can be modified and added:

{
  "patches": [
    {
      "file": "/data/paper.yml",
      "ops": [
        {
          "$set": {
            "path": "$.verbose",
            "value": true
          }
        },
        {
          "$set": {
            "path": "$.settings['velocity-support'].enabled",
            "value": "${CFG_VELOCITY_ENABLED}",
            "value-type": "bool"
          }
        },
        {
          "$put": {
            "path": "$.settings",
            "key": "my-test-setting",
            "value": "testing"
          }
        }
      ]
    }
  ]
}

NOTES: Only JSON and Yaml files can be patched at this time. TOML support is planned to be added next. Removal of comments and other cosmetic changes will occur when patched files are processed.