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docker-compose wrapper allowing for user data and env. var defaults

Project description

docker-compose wrapper allowing for user data and env. var defaults

dcw can be invoked with exactly the same commands as docker-compose. If -f/--file is specified as the first option the filename to be read will be taken from the argument. Otherwise docker-compose.yml is assumed (currently the alternative default files are not tested/read) the YAML file.

The YAML file has to be in the version: “2” format.

dcw loads and processes the YAML file and invokes docker-compose --file=- and the original commandline arguments, then pipes in the processed data (as YAML).

The examples in usage refer to a container smtp that implements an SMTP “filter”, accepting material on an incoming port, filtering spam and viruses, and sending mail on to a relay for delivery.


Tested version combinations: - docker-compose 1.8.1, dcw 0.5.0


The following subsections describe the processing that dcw does and the reasons for doing so.

reading defaults

The key value pairs under user-data -> env-defaults are taken to populate os.environ, unless the key already exists. This means you can set defaults for the environment variables used in the docker-compose YAML file.

For smtp this allows you to have specific ports that will be used on the deployment machine “backed in”, while on the development machine (where there is the need for uninterrupted “normal” mail during testing) some environment variables can be set, for port-numbers and directories, that override the values in the YAML file.

stripping user-data

Any existing top-level key “user-data” or starting with “user-data-” is removed. This allows for storing additional data in the file (which would require yet another configuration file or extraction from YAML comments).

including defaults in Dockerfile actions

All of the environment variables specified in env-defaults are written to the file (if the YAML file is newer than that file). They are written with the environment variable value if that is available, otherwise with the default as specified in the YAML file.

The file looks like:

export DOCKER_SMTP_HOST="some_host_name"

and this file can be mapped in the Dockerfile with:

ADD /tmp/

and then used in some script in later (RUN) stanzas with:

source /tmp/


This is the first part of one of my docker-compose.yml files:

version: '2'
  author: Anthon van der Neut <>
  description: postfix container
    NAME: submission
    PORT: 587    # external port, overridden during development
    DOCKER_HOST_IP: <hostip
    container_name: ${NAME}
    build: .
    - docker-host:${DOCKER_HOST_IP}
    - ${PORT}:587

The “author” and “description” information can easily be extracted and used by other processes.

While developing I cannot use the submission port (587) as that is already taken, and there I do export PORT=10587. On my deployment machine I don’t want to have to set PORT to the default value. With dcw the PORT env. var is set to 587 because there is no environment var “PORT” defined on that machine.

Volumes: directories vs files

If the volume you create doesn’t exist yet on the host, Docker will create a directory even though you might prefer an empty file.

If dcw finds that any of the host parts of the volumes list end in / (i.e. has a slash before the ‘:’) it will assume all directories have such a slash, and any files will not have that. Based on that information each host based path will be checked (and reported if not correct) and each non-existing path will be created (i.e. empty directory or empty file, as appropriate).

Extra commands

The wrapper adds a few extra commands to docker-compose:

bash               run bash in container
generate           generate systemd/upstart auto startup file
templates          list files used as template
expand             show expanded YAML and
truncate           truncate log file (needs sudo)

Limiting log size in 2.0

Truncating is rather cruel. A better way to restrict output is by adding a few lines to the docker-compose.yaml file (in the container section):

logging:   #
  driver: json-file
    max-size: 100k
    max-file: "10"  # this is a number, has to be a string for Go


The templates and generate option are used to generate systemd/upstart templates. The templates are expanded and use \ to limit the line length a bit.

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