Add ${variable-word} parameter substitution modifier to dockerfile environment replacement documentation

[gnowland]

Is this a docs issue?

• [x] My issue is about the documentation content or website

Type of issue

Information is incorrect

Description

Environment replacement documentation is missing the following bash string parameter substitution modifier:

${variable-word}

Per 10.2. Parameter Substitution - The Linux Documentation Project

"${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null.

Meaning that if you want to set a default for an environment variable ONLY if it is not declared, but not null, you must use - without the :. For example:

• -

  # Dockerfile
  ARG HTTP_PROXY
  ENV http_proxy=${HTTP_PROXY-http://my-proxy.com:1234}
  RUN echo $http_proxy
  
  # No build arg
  $ docker build -f Dockerfile .
  > http://my-proxy.com:1234
  
  # Build arg null (this is the difference!)
  $ docker build --build-arg HTTP_PROXY="" -f Dockerfile .
  > 
  
  # Build arg value
  $ docker build --build-arg HTTP_PROXY="override" -f Dockerfile .
  > override
  

• :-

  # Dockerfile
  ARG HTTP_PROXY
  ENV http_proxy=${HTTP_PROXY:-http://my-proxy.com:1234}
  RUN echo $http_proxy
  
  # No build arg
  $ docker build -f Dockerfile .
  > http://my-proxy.com:1234
  
  # Build arg null (this is the difference!)
  $ docker build --build-arg HTTP_PROXY="" -f Dockerfile .
  > http://my-proxy.com:1234
  
  # Build arg value
  $ docker build --build-arg HTTP_PROXY="override" -f Dockerfile .
  > override
  

Tested and confirmed locally.

Location

https://docs.docker.com/reference/dockerfile/#environment-replacement

Suggestion

Update documentation to include the ${variable-word} bash string parameter substitution modifier.