What does this mean in docker-compose.yml directive?

${{ variable }}

And how is it used?

CodePudding user response：

In a pipeline, template expression variables (${{ variables.var }}) get processed at compile time, before runtime starts. Macro syntax variables ($(var)) get processed during runtime before a task runs. Runtime expressions ($[variables.var]) also get processed during runtime but were designed for use with conditions and expressions. When you use a runtime expression, it must take up the entire right side of a definition.

In the following example, you can see that the template expression still has the initial value of the variable after the variable is updated. The value of the macro syntax variable updates. The template expression value does not change because all template expression variables get processed at compile time before tasks run. In contrast, macro syntax variables are evaluated before each task runs.

variables:
- name: one
  value: initialValue 

steps:
  - script: |
      echo ${{ variables.one }} # outputs initialValue
      echo $(one)
    displayName: First variable pass
  - bash: echo "##vso[task.setvariable variable=one]secondValue"
    displayName: Set new variable value
  - script: |
      echo ${{ variables.one }} # outputs initialValue
      echo $(one) # outputs secondValue
    displayName: Second variable pass