Variables and parameters in Azure Pipeline

Variables

Used to store and manage data during the pipeline execution.
Value can only be string.
Are mutable and value can change from run to run or job to job.
Can be overwrite, the most locally scoped variable wins.

Definition example

variables:
    - name: myImportantVariable
      value: contoso
    - name: SECONDVARIABLE
      value: "LongTextPRDEnv"

Usage example

steps:
    - bash: echo $(myImportantVariable)
    - powershell: echo $(myImportantVariable)
    - script: echo $(myImportantVariable)

Parameters

Used to provide input values to a pipeline when it is triggered.
Value could be string, number, bool or object (and more complex types).
Can only be used in template expressions.

Definition example

parameters:
    - name: myString
      type: string
      default: a string

    - name: myMultiString
      type: string
      default: default
      values:
          - default
          - ubuntu

parameters:
    - name: myNumber
      type: number
      default: 2
      values:
          - 1
          - 2
          - 16

    - name: myBoolean
      type: boolean
      default: true

    - name: myObject
      type: object
      default:
          foo: FOO
          bar: BAR
          things:
              - one
              - two
          nested:
              one: apple
              two: pear
              count: 3

All available data types could be found in Azure documentation

Usage example

parameters:
    - name: image
      displayName: Pool Image
      type: string
      default: ubuntu-latest
      values:
          - windows-latest
          - ubuntu-latest
          - macOS-latest

trigger: none

jobs:
    - job: build
      displayName: build
      pool:
          vmImage: ${{ parameters.image }}
      steps:
          - script: echo building $(Build.BuildNumber) with ${{ parameters.image }}

How to use it

It depends ;)

First of all, we need to know that there are 3 different syntaxes for using a variables and parameters.

Syntax

A syntax how the variable or parameter is referenced, determines when the value will be processed.

Type	Example	When is it processed?
template expression	`${{ variables.var }}`	compile time
macro	`$(var)`	runtime before a task executes
runtime expression	`$[variables.var]`	runtime

Template expression

Are process at compile time, and get replaced before runtime starts.
Are designed for reusing parts of YAML as templates.

Macro

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Runtime expression

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