azure-pipelines.yaml

# Starter pipeline
# Start with a minimal pipeline that you can customize to build and deploy your code.
# Add steps that build, run tests, deploy, and more:
# https://aka.ms/yaml

trigger:
- master

pool:
  vmImage: 'ubuntu-latest'

variables:
   major: 2
   minor: 0

stages:

# Versioning master branch builds

- stage:
  displayName: Build_Master_Version_Number
  condition: eq(variables['Build.SourceBranch'], 'refs/heads/master')
  jobs:
  - job: Build_Master_Version_Number
    variables:
       patch: $[counter(variables['minor'], 0)]
    steps:
      - bash: |
           echo "##vso[build.updatebuildnumber]$(major).$(minor).$(patch)"
        name: SetMasterBuildName

# Versioning feature branch and PR builds

- stage:
  displayName: Build_Branch_Version_Number
  condition: ne(variables['Build.SourceBranch'], 'refs/heads/master')
  jobs:
  - job: Build_Branch_Version_Number
    variables:
       prpatch: $[counter(variables['system.pullrequest.pullrequestid'], 0)]
       brpatch: $[counter(variables['build.sourcebranchname'], 0)]
    steps:
      - bash: |
           echo "##vso[build.updatebuildnumber]$(major).$(minor)-PullRequest.$(prpatch)"
        condition: eq(variables['Build.Reason'], 'PullRequest')
        name: SetPRBuildName
      - bash: |
           echo "##vso[build.updatebuildnumber]$(major).$(minor)-$(Build.SourceBranchName).$(brpatch)"
        condition: ne(variables['Build.Reason'], 'PullRequest')
        name: SetBranchBuildName


# Stage for building your application

- stage: Build_Steps
  displayName: Build_Steps
  condition: always()
  jobs:  
  - job: Build_Steps
    displayName: Build_Steps
    steps:
    - script: echo Hello, world!
      name: 'Run_a_one_line_script'
    - script: |
          echo Add other tasks to build, test, and deploy your project.
          echo See https://aka.ms/yaml
      name: 'Run_a_multi_line_cript'

No worries, just wanted to point out those tips in case they could be of use to anyone else. after writing that comment, I actually had to revert some of my work trying to move down to specific tasks/jobs. I can't specify variables: on a task it seems, only on a job. But jobs can apparently run in parallel, so I actually hit a situation where my build ran before the script to get build number. It's possible to specify a dependsOn:, but I don't think that supports an 'or' operator to specify one job OR another. So, I had to go back to separate stages so that I had a parent stage to use as a dependsOn:

My use it actually for .NET Core, but you can easily see the similarities
https://gist.github.com/nickalbrecht/51aa4a9ae2535ccc1b74110d1cf98d14

Thank You for this !! It is very helpful