Get content from text file and save it into a variable - Azure YML

How about this:

variables:
  - name: MY_TEXT_FILE
    value: 'greetings.txt'
    readonly: true

  - name: GREETINGS_CONTENT
    value: ''

steps:
  - task: Bash@3
    displayName: 'Save text file content in a variable'
    inputs:
      targetType: 'inline'
      script: |
        testcontent=$(cat $MY_TEXT_FILE)
        export encoded=$(echo "$testcontent" | base64 -w 0)
        echo "##vso[task.setvariable variable=GREETINGS_CONTENT]$encoded"

  - task: Bash@3
    displayName: 'Decode and print variable'
    inputs:
      targetType: inline
      script: |
        decoded=$(echo "$GREETINGS_CONTENT" | base64 -d)
        echo "My greetings are: $decoded"