What is the different between $, ${}, and ${{}}

I see the use of $env, ${ env }, and ${{ env }}. What is the difference between these three?

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Thank you for your question.

$env is used to use environment variable in bash script.

${env:name} is used to use environment variable in PowerShell Core script.

Please refer to this document: https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#using-a-specific-shell

${{ }} is used to evaluate an expression and use contexts. Please refer toAbout context and expressions.