About expressions
You can use expressions to programmatically set environment variables in workflow files and access contexts. An expression can be any combination of literal values, references to a context, or functions. You can combine literals, context references, and functions using operators. For more information about contexts, see Contexts reference.
Expressions are commonly used with the conditional if keyword in a workflow file to determine whether a step should run. When an if conditional is true, the step will run.
您需要使用特定语法指示 GitHub 对表达式求值，而不是将其视为字符串。
${{ <expression> }}
注意
The exception to this rule is when you are using expressions in an if clause, where, optionally, you can usually omit ${{ and }}. For more information about if conditionals, see GitHub Actions 的工作流语法.
警告
创建工作流和操作时，应始终考虑代码是否可能执行潜在攻击者的不受信任的输入。 某些上下文应被视为不受信任的输入，因为攻击者可能会插入自己的恶意内容。 有关详细信息，请参阅“GitHub Actions 的安全强化”。
Example setting an environment variable
env:
MY_ENV_VAR: ${{ <expression> }}
Further reading
For technical reference information about expressions you can use in workflows and actions, see 对工作流和操作中的表达式求值.