Some of these reserved keywords can be directly used in the shell, so for example $?
其中一些保留关键字可以直接在 shell 中使用，例如 $?
英语百科
Reserved word
In a computer language, a reserved word (also known as a reserved identifier) is a word that cannot be used as an identifier, such as the name of a variable, function, or label – it is "reserved from use". This is a syntactic definition, and a reserved word may have no meaning.