使用defined?关键字（文档）。它将返回一个带有项目类型的字符串，或者nil如果它不存在。

>> a = 1
 => 1
>> defined? a
 => "local-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"

正如 skalee 评论的那样：“值得注意的是，设置为 nil 的变量已被初始化。”

>> n = nil  
>> defined? n
 => "local-variable"