假设使用了bash：

对于更一般的情况，如果输入中可能包含任意字符，包括引号，请考虑使用（特定于bas的）${var@A}创建分配：

ssh root@localhost ${KEY@A} echo 'hi'

更新1：bash（4.4版）手册页：在“ @A”（和其他操作）上：

  ${parameter@operator}
          Parameter  transformation.   The  expansion  is either a transformation of the value of parameter or information
          about parameter itself,depending on the value of operator.  Each operator is a single letter:

          Q      The expansion is a string that is the value of parameter quoted in a format that can be reused as input.
          E      The expansion is a string that is the value of parameter with backslash escape sequences expanded as with
                 the $'...' quoting mechansim.
          P      The  expansion  is a string that is the result of expanding the value of parameter as if it were a prompt
                 string (see PROMPTING below).
          A      The expansion is a string in the form of an assignment statement or declare command that,if  evaluated,will recreate parameter with its attributes and value.
          a      The expansion is a string consisting of flag values representing parameter's attributes.