readonly: readonly [-aAf] [名称[=值] ...] 或 readonly -p
    Mark shell variables as unchangeable.

    Mark each NAME as read-only; the values of these NAMEs may not be
    changed by subsequent assignment.  If VALUE is supplied, assign VALUE
    before marking as read-only.

    Options:
      -a    refer to indexed array variables
      -A    refer to associative array variables
      -f    refer to shell functions
      -p    display a list of all readonly variables or functions, depending on
            whether or not the -f option is given

    An argument of `--' disables further option processing.

    Exit Status:
    Returns success unless an invalid option is given or NAME is invalid.

readonly命令用于定义只读shell变量和shell函数。readonly命令的选项-p可以输出显示系统中所有定义的只读变量。

什么参数不加和加p选项是一样的效果

-f：定义只读函数；

-a：定义只读数组变量；

-A: associative array 关联数组，类似map！

-p：显示系统中全部只读变量列表。