If no name arguments are given, or if the-p option is supplied,
a list of all Readonly names is printed.
The exit status is zero, unless end-of-file is encountered, read times out(in which case the status is greater than 128),
a variable assignment error(such as assigning to a Readonly variable) occurs,
or an invalid file descriptor is supplied as the argument to-u.
The return code is zero, unless end-of-file is encountered, read times out(in which case the return code is greater than 128),
a variable assignment error(such as assigning to a Readonly variable) occurs,
or an invalid file descriptor is supplied as the argument to-u.