How to test if a variable is declared?

[Peng Yu]

Hi,

-v only test if a variable has a nonempty value. What is the best way
to test whether a viable is declared? (In the following example, it
should return 0, even the value of x is an empty string.) Thanks.

$ declare x; [[ -v x ]]; echo $?;
1


       -v varname
              True if the shell variable varname is set (has been assigned a
              value).

-- 
Regards,
Peng