Re: Error message garbage when parameter expansion used inside (()) and variable unset

[Greg Wooledge]

On Mon, Apr 02, 2018 at 04:16:54PM -0500, PRussell wrote:
> The above tells us what happens to an unset variable if not using parameter
> expansion.
> 
> But if a shell variable uses parameter expansion and is null or unset, what
> does it evaluate to inside (()) syntax?  

The parameter expansion is performed exactly as you would expect,
resulting in an empty string, and most likely a syntax error.

> ./tt: line 18: var1: var1 ==  : syntax error: operand expected (error token 
> is "==  ")

Yes, like that.  So don't do that.  This is a bug in your script, not
in bash.