regading detection of stdin

[Hugo Florentino]

On Fri, 06 Dec 2013 01:26:28 +0100, Adam D. Ruppe wrote:
>
> On Friday, 6 December 2013 at 00:24:22 UTC, Hugo Florentino wrote:
>> if nothing is passed from stdin and no parameter is provided,
>> the application freezes:
>
> Does it freeze or just wait for you to press enter on the
> keyboard?
>

I see... so the "problem" simply was that function readln was expecting 
user input. In that case, this is not what I intended.
I want the application to receive the stdout of another command as 
stdin, and to detect if nothing is sent.
How coud I accomplish this?