way to find out if variable has been defined

[<pl...@pi...>]

Hi,

I have a number of gnuplot scripts that have various options set. I 
would like to set a default value in the script and optionally determine 
the value from the calling script or command.

This does not seem to be possible.

eg.

if (doPNG) { set terminal png; set output pngfile}
else { set terminal wxt }

plot ....


called as :

(echo 'doPNG=1;'  && cat cos_fit.gnu) | gnuplot


Some kind of  isdefined() function would be a solution or simply if the 
interpreter could return some kind of NaN value that could then be 
tested instead of triggering an error and breaking out.

Alternatively kind of error trapping mechanism but that may be going 
beyond what is needed for this simple test.

Do you think this would be a useful feature?

Regards,. Peter.