Use the eval command. The eval command is designed to allow execution of a string that is built at runtime.

For example:

switch($timeOfDay) {

 case "morning":

 $shape = "circle";

 break;

 case "afternoon":

 $shape = "sphere";

 break;

 case "evening":

 $shape = "cone";

 break;

 default:

 $shape = "cylinder";

}

eval $shape -r 5; // create specified shape with radius 5.

Alternatively, you could use eval with function syntax:

eval ($shape+" -r 5");

You can also use the evalEcho and evalDeferred commands.