Verifying Variable Existence

Sometimes you might want to create an expression that checks only for the existence of a variable in the pipeline or checks to see whether a variable is null. The following table describes the syntax used to check the pipeline for variable existence.

To check if...	Use this syntax...	Description
Variable exists	variableName	Evaluates to true if the specified variable exists in the pipeline and has a non-null value.
Variable does not exist	!variableName	Evaluates to true if the specified variable does not exist in the pipeline or is null.