[Help-bash] How to tell if an environment variable exists or not

[Bill Gradwohl]

Assuming set -u, how does one test to see if an environment variable exists without creating it in the process? Specifically, if the variable is only unset, not null. I'm aware of :=, but that creates the variable if it didn't exist before. At the end of the test, the variables existence should be unchanged from before the test.

Thank you

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Bill Gradwohl