Are べき and べきだ one and same thing?

I found many grammars where べき is defined, but examples with べきだ are provided.

知るべき。

両親には従うべきだ。

Or is だ a copula? But then why do we use a copula in such case?
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2021-01-24T14:01:13.757
83871
2021-01-24T14:01:13.757
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question
particles copula modality
What is the difference between べき and べきだ