I think part of the point may be to select for people who don't consider "being interrogated" to be a contest. There are plenty of folks who are just like "Here's what I know, here's what I don't know, you decide whether you would want me on your team. If you don't, that's fine, I'll find another team to join." Then there are other folks who, when they're asked a question and don't know the answer, feel like their self-worth is under attack. The latter group can be really hard to work with, and can do a lot of damage to a team.

I had a PM friend who would try to push the bounds of a candidate's knowledge until they gave up and said "I don't know". The actual knowledge wasn't the point of the question: it was that they could say "I don't know", because PMs who can't tend to make life miserable for the engineers & other teammates who work with them.

Yes, exactly. I think to be a good developer, you have to be willing to keep learning. And to be a good developer in a team context, you have to be willing to learn from your teammates, ask for help, and share knowledge in a way that's supportive, not prideful.

And I agree with your PM friend; I think willingness to admit ignorance is even more valuable there.

If somebody goes into an interview expecting never to be asked a question they might struggle with, I'm not sure they're somebody I want to hire.

But no, I don't think they walk away feeling interrogated. My style is pretty conversational, and when people don't have all the answers, I definitely work to make them feel comfortable with that. E.g., for this question I'd close with something like, "Of course nobody knows all the arguments to ls. I sure don't. So you did fine."