Not always. ;) Specifically, if my_program.sh outputs some 1024 lines then it is still nonzero probability that your behave_differently.sh does not in fact behave differently.

One could try to guarantee unique outputs with

  sleep 1
  date

but the intended requirement of behave_differently is that it produces different output when run on a computer in exactly the same state. Or equivalently, that the program doesn't depend on the computer state, which is true in the more formal setting of universal Turing machines that start from a fixed state.

This does not solve the issue. behave_different.sh has to behave different *all* the time *on every input*. Not just 99.999999% of the time.

Imagine my_program.sh says: echo "2022-02-26 10:30:00" and you run your version of behave_differently.sh at 10:29:59 on the 26/2. Then the output is the same.

If you want something that always behaves differently, why not just concatenating something to whatever the output of the other script would be? I'm familiar with the halting problem, but considering the quick description inside this post I don't see which part of it would forbid this

If my_program.sh runs forever - how will you obtain its output?

I think that's the problem here.

You don't need to wait for it to terminate if you just print something and then invoke my_program.sh

No, there's a 0% chance my implementation outputs 1 as that is only 1 byte long. If you outputted 1023 0s and then a 1 there would be a 2^-1024 chance. That probability is too small to practically happen. The heat death of the universe will happen first.

> The heat death of the universe will happen first.

No, the heat death might be _more likely_ to happen first, but it's not guaranteed.

You're right that my example was flawed. I've updated it. There is however still nonzero probability that ./behave_differently.sh some_peogram.sh behaves the same as ./some_program.sh

Ah, but determinism was implied. Remember, this argument was confirmed considering a Turing Machine.

Also remember that we are considering a bash interpreter which implies we at least have access to $RANDOM. The section behave_differently.sh comes from is only focusing on bash. It does not mention Turing machines.

The thing is, the two instances do not "behave" differently. They only output different values based on the same process. Behaviorally they are identical.

You just brought an engineer to a scientist fight. In theory, what you're saying is wrong ; in practice, it's open to debate.