Question

Why do these have an exit code of 1?

n=1
let --n

And why does this have exit code of 0?

n=1 
let n--

This has exit code of 1 too… why?

n=1
let "n = n - 1"

Answer 1

man bash says:

let arg [arg ...]
          Each arg is an arithmetic expression to be evaluated (see ARITH‐
          METIC EVALUATION above).  If the last arg evaluates  to  0,  let
          returns 1; 0 is returned otherwise.

1 – 1 = 0, therefore the exit code is 1.