Explain BASH code line let RETVAL=$((RETVAL|$?))

I'm looking for some explanation of the below line of BASH from a shell script. I'm generally familiar with all the individual builtins and the use of $(( )). I'm a bit confused about what the bitwise OR operator is comparing in this line.

let RETVAL=$((RETVAL|$?))

Inside double parentheses, the expression is evaluated not as a command, but as an arithmetic expression.

Consequently, this is not a pipe operator but a bitwise or operator.

In this case, it will set any bits in the variable RETVAL which are set in the result of the last statement (as contained in the special variable $?).

It's possible that this is just abusing bitwise or as a logical operator, since the result will be zero (generally interpreted as success / true) only if both the previous value of RETVAL and $? are zero. In other words, repeatedly using this after multiple commands gives you a zero result only if they all returned zero.

This generally means someone is ORing together the exit status of several different operations to come up with a combined exit status.

let is an antiquated way of entering an arithmetic context; (( )) and $(( )) are the modern alternatives; the code below is thus going to prefer the more modern bash-only syntax ($(( )) is both modern and portable but makes you responsible for ensuring that the value it evaluates to has no unintended side effects, whereas (( )) is modern but not portable, and let is neither portable nor modern).

Let's say that you want to always run functions foo1, foo2, and foo3, and report whether any of them failed:

main() {
  local retval=0 # local is the only non-POSIX thing in this example
  foo1 || : $(( retval |= $? ))
  foo2 || : $(( retval |= $? ))
  foo3 || : $(( retval |= $? ))
  return "$retval"
}

...will return an exit status that determines whether any of foo1, foo2, or foo3 failed; and if their exit status works as a bitmask, will set high bits to indicate which failure modes took place.

Using || instead of ; is done to make it explicit that the exit status of the commands on the left-hand side are "checked", so set -e or ERR traps will not trigger should they be nonzero.

Using lowercase variable names is done for compliance with the conventions specified in https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html, whereby all-caps names are used for variables meaningful to POSIX-specified tools, whereas names with at least one lower-case character are reserved for application use and guaranteed not to have unintended side effects on POSIX-specified tools. (Because setting a regular shell variable overwrites any like-named environment variable, the same conventions apply to both types).