But you can do

    foo=0 # true
    bar=1 # false

    [ $foo = 0 ] && [ $bar = 0 ]
    baz=$?
    echo $baz

The syntax is a bit unusual coming from more modern languages, as is the use of 0 as true. But you can express whatever conditionals you want. Remember that C originally had no dedicated boolean type either.

You could also use numerical expressions although I don't recommend it due to being even less readable (even more so if you have expression complex enough that you need to protect against overflow):

    baz=$((foo + bar)) // foo && bar
    baz=$((foo * bar)) // foo || bar

That still doesn't make it look nice, since if you use false / true, you can use variables directly in if checks. For example:

    foo=true

    if $foo; then
      echo yes
    fi

But with 0 / 1 - that won't work. I.e. you can get some, but not all features of a normal boolean type in various ways.

Yeh you'd have to do something like

    foo=true
    bar=false

    $($foo) && $($bar); baz=$?
    case $baz in
    1) echo false;;
    0) echo true;;
    esac

It just ain't worth it.

Yeah, that's very convoluted. It would be useful to have some context for boolean expressions, similarly how $((...)) works for numeric ones.