I had the misfortune of having to maintain an 18'000 line bash 3 script. Array handling broke in many subtle ways between bash 3 and 4.

> If your scripts do that much work that your shell's speed matters, you should reconsider writing shell scripts and start thinking about using something like perl, python or ruby.

Usually yes.

But there is a small subset of use cases which need a really portable solution. Once I would have recommended to just write it in perl, because it was on virtually all systems. But now perl is getting phased out on some OSs/distros, and with python you never know if you get 2 or 3. Or embedded systems which don't have python/perl in the first place.

My go to solution for slow scripts is usually to limit subshells/forks as much as possible, and/or run it with busybox with the "exec prefers applets" option

edit: it's actually the "run 'nofork' applets directly" option. Can give quite a speed boost compared to bash if you have to call "external" utilities like grep/head/etc.