It is easier to understand the POSIX standard with a ksh focus, particularly ksh88.
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V...
The ksh family is my personal favorite, in particular mksh and ksh93. Both with excelent feature sets. I even made a platformer game that works on them https://github.com/alganet/tuish/blob/main/examples/game.sh (and zsh/bash/busybox too) to show how feature-complete they are.
Shell interpreters are such a broad subject, I could go all day talking about cool things that can be done with them.
I want to get people focusing less on the spec. It's for whoever implements interpreters, not people who write scripts. And there's a gap on that, which I'm trying to cover (with full ksh support, much more than you think).
After David Korn retired, an attempt was made to bring out a new version which failed dramatically, and AT&T reverted to Korn's final release.
SmartOS uses ksh93 as root's shell.
Last commit was 11 hours ago, still maintained.
https://www.reddit.com/r/openbsd/comments/jvbj4s/a_new_appro...
There were core performance problems in the 2020 effort.
It's what Debian and Fedora use, so AT&T doesn't matter anymore. Real-world upstream is the repo I linked, which is maintained. All the 2020 drama is now obsolete.