The pitfalls documented here show that it's really, really hard to write portable shell programs. You might as well not try at all.

Also, I find it funny that they say

> ... to prevent trouble, do not take advantage of features that were added after Unix version 7, circa 1977.

And then they go on to call some systems ancient, not worth considering for portability.

> A few ancient 4.2BSD based systems ... required <some obscure behaviour>. However, these ancient systems are no longer of practical concern.

I feel sorry for the developers (including the Postgres community, which I count myself part of), have to deal with tooling (Autoconf, and family) that continues to prioritize supporting 45+ years (almost half a century!!) old systems. I am sure that the sysadmins of those old systems do not care about running the new software we're developing today; they have some reasons to keep those systems running, primarily monetary reasons.