The How and Why of User Private Groups in Unix (opens in new tab)

i came across this exactly as the author did, as i’m sure very many others did. he doesn’t say if the RH manual describes the reason for it, or just names it.

i had to learn the reason for myself experimentally just as the author apparently did, so i think the RH manual was probably lacking.

i do find it clever but also arcane. perfect greybeard material

tldr: It's part of a solution to make adminstering shared machines (machines with more than one user) easier by using the setgid bit and a umask that allows rw for groups.

Interesting, I've occasionally wondered why UPGs are a thing, but never enough to research it. Like so many things nowadays, I take a Chesterton's Fence approach and assume things are that way because at some point it became a best practice for some reason.