Re: xbps-triggers:system-accounts: use grep to check for user/group existent

[ahesford <ahesford@users.noreply.github.com>]

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New comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/24754#issuecomment-690300970

Comment:
The more I look at this, the more I see breakage when populating an alternate root. Won't `useradd`, `usermod` and `groupadd` always act on the system root? These commands provide a `--root` option that seems to redirect their actions in the way we want, but we aren't using that option now. It's also not clear from their man pages how well these commands get along with directory services.

Ignoring `system-accounts` unless `$ROOTDIR = /` like @ericonr suggests is a reasonable way to avoid the worst of these issues. If you go that route, the hook should at least print all of the users and groups that should exist (along with ID numbers, if provided) for the package to function as expected.

If we really want to do this the "right" way, it's not obvious to me that the existence tests add value. For groups, just `groupadd --root ...` and catch the return code to know the difference between a real failure and an attempt to create a group that already exists. For users, just `useradd --root ...` and, if the return code indicates that the username already exists, try `usermod --root ...` instead.